Preparation of flood risk and vulnerability map final report ktm sept_17
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Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley

Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley

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Preparation of flood risk and vulnerability map final report ktm sept_17 Preparation of flood risk and vulnerability map final report ktm sept_17 Document Transcript

  • Government of Nepal Ministry of Water Resources Department of Water Induced Disaster Prevention Pulchowk, Lalitpur, Nepal FINAL REPORT THE PREPARATION OF FLOOD RISK AND VULNERABILITY MAP OF THE KATHMANDU VALLEY May 2009 Submitted by the Joint Venture of: FBC Full Bright Consultancy (Pvt.) Ltd. GEO Consult (P.) Ltd. P. O. Box 4970, Maitidevi and Sankhamul, Kathmandu Kathmandu, Nepal Tel: 47 82758 Tel: 44 33149 and 44 11780 E-mail: info@geoconsultnepal.com Fax: ++ 977-1-44 13331 Homepage: www.geoconsultnepal.com E-mail: fbc@mos.com.np
  • EXECUTIVE SUMMARY The assigned work entitled “Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley” is carried out by the joint venture of Full Bright Consultancy (Pvt.) Ltd. and Geo Consult (P) Ltd. based on the scope of works. The study area is situated within the Kathmandu Valley between latitude 27°32' 00" N to 27°49'16" N and longitude 85°13'28" E to 85°31'53" E. The area has covered all the major sub-basins of Bagmati River Basin, namely Bishnumati, Hanumante, Manohara, Dhobi Khola, Balkhu, Nakhu, Kodku, Kalimati Khola (Manamati Khola) and Samakhushi Khola Sub-basins. These rivers flow towards valley center to join with the Bagmati River that eventually drains out of the study area through the Chovar gorge. Main objective of the study is to prepare the flood risk and vulnerability map of the Kathmandu valley by utilizing satellite image and GIS. Geomorphologically, the Kathmandu Valley and its surrounding mountainous parts can be subdivided into hill and hill slopes, rocky outcrop, terraces, floodplain and riverbed. Most of the rivers originate (except Samakhusi) at the hilly areas and traverses through the terraces developing flood plain. Geologically the Kathmandu valley and its surrounding area can be subdivided into two major geological units as: basement rocks having the rocky areas and the overlying basin-fill sediments. To prepare preliminary flood maps according to scope of work the data on water-induced disaster, flood risk and vulnerability of Kathmandu valley were acquired from aerial photographs (1978, 1985, and 1992), recent ALOS image (spatial resolution of 2.5 m), QuickBird image (0.6 m spatial resolution), all available hardcopy and digital maps like topographical, geological, and engineering geological of the Kathmandu valley, available relevant literature and reports on in the internet, local libraries, government institutions (like Ministry of Home Affairs, Department of Mines & Geology, DWIDP, DHM etc.), ICIMOD, Nepal Red Cross Society and NGOs etc. Similarly the Consultant had also collected the precipitation/rainfall data from DHM as recorded at different rain gauge stations within the Kathmandu Valley and its adjacent areas, and the hydrological data of the rivers i.e. stream flow summary under present study as far as their availability. All the data were spatially linked for analysis purpose in the GIS environment. Once the DEM was generated then the river network was generated from the DEM in the GIS environment. The HEC GeoHMS was used to evaluate the raster data of the DEM. The stream network data were then overlaid with satellite images and were verified the accuracy of the network that is really in the filed and that generated. Based on the calculation made from the rainfall-runoff model and other empirical methods, the discharges of the rivers under consideration for various return periods have been obtained at the desired points and locations. The computed discharge has been used as input parameter while running the GIS-based flood model using HEC-RAS and with geometric data generated from HEC GeORAS to prepare preliminary hazard map. The hydrological analysis is carried out based on the empirical formula and observed data. The discharge data of the Bagmati River at Chovar gorge is used, which is available for the period between 1963 and 1980. This observed data has been used to estimate the Bagmati River discharge at various return i
  • periods. The values obtained from the frequency analysis using the observed data has been compared with the one obtained from the empirical methods. As the other rivers are ungauged, the regional and empirical formulae have been adopted in estimating the flood magnitudes. The flood estimation at various locations for the return periods has been calculated by adapting the best fit method i.e. Snyder’s method, which resembles close to the observed data for the Kathmandu Valley. Primary data relating to past flood events and its disasters in terms of loss of property and lives has been collected as far as possible from direct field survey by mobilizing a team comprising by Team Leader/Water Resources Engineer, Engineering Geologist, GIS modelling Export, Remote sensing Export, Hydrologist, Socio-economist, Environmentalist, Surveyor with Technical Assistants. The field study team verified and checked all the data collected during the desk/inception phase and the result of preliminary flood hazard map. The verification had been made in consultation with local people, walkthrough along the river and other areas of influence as required and socio-economic data were also collected. Field based flood hazard map has prepared based on historical data, other field data like bankfull stage, bank failure, existing protective measures, sites of sand mining from the river channel and its surroundings, former and new channel courses, area of river bank encroachment, property loss etc., that gathered on the hard copy of the QuickBird image at 1:2000 scales during the field survey of the rivers in the Kathmandu Valley. Benchmark survey of the bridges as mentioned in the scope of work is carried out to determine the level of the bridges during the period of flooding. Based on the all primary, secondary and hydrological model runs and their outputs, the Consultant has prepared the flood-inundation maps for all of the rivers under study from the combination task of HEC-RAS, HEC-GeoRAS and Arc View GIS software. The discharge obtained from Syder’s method for the return period of 50 years has been used for the computation of inundation map. We have developed hazard classification in term of flood depth covering flooded area minimum up to less than of 1 m depth to that of the total flooded area for particular flood return period. Thus, the flood depths are classified as: up to 1 m, 1-3 m, and above 3 m, which are used respectively, for distinguish low, moderate and high hazard levels. Certain causative factors of flood hazard that based on field study are determined for rating and also weighting according to their significance by certain values to get hazard level for the field based hazard map. Thus flood hazard maps based on field have been prepared and the model based flood hazard maps have been validated as well as upgraded from the field observation. According to the result the areas around Gokarneshwar and Guheswori along the Bagmati River is having moderate flood hazard, while the downstream from the confluence of Manohara and Bagmati, the main water course increases to attain deep water level. Hence the small area in this river stretch falls within the moderate to high hazard level. It is consistent with the field based hazard map, although the return period of the field based hazard could be smaller than the model based. The high hazard area increases to the downstream from the Bishnumati confluence, where the aerial coverage of deep water depth increases e.g. near Nakhu dovan. The upper reach of Manohara River is dominated by low flood hazard, however, aerial coverage of the area is higher then in the downstream area. Moderately hazardous area gradually increases towards downstream and the area near the confluence ii
  • of Manohara and Kodku falls remarkably in high hazard level. The area is also consistent with field condition. For the case of the upper reaches of the Bishnumati River, dominant area lies in shallow water depth with little area having moderate water depth. To the lower reaches of the Bishnumati River, moderate water depth gradually increases towards downstream and moderate hazard level covers higher area than its upstream. Distribution of moderate hazard level in the little area to upstream from the Ring Road bridge at Chabahil and in the area around the Anamnagar of Dhobi Khola is consistent with the field observation. Deeper water level is also found in the upstream area of Godavari Khola around the Dharmeshwar village, Gwarko area of Kodku Khola, upstream of Nakhu Dovan in Nakhu Khola and Balkhu area of Balkhu Khola indicating the high flood hazard probability. Most of the area along the river corridors of the rivers are covered by low hazard zone, however, parts of the river corridors with small area fall in moderate hazard zone e.g. in Thimi area of Hanumante Khola, Kalimati area of Karakhusi Khola etc. Major concrete bridges located over these rivers e.g. at Gongabu and Balaju of Bishnumati River, at Sinamangal and Subidhanagar of Bagmati River, etc. are highly vulnerable condition because of deep exposure of foundation of the bridge piers due to intense river bed incision. The cross-sections obtained from benchmark survey across the river at the bridge foundation, in contrast, depict the discharge level for the return period of 50 years flood at lower depth than the bridge height, which indicate no hazardous condition during inundation. To prepare vulnerability map of the river corridors, the assessments are concentrated on the following elements that can have direct impact of the hazard: • Infrastructure: bridge, road and buildings • Land use: settlement area, agriculture area and barren area, and • Population: above 65 and children, female and male The ranking value at a range of 0 to 1 is allocated for the factor class at risk in accordance with their importance. The Flood Risk and Vulnerability Map at the scale of 1:10,000 for the catchment and 1:5,000 for the key locations of the rivers in the Kathmandu valley depending on the scope of the study have been prepared. The vulnerability map shows that about 10%, 33% and 57% of the surveyed area (15262200 sq. m) is occupied, respectively by high, medium and low vulnerable area. High vulnerable area are situated near Jorpati, Gaurighat, Tilganga, Subidhanagar, Thapathali, Teku confluence and Balkhu confluence of Bagmati River. Likewise, the high vulnerable area is found near the confluence of Manohara and Hanumante rivers, Katubahal and Anamnagar of Dhobi Khola, around the confluence of Mahadev Khola and Bishnumati, Mhaipi and Khusibu of Bishnumati River, Kalanki-Bulkhu area of Balkhu Khola. The concrete bridges located at Tilganga, Sinamangal, Subidhanagar of Bagmati River, bridge at Jadibuti of Manohara River, at Bijulibazar of Dhobi Khola and bridges at Gongabu and Balaju are highly vulnerable because of extreme exposure of piers foundation due to intense river incision. However, these bridges are situated at higher level from the water level during the flood of 50 years return period. Low vulnerable areas are noted at upstream from Gothatar of Manohara River, upstream from Jorpati of Bagmati River, upstream from Katubahal of Dhobi iii
  • Khola, upstream of Mahadev and Bishnumati rivers, upstream from Kalanki of Balkhu Khola, and the river corridors of Nakhu and Godavari. Wide coverage of moderate vulnerable areas is noted in Guheswori-Gokarna segment and area around Sankhamul of Bagmati River, river sector between Magargaon to Chhapro of Manohara River, around Maitidevi in the Dhobi Khola sector, around Balaju, Banasthali and downstream from Dhalko of Bishnumati and Thimi to Lohakinthali of Hanumante Khola. Normally agriculture areas are the dominant element in the zone of low and moderate vulnerability. The risk map shows about 90%, 6% and 4% of the total surveyed area (14945875 sq m), respectively, are located in the low, moderate and high risk level. The high risk area are situated at Thapathali, around Bishnumati and Balkhu confluences along the river corridor of Bagmati River, small strip at downstream from confluence of Manohara and Hanumante, downstream from confluence of Karakhusi and Bisnhumati, while the moderately risk areas occur at Gaurighat, Tilganga, Shankhmul, Thapathali along the sector of Bagmati River, around and downstream from confluence of Manohara and Hanumante, around Anamnagar of Dhobi Khola, downstream of Mhaipi and around Khusibu of Bishnumati, around Balkhu of Balkhu Khola and Thimi to the confluence of Godavari Khola along the Hanumante. Snyder’s method has been used for the computation of design discharge and for proposing the waterway and hydraulic modelling. Since the natural waterway of the river should be left undisturbed with the hydraulic consideration for the river to allow its flood to pass unhindered with ample space for the meandering as well, different waterway with respect to the position of the river in the anticipated floods at 50 years return period has been proposed. This indicates that if the flood of 50 years return period occurs, the area behind the respective waterway will get flooded. The waterway in the case of 50 years flood for the Manohara, Bagmati, Dhobi Khola, Bishnumati, Mahadev Khola, Karakhusi, Balkhu, Nakhu, Kodku, Godavari and Hanumante are assigned, respectively as 42-130, 51-205, 31, 28-83, 16-31, 30, 50, 56, 34, 13-51, and 78 m. Under this scenario construction of engineering structure for the protection of concrete bridges in several places is most essential. As the urbanization in the Kathmandu valley is rapidly increasing, route for access from the hazard zone to safer zone has been increasing its important although such access routes have generally small width. These routes are marked in the map for rescue route during the flooding events. The major road like Ring Road can be used for other facilities during the flood calamities. Nearest school buildings of the possible flooding sites are marked in the map with levels of flood hazard for the use of temporary shelter. iv
  • ACKNOWLEDGEMENT We are thankful to the Department of Water Induced Disaster Prevention for giving us an opportunity to undertake the study entitled "Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley". We sincerely express our gratitude to Mr. Mahendra Gurung, Director General, on entrusting the job and cooperation extended to us during the study period. We gratefully acknowledge the support, cooperation and encouragement provided by Mr. Basistha Raj Adhikari, Senior Divisional Engineer and Mr. Shreekamal Dwibedi, Engineering Geologist during various stages of the study. Furthermore, we are sincerely thankful to the people residing along the studied river corridors for extending the support and providing valuable information during the field investigation. Full Bright - Geo Consult JV Kathmandu, Nepal May, 2009 v
  • TABLE OF CONTENTS EXECUTIVE SUMMARY ......................................................................................................... I  LIST OF FIGURES............................................................................................................... VIII  LIST OF TABLES .................................................................................................................. IX  1.  INTRODUCTION.............................................................................................................. 1  1.1  GENERAL .................................................................................................................    1 1.2  OBJECTIVES OF THE STUDY ...........................................................................................    1 1.3  SCOPE OF WORKS ......................................................................................................    1 1.4  STUDY AREA .............................................................................................................    2 1.4.1  The Kathmandu Valley ................................................................................... 2  1.4.2  Flooding Situation in Kathmandu Valley......................................................... 3  1.5  REVIEW OF RELEVANT LITERATURES ...............................................................................    3 1.5.1  Past Studies within Kathmandu Valley Related to the Present Study ............ 5  1.5.2  Satellite Images, Aerial Photographs and Maps ............................................ 6  1.5.3  Geomorphology and Geology of Kathmandu Valley ...................................... 7  2.  METHODOLOGY........................................................................................................... 10  2.1  PRE‐FIELD STUDY .................................................................................................... 10  2.2  FIELD WORK AND OTHER ACTIVITIES  ............................................................................ 15  . 2.3  POST FIELD ACTIVITIES .............................................................................................. 18  3.  DEM PREPARATION FROM SATELLITE IMAGE ....................................................... 22  3.1  THE ALOS STEREO‐PAIR IMAGE .................................................................................. 22  3.2  DEM PREPARATION AND FLOOD HAZARD MAPPING ....................................................... 22  3.3  UPDATING THE DEM ALONG THE RIVER CORRIDOR  ......................................................... 22  . 4.  SOCIO-ECONOMIC CONDITION ................................................................................. 24  4.1  BAGMATI RIVER ...................................................................................................... 25  4.2  BISHNUMATI RIVER .................................................................................................. 28  4.3  MANOHARA RIVER .................................................................................................. 30  4.4  HANUMANTE KHOLA ................................................................................................ 32  4.5  GODAVARI RIVER..................................................................................................... 33  4.6  DHOBI KHOLA ......................................................................................................... 33  4.7  SANGLA KHOLA ....................................................................................................... 34  4.8  SAMAKHUSI KHOLA .................................................................................................. 35  4.9  BALKHU KHOLA ....................................................................................................... 35  4.10  KARAKHUSI KHOLA .................................................................................................. 36  4.11  NAKHU KHOLA ........................................................................................................ 36  4.12  KODKU KHOLA ........................................................................................................ 38  5.  FIELD CONDITION ALONG RIVER CORRIDORS....................................................... 40  5.1  BAGMATI RIVER ...................................................................................................... 41  5.2  MANOHARA RIVER .................................................................................................. 44  5.3  BISHNUMATI RIVER .................................................................................................. 51  5.4  HANUMANTE KHOLA ................................................................................................ 54  5.5  DHOBI KHOLA ......................................................................................................... 56  5.6  SAMAKHUSI KHOLA .................................................................................................. 58  5.7  BALKHU KHOLA ....................................................................................................... 58  vi
  • 5.8  KARA KHUSHI KHOLA (MANAMATI KHOLA) ................................................................... 62  5.9  NAKHU KHOLA ........................................................................................................ 62  5.10  KODKU KHOLA ........................................................................................................ 64  5.11  GODAVARI KHOLA ................................................................................................... 67  5.12  WATER LEVEL AT CONCRETE BRIDGES FROM BENCHMARK SURVEY ....................................... 69  6.  HAZARD, VULNERABILITY AND RISK ASSESSMENT ............................................. 70  6.1  FIELD BASED HAZARD ASSESSMENT CRITERIA  ................................................................. 70  . 6.2  FIELD BASED FLOOD HAZARD MAP............................................................................... 74  6.3  FIELD BASED VULNERABILITY ASSESSMENT CRITERIA ........................................................ 75  6.4  FIELD BASED FLOOD VULNERABILITY MAP ..................................................................... 79  6.5  FIELD BASED RISK ASSESSMENT CRITERIA ...................................................................... 80  6.6  FIELD BASED FLOOD RISK MAP ................................................................................... 80  6.7  RAINFALL RUNOFF AND HYDROLOGICAL MODEL BASED FLOOD HAZARD MAP  ....................... 80  . 6.8  MODEL BASED FLOOD VULNERABILITY MAP .................................................................. 87  6.9  MODEL BASED FLOOD RISK MAP ................................................................................ 87  6.10  MINIMUM WIDTH REQUIRED FOR NATURAL WATERWAY OF RIVERS ................................... 88  6.11  VULNERABILITY AND RISK OF MAJOR CONCRETE BRIDGES.................................................. 89  6.12  RESCUE ROUTE........................................................................................................ 90  7.  LIMITATION OF THE PRESENT STUDY ..................................................................... 91  8.  CONCLUSIONS AND RECOMMENDATIONS ............................................................. 92  8.1  CONCLUSIONS ......................................................................................................... 92  8.2  RECOMMENDATIONS ................................................................................................ 93  REFERENCES ...................................................................................................................... 95  ANNEX Annex-1: Field Photographs Annex-2: Field Based Flood Hazard, Vulnerability and Risk Map Annex-3: Model Based Flood Hazard, Vulnerability and Risk Map Annex-4: Sample Rescue Route Map for Flood Risk Area vii
  • LIST OF FIGURES Figure 1-1: Bagmati River catchment with its major tributaries in the Kathmandu Valley ....... 3  Figure 1-2: Geomorphological map of the Kathmandu Valley................................................. 8  Figure 1-3: Geological section in north south direction passing through the Kathmandu Valley (adopted after Sakai et al., 2002) (S=Siwalik group, B=Bhimphedi group, P=Phulchauki group, N=Nuwakot group, G=Granite, Gn=Gneiss and Granite ............... 9  Figure 1-4: Geological map of the Kathmandu Valley (adopted after Sakai, 2001) ................ 9  Figure 2-1: Flow chart of the study ........................................................................................ 11  Figure 2-2: Application of HEC-RAS and HEC-GeoRAS ...................................................... 21  Figure 3-1: Surface generated from the DEM ....................................................................... 23  Figure 4-1: Location of Social Survey ................................................................................... 24  Figure 5-1: River shifting and encroachment within the Kathmandu Valley (Alos satellite image in the background) .............................................................................................. 41  Figure 5-2: Manohara River shifting between Godar and Gothatar area. Blue line shows the river course as of DoS Topomap and the present river course can be observed on the satellite image in the background. ................................................................................. 45  Figure 5-3: Manohara river shifting around Phuyalgau-Changunarayan area. Present river course can be seen on the satellite image (Alos) while the blue area represents the river course as of topographical map of DoS. ........................................................................ 46  Figure 5-4: Manohara river shifting around KLhulatar Bramhakhel area. Present river course can be seen on the satellite image (Alos) while the blue area represents the river course as of topographical map of DoS. .................................................................................... 46  Figure 5-5: Balkhu River course on topographical map (blue area) and on Alos satellite image between Balkhu Chowk and Kalanki) The river course is not so clear in this image. ............................................................................................................................ 59  Figure 5-6: Balkhu River course on topographical map (blue area) and on Quickbird satellite image between Balkhu Chowk and Kalanki) The river course is distinct in this image showing the impact of human activities. ........................................................................ 60  Figure 5-7: Flash flood of 23 July, 2002 in Balkhu River near Kalanki (Photograph adopted from Kantipur Publication, 2002) .................................................................................... 61  Figure 5-8: Kodku River confined to narrow channel due to River training structures (Information is not clear on Alos image). ....................................................................... 65  Figure 5-9: Kodku River confined to narrow channel due to River training structures (Information is clear on Quickbird image). ..................................................................... 65  Figure 5-10: Comparison of the information obtained to study the impact of human encroachment as well as natural process on the Kodku River through satellite images of viii
  • different resolution (Alos on the right and Quickbird on the left).The blue area is river course as of topographical map. .................................................................................... 66  Figure 6-1: Flow chart showing the collection of field level information for hazard mapping. 73  Figure 6-2: Flow Ward-wise population distribution of females (data from NPC, 2001 and present study). ............................................................................................................... 77  Figure 6-3: Ward-wise population distribution of male (data from NPC, 2001 and present study). ............................................................................................................................ 78  Figure 6-4: Ward-wise population distribution (data from NPC, 2001 and present study) .... 78  Figure 6-5: Cross-section lines on the studied rivers as part of data preparation for HEC- RAS................................................................................................................................ 85  Figure 6-6: model based flood hazard map of the studied rivers (50 yr. return period). ....... 86  LIST OF TABLES Table 1-1: List of DoS topo-sheet used in the study. .............................................................. 7  Table 2-1: Location of rainfall station within the Kathmandu valley with their geographic characteristics ................................................................................................................ 12  Table 2-2: Location of hydrometric stations within the Kathmandu valley with their geographic characteristics ............................................................................................. 13  Table 4-1: Bagmati River (Flood hazard data) ...................................................................... 26  Table 4-2: Bagmati River (Demographic features) ................................................................ 26  Table 4-3: Bagmati River (Awareness level and institutional framework) ............................. 26  Table 4-4: Bishnumati River (Flood hazard data).................................................................. 28  Table 4-5: Bishnumati River (Demographic features) ........................................................... 28  Table 4-6: Bishnumati River (Awareness level and institutional framework)......................... 29  Table 4-7: Manohara River (Flood hazard data) ................................................................... 30  Table 4-8: Manohara River (Demographic features) ............................................................. 31  Table 4-9: Manohara River (Awareness level and institutional framework) .......................... 31  Table 4-10: Hanumante River (Flood hazard data) ............................................................... 32  Table 4-11: Hanumante River (Demographic features) ........................................................ 33  Table 4-12: Hanumante River (Awareness level and institutional framework) ...................... 33  Table 4-13: Godavari Khola (Flood Hazard data) ................................................................ 33  Table 4-14: Godavari Khola (Demographic features)........................................................... 33  Table 4-15: Godawori Khola (Awareness level and institutional framework) ........................ 33  Table 4-16: Dhobi Khola (Flood hazard data) ...................................................................... 34  Table 4-17: Dhobi Khola (Demographic features) ................................................................. 34  Table 4-18: Dhobi Khola (Awareness level and institutional framework) .............................. 34  ix
  • Table 4-19: Samakhusi Khola (Flood Hazard data) .............................................................. 35  Table 4-20: Samakhusi Khola (Demographic features) ........................................................ 35  Table 4-21: Samakhusi Khola (Awareness level and institutional framework) ...................... 35  Table 4-22: Balkhu Khola (Flood hazard data)...................................................................... 35  Table 4-23: Balkhu Khola (Demographic features) ............................................................... 35  Table 4-24: Balkhu Khola (Awareness level and institutional framework) ............................. 36  Table 4-25: Karakhusi Khola (Flood hazard data)................................................................. 36  Table 4-26: Karakhusi Khola (Demographic features) .......................................................... 36  Table 4-27: Karakhusi khola (Awareness level and institutional framework) ...................... 36  Table 4-28: Nakhu Khola (Flood hazard data) ...................................................................... 37  Table 4-29: Nakhu Kkhola (Demographic features) .............................................................. 37  Table 4-30: Nakhu Khola (Awareness level and institutional framework) ............................. 37  Table 4-31: Kodku Khola (Flood Hazard data) ...................................................................... 38  Table 4-32: Kodku Khola (Demographic features) ................................................................ 38  Table 4-33: Kodku Khola (Awareness level and institutional framework) ............................. 38  Table 6-1: Ranking value for river morphology and bank instability ...................................... 72  Table 6-2: Ranking value for bankfull level and relief of surrounding area ........................... 72  Table 6-3: Ranking value for infrastructure and buildings ..................................................... 73  Table 6-4: Ranking vulnerability value for infrastructure and buildings ................................. 76  Table 6-5: Ranking vulnerability value for land use element class........................................ 76  Table 6-6: Ranking vulnerability value for population element class..................................... 77  Table 6-7: Values for Standard Normal Variate for various return periods ........................... 81  Table 6-8: Measured discharge data of Bagmati at Chovar .................................................. 82  Table 6-9: Flood flow estimation by various methods ........................................................... 82  Table 6-10: Calculation of extreme rainfall for different return period ................................... 83  Table 6-11: Reach-wise discharge distribution for various return periods ............................ 83  Table 6-12: Waterway for different return periods at various locations of the rivers. ............ 89  x
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 1. INTRODUCTION 1.1 GENERAL In accordance with the agreement signed on June 6, 2008 between Full Bright Consultancy (Pvt.) Ltd. and Geo Consult (Pvt.) Ltd. JV (Consultant) and Department of Water Induced Disaster Prevention, this Final Report has been prepared and presented for “Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley” based on the scope of works as stated in the Terms of Reference (ToR). The Final Report includes the details of the output obtained from the analysis of the data collected from field, literature and other through interpretation of satellite images. In addition, the outputs of the GIS based analysis of such data are also incorporated in the present report. The comments on the Draft Report from the side of client are also incorporated in this report. 1.2 OBJECTIVES OF THE STUDY The overall general objective of the study is to prepare the flood risk and vulnerability map of the Kathmandu valley. While the specific objective is to utilize the state-of-the-art technology in the field of satellite image processing and GIS and carry out GIS-based impact and vulnerability assessment of the Kathmandu valley. 1.3 SCOPE OF WORKS In order to fulfil the objectives of the study the scope of works, which is outlined in the Terms of Reference (ToR) are as mentioned below: • Study and data collection of the water-induced disaster susceptible areas. • Use aerial photo, satellite imageries and topographic maps to extract relevant data. The satellite image to be acquired within past six months having the spatial resolution of 2.5 meter or less. • Conduct benchmark survey to determine the level of all the bridges in Ring-Road and other critical locations within the city area of the valley. • Use GIS to analyse and prepare flood maps using both the rainfall runoff model and hydrological model. The work includes the following designated areas: • The rivers under the proposed consulting job shall be all the rivers within the Kathmandu valley upstream from Chobhar gorge. All the sub-basins of the primary tributary of Bagmati River shall be delineated. • DEM should be generated from the stereo-pair image. The data shall be used to assess the flood and inundation. • Data acquired during previous studies by the Department should also be utilized in this study. 1
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report • The vulnerability assessment of the infrastructure and settlement should be carried out in the field at specific vulnerable communities as identified during the pre-field study (scale 1:10,000). While carrying out the vulnerability assessment, the population should be divided into three groups: Male, Female, above 65 and children. Likewise, awareness level and the rescue routes and availability of evacuation centre should be shown on the map. • Propose the minimum required width of all the rivers under natural condition inside the urban and sub-urban areas. • Each map should clearly indicate the existing settlements and infrastructure under threat delineating the administrative boundaries. 1.4 STUDY AREA 1.4.1 The Kathmandu Valley Kathmandu Valley is situated between latitude 27° 32' 00" N to 27° 49'16" N and longitude 85°13'28" E to 85°31'53" E. The study area is urban centre and the surrounding area in the Kathmandu valley which covers the parts of the areas of Kathmandu, Lalitpur and Bhaktapur districts of Central Development Region of Nepal. The study area shall cover all the major sub-basins of Bagmati River Basin, namely Bishnumati, Hanumante, Manohara, Dhobi Khola, Balkhu, Kodku, Kalimati Khola (Manamati Khola) and Samakhushi Khola Sub-basins etc. The Kathmandu Valley is an intermountain valley (Figure 1-1) that occupies an approximate catchment area of 625 km2. The major rivers traversing the valley are Bagmati, Bishnumati and Manohara. Apart from these major rivers, other rivers like Dhobi Khola, Tukucha, Samakhusi, Kodku (Karmanasa), Balkhu, Nakhu, Mahadev, Hanumante are also flowing through the valley. The drainage pattern inside the Kathmandu valley is such that all the rivers flow towards valley center to join with Bagmati River that eventually drains out of the study area through the Chovar gorge. The average annual precipitation in the valley is around 1600 mm of rain. The rainwater is drained through a number of rivers, streams and rivulets that discharges to the Bagmati River. The occasional torrential rains within the valley have caused flooding problems in the core areas of the city causing loss of life, and damage to private properties, especially to the areas in close proximity to the rivers. The floods and inundation problem along the river banks is common during high intensity and long duration precipitation. The fear of damage due to flood have also been prominent because of rapid growth of settlement near the river floodplains and encroachment of the floodplain of rivers. The flood situation is severe especially due to excessive deforestation in the hilly areas around the valley hills and also because of the natural climatic changes. In addition, in the urban areas, the decreased infiltration capacity of soil due to surface sealing results in the increased rainwater drainage into nearby rivers and streams. Thus the flood situation within Kathmandu Valley is not friendly to the urban dwellers and there is a need to take steps for the protections of river regimes to control the effects due to rivers flooding. 2
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 1.4.2 Flooding Situation in Kathmandu Valley The valley contains three main historical cities: Kathmandu, Lalitpur and Bhaktapur. Kathmandu valley is a fast growing city with a population of approximately 1.8 million people (CBS, 2001). With the rapid urban expansion in the Kathmandu Valley, the settlements have expanded even along the banks of rivers and streams, because of scarcity of residential lands. This has also affected the natural path of river by altering the river channels and diverting the flow. Though there seems to be nonexistent of the flooding problem during the low flow period in pre-monsoon season, the flooding situation in the monsoon period has threatened the lives and properties. Therefore, there is an urgent need to carry out a detailed study and prepare the flood risk and vulnerability map of the Kathmandu Valley. Such study is extremely needed in order to manage the present as well as the future growth of the Kathmandu Valley. Therefore, it is realized to carry out study of the flooding problem due to the rivers within the valley upstream from Chovar gorge. Figure 1-1: Bagmati River catchment with its major tributaries in the Kathmandu Valley 1.5 REVIEW OF RELEVANT LITERATURES National Water Plan for Water Induced Disaster Management The National Water Plan has set following targets for water induced disaster management: • By 2007, identification of potential disaster zone by its type and location in district map 3
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report • By 2007, availability of emergency relief materials in all the five development regions • By 2017, establishment of infrastructures for mitigating predictable disasters in 20 districts • By 2017, establishment of warning systems in all over the country and bringing them in functional stage • By 2027, reducing the level of economic losses due to water induced disaster to the levels experienced in other developed countries. The whole program has been envisaged to address the needs of poor people with a view to improving the living condition. The focus of Water Induced Disaster Management during the first five years has targeted to enhance the institutional capabilities. Then for the following 10 years it has targeted to mitigate the adverse effect of the disasters. In the next 10 years the long term goal of the plan is to make the water disaster management fully functional effective and responsive to people’s need. The following seven action programs have been identified and prioritized: • Water Related Disaster Management Policy and Program • Risk/ Vulnerability Mapping an Zoning Program • Disaster Networking and Information System Improvement Program • Community-level Disaster Preparedness program • Program for Relief and Rehabilitation Measures • Activation of Inundation Committee • Flood, Drought, Landslides/Debris Flow, GLOF and Avalanches Mitigation Program Twenty-six activities have been envisaged to fulfill the targets of above seven programs. Integrated Water Resources Management and River Basin Concept The important issues identified by WRS for Integrated water resources management and river basin planning in Nepal are as follows: General issues: • Need for comprehensive water resources policy and Lack of river basin planning and management. Legal issues: • Lack of specific water rights and ownership provision Database issues: • Inadequate hydro-meteorological network Institutional issues: • Absence of an effective institutional framework for coordinated and integrated development • Indistinct responsibilities between policy, implementation, operational and legitimate institutions. • Carrying out independent planning and implementation work of project to fulfill the target of individual departmental goal. 4
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Environmental issues: • Lack of Environmental database and mapping • Lack of practice for integration of environmental consideration into the planning of water resources development. International issues: • Absence of basic legal framework in the country for the development of trans-boundary rivers • Lack of legal mechanisms for institutional cooperation between riparian countries. 1.5.1 Past Studies within Kathmandu Valley Related to the Present Study Flood Hazard map of Bagmati River Basin SILT Consultants (P.) Ltd., ERMC (P) Ltd., and TECHDA JV, carried out study and prepared water induced hazard map of Bagmati watershed during 2005-2006. The job was assigned by DWIDP. The study was carried out using GIS tools and coarse resolution satellite images. Though the study covered entire Bagmati River Basin, there is some useful information on the upper catchment of Bagmati River that covers the Kathmandu Basin. Detailed feasibility Study of Manohara River Training Works Masina Consultancy carried out the Manohara River Training works through the RTP/DWIDP. Landlside Study by DWIDP in and around the Valley DWIDP has carried out landslide study under DMSP projects at some disaster sites, out of which some lies in and around the Kathmandu Valley. These are the Chalnakhel, Okharpauwa model and Matatirtha rehabilitation sites. Among these three sites, the Matatirtha rehabilitation site falls within the area to be covered in the present study. The Matatirtha debris flow was a complicated failure that occurred in 23 July 2002. Bioengineering and civil construction works were implemented in the site in order to mitigate the effect of debris flow hazard. Kathmandu Valley Flood Study Full Bright Consultancy carried out the study of flood prone regions of Kathmandu Valley in 2007. The main objective of the study was to establish setbacks for some of the tributaries of Bagmati River, namely Kodku, Nakhu, Balkhu, Karakhusi, Mahadev, Samakhusi and Tukucha. Hydrological, social and geological/geomorphological studies were carried out during the study. Topographical survey at the river stretches and use of high resolution satellite image followed by one dimensional flood modeling using HEC-RAS were main approaches of the study. The experience of this work is highly relevant in the context of present project. 5
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 1.5.2 Satellite Images, Aerial Photographs and Maps Satellite Images The Consultant appreciates that the use of latest technologies are being encouraged to use for the various study for verification, authentication of various data and information that are difficult to get during the field survey by using technologies like remote sensing etc. The ToR though has encouraged using satellite imageries that is acquired within past six months and having the spatial resolution of 2.5 meter or less. As we had proposed in our Technical proposal, the ALOS (Advanced Land Observing Satellite) image having spatial resolution of 2.5 m have been utilized for the study purpose. The technical specification the satellite is presented below: • Scenes: Nadir and Backward • Swath width: 35 km • Spatial resolution : 2.5 m • Date Acquired: 11 April 2008 The objective of the present assignment is to prepare the flood maps at the scale of 1:10,000 (for the catchment) and 1:5,000 (for the key locations). Since the ALOS image is the panchromatic image with the spatial resolution of 2.5 m, there is a possibility to miss the important information necessary for the flood hazard and detailed social vulnerability assessment. Secondly and more importantly, the study intends to use the DEM prepared by the satellite image for the generation of flood hazard map. The studies has shown that this image is supposed to use to prepare 1:25,000 topographic map (ALOS web page; Izumi K.) and the DEM prepared from this image has the RMS error of around 8.0. This image has been useful for extraction of additional information that is required for vulnerability assessment. For this purpose too, if the provision for purchasing high resolution image like Quickbird is made, more precise information can be obtained. Further, the high resolution image can be used as a base map while carrying out the field observation. Aerial Photographs The aerial photographs of the Kathmandu valley (1978, 1985, and 1992) have also used in the present study. The information extracted from the photographs was about the trend of changes in the river morphology, land use condition as well as urbanization in the Kathmandu valley that has links to present study. Topographical Maps The topographical maps of Kathmandu valley are available in 1:25,000 scale maps prepared by Department of Survey, Topographical Branch. These maps are having the Modified UTM projection system and the Everest 1830 Datum. These maps were compiled from 1: 50,000 scale aerial photography of 1992 with the field verification in 1995. The following topo-sheets cover the Kathmandu Valley (Table 1-1) 6
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 1-1: List of DoS topo-sheet used in the study. S. No. Topo-sheet number 1 2785 06A; 2785 06B; 2785 06C; 2785 02C; 2785 02D; 2785 05B Geological Maps The following geological and engineering geological maps have been relevant to the present study: • Geological sketch map of Kathmandu area (72 E/6), Scale 1: 63.360, HMG/UNDP Mineral Exploration Project, 1978. • Engineering and environmental geological map of Kathmandu Valley, Scale 1:50,000. Department of Mines and Geology, Kathmandu, 1998. 1.5.3 Geomorphology and Geology of Kathmandu Valley Since the geology and geomorphology of the region plays a dominant role in the development of particular type of river system and also for the morphology of the river, it is necessary to have a brief understanding of the geomorphology and geology of the regions as well as of the study area. Geomorphology Kathmandu Basin is an intermontane basin that lies in central Nepal. The basin is surrounded by mountains ranging in altitude up to 2,700 m. The hills are of low altitude in the eastern and western part while the northern and southern part of the valley is having high hills (Shivapuri and Mahabharat Lekh). The average altitude of the valley floor is 1330 m, the minimum being around 1200 m at the southern margin of the valley. Geomorphologically, the Kathmandu Valley and its surrounding mountainous parts can be subdivided into hill and hill slopes, rocky outcrop, terraces, floodplain and riverbed (Figure 1-2). Hills and hill slopes can be observed in the periphery of the valley while the rocky outcrops can be observed in some of the areas within the valley such as in Gokarneswor, Pashupatinath, Swayambhu, Balkhu and Chobhar area. The terraces are widely distributed in the basin with flat to gentle topography. There are different levels of terraces in the valley. The terraces that are distributed around the central part of the valley are having height of 20 m to 50 m from the present riverbed. The terraces, which are distributed further outwards from the center of the valley, are situated at the higher level with the height ranging from 50 m to 80 m. Similarly, the terraces that are distributed along the fringe of the valley near the hilly parts are much higher reaching up to 160 m from the present riverbed. As far as the rivers considered in the present study is concerned, most of the rivers originate (except Tukucha and Samakhusi) at the hilly areas and traverses through the terraces developing the flood plain. 7
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 1-2: Geomorphological map of the Kathmandu Valley Geology The Kathmandu valley and surrounding area can be subdivided into two major geological units, i.e. the rocks exposed around the Kathmandu valley comprising Basement Rocks and Basin-fill Sediments overlying the basement rocks (Figure 1-3). The section shows that Kathmandu Basin is a tectonic basin lying above thrusts. The basin sediments are consisting of thick fluvio-lacustrine origin underlain by the rocks of Phulchauki Group that is predominantly consisting of limestone. Kathmandu valley lies on the Kathmandu Complex that is consisting of metamorphic rocks overlain by the fossiliferous Tibetan Tethys sediments (Stocklin and Bhattarai 1981). The valley floor is basically filled with the sediments derived from the surrounding mountainous regions. Therefore, the valley floor is filled with the late Pliocene to Pleistocene thick basin- filled sediments (Yoshida and Igarashi, 1984). The maximum thickness of the basin filled sediments is around 650 m. Many drill core data suggests that more than 300 m thick muddy and sandy sediments are extensively distributed under the basin. Further, there is extensive distribution of thick black clayey sediments of lacustrine origin. The northern part of the valley is basically consisting of fluvio-deltaic deposit, lacustrine deposit (consisting of Kalimati Clay), which is distributed in the central and south-western part, while in the southern part, distinct fan deposits dominantly consisting of coarser materials are well distributed (Figure 1-4). There are many faults running in WNW-ESE direction. Alluvial fan and recent flood plain deposits are basically associated with the rivers flowing through the valley while the colluviums are distributed along the margins of the valley. The residual soils can be observed on the areas where rocks are highly weathered forming the soils e.g. on the southern slope of Sivapuri mountain. The talus deposits are the sediments deposited on the mountain slopes. 8
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 1-3: Geological section in north south direction passing through the Kathmandu Valley (adopted after Sakai et al., 2002) (S=Siwalik group, B=Bhimphedi group, P=Phulchauki group, N=Nuwakot group, G=Granite, Gn=Gneiss and Granite Figure 1-4: Geological map of the Kathmandu Valley (adopted after Sakai, 2001) 9
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 2. METHODOLOGY The methodology for the preparation of “Flood Risk and Vulnerability map of Kathmandu valley” in GIS environment for flood of different return periods of the specified region on a scale of 1:10,000 and detailed social vulnerability assessment at specified areas (1:5,000) is described in this section. In order to generate the flood hazard, the rainfall-runoff and hydraulic modeling has carried out apart from other activities. The Methodology has been developed considering the following facts based on the Terms of Reference (ToR): • the nature of the assignment/study including understanding, • additional information collected by the Consultant from various sources, including those from persons contacted at DWIDP, similar previous studies; and previous experience of the JV Firms in undertaking similar studies. The study work has been grouped in the following major four steps: • Project Start up Activities leading to Tem Mobilization • Pre-field study leading to Inception Report • Fieldwork and Other Activities leading to Field Report • Post Field Activities leading to Draft Report and Final Report Flow Chart showing major Tasks/Activities with deliverables is presented in Figure 2-1. The respective activities to be carried out in each steps, are described in the following sections: 2.1 PRE-FIELD STUDY The activities/tasks that were planned and that needed to be carried out during the pre-field study are described herein in the subsequent sections. Collection of Reports, Maps and Satellite Imageries and Review Soon after the mobilization, all the relevant reports, available hardcopy and digital maps, data and satellite imageries have been acquired. The maps include recently published topographic maps, aerial photographs, geological maps, land utilization maps, land classification maps, soil maps, and other relevant thematic maps of the study area. Apart from the maps, recent satellite imageries, which is the most important aspect of the study, that covers Kathmandu Valley (the study area) has been acquired from concerned agencies. The collected maps and photographs have been studied in depth by the study team and collated them with respect to the study objectives, scope of works, and final deliverables. Available relevant literature on in the internet, local libraries, government institutions (like Department of Mines & Geology apart from DWIDP, DHM etc), ICIMOD etc. on “flood risk and vulnerability mapping “ has been reviewed in the context of the study framework and desired outputs. 10
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Pre-Field Study Collection and Review of Relevant Collection and Study of Maps and Literature Images, Prepare DEM Historical Data on Water Induced Define Hazard Classification Criteria Disasters Identify the Areas for Detailed Generate Preliminary Flood Maps Technical and Social Survey Develop Field Study Format and Inception Report Questionnaire Field Study Gather Historical Disaster Data Verify and Update Preliminary Flood (Questionnaire, FGD, Interview) Maps Check Threat of Flooding to Conduct Benchmark Survey (to Settlement, Infrastructure, Historic Measure the Level of Bridge) Important Place Check the Means of Communication, Collect Information on Geology, Soil Transportation and Evacuation Depth, Bank Cutting and Other Routes Hydrological Data Hydrological Analysis; Run Rainfall Run off Model Field Report Post-Field Activities Run 1 D GIS Based Flood Simulation Model using HEC-RAS & HEC-GeoRAS Prepare Updated Flood Risk and Vulnerable Maps and Validate Draft Report & Presentation Correction and Final Report Figure 2-1: Flow chart of the study 11
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report In addition, the data were collected from the following institutions; • Water and Energy Commission Secretariat • Department of Water Induced Disaster Prevention • Department of Irrigation • Department of Hydrology and Meteorology • Department of Survey/Topographical Branch • Central Bureau of Statistics • Department of Mines and Geology • Mountain Risk Engineering Unit of Tribhuvan University For the purpose of the study the Advanced Land Observation Satellite (ALOS) images of the resolution within the permissible limit as given in the ToR i.e. less than 2.5 m has been utilized. The images have been handed over to DWIDP after completion of the study period. Collection of hydro-meteorological data The Consultant had collected the precipitation/rainfall data from DHM as recorded at different rain gauge stations within the Kathmandu Valley and its adjacent areas. The precipitation can contribute to the flow regime of the rivers within the valley namely; Bagmati and its sub-basin like Bishnumati, Hanumante, Manohara, Dhobi Khola, Balkhu, Kodku, Kalimati Khola (Manamati Khola), and Samakhushi Khola. These data were analyzed statistically in order to obtain the monthly average, rainfall intensity, maximum probable rainfall etc. for use in the rainfall runoff model that has been used in the analysis for runoff and hydrological model. The various rainfall stations that are located within the Kathmandu Valley are given in the Table 2-1. Table 2-1: Location of rainfall station within the Kathmandu valley with their geographic characteristics Index Elevation District Location No. Latitude Longitude (m) Lalitpur Godavari 1022 27° 35’ 00” 85° 24’ 00” 1400 Lalitpur Khumaltar 1029 27° 40’ 00” 85° 20’ 00” 1350 Kathmandu Kathmandu Airport 1030 27° 42’ 00” 85° 22’ 00” 1336 Kathmandu Panipokhari 1039 27° 44’ 00” 85° 20’ 00” 1335 Likewise, hydrological data of the rivers i.e. stream flow summary under present study had been collected as far as their availability for the rivers. The collected data has been analyzed and the possible flood magnitude of various return periods has been estimated by using empirical/rational methods. The hydrometric stations situated within Kathmandu Valley are given in the Table 2-2. 12
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 2-2: Location of hydrometric stations within the Kathmandu valley with their geographic characteristics River/Location/Station Latitude Longitude Elev. Drainage No. (m) area (km2). Bagmati/ Sundarijal/505 27° 06’ 03” 85° 27’ 40” 1600 17 Nagmati/ Sundarijal/507 27° 46’ 20” 85° 26’ 10” 1660 13 Sialmati/ Shyamdada/ 27° 46’ 10” 85° 25’ 10” 1660 3 510 Bagmati/ Gaurighat/530 27° 42’ 30” 85° 21’ 00” 1300 68 Bishnumati/ 27° 06’ 03” 85° 27’ 40” 1454 4 Budhanilkantha/536.2 Nakhu/Tika Bhairav/540 27° 34’30” 85° 18’ 50” 1400 43 Bagmati/ Chovar/550 27° 29’ 40” 85° 17’ 50” 1280 585 Collection of Historical Data on Water-induced Disaster The Consultant has collected historical data on water-induced disaster from various previous reports. As far as possible all the data has been referenced spatially i.e. with location in terms of latitude and longitude so that they make easy to overlay in the GIS environment and has been documented for further use in the study process. In addition, collection of data from the following agencies has been performed: • Department of Home • Central Bureau of Statistics • ICIMOD • Department of Forest and Soil Conservation • Bagmati Watershed Project, • NSET etc. Delineation of Study Area and Preliminary Flood Maps The each collected topographic maps, thematic maps etc. were combined together (mosaic in GIS environment) as they were available in different sheets/format and or projections. The area was delineated in the maps with respect to the area of interest (catchment of Bagmati River upstream of Chovar gorge). The maps obtained in hard copies were digitized covering the study areas by using standard GIS software as well. Together with the historical data on water-induced disasters collected from the relevant reports and the output from the study and analysis of the collected maps and imageries, a preliminary flood maps was generated. Collection of Socio-economic Data The socio-economic data of the study area particularly, population, agriculture system, ethnicity, occupation etc. has been collected from secondary sources that includes CBS, VDCs and Municipalities, the three district development committees and village profiles prepared by different agencies. In addition, the data relating to past flood events and its disasters in terms of loss of property and lives has been collected as far as possible from direct field survey by mobilizing sociologist and also from various reports and secondary information published by DWIDP, Ministry of Home Affairs, Nepal Red Cross Society and 13
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report NGOs. Such data were used to assess the flood risk and vulnerability mapping. All the data were spatially linked for analysis purpose in the GIS environment. Interpretation of Satellite Imageries The ALOS satellite imageries of the study area have been interpreted to delineate the riverbank lines, flow lines, and morphological features of the rivers under study. The information has been exported to the GIS environment to collate for preparing base maps and for GIS based information for hydraulic modeling. Preparation of Flood Hazard Classification Criteria Based on the secondary data, satellite images and other document review, the Consultant prepared flood hazard classification criteria for different return periods in the analysis phase. This was based on various parameters or combination of the same like; flood magnitude, topographical feature, past flood events, past flood damages, frequency of occurrence, flood depth, inundation area output of flood hazard model etc. This will precisely describe in the later respective chapter. In general the flood depths are classified as up to 1 m, 1-3 m, and above 3 m. Once the flood depth map is prepared; the area in the three classification zones as stated above can be worked out. Hazard classification is developed in term of flood depth covering flooded area minimum up to of 1 m depth to that of the total flooded area for particular flood return period. Hence, based on the criteria the hazard level is classified to low, moderate, and high depending upon the depth. Here, example of 1 m is taken as it is considered that flood depth > 1 m can cause severe damages to infrastructures including roads, buildings, drainage etc. Similarly, these outputs can be linked to the population served for the physical boundary that has been considered. Further, the soil type can be taken into considerations for interlinkage between the flood magnitude, depth and hazard. With ranking of various parameters raster analysis was performed to overall classification for the flood risk zone and preparations of the flood risk and vulnerability maps were performed based on the classification. Identification of Areas for Detail Study Based on the preliminary flood map, the secondary information on flood and damage scenario, the physical location of such area depending on the severity of flooding has been identified and the detail information were taken from these areas during the fieldwork. Preparation of Questionnaire, Form and Formats Questionnaire for field survey was prepared during the inception phase to cover engineering, socio-economic and environment aspects for the purpose of conducting Key Informant Survey, Focus Group Discussion, RRA in order to know Flood and Inundation in the study area. In addition it contained format for verification of preliminary flood maps in the field itself as well. Preparation of Inception Reports Based on the findings of the desk study, Inception Report was prepared. The Inception Report primarily contained the findings based on the desk study, detailed work plan work methodology to carryout the work including schedule of manpower assignment in the field/office and the schedule of activities for the remaining phases of the study. Likewise, it 14
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report contained the preliminary flood hazard map and the identification of the areas to be studied in detail in terms of the severity of flood. 2.2 FIELD WORK AND OTHER ACTIVITIES The field work was conducted in a systematic manner to gather the required information from field observation as well as with the public interaction using various tools. Different components of the fieldwork phase are as mentioned: Preparatory Work After the submission and acceptance of the Inception Report, the Consultant’s team comprising of a Team Leader/Water Resources Engineer, Engineering Geologist, GIS modelling Expert, Remote sensing Expert, Hydrologist, Socio-economist, Environmentalist, Surveyor with Technical Assistants were mobilized to the field. The team hired sufficient field enumerators and train them on the use of the information/data collection format as well as methodology for primary data collection in the field as well. Verify and Update Preliminary Flood Map The field study team verified and checked all the data collected during the desk/inception phase and the result of preliminary flood hazard map was checked and updated. The verification had been made in consultation with local people, walkthrough along the river and other areas of influence as required. The new area, which is potentially hazardous, had been marked and later the relevant data were incorporated in the map. Bench Mark Survey Bench mark survey has been conducted to determine the level of all the bridges in Ring Road and other critical locations within the city area of the valley. Flood Mark Survey The Consultant has carried out the flood mark survey of the various rivers at locations that are more vulnerable as identified in the Inception Phase. GPS instrument was used for the location of the flood mark. This was verified with the ground realties and with the information obtained during the Focus Group Discussion and from the Key Informant. Collect Additional Information The field work has also focused to collect the information on geology, soil depth, trend of bank cutting and other locally available hydrological data. The settlements, land use, infrastructures such as roads, bridges, irrigation canals, drinking water and communication related infrastructures were carefully observed and noted on topomap and diary. The team has also identified the areas of settlement, infrastructure, historic important places etc. considering the threats from water-induced disasters. The attempts have been made to explore such infrastructures, which are under threats of water- induced hazard. In addition, the existing communication and transportation system including routes, which are very important for evacuation in case of emergency and rescue operation, has been 15
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report verified and marked on the map. Similarly, the communication systems such as telephones and other means have been marked. All the other public facilities available in the area have been shown on the maps. Socio-economic Survey The social survey has been carried out at the key locations as identified during the pre-field study to gather the historical disaster data. The Socio-economic impact assessment due to flood and erosion in the past 10 years has been conducted by RRA method. The socio- economic survey of the study area, which primarily contains population, occupation, infrastructures, ethnicity etc., has been collected with the help of Participatory Rural Appraisal Methods. These data has been verified and triangulated in consultation with local government bodies such as Village Development Committees and Municipalities. The actual field survey consisted of the following activities: After arriving at the sample locations survey team conducted the survey using structured and semi-structured questionnaire thoroughly following the survey guidelines. Before the start of the survey, the Consultant selected enumerators to conduct the survey with minimum of intermediate level as their basic education. The enumerators were adequately trained to conduct field surveys. The issues that need to be addressed during the training were: locating and enlisting & soliciting cooperation from respondents, motivating the respondents to properly provide information, clarifying any confusions/concerns, observation of quality of responses, and conducting interview etc. Similarly, the other important aspects during the field socio-economic survey were: describing the entire study, stating who is the sponsor of the study, educating about the survey tools, explaining sampling logic and process, explaining interviewer bias, walking through the interview, explaining respondents selection procedures, rehearsing the interview, explaining about the supervision and explaining the schedule by which the exercise has to be completed. Key Informant Survey A checklist was prepared for collection of information from the relevant key informants. The checklist of the key informants has given guidelines to collect the information with regard to perception of people towards the Water Induced Disaster and historical record. Similarly, it consisted of obtaining the socio-economic status of the area. Focus Group Discussion Focus group discussion (FGD) was conducted in different sections of the river (Head, Middle and Tail) on both Banks and settlements (affected, partially affected, not affected due to 16
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report flooding, inundation etc.). The FGD session concentrated to collect information on water induced historical disaster data, socio-economic condition of the people in general, past flood and disaster including damages, flood and disaster management practices. Vulnerability Assessment The vulnerability assessment of the infrastructure and settlement was carried out in the field at specific vulnerable communities as identified and validated during the pre-field study. While carrying out the assessment, the population group was divided as male, female, above 65 years of age and children. Collection of Digital Photographs Various digital photographs were collected for different stretches of the rivers and preliminary assessment was done to match with the information collected in the field and to generate flood related data as well as to verify. Run Rainfall Runoff Model For various reasons, it is not possible to take the continuous measurements of hydrological variables. However, there are some approaches, which can be applied with reasonable accuracy to arrive at a rainfall runoff model. A particular approach to be adopted depends on the data availability and the purpose of water use and analysis. Various known approaches that can be used for the study area have been described in the following sections. Rainfall Analysis From the observed rainfall data in and near the study area at various rain gauge stations, the maximum 24 hour observed rainfall data were extracted. A frequency analysis adopting Gumbel distribution was conducted. The result of this analysis is the predicted 24 hour maximum rainfall for 2, 5, 10, 20, 50 and 100 year return periods. The point rainfall can be converted into the aerial average rainfall by methods like, Theissen polygon, isohyetal, arithmetic mean, etc. In this study, Theissen polygon method is used to estimate the aerial average rainfall. Since the extents of the basins are not so large, it is assumed that single Theissein polygon would be applicable to compute the aerial average rainfall of the entire study area. Flood Estimates The following approaches have been examined to find out the best method for analyzing the runoff quantity in the rivers of Kathmandu Valley. Empirical Approaches o Modified Dicken's Formula o Regional Flood Relationship (WECS, 1989 ) Method o Tahal (2002) Method o Sharma and Adhikari Method o Snyder Method Frequency Analysis and Observed Flood 17
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Based on the approaches as mentioned above rainfall-runoff model was run to estimate the flood magnitudes of different return period for the various rivers at required locations in the analysis phase. 2.3 POST FIELD ACTIVITIES All the data collected during inception phase and fieldwork has been analyzed and interpreted and the final work has been presented in this report. The detail analysis is being carried out and the outcome is presented in the report as per ToR. All the data, information, and maps collected/studied have been organized for easy references and uses. Further, they have been grouped/classified under different headings for meaningful use and interpretation based on the each sub-watershed. In order to make the data useful to the present analysis a careful review of the data and maps were performed in accordance with the necessity in relation to their adequacy, consistency, and reliability. The following section describes various steps undertaken during the office works stage. Compilation and Analysis of Socio-economic Data The secondary data collected from literatures and primary data collected using RRA, FGD, interview with key informants and field observation has been analyzed and grouped in order to assess the damage scenarios in the past as well as in future. The information that would of importance is that on population, agriculture system, ethnicity, occupation etc. The data relating to past flood events and its disasters in terms of loss of property and lives have been considered during the analysis and interpretation. Analysis of Attribute Data and Maps The attribute data collected during the filed works has been incorporated in the respective GIS themes or appended by creating new themes. These Themes have been used to prepare flood, inundation, and flood hazard and vulnerability & related maps. Preparation of DEM/TIN from the Stereo-Pair Images The workflow of DEM Preparation begins with input of a stereo Input ALOS Stereo image pair that contains or has associated RPCs (Rational Pair Images Polynomial Coefficients). Next, GCPs (ground control points) has to be entered that ties the DEM to a planar map projection. Add/Edit GCPS The result is an absolute DEM. An absolute DEM uses ground control and has horizontal and vertical references systems tied Select Required Parameters to these geodetic coordinates. Next, the relationship between the stereo images must be defined by selecting or generating Output DEM and Examine Results tie points. The tie points are used to define the epipolar geometry and create epipolar images, which are then used to Edit DEM extract the DEM. Once the epipolar images are created, need to specify the output projection parameters for the DEM and then specify the DEM extraction parameters. The resulting DEM can be examined and/or edited, if required. Once the error (CE-circular error for horizontal control, LE-linear error for vertical control, RMS-root mean square error for overall validation of DEM generated) is 18
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report within acceptable limit in line with the study objectives and map scale, it can be used for the further works relating to the preparation of flood map of the study area as result of river flood. The required DEM has been generated from the above mentioned procedure and has been used in the flood hydrological computation and hydraulic modeling with integration to GIS. Generation of River Network and Basin and Sub-basins Once the DEM was finalized then the river network was generated from the DEM in the GIS environment. The HEC GeoHMS was used to evaluate the raster data of the DEM and downward slope grid flow accumulation process was traced the river network. Similarly, the basin and sub-basin area were delineated from it as per the requirement. The stream network data were then overlaid with satellite images and was verified the accuracy of the network that is really in the filed and that generated. Depending upon the accuracy of the generated river network the confidence level of DEM was ascertained. The sub-basin areas were useful in flood estimation using empirical models that has been used for the rivers and basins in Kathmandu Valley. Hydrological Modelling and Preparation of Updated Flood Risk Map Based on the calculation made from the rainfall-runoff model and other empirical methods, the discharges of the rivers under consideration for various return periods have been obtained at the desired points and locations. The computed discharge has been used as input parameter while running the GIS-based flood model using HEC-RAS and with geometric data generated from HEC GeORAS to prepare preliminary hazard map. This hazard map has identified the locations for detailed technical and social survey. Based on the data obtained from the field work and a detailed refined hydrological analysis combined with GIS based hydraulic modeling, the preliminary flood map was updated. Following methods have been applied to prepare flood maps in general: Preparation of digital elevation model (DEM) from the stereo-pair satellite images using appropriate software. Use of satellite imageries of past in order to delineate the river bank lines, flow path etc. Use of satellite image and topographical maps to identify the possible debris flow deposits, and likely areas of landslides Depiction and assessment of human activities like change in land use, intervention in river in the form of river related works (like construction of embankment, dams/check dams, barrages, river training works) Assessment of floodplains and inundation depth using appropriate simulation models for steady state flow conditions The HEC-RAS model can be used for backwater analysis and rating curve preparation. The water surface profiles computed by the model had been compared with the real field data and accordingly, the model was fine tuned. The updated flood map is prepared for different return period at the scale of 1:10,000 for the catchment and 1:5,000 for the key locations. The Steps to be carried out during the application of HEC-GeoRAS in conjunction with HE- RAS are as follows: 19
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Starting a New Project: Start up ARCVIEW and change directories to the workspace containing DTM. Creating a Contour Coverage: A Counter coverage must be created next. It will be used to help to create the RAS Coverages later. Creating RAS Coverages: Main channel Invert, main channel Bank, Over bank Flow paths and cross section cut lines. Creating a HEC-RAS import File Running HEC-RAS: To use HEC-RAS in concert with the GIS, perform the following steps: o Import the RAS Import File into HEC-RAS from the Geometric Data Editor. o Complete the following hydraulic data: roughness coefficient, expansion and contraction coefficients, and hydraulic structure data, if any o Run simulation in HEC-RAS and review the output. o Export the water surface profile results back to the GIS. Importing a HEC-RAS Export File - Inundation Mapping Classification of hazard based on the approved criteria Raster Analysis of map or map calculation based on the approved methodology and criteria for further analysis Preparation of flood risk and vulnerability maps based on the raster analysis Printing Map Results The general process for Using HEC-RAS and HEC-GeoRAS is presented in Fig. 2.2. Preparation of flood hazard, vulnerability and risk map Based on the all the primary, secondary and hydrological model runs and their outputs, the Consultant has been prepared the Flood Risk and Vulnerability Map of the Kathmandu valley for the rivers within the scope of the study. All the maps will be in GIS format. Validation of Flood Modelling Result The flood-prone areas were identified based on the spectral properties of satellite imageries, which had been verified in the field as well. Minimum Width of River Analysis was made to come out with the figure in recommendation of minimum width of river that is required to pass the flood magnitude of different return period floods. The recommendation figure is varied for different stretches of rivers as well as depending upon the urban and sub-urban areas. 20
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 1. Create Stream Centerline Start an ArcView - Label river and reach names Project - Atribute theme - Extract elevations 2. Create Banks theme GIS Data Development 3. Create Flow Path Centerline preRAS menu - Label f low paths 4. Create/Edit land Use theme - Estimate n-value 5. Create Level Alignment Generate RAS GIS - Extract/input elevations Import File 6. Create Cross Secton Cut Lines - Attribute theme - Extract elevations 7. Create Inef f ective Flow Areas Run HEC-RAS 8. Create Storage Areas - Extract elevation-volume Enough No Cross 1. Create new project Sections 2. Import RAS GIS Import File 3. Complete geometric, hydraulic structure and f low Yes data 4. Compute HEC-RAS results 5. Review results f or hydraulic Generate RAS GIS correctness Export File 1. Import RAS GIS Export File RAS Results Processing 2. Generate water surf ace TIN postRAS menu 3. Generate f loodplain and Depth grid 4. Generate velocity TIN 5. Generate velocity grid Correct No inundated area? Reduce grid Yes Yes Enough Suf f icient No Cross No map Yes Detailed f loodplain Section? detail? Figure 2-2: Application of HEC-RAS and HEC-GeoRAS 21
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 3. DEM PREPARATION FROM SATELLITE IMAGE 3.1 THE ALOS STEREO-PAIR IMAGE The Alos stereo pair images (Prism sensor) of Japanese satellite acquired during April 11, 2008 was purchased and used during the present study. The images were captured from Nadir and Backward positions. This image is having an areal coverage of 35 km x 35 km that covers the Kathmandu valley. The spatial resolution of the image is 2.5 m. One of the objectives of selecting this image is to assess its usefulness for the purpose of flood hazard mapping in urban area. 3.2 DEM PREPARATION AND FLOOD HAZARD MAPPING A preliminary Digital Elevation Model was generated using the above mentioned stereo-pair satellite images in order to prepare preliminary flood hazard map. Thus generated DEM was used to generate preliminary flood hazard map of the valley. The preliminary flood hazard map of the valley has shown some vulnerable locations, which was verified during the field investigation and also used to identify the locations to conduct social survey. It had been realized that the preliminary DEM needed to be improved as some errors were realized during the field study. At the second stage, the preliminary DEM was further improved at the areas noted to have erroneous elevation. During this stage of digital image processing, all together 41 Ground Control Points (GCPs) were used, out of which 15 points were check points. While preparing DEM, 9861 auto generated object points (tie points in the respective images) were generated by the program. RMS error for X, Y and Z coordinates were obtained as 4m, 6m and 7m, respectively. This much of error is considered as an acceptable one in generating DEM with satellite image having 2.5 m spatial resolution. 3.3 UPDATING THE DEM ALONG THE RIVER CORRIDOR It has been realised that the DEM generated from the satellite image gave a good result at the mountainous areas. Whereas, in the valley areas, the river network generated from the DEM could not delineate the river course at some locations. Basically, the areas having high meandering condition, the stretch located at the highly urbanized areas and locations with narrow river widths were having problems. This is obvious with respect to the image resolution that has been used in the present study. The present satellite image is recommended for the preparation of 1:25,000 scale topographic maps and the vertical accuracy is suggested as 7 m. In this regards, the problematic areas as mentioned above is not an unusual. Further, several studies have also shown that editing of the image generated DEM is needed to represent the meandered segment of the river and also to best represent the narrow river valleys located in urbanized areas. For this purpose, the consultant has carried out the following activities: ­ Use Quickbird (0.6 m) image to demarcate the river courses and banks. It was necessary because this information could not be obtained well from the Alos image, as recommended for this study. 22
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report ­ Add spot heights so as to re-align the river course. For this purpose, the relative bank heights as measured in the field were used. ­ The ortho image has been used to exactly locate the river courses. Thus prepared DEM well reflects the terrain of Kathmandu Valley and is presented as hill shade in Figure 3-1 below: Figure 3-1: Surface generated from the DEM 23
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 4. SOCIO-ECONOMIC CONDITION Social survey was furnished by collecting key information from interview as well as focus group discussion with the people residing nearby the studied river banks. The team of social scientist carried out the study at the areas identified during the preliminary study and technical survey along the river banks. These locations are identified as flood prone areas where regular impact of flood and also human encroachment to the natural regime of river has been taking place (Figure 4-1). Figure 4-1: Location of Social Survey Because of very dense population and complete river capturing in the downtown areas of the Karakhusi, Tukucha and Samakhusi Kholas by residential houses and roads, it was not possible to interact with the local people. During the survey two to three persons were sampled in each site for interview using the pre-developed questionnaire. Likewise, focus group discussions were made at each selected sites. Besides interacting with the local people, the team made their own judgement on the social impact due to flood and vice versa. The following are some of the issues related to social survey in the Kathmandu Valley: • difficulty to find out the key informant as the people are very busy • people have negative impression about the studies to be carried out by different organization and hence are unsupportive in some cases 24
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report • since the migration in the valley is more, many new residents are not familiar about the past history of flooding. Only recent events have been reported. In spite of above mentioned obstacles, the team of social scientist did their best to gather as much information as possible utilizing different tools, techniques and approaches that is generally adopted in such circumstances. The sample filled questionnaire and focal group discussions are presented in Annex. The data will be analyzed in detail and presented in the draft report. At present, the outcome of the social survey is briefly summarized below and elaborated for the major rivers: 4.1 BAGMATI RIVER Eight locations were identified along the Bagmati River for social survey and the reasons for the selection of these sites are given below: • Bag-Soc 1 (Uttar Bahini area): Gorge at Gokarneswor Mahadev blocks the full discharge of the river then occasionally inundation occur in the upstream area particularly in Uttarbahini. • Bag-Soc 2: Area between Bagmati bridge at Jorpati and Gaurighat. Inundation during monsoon due to subsurface discharge. • Bag-Soc 3: Stretch between old Baneswor bridge and Bagmati bridge at Subidhanagar. Flooding problem in the area occupied by landless people. • Bag-Soc 4: Koteshwor Mahadev Sthan. The area is having bank erosion problem. • Bag-Soc 5: Sankhamul area that experience flooding during monsoon. • Bag-Soc 6: 200 m upstream from the confluence of Tukucha that is having flood problem. • Bag-Soc 7: 500 m upstream and downstream from confluence of Bishnumati and Bagmati rivers. Inundation and river blockade from solid waste disposal narrowing the river channel. • Bag-Soc 8: 150 m upstream and downstream from the confluence of Balkhu Khola. Inundation taking place up to Kumari Club area. The result of the field survey is tabulated and briefly summarised in Table 4-1, Table 4-2, and Table 4-3 25
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 4-1: Bagmati River (Flood hazard data) S VDC/ Ward Area (ropani) Vulnerable Vulnera Damage Damage N Municipal Reside Agricu Infrastructure/histor ble value of ity ical houses value of crops ntial lture places/industries land + (Rs) house (Rs) 1 Gokarnes 6 10 250 Water intake building-1 10 39,000,000 5000,000 hwor Private school-1 2 Gothatar 2 - - - - - - 3 Kathmand 34 Corridor is in risk 108 3,000,000 u 4 Kathmand 35 10 Madhev than temple 100,000,000 u Road – 10m 5 Kathmand 32 - - Bridge - 1 7 20,000,000 u 6 Kathmand >400 - Kaal mochan ghat 400 carores u 7 Lalitpur 1 - - Teku ghat 200 2carores 8 Kirtipur 14 - - - 30 600,000 Table 4-2: Bagmati River (Demographic features) SN. Section No.of HHs Male Female Above 65 Children 1 Bag soc 1 10 32 28 7 5 2 Bg soc 2 - - - - - 3 Bag soc 3 108 290 250 78 220 4 Bag soc 4 - - - - - 5 Bag soc 5 7 38 32 14 18 6 Bag soc 6 - - - - - 7 Bag soc 7 200 550 450 135 380 8 Bag soc 8 30 110 80 20 45 Table 4-3: Bagmati River (Awareness level and institutional framework) S Section Cause of Protection work to be Existing Supporting N flood/Inundation adopted organization organization 1 Bag soc 1 Narrow river channel at Wide outlet No DDC Mahadev than Ghatte Retention of wall Bagmati Khola block the flow of development Bagmati. planning Khan pani 2 Bag 2 Pashupati development Sand mining should be Pashupati fund has constructed stoped because it will development wall on either side of damage the walls. fund river after 2054 so there is no problem of flood. 3 Bag soc 3 Narrow river Retention of wall lumanti 4 Bag soc 4 Deep river so bank Retention of wall No cutting 5 Bag soc 5 Narrow river channel Bigger wall should be No constructed 6 Bag soc 6 Narrow river channel Construction of wall. No 7 Bag soc 7 Bagmati is blocked by Wider river channel No bishnumati solid waste Wall construction 8 Bag soc 8 Narrow river channel Retention of wall Kumari club solid waste Management of solid waste 26
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Damages from flood/inundation The residents nearby the Bagmati River experienced flood problem in the past and damages to houses, agriculture land and other properties was frequent (Plate 4.1). However, the construction of river training structures at some location have solved the problem but the need to protect these structures from human activities at river channel (especially sand mining) is deeply realised. In addition, the residents opined that the tendency to confine the river within narrow channel through the construction of structures needs to be controlled. The event during 2005 and 2006 AD, principally affected at the locations Bag-Soc 1 and Bag-Soc 3. One school and the intake at Bag-Soc 1 site is at risk. Likewise, 10 buildings are vulnerable and 10 ropanis of residential land were affected. Because of river training structures from Gokarneshwar to Pashupati in 1988, the inundation problem around Bag-Soc 2 location has been solved. However, the continued sand mining has threatened the structure. Bag-Soc 3, total of 108 households are vulnerable because of bank erosion by the river. Likewise, at Sankhamul (Bag-Soc 5), seven houses are vulnerable and one bridge is tilted because of under cutting by river. There is bank erosion causing the Teku Ghat and 200 squatters vulnerable at location Bag- Soc 7. Around 100 m road corridor is at risk and 30 houses are vulnerable at location Bag- Soc 8. Awareness level regarding flood vulnerability Regarding the different questions raised to explore the awareness level of the residents, it has been realised that the people in the survey area are moderately aware of the causes of flood/inundation, the appropriate flood protection measures, rescue route and evacuation centres. Institutional framework District Development Committee, Bagmati Development Planning, Pashupati Development Fund, Lumanti (Saving and credit organization established by squatters) and local club are found to be involved in working towards development of corridor over the solid waste, construction of dykes/embankments etc. At some localities, the local people are involved towards flood control measures through local club or individually. Flood control measures and their effectiveness The local people said that the construction of flood control measures at some location is effective but not enough. Likewise, there have been no further activities to protect the road constructed along the Bagmati corridor even if it is vulnerable and damaged at some places. In addition, the effectiveness to control the activities (e.g. sand mining) that affect natural regime of the river and also stability of the structure is very poor. Rescue routes and evacuation centres The Bagmati River flows through the urban area that good accessibility of road so the rescue route is not the problem during the event of flood. However, the people strongly feel the need of evacuation centres. They feel that the identification and necessary preparation to develop the open space and other public places as evacuation centre is required. In addition, the information dissemination about such places is also lacking. Almost all the 27
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report participants of focus group discussion expressed their willingness to participate in any activities of flood control along the Bagmati River corridor. Support necessary to cope with flood and other issues The people from the affected communities pointed out the need to implement further river training structures at the critical locations to avoid the possible damage due to flood. In addition to the hardware part, they equally understand the importance of awareness rising activities. This is important factor to reduce the human encroachment to the river regime and also unlimited use of the resources (sand mining). They also feel the strictly implement the rules and regulations against such activities. 4.2 BISHNUMATI RIVER The five locations selected for the social survey along the Bishnumati River are having different problems, as mentioned below: • Bis-soc 2: There is inundation problem in the area lying around 500 m upstream from the confluence of Sangla Khola. • Bis-soc 3: Inundation and bank cutting problem observed around the confluence of Sangla Khola and Bishnumati River. • Bis-soc 4: Confluence of Mahadev Khola and Bishnumati River. The Gongabu area is having inundation problem. • Bis-soc 5: Around 300 m upstream from Sovabhagawati Temple is experiencing the inundation during monsoon season. • Bis-soc 6: Flood problem around Tankeshwor. The public opinion in different flood related aspects of Bishnumati River is briefly summarised in Table 4-4, Table 4-5, and Table 4-6. Table 4-4: Bishnumati River (Flood hazard data) S VDC War Area(ropani) Vulnerable Vulne Damage Damage N d Resi Agricu Infrastructure/hi rable value of value of denti lture storical house land + crops al places/industrie s house(Rs) (Rs) s 1 Sangla 9 80 Road -100m 2 40,000,000 400,000 Bridge - 1 2 Gongabu 3 5 45 Road -20m 6 30,000,000 200,000 3 Gongabu 4 10 100 Bridge -3 10 50,000,000 500,000 4 Kathmandu 13 - - Temple - 1 - - - 5 Kathmandu 17 - - Sobha bhagwati - - - Table 4-5: Bishnumati River (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Bis soc 1 2 7 5 1 3 2 Bis soc 2 6 38 32 11 17 3 Bis soc 3 10 42 38 16 23 4 Bis soc 4 - - - - - 5 Bis soc 5 - 28
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 4-6: Bishnumati River (Awareness level and institutional framework) SN Section Cause of Protection work to be Existing flood/Inundation adopted organization 1 Bis soc 1 Heavy sand mining Sand mining should be No stopped. Retention of wall 2 Bis soc 2 Heavy sand mining Sand mining should be No Narrow river channels stopped. Retention of wall 3 Bis soc 3 Excessive sand mining Sand mining should be No Narrow river channels stopped. Retention of wall 4 Bis soc 4 No flood n inundation 5 Bis soc 5 No flood n inundation Damages from flood/inundation The people have remembered the 2006 AD flood in Bishnumati River that has damaged the houses, agricultural land, infrastructure (bridges), road, religious places like temple and Gumba (Plate 4.2). The number of vulnerable houses are 6 in Gongabu-3 (Bis-soc 2) and 10 in Gongabu-4 (Bis-soc 3). Around 15 ropanies of residential land and 145 ropanies of agricultural lands are vulnerable to flooding impact in these two locations. There had been significant damage to crops in the past. The above figure indicates that though the river has affected moderate area of land, it has significant amount in monetary aspect regarding the high land value in the valley. The areas around the confluence of Sangla Khola and Bishnumati River always suffer from inundation. These two locations are vulnerable to possible impact of flood because of insufficient river training structures and narrowed river channel. However, the sites that used to be frequently affected by the river in the past (Bis- soc 4 and Bis-soc 5) have become safer after construction of road corridor and flood walls. Awareness level regarding flood vulnerability The people have indicated the excessive sand mining and narrow river channel as one of the reason of flood problem. The people think that in order to reduce the impact of flood, construction of flood wall, restriction to sand mining and encroachment to river bank should be stopped so as to keep the river under its natural regime. Similarly, the public buildings and schools are identified as evacuation centres and there is rescue route is available in the area. Institutional framework In the surveyed area, no institutions are involved towards working in the flood protection measures. The individual residents and housing companies have constructed the mitigation structures (Bis-soc 2 and Bis-soc 3). This is to be noted that the people from these two localities have even not made any approach to work for flood control measures through the coordination with other organizations. 29
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Flood control measures and their effectiveness Though the construction of river training structures are working to some extent and flood problem does not exist at present, the place remains vulnerable because of narrowing the river channel in order to occupy more lands by the housing companies. Rescue routes and evacuation centres Rescue routes are available in all site, however, the necessity of evacuation centre is strongly felt. They suggested that it is necessary to identify and declare evacuation centres in the vicinity of their locality. The people are willing to participate in any flood control activities to be conducted by any organization. It can be justified from their involvement in the past during the construction of flood wall in their area by the housing company. Support necessary to cope with flood and other issues Construction of river training structures, control to sand mining, conducting awareness program, maintaining the natural river width are some of the works that the people have identified in order to cope with the flood. 4.3 MANOHARA RIVER The following four locations were identified along the Manohara River for social survey: • Man-soc 1: 250 m upstream and downstream from Khulal Tar located at north of Indraini • Man-soc 2: Upstream area of Manohara bridge at Mulpani-Nilbarahi. Severe bank erosion problem affecting road section and bridge. • Man-soc 3: East of Gothatar planning area that is having bank erosion problem resulting from river entrenchment due to extensive sand mining. • Man-soc 4: 150 m upstream and 200 m downstream from the confluence of Hanumante at Mattitar. Inundation problem due to river blockade. The social issues identified due to the effect of Manohara River are briefly summarised in Table 4-7, Table 4-8, and Table 4-9. Table 4-7: Manohara River (Flood hazard data) S VDC Ward Area(ropani) Vulnerable Vulner Damage Damage N Reside Agricu Infrastructure/h able value of value of ntial lture istorical houses land + crops places/industri house(Rs) (Rs) es 1 Indrayani 5 17 300 Road – 30m 4 5,440,000 1000,000 Electric poles – 5 Govt.School - 1 2 Dubakot 9 200 Road -10m 600,000 Bridge Electric poles - 4 Water intake building -1 3 Gothatar 8 30 20 5 15,000,000 100,000 4 Imadole 3 50 Mahadev temple 5 40,000,000 200,000 30
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 4-8: Manohara River (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Man soc 1 4 11 13 5 10 2 Man soc 2 3 Man soc 3 5 12 18 8 7 4 Mansoc 4 5 14 16 7 8 Table 4-9: Manohara River (Awareness level and institutional framework) SN Section Cause of flood Protection work to Existing Supporting be adopted organization organization 1 Man soc Regular change of river Retention of wall No DWIP 1 channel. 2 Man soc Excessive sand mining. Sand mining should No 2 Narrow river channel at be stopped. bridge. Retention of wall. 3 Man soc Excessive sand mining Sand mining should No Private 3 be stopped. (Planning) Retention of wall. 4 Man soc It is the junction of The island like Tripur DWIP 4 Manohara, hanumante structure should be Sangam Bhu and godawari. removed. samrachhan Retention of wall. samittee Damages from flood/inundation All the sites falling around the Manohara River had been affected by the floods in the past, notably during 2006 BS in the recent past (Plates 4.3 and 4.4). The sites Man-soc 1 and Man-soc 3 have 4 and 5 vulnerable houses, while 17 and 30 ropanies, respectively residential land is vulnerable. The affected agricultural land was 300, 200 and 20 ropanies in sites Man-soc 1, Man-soc 2 and Man-soc 3, respectively. The flood affected roads, bridges, electric poles and school. The worse case happened in Man-soc 3 during 1996 monsoon when 2 storied pillar house was completely swept away in Maddhepur thimi municipality-1, Lachhi village. The above mentioned losses and vulnerability accounts billions of rupee indicating the severity of the flood in the river. Awareness level regarding flood vulnerability The people in the surveyed area are quite familiar about the causes of flood/inundation in their surrounding and have reported that the regular change of river channel (river shifting), sand mining, narrowed river channel together with meteorological factors are the causes of flood impact. In order to lessen such problems, they also proposed to construct flood walls/dykes, control to sand mining and provide necessary width for the river. Institutional framework Some organizations are involved in mitigation activities in the area, namely DWIDP, World Vision and private sector (Nilgiri Housing), respectively in Man-soc 1, Man-soc 2 and Man- soc 3 sites. However, no local institutions are set to cope with the flood in the area due to lack of social network. 31
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Flood control measures and their effectiveness There have been no sufficient flood control measures implemented by the government organization except at few places. However, the private housing companies are protecting the areas occupied by them by constructing structures only to protect their properties and not thinking the probable impact to the surroundings. The people are looking for further government initiatives in this matter. Rescue routes and evacuation centres The people feel that they have enough rescue routes during the time of flood and think that the schools, public places/open areas and community buildings can be used as evacuation centres. Support necessary to cope with flood and other issues The affected people are looking for the support to construct flood walls, making availability of gabion wires and conducting awareness rising programs from government and non- government institutions. They are also looking for short term and long term planning towards flood control to reduce the vulnerability in their locality. They are also equivocal in the matter that if any activity directed to flood control is being implemented in the community, they will participate and provide full support in every aspect. 4.4 HANUMANTE KHOLA Social survey was carried out at the following two locations in the vicinity of Hanumante Khola: • Han-soc 1: Area between Suryabinayak and Jagate. The area is having inundation problem • Han-soc 2: Area between Shyamashyama Dham and Thimi was selected as it is having inundation problem. The Hanumante River has also inundated significant area of agriculture field as well as houses located at the lower elevations. The people around these locations are also looking for effective mitigation measures to be conducted from the government level. The social issues identified due to the effect of Hanumante River are briefly summarised in Table 4-10, Table 4-11, and Table 4-12. Table 4-10: Hanumante River (Flood hazard data) S VDC Ward Area(ropani) Vulnerable Vulnerab Damage Damage N Resid Agric Infrastructure/histo le value of value of ential ulture rical houses land + crops places/industries house(Rs) (Rs) 1 Bhaktapur 7 - 150 Hanumante ghat - 7000,000 Maheshwori temple Pardeshi bhimsen 2 Thimi 7 - 200 Saw mill - 15,000,000 Pen and TV industries 32
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 4-11: Hanumante River (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Han soc 1 - - - - - 2 Han soc 2 - - - - - Table 4-12: Hanumante River (Awareness level and institutional framework) SN Section Cause of flood/Inundation Protection work to be Existing Supporting adopted organization organization 1 Han Narrow outlet at bridges Retention of wall No Bkt soc 1 Narrow river channels Wider river channels Nagarpalika River of bank fall down and block the river 2 Han Narrow river channel Retention of wall No soc 2 4.5 GODAVARI RIVER The only one location selected for social survey lying along the Godavari River is lying at 250 m upstream from the confluence of Manohara at Balkot. The area is having inundation problem due to river blockade (Location: God-soc 1). The people reported that the problem is basically due to surging effect in the river during the monsoon period. In order to reduce such impact, it is necessary to maintain the necessary width of the river during high flow period and any action towards narrowing the river channel should be immediately controlled. The output of the study is tabulated in Table 4-13, Table 4-14, and Table 4-15. Table 4-13: Godavari Khola (Flood Hazard data) S VDC Ward Area(ropani) Vulnerable Vulnerable Damage Damage N Residenti Agricult Infrastructure/his houses value of value of al ure torical land + crops places/industries house(Rs) (Rs) 1 Imadol- 2 500 - 14 250,000 Balkot Table 4-14: Godavari Khola (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 God soc 1 15 40 44 16 22 Table 4-15: Godawori Khola (Awareness level and institutional framework) SN Section Cause of Protection work to Existing Supporting flood/Inundation be adopted organization organization 1 God River blockade by Retention of wall Motishwor sudhar DWIP soc 1 hanumante samittee 4.6 DHOBI KHOLA There are four locations where social survey was conducted. The areas and problems are as follows: • Dhob-soc 1: located at 500 m upstream from Gopikrishna Hall is having the problem of bank stability. 33
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report • Dhob-soc 2: Dhobi Khola at Kalopul. The area is also having the problem of bank stability as well as inundation. • Dhob-soc 3: 500 m upstream from the bridge at Anamnagar. There is inundation problem in the settlements of landless people. • Dhob-soc 4: The Buddhanagar area located at 200 m downstream from bridge at Babarmahal is experiencing inundation problem. The upper reaches of Dhobi Khola are having some problems of bank cutting, inundation. However, the vulnerability at lower reaches is mostly due to the settlement in the river floodplain. Therefore, the people feel that the issues should be properly addressed in order to reduce the human encroachment. The output of the study is tabulated in Table 4-16, Table 4-17, and Table 4-18. Table 4-16: Dhobi Khola (Flood hazard data) S VDC/ Ward Area (ropani) Vulnerable Vulnera Damage Damage N Municipal Reside Agricu Infrastructure/histori ble value of value of ity ntial lture cal places/industries houses land + crops house(Rs) (Rs) 1 Kathmand 6 - - Bridge -1 - - - u 2 Kathmand 7 - - Bridge – 1 5 300,000 u Corridor 3 Kathmand 32 - - Road 60 300,000 u 4 Kathmand 10 - - Road - u Table 4-17: Dhobi Khola (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Dhob soc 1 - - - - - 2 Dhob soc 2 5 14 16 4 7 3 Dhob soc 3 60 160 140 75 100 4 Dhob soc 4 - - - - - Table 4-18: Dhobi Khola (Awareness level and institutional framework) SN Section Cause of flood/Inundation Protection work to be Existing Supporting adopted organization organization 1 Dhob No problem of flood soc 1 2 Dhob Narrow outlet at kalopul Outlet should be wider No soc 2 Narrow river channel 3 Dhob Narrow river channel Retention of wall No soc 3 4 Dhob Blockade by solid waste Management of solid No soc 4 waste 4.7 SANGLA KHOLA Sangla Khola is the tributary of Bishnumati River. There is extensive sand mining from river bed in area north of Gongabu resulting the river banks vulnerable with respect to bank stability (Location: Bis-soc 1). 34
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report People have also reported that the bank erosion by Sangla Khola is one of the major problems. 4.8 SAMAKHUSI KHOLA The area lying between Samakhusi and Ringroad is having inundation problem due to this river (Location: Sam-soc 1). No other places have been noticed as having severe problem due to this khola. The output of the study is tabulated in Tables Table 4-19, Table 4-20, and Table 4-21. Table 4-19: Samakhusi Khola (Flood Hazard data) S VDC Ward Area(ropani) Vulnerable Vulnerable Damage Damage N Reside Agricu Infrastructure/his houses value of value of ntial lture torical land + crops places/industries house(Rs) (Rs) 1 Ktm - - - 50 100,000 Table 4-20: Samakhusi Khola (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Sam soc 1 50 170 130 25 85 Table 4-21: Samakhusi Khola (Awareness level and institutional framework) SN Section Cause of flood/Inundation Protection work to be Existing Supporting adopted organization organization 1 Sam River encroachment Construction of walls No soc 1 4.9 BALKHU KHOLA The two locations lying along the Balkhu Khola selected for social survey are: • Bal-soc 1: Downstream and upstream of Khasibazar area affected from bank cutting and inundation from Balkhu Khola. • Bal-soc 2: The area around the Balambu bridge is having inundation problem. The output of the study is tabulated in Table 4-22, Table 4-23, and Table 4-24. Table 4-22: Balkhu Khola (Flood hazard data) S VDC Ward Area(ropani) Vulnerable Vulnera Damage Damage N Reside Agricu Infrastructure/ ble value of value of ntial lture historical houses land + crops places/industries house(Rs) (Rs) 1 Ktm 14 4 12 Bridge - 1 12 40,000,00 60,000 0 2 Balambu 4 15 120 Bridge-1 4 80,000,00 600,000 0 Table 4-23: Balkhu Khola (Demographic features) Sn. Section No.of HHs Male Female Above Children 65 1 Bal soc1 12 70 50 18 20 2 Bal soc 2 4 12 11 8 6 35
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 4-24: Balkhu Khola (Awareness level and institutional framework) SN Section Cause of Protection work to be Existing Supporting flood/Inundation adopted organization organization 1 Bal soc 1 Narrow river channel Retention of walls No 2 Bal soc 2 Narrow outlet at bridge Retention of wall No 4.10 KARAKHUSI KHOLA The two locations selected for the social survey are as follows: • Kar-soc 1: There is bank erosion and inundation problem around the Soaltee area. • Kar-soc 2: The area lying in the Siuchatar area having flood problem. The social issues identified due to the effect of Karakhusi Khola are tabulated in Table 4-25, Table 4-26, and Table 4-27. Table 4-25: Karakhusi Khola (Flood hazard data) S VDC/Muni Ward Area(ropani) Vulnerable Vulnerable Damage Damage N cipality Reside Agricu Infrastructure/his houses value of value of ntial lture torical land + crops places/industries house(Rs) (Rs) 1 Kathmand 20 - 20carores u 2 Suichatar 1 10 70 - - - 300,000 Table 4-26: Karakhusi Khola (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Kar soc 1 - - - - - 2 Kar soc 2 - - - - - Table 4-27: Karakhusi khola (Awareness level and institutional framework) SN Section Cause of Protection work to be Existing Supporting flood/Inundation adopted organization organization 1 Kar soc Narrow river channel Retention of wall No Private initiation 1 2 Kar soc Narrow river channel Retention of wall No Private planning has 2 constructed 8fit tall wall 4.11 NAKHU KHOLA The two locations selected for the social survey are as follows: • Nak-soc 1: 300 m upstream from confluence of Bagmati Nadi. Problem: Inundation and bank erosion. • Nak-soc 2: 400 m upstream from Nakhu Bridge- Nakhipot. Problem: Bank erosion and inundation. The social issues identified due to the effect of Nakhu Khola are summarised in Table 4-28, Table 4-29, and Table 4-30. 36
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 4-28: Nakhu Khola (Flood hazard data) S VDC Ward Area(ropani) Vulnerable Vulnera Damage Damage N Reside Agricu Infrastructure/his ble value of value of ntial lture torical houses land + crops places/industries house(Rs) (Rs) 1 Saibu 4 10 200 Road 50m 6 10,000,000 1,000,000 Water supply building – 1 Magar samaj kendra 2 Saibu 6 20 100 Pig farm 14 20,500,000 100,000 Table 4-29: Nakhu Kkhola (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Naok soc 1 6 20 16 8 15 2 Nok soc 2 14 35 25 18 30 Table 4-30: Nakhu Khola (Awareness level and institutional framework) SN Section Cause of flood/Inundation Protection work to be Existing Supporting adopted organization organization 1 Nok soc River blockade by bagmati Narrow bagmati river No 1 channel should be widened 2 Nok soc Narrow river channel Retention of wall No 2 Damages from flood/inundation The recent damage due to flood was of 2063 BS when six houses at Nak-soc 1 site and 14 houses at Nak-soc 2 site became vulnerable. The event damaged 50 m road, six electric poles endangering the Magar community at Nak-soc 1. Similarly 15-20 pig hut were damaged, tens of ropani of residential land and agriculture field were affected due to the flood. Such event repeated during the 2007 AD flood when several smaller farmers lost their income source due to flood e.g. pig farm around the Bhaisepati village. This is only one scenario and such event could take place any time during the monsoon when there is continuous rainfall in the catchment. Awareness level regarding flood vulnerability The awareness level of the people at these localities is quite high as they are familiar about the nature of river. They know that during high flood period, Bagmati River blocks the Nakhu Khola causing back surging and inundating and cutting the adjacent land. They propose that the narrowed channel of Bagmati should be rehabilitated so as to allow more space for discharge of water thereby reducing the impact of Nakhu Khola. Similarly, necessary construction of flood protection measures also reduces vulnerability. Institutional framework Besides the vested interest of housing companies, none of the organizations have carried out significant measures to reduce the impact of Nakhu Khola flood. 37
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Flood control measures and their effectiveness Since no systematic measures are taken to control the flood, the effectiveness can’t be evaluated at these locations. Rescue routes and evacuation centres No evacuation centres are informed to the local community, which is their need. Support necessary to cope with flood and other issues Construction of retaining wall and conduct awareness rising activities are some of the aspects to be incorporated in any mitigation works. 4.12 KODKU KHOLA The two locations selected for the social survey in the Kodku Khola are as follows: • Kod-soc 1: Kodku Khola at Imadol. Inundation and bank erosion problem. • Kod-soc 2: Hattiban area. Inundation and bank erosion problem. The social issues identified due to the effect of Kodku Khola are summarised tabulated in Table 4-31, Table 4-32, and Table 4-33. Table 4-31: Kodku Khola (Flood Hazard data) S VDC Ward Area(ropani) Vulnerable Vulnerable Damage Damage N Reside Agricu Infrastructure/ houses value of value of ntial lture historical land + crops places/industries house(Rs) (Rs) 1 Imadol 5 8 - Bridge-1 14 10,000,000 - Road 15m Electric poles 3 Saw mill-1 2 Dhapakhel 7 10 22 Wall-70m 5 20,000,000 10,000 Road 120m Table 4-32: Kodku Khola (Demographic features) Sn. Section No.of HHs Male Female Above 65 Children 1 Kod soc 1 14 46 38 10 22 2 Kod soc 2 5 17 13 4 9 Table 4-33: Kodku Khola (Awareness level and institutional framework) SN Section Cause of flood/Inundation Protection work to Existing Supporting be adopted organization organization 1 Kod soc Too much narrow river River area should be No 1 widened 2 Kod soc Narrow river channel Retention of wall No Private 2 initiation Damages from flood/inundation Like in other rivers, there had also been damages to the houses, agriculture field, residential area and other infrastructure nearby the Kodku Khola due to the flood of 2006 AD and 38
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report before (Plate 4.5). One house was completely damaged and 14 houses are vulnerable at Imadol (Kod-soc 1) and five houses are vulnerable at Dhapakhel (Kod-soc 2). Likewise, 8 ropani of residential land affected and one saw mill was inundated that caused damage to timber at Kod-soc 1. Similar damage happened at another location (Kod-soc 2) where 10 ropani of residential land is vulnerable, the inundated water swept away all fishes from the fishery, around 120 m stretch of gravel road as well as crops were fully damaged. The main causes of the damage due to the river is because the river is artificially made narrow than it used to be in natural condition. Awareness level regarding flood vulnerability The level of awareness of the people in these localities is of satisfactory level as they are familiar to causes of flood/inundation in their localities. Institutional framework There are no government and non-government institutions working for flood management in the area surrounding this river. However, the people have initiated themselves to protect their land and houses by constructing the flood walls at vulnerable locations. Flood control measures and their effectiveness The perception of the people is that the structures constructed to protect the surrounding area normally works as far as it is technically sound. Rescue routes and evacuation centres The people think that there is no problem of rescue route but identification and capacity building of evacuation centres and the information dissemination is necessary. Support necessary to cope with flood and other issues Awareness program is essential in the area together with the construction of mitigation structures. The trend of land encroachment should be effectively checked. 39
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 5. FIELD CONDITION ALONG RIVER CORRIDORS The field condition with respect to river morphology, river hydrology, location and effectiveness of existing engineering structures and engineering geological observations has been dealt in this chapter. Most of the major rivers and their tributaries have been originated from the surrounding hills of the Kathmandu Valley. The major hilly areas are Nagarjung, Sivapuri, Nagarkot and Phulchauki representing, respectively, in clockwise direction from west, north, east and south. Rivers flowing in the vicinity of hilly area have high gradient, in contrast the gradient of river and velocity of flow is law in the valley flow areas. The discharges of these rivers were significantly lower during the survey period (November to January). As the peak flow data of gage stations of the rivers are not available for determination of the bankfull stage, the maximum stream flow imprints in the river channel area of surveyed rivers is used as an index of high flow level of the monsoon discharge. Bankfull discharge is associated with a momentary maximum flow that has an average recurrence interval of 1.5 years as determined using a flood frequency analysis (Dunne and Leopold 1978). Bankfull discharge defines the morphological characteristics of the channel such as bars, meanders, and bends. The bankfull states were accurately identified in the field and these states were used as a common reference point to describe quantitatively the channel and to assess river condition. The bankfull concept, historical data, other field data, property loss etc., are used for vulnerability assessment of the rivers in the Kathmandu Valley. In some stream segment, the floodplains are developed within the incised river channel, while in others the flow just fills the channel to the top of its banks where flooding begins. In such case the water begins to overflow onto to settlement or cultivated areas and inundation occurs. Kathmandu valley got flooded in past due to massive downpour of monsoon. Other causes of the flood in the valley are the river bank encroachment, high sinuosity of river, narrow outlet, increase in riverside settlements etc. The surveyed river includes Bagmati, Manohara, Bishnumati, Hanumante, Dhobi Khola, Samakhusi Khola, Manamati Khola, Balkhu Khola, Nakhu Khola, Kodku Khola and Godavari Khola. The main data of the individual rivers are presented below: It has been observed that the rivers within the valley are having natural shifting as well as trained. Thus, the scenario can be divided as river shifting and river encroachment. The use of ALOS satellite image (2008), QuickBird satellite image (2005), Topographical map of Department of Survey (1995) and field verification (2008) has been carried out in order to study the temporal river course changes. Fig. 5.1 shows the locations of river shifting and river encroachment within the Kathmandu valley. Details are given in description of respective river. 40
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 5-1: River shifting and encroachment within the Kathmandu Valley (Alos satellite image in the background) 5.1 BAGMATI RIVER The features observed during field survey are described in the following sub-headings, where each described content in the sub-headings has been arranged in an order from upstream to downstream of the river: River morphology Bagmati River originates from the hilly area of Sivapuri Range and flows towards southwest to south before merging with Manohara River. River gradient in the mountainous area of Sundarijal is high and the river carries large sediment load consisting of large blocks, boulders and pebbles in the upper reaches. River has straight to highly meandered morphology to downstream from the Sundarijal Bazar area. Mid-channel bars were also observed in several places. Small floodplain areas are alternatively found along the stretch of the river. When the river descends from the Sundarijal Jail area, river gradient decreases and the channel materials are comprised of little small boulder, cobble and gravel with sands. At the central part of the valley, river gradient reduces much more and river transports smaller size of bed load material comprising small gravel and sands that are deposited as point bars. The river has straight channel morphology in the sector between Gokarneswor gorge, where hard rock is exposed, and Gothatar village with mid-channel 41
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report bars in several places and the river shows meandering channel type with lower sinuosity in the stretch downstream from the Gothatar village. River terraces on both sides of the river are used for agriculture purpose to the upstream of the Gokarneswor Temple. To the downstream area from the Gokarneswor gorge river terraces are mostly covered by settlement area that is expanding rapidly towards the river side. The river again passes through the gorge of Pashupati area, where hard rocks are cropped out. The river sides in the area between Pashupatinath and the confluence of Bagmati and Manohara rivers is densely occupied by settlement area. In this sector sinuosity of the meander channel is higher than in the upstream area. Downstream from the confluence of Manohara River, Bagmati River flows towards west and shows slight meandering morphology. It abruptly change its direction of flow to south ward direction from Teku Dobhan. River shifting Bagmati River to downstream from Gokarneswor gorge is mostly confined to a definite channel course by bank protection work as well as from human encroachment activities to the river channel and floodplain area. Minor shifting of river channel is found to the upstream of Uttarbahini, Nayanpati and Ruksetar area. Flooding area To downstream from Sundarijal, high flood level mark was noticed at 1 m above the current river bed. Channel materials are composed of small boulder, cobble and gravel with little sands. Estimation of Current channel flow shows that depth and width of the river in the area are, respectively 0.25 to 0.4 m and 7 to 11 m. According to local people, flood occurs each 3 to 4 years for about one hour and damage paddy field from each flood. The area downstream from steel bridge located near Sundarijal market was inundated in 2007. Sediments of about 5 to 10 cm thick sands were accumulated from the flood on the cultivated land. Although sand mining activity from the river bed have been reducing channel aggradation in the area and aiding to reduce overflow of monsoon water through top of banks, bank stability in the area is being reducing by stream flow erosion at toe of banks because of sand extraction. At Uttar Bahini area the active channel depth and width were estimated at a range from 0.2 to 0.4 m and 6 to 11 m, respectively. Likewise high flood level mark was observed at 1.75 m above from river bed. According to villagers, inundation in the area was occurred in1967 for 4-5 hours and water level was raised up to 1 m from the top of river bank. About 5 to 10 cm thick fine sediments were accumulated on the cultivated land after inundation. The causes of inundation in the area could be blockade of high river discharge from narrow width of bridge over the river at Uttar Bahini, high river meandering morphology and narrow natural gorge at Gokarneswor Galchi, where basement hard rocks are exposed. Flood prone area in this reaches of the river is wider than the upstream area. To upstream of Gokarneswor Gorge about 5-10 cm thick sand layer was accumulated on surface of lower terrace from the flood of monsoon in 2008. River side encroachment from construction of settlement area is more to the downstream direction of the river from Pashupatinath, which is an another major cause of flooding because of insufficient spaces for natural flow regime consequently tends the stream flow blockade during high level of discharge in monsoon period and result in inundation 42
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report surrounding the area as well as trigger to river incision. Both sand extractions from river bed and river encroachment have damaged the infrastructures located at the surrounding of the river. Sukumbasi Tole settled between Sinamangal and Minbhawan is an example, where floodplain and flood prone area of the river are encroached by constructing road and slums (Plates 5.1 and 5.2). Capturing of river by slum area is still continuing along the river corridor (Plate 5.3 and 5.4). Such activities also trigger inundation in the surrounding area because of narrow river channel width. The active channel of the river in the stretch between the confluence of Dhobi Khola and the confluence of Manohara, downstream to the confluence of Bishnumati River, upstream and downstream from the confluence of Nakhu Khola varies from 20 to 30 m. Width of the channel increases up to 40 m at upstream of Thapathali bridge and downstream to Ring Road bridge at Balkhu. Width of floodplain of the river increases up to 50 m in these areas. The channel width is highly reduced in some places by depositing waste materials, which can trigger flooding at the upstream as well as confinement of channel at the location of infrastructure can damage the structure due to a definite site erosion (Plate 5.5). The flooded areas in the past along the corridor of the Bagmati River are Sankhamul in 1988, Thapathali in 1991, Minbhawan-Subidhanagar in 1995, Balkhu and Tribhuvan University Main Gate in 1999 and 2002, and around Modern Indian School at Chobhar in 2002. Bank stability Material in the bank section downstream to Sundarijal Bazar area consists of about 0.5 to 1 m thick layers of sand and silt and about 0.3 to 0.5 m thick gravel, respectively, at top and bottom (Plate 5.6). Slope of banks varies from 20 to 80 degree and the average value represents 40 degrees. Sparse bushes and grasses are the riparian vegetation on the banks in the area. Riparian vegetation like trees and bushes along the river section around the Gokarneswor village area were found denser than in the upstream area. River banks in the stretch between Sundarijal and Gokarneswor Galchi were failed particularly at river bending side because of loose sediment compaction and lack of riparian vegetation. Depth of the river channel from its lower terrace has been increasing up to 3.5 m to downstream area from the Pashupatinath Temple due to illegal sand mining from river channel at its lower reaches. Channel incision has also exposed semi-consolidated mud of Kathmandu Basin deposit at bottom in some places along the river channel. Bank failures were observed around area at downstream of Basnetgaun near Tribhuvan University Gate at Kirtipur. Flood control structure Banks at Uttar Bahini area are partly protected by engineering structures. Left bank of the river at upstream from bridge near Gokarneswor village has also protected by Gabion structure of 2 m high and 1 m wide. Flood walls on both sides of river are continuously constructed in the river sector between Gokarneswor gorge and Guhewsori Temple (Plate 5.7). To downstream direction from Pasupathinath Temple area banks are partly protected in several places e.g. in Tilganga, Bhimsengola, Tinkune, Sankhamul, Budhanagar, Thapathali, Teku, Balku etc. Such structures are collapsed in few places e.g. in Budhanagar area (Plate 5.8). 43
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Vulnerable infrastructure Pipeline for water supply to the Kathmandu city from Sundarijal aligned near the river course can be damaged from the impact of big boulder during extreme monsoon discharge (Plate 5.9). Area along the river corridor to the downstream from the Sundarijal market to Gokarneswor is used for agriculture purpose except in Uttar Bahini and Gokarneswor Galchi, where religious and settlement areas are located. Demarcation wall of a floriculture centre at right side of the river downstream from Sundarijal jail is constructed at about 2 to 100 m distance from the river bank, hence some territory of the centre is vulnerable to flood. Bridge made up of brick and cement at Gokarneswor village over the river has higher opening than upstream width of the river; however, sedimentation near the foundation of the bridge has strongly reduced opening of the bridge, which can confine localized channel flow below the bridge and can trigger channel erosion at foundation. A deep tube well located at right bank of the river near the bridge is in safer position because of the protection from bridge abutment, nonetheless reducing bridge span can block high discharge and can have possibility of inundation in the area. At upstream of Gokarneswor gorge, foundation of a small steel bridge at right bank of the river is placed on floodplain, where river current in each monsoon hits the area. Foundations of the concrete bridge over the river at Ring Road near Pashupatinath, Tilganga are deeply exposed because of high rate of river incision triggered by extensive sand mining at downstream direction. Likewise localised channel erosion at foundation of concrete bridge over the river near Bhimsengola, Baneswor owing to river encroachment and sand mining has recently damaged the bridge and circulation of transportation through the bridge is complete prohibited (Plate 5.2). Foundation of the concrete bridge over the river at Subidhanagar, Tinkune is also in worse condition (Plate 5.10). Although check dam across the channel has been constructed at little downstream from bridge site, river bed scouring has not still been reduced. 5.2 MANOHARA RIVER The Manohara River corridor from its mountainous area to valley floor is described in the following sub-headings: River morphology The valley of Manohara River at its upstream from the Salinadi lies in the mountainous area, where the valley is narrower than downstream area. Land with sparse vegetation cover and barren soil terrain in the hilly areas have been contributing sediments from mantle erosion of surficial soil and also from landslides to the Manohara River via small gullies and tributaries. River gradient in the mountainous area is high and sediment load of the river comprised of boulders to cobbles with gravel and sandy materials. Gradient is gradually decreased to downstream direction from Salinadi Temple and river valley also becomes wider. River channel has been aggrading from accumulation of bed load sediments as point bar and also as occasional mid channel bar, which are comprised of sandy gravels. Morphology of the river shows partly straight to meandering type in the area between Salinadi Temple and Susma Memorial Hospital area. 44
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report The river is strongly meandered in the stretch between the concrete bridge at Nilbarahi- Danchi road and Susma Memorial Hospital. Alternating point bars are normally developed in many places that of gravel and sand. Sinuosity of the meandering river in the area is found high and there is also several abandoned paleo-channel area, which have already been used for cultivation in most of the places. Sinuosity of the river is the highest to downstream up to about 3 km from the concrete bridge over the river at Nilbarahi-Danchi road, while the river has partly meandered to mostly straight channel morphology to downstream section up to its confluence with Hanumante Khola. River gradient gradually starts to increase due to extensive sand mining activities from river channel area. Large area of abandoned meander channel and extensive coverage of mining from channel area are the characteristics feature near Gothatar village (Plate 5.11). The terraces on both sides of the river to upstream from Gothatar village are occupied by agriculture lands. Hence, the risk of flood is only on crops and agricultural lands. The river terrace height near Gothatar village is rapidly increasing due to extensive continuous sand mining from the channel and its surrounding areas. The area to the right side of river is planned for new settlement. Area around Jadibuti is also covered by significant settlement. To downstream from Jadibuti, land to the left side of the river is separated for new settlement, while the right side is used for agriculture purpose. River shifting Sinuosity of the meandering river in the area between the concrete bridge at Nilbarahi- Danchi road and Nepal Engineering College is found high and there are several abandoned paleo-channel. Higher degree of sinuosity indicates the higher degree of lateral river migration in the area. Higher amount of river shifting is found in the area between Gothatar and Changunarayan villages, where maximum shifting is about 400 m (Figure 5-2 and Figure 5-3). Figure 5-2: Manohara River shifting between Godar and Gothatar area. Blue line shows the river course as of DoS Topomap and the present river course can be observed on the satellite image in the background. 45
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 5-3: Manohara river shifting around Phuyalgau-Changunarayan area. Present river course can be seen on the satellite image (Alos) while the blue area represents the river course as of topographical map of DoS. Figure 5-4: Manohara river shifting around KLhulatar Bramhakhel area. Present river course can be seen on the satellite image (Alos) while the blue area represents the river course as of topographical map of DoS. 46
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report In between Khulaltar and Brahamakhel area, river has shifted up to 200 m (Figure 5-4). Manmade river entrenchment and migration is also significant in the Gothatar area (Plate 5.12). Lateral migration of river channel erodes the agriculture land (Plate 5.13). A new channel is opened at downstream of Manohara Bridge near Jadibuti according to the cadastral map of the government. Obviously, the area illustrates the shifting reach of the river. Further, natural channel flow pattern in the area has already started modification on both banks of the river by bank under cutting (Plate 5.14). Flooding area Width of active channel between Salinadi Temple and Susma Memorial Hospital area varies from 6 to 10 m and depth of channel flow ranges from 0.15 to 0.3 m during the survey time. High Flood Level mark was observed at a range from 1 to 1.5 m above from the current river bed. Height of floodplain varies from 1 to 1.5 m. Lower river terrace ranges in height from 2 to 3 m from river bed. A remarkable flood was occurred in 2003 near Khulaltar village. According to local people the flood had covered an area of about 25, 000 sq m for 3-4 hours and one local farmer was killed in the flood. In 2007, inundation was again occurred for half an hour at downstream of the area. Depth of inundated water was about one meter and about 5 to 25 cm thick fine sand, silt and clay were deposited in an area of about 60,000 sq m of left bank. Floodplain and flood prone areas are generally wide. The width of the active channel flow ranges from 6 to 13 m and depth varies from 0.15 to 0.3 m between the concrete bridge at Nilbarahi-Danchi road and Susma Memorial Hospital. High flood level mark was found from 0.55 to 1.5 m above river bed and floodplain height is situated at 0.55 to 1 m above the river bed. Bankfull width in the area have a range from 15 to 35 m. Lower terrace in the area between the concrete bridge at Nilbarahi-Danchi road and Susma Memorial Hospital is from 1.5 to 2 m high from river bed. Floodplains are well developed in the area (Plate 5.15). In the river stretch up to about 1 km downstream from the concrete bridge at Nilbarahi- Danchi road, the active channel flow width varies from 5 to 12 m and depth is from 0.2 to 0.3 m. High flood level was noticed at 0.8 to 1.5 m above the river bed and the width of bankfull stage is from 10 to 30 m. Floodplain height was found at 0.8 to 1 m above the river bed. Lower terrace are positioned at 1.5 to 2.5 m. To downstream up to the concrete bridge at the road between Sinamangal and Thimi the width and depth of active channel flow vary, respectively, from 5 to 12 m and from 0.15 to 0.5 m. High flood level was noticed from 0.75 to 1 m above the river bed. Bankfull stage has the width range from 10 to 80 m. Floodplain and lower terrace height vary, respectively, from 0.5 to 1 m and 1.75 to 4 m above the river bed. Floodplain areas are also cultivated in some places (Plate 5.16). The river sector between the concrete bridge over the river at the road between Sinamangal and Thimi and the bridge over the river at the road joining Koteswor and New Thimi has the channel flow width and depth, respectively, from 7 to 10 m and from 0.25 to 0.4 m. High flood level in the stretch was found at 0.75 to 1 m from river bed and the bankfull width 47
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report varies from 12 to 20 m. Floodplain height in the area is at 0.75 to 1 m from river bed. Lower terraces are situated at 1.5 to 2.5 m above from the river bed. Illegal sand mining from the river bed to downstream area from the concrete bridge at the road between Sinamangal and Thimi has been degrading bank stability as well as monsoon flood of few years back lost cultivated land of about 2 ropani due to toe cutting (Plate 5.17). Floodplain areas are also cultivated, which further increase obstacle to monsoon flow and trigger inundation. In little area height of natural levee is lower than flood prone height, hence overtopping of extreme weather discharge through the levee can trigger inundation. The area located at right bank of the river to the north of Jadibuti is an example, where levee height is lower than the flood prone height. Width and depth of channel flow in the corridor to downstream direction from the concrete bridge at Jadibuti to up to the confluence of Manohara River and Hanumante Khola vary, respectively, from 3.5 to 9 m and from 0.2 to 0.5 m. High flood level was found at 1 to 1.5 m above the river bed and width in bankfull stage is about 15 m. Height of lower terrace of the river ranges from 2 to 2.8 m from the river bed. The area little upstream and downstream from the confluence of Hanumante and Manohara rivers was inundated in 2007 due to blockade of stream flow at the confluence. Settlement area at the left bank was partially affected by the event. According to local people, water level was raised up to 30 cm from the existing top surface of bank. Height of existing flood protection wall is not enough. In addition, the walls are partially damaged. Further increment in water level during extreme weather event can cause more damage to life and property. Flood was also occurred in 1993 near by the Arniko Highway of Jadibuti area. Active river channel in the belt between the confluence of the Manohara River at Hanumante Khola and at Bagmati River shows the depth and width range, respectively, from 0.3 to 0.4 m and from 9 to 12 m. High flood level was noted at 1.8 to 2.5 m above the river bed. Width in bankfull stage varies from 7 to 25 m. Lower terrace height were found at 2 to 4.8 m from river bed. In the area, floodplain is wide (Plate 5.18). The area around the concrete bridge at Balkumari was flooded in 2000. Bank stability River banks to downstream from Salinadi Temple are covered by riparian vegetation of sparse trees, bushes and grasses. Vegetal cover on the banks of the river reduces bank erosion and finally helps to protect agriculture land from river erosion. Bank materials to upstream direction from concrete bridge located at Nilbarahi-Danchi road are comprised of 1 to 2 m thick silt and fine sand at top and 0.3 to 0.5 m thick sandy gravel at base. At few locations bank is composed of 2 m thick loose gravelly sand of channel deposit. Major problem of the area is the instability of vertical bank at river bend, where toe erosion is intense. In addition, when radius of curvature of the outer bend of river is very low, bank erosion is found more intense. In contrast, banks at a segment of straight channel are rather stable. Field evidence validates such property of satellite image and help to point out other failure sites on the image. Bank with a slope of 45 to 90 degrees are found stable when the bank material is comprised by cohesive soil and covered by grasses. 48
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Around the area at about 300 m upstream from the concrete bridge over the river near Nepal Engineering College, river has started to incise its bed due to illegal sand mining from river channel. River channel incision along the stretch of the river has manifested by development of new stepping terrace. Blocks are failed from the banks comprised by thick mud layer with interbedded silt and sand. Bank stability is found to be very low if the banks are composed of loose sediments of alternating sand and sandy gravel layer of old channel deposit (Plate 5.19). Such paleo- channels are frequently observed in the sector of river corridor up to 1 km downstream from the bridge located at the road connecting Danchi to Nilbarahi. Loose sediments of the old channel deposit at steep slope are sliding down to the river channel. Generally steep banks with composition of 1.5 to 2 m thick alternating silt, sand and gravelly sand located at the outer bend of the river with low radius of curvature were also collapsed in this sector due to toe erosion by river current. In some places bank erosion is extended for about 100 m along the river. Although high flow discharge during monsoon is confined within the channel when the river bed is deeply entrenched, possibility of bank failure is high owing to intense bank toe cutting. Such events are characteristics in many places at right and left banks of the river up to the bridge over the river near Sanothimi. According to local people lower terrace (former floodplain) with a height of 1.75 m from river bed was developed in 2-3 years period. River channel is brutally incised in a distance of 150 m. Rapid river entrenchment has triggered intense bank failure in the area. Bank material is comprised by Manohara River deposit and Kathmandu Basin deposit. These deposits are found together in some places at bank sequence. Manohara River deposit includes 0.5 to 1 m thick silt-clay and 1 to 1.75 m thick loose sand-gravel. Kathmandu Basin deposit consists of 1 to 2.5 m thick sand, silt and clay and 7 m thick gravely sand. Kathmandu Basin deposit shows higher degree of compaction than Manohara River deposit. The extensive sand mining site is situated to the northeast side of Gothatar, where the channel cross-section clearly illustrates increase in depth and width of the channel (Plate 5.11 and 5.20). Local people have started cultivation on the floodplain area of sand mining site (Plate 5.16). Such cropping practice lies in the area, where the probability of occurrence of flood each year during monsoon is very high. Stream flow during high flow period confines only within the sand mining area, nonetheless, higher bank height with steep slopes are more unstable and slumped in several places. In addition, riparian vegetations are also absent on the banks of the area. Sudden impact of river modification attributed to sand mining activities is also seen on agriculture product due to lack of surface irrigation because of increased bank height sometimes up to 5 m from stream channel. Bank materials to downstream from the concrete bridge over the river near Sanothimi consist of 0.5 to 2 m thick silt, sand and sandy gravel underlain by 1 to 1.5 m thick semi- consolidated mud of Kathmandu Basin deposit. Riparian vegetations are better in the area than the upstream sector. At upstream and downstream from the confluence of Hanumante and Manohara rivers bank materials are also comprised of silt, sand and gravel of the river deposit at top and semi- consolidated mud of Kathmandu Basin deposit at bottom. 49
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Area around Jadibuti is covered by significant settlement. A new channel is opened at downstream of Manohara Bridge near Jadibuti according to the cadastral map of the government. Distance between two banks is about 9 m and the channel is deeper and narrower than the earlier one (Plate 5.14). The morphology increases the hydraulic mean depth rapidly to reduce stream overflow, nonetheless, toe erosion of vertical banks on both sides has triggered block failure. Bank material is comprised of 0.7 m thick gravelly sand and sandy silt deposit of Manohara River at top and semi-consolidated fine-sand, silt and clay deposit of Kathmandu Basin at base. Land to the left side of the channel is separated for new settlement, while the right side is used for agriculture purpose. Upstream from the confluence of Manohara and Bagmati rivers, the height of lower terrace is about 5 m, which clearly indicates higher amount of river channel incision in the area. Semi- consolidated mud layer of Kathmandu Basin is exposed on the banks. Flood control structure River shifting to eastward near the road connecting Danchi village and Nilbarahi has brought stream flow current near the base of the road, where the area is protected by Gabion boxes, however lateral scouring at left bank of the river has been continuous and increasing risk to the bridge over the river as well as to road. The protecting structure is also tilted at one place due to toe erosion (Plate 5.21). In the Jadibuti area right bank of the river is protected by engineering structures, while the bank of the slum area is partly protected (Plate 5.22). The left and right banks of the river to upstream from the confluence of Manohara and Hanumante rivers are protected by Gabion structures. These structures also protect gravel road to left and earthen road to the right side of the river at downstream area. Height of the structures is at least two times than height of high flood level mark in the area. Foundation of the concrete bridge over Manohara River at Ring Road is protected by concrete slab, which inhibits river bed erosion. To upstream direction from the bridge lower terrace height is about 4 m. Right bank of the river is protected by 3 m high Gabion wall. It also protects blacktopped road and settlement area of the side. To upstream from the confluence of Manohara and Bagmati rivers, right bank of the river is partly protected by Gabion wall of 3 m height, which also protects blacktopped road. In this area high flood level mark was found at 2.5 m above the river bed. Riverbed in the area is in very deep depth (Plate 23). Vulnerable infrastructure Close to Danchi village river channel is near to the road linking Gothatar-Danchi area. Steeply sloping river bank of the road side is protected by 2 to 3 m high Gabion structure. A deep tube well located near by the road and river bank in the area is also protected by Gabion structure, however, very steep bank has already been started to erode owing to toe cutting by the stream current. In one site to the north of Gothatar village an electrical pole for three phase lines is highly tilted due to rapid changes in river morphology (Plate 5.24). Foundation of demarcation wall 50
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report of a housing colony in new settlement area in the Gothatar area has placed at the right bank of river, where floodplain of the river is encroached (Plate 5.25). Such activities increase vulnerability of new structure to flood. The area to the left side of river at upstream of newly constructed concrete bridge over the river near Sanothimi-Sinamangal is cultivated and the area to the right side of the river is allocated for new settlement planning. A new blacktopped road passing along the right bank of the river to access the settlement area is protected by masonry wall of 1.75 m height and 0.4 m width. However, sand mining activities in several places from the bank area have been deteriorating the river bank condition and making the structure more danger from flood in several places. Further, sewerage treatment structures of the area constructed near the right bank of the river are also now in more danger condition from flood because of sand extraction (Plate 5.26). In some places right bank of the river is lowered to the level of river bed owing to sand mining. Current of channel flow during monsoon period can easily hit the structures. Blacktopped road joining Jadibuti to Gothatar, sand cleaning industry and settlement area at Jadibuti are protected by Gabion revetment and Gabion structure from flood of Manohara River, while the Sukumbasi slums area situated at left bank of the river in the area is partly protected by sandbags and Gabion wall. Sheds of the homeless people have been installed at a distance of about 10 to 100 m from the river bank. Sand mining activities in the river bed of the area is also continuous and the activities have gradually degrading the engineering structures constructed for bank protection of the area. Foundation of the concrete bridge over the river at the road joining Kathmandu and Bhaktapu at Jadibuti is badly exposed because of vertical river erosion. Gabion check dams have been constructed to reduce further scouring at little downstream. Nonetheless, at about 25 m downstream from the foundation of bridge, a sudden jump of about 3 m in river profile is a remarkable evidence of river entrenchment. 5.3 BISHNUMATI RIVER The Bishnumati River has a large tributary called as Sanla Khola. Shangla Khola joins with Bishnumati River at Manohar Tirtha Ghat. Characteristics of both rivers are described in the following sub-headings: River morphology Bishnumati and Sangla Khola originate from hilly area of Shivapuri Range. When Sangla Khola flows in its upper reaches through the Kathmandu Basin sediments, it has very low sinuosity of meandering river morphology. The river channel gradient is high with small floodplain area. Lateral point bar and mid-point bars are composed of cobbles, gravel and sand. Sands have been extracting from the river bed at its upper reaches too. However, river channel aggradation in the area is high (Plate 5.27). Due to extensive sand mining from river channel area around Phutung village, river gradient is dramatically increased and channel incision is found higher toward downstream. The river near Karkigaon area has been scouring the channel substratum comprised of semi-consolidated Kathmandu Basin fill sediments. Lignite and hard iron pan appeared at the top of the deposit resist for erosion in certain degree and develop nick point in the river profile (Plate 5.28). However, headward channel erosion has been propagating the nick point toward upstream direction. Extensive sand mining activities from the river channel and its surrounding near Fhutung village has 51
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report enlarged the river valley width up to 75 m. Likewise depth of the river is also tremendously increased. River terraces of the area are used for agriculture purpose. Due to change in river morphology it is not possible to irrigate agriculture lands surrounding the river area consequently crop production is highly affected. In fact farmers had surface irrigation facility from water of Sangla Khola for these lands. River morphology to downstream from Manamaiju area shows a straight channel pattern, in contrast Bishnumati River to upstream from its confluence with Sangla Khola at Manohar Tirtha Ghat reveals a meandered channel pattern. River sides of the Bishnumati River at upstream from Manohar Tirtha Ghat is densely occupied by new settlement than in the Sangla Khola area. The river stretch to downstream from Manohar Tirtha Ghat shows straight to slightly meandered morphology. Small disconnected strip of floodplains are developed in the channel sides. Occasionally mid-channel bars with grass and bushes cover are formed. River channel is highly polluted by solid waste disposal. Point bars are comprised by gravel and sands. Depth of the river channel to downstream from the concrete bridge over Bishnumati River at Ring Road near Gongabu Bass Park has been increasing due to illegal sand mining from the channel area. Floodplain to the right bank of the river from the Gongabu Bass Park to the concrete bridge at Sohrakhutte-Balaju road has been encroached by Sukumbasi slums. Lower terrace on both side of the river are densely populated particularly from Kangleswori to downstream area. River shifting River channel is highly modified in the area between Baluwapati and Phutung, where extensive sand mining is in operation from the surrounding of the river channel area. In this area river channel is highly shifted. River channel is also modified in Maijuhiti and Baniyatar area. Flooding area Sangla Khola upstream from Khatrigaon has channel width from 2.5 to 5 m and depth from 0.05 to 0.07 m. High flood level varies from 0.5 to 0.8 m above the river bed. Floodplains are also situated at 0.5 to 0.8 m above the river bed. Width of bankfull stage ranges from 7 to 9 m, while lower terrace height varies from 1 to 1.5 m. Sands have been extracting from the river bed in the area of Sanglabazar. Sand vendors have been using bamboo check dams to accumulate sand on channel. If the accumulated sediments are not removed in time, channel aggradation reduces the cross-section area of the channel and monsoon flow can certainly overtop the river banks during high discharge. According to villagers these obstacles has already caused flood in 2007 and destroyed paddy field in the surrounding area of the river. River stretch between Khtrigaon and Manohar Tirtha Ghat has river width from 0.75 to 6 m and depth from 0.05 to 0.35 m. High flood level mark is at 0.6 to 1 m above the river bed and width of bankfull stage is up to 15 m. Lower terrace height varies from 1.7 to 4.5 m. Bishnumati River upstream from the Manohar Tirtha Ghat has 3 to 5 m channel width and 0.1 to 0.15 m channel depth. The river at Maijuhiti area has high flood level marks at 0.8 to 1.7 m above river bed and floodplain was found more or less at the same height. Lower 52
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report terrace height ranges from 1.2 to 3 m. Settlement area was inundated in 2007 and again in 2008 for 2-3 hours and some reinforced buildings were affected from the flood. Width and depth in the river stretch from the Manohar Tirtha Ghat to downstream at confluence of Bishnumati and Bagmati rivers vary, respectively, from 8 to 15 m and 0.2 to 0.5 m. Height of high flood level and lower terrace from the river bed range from 1 to 1.5 m and from 2 to 2.5 m, respectively. Owing to narrow span of the concrete slab bridge over the Bishnumati River at Manohar Tirtha Ghat high monsoon discharge occasionally overtops 2.5 m high lower terrace of the area and buildings near by the river are affected by inundation. Naya Bazar and Balaju area, Sobhabhagwati area, and the area near National Trading Corporation were hit, respectively, by the flood of 1994, 1989 and 1995 respectively. Bank stability Riparian vegetations are sparse and represented by trees, bushes and grasses. Bank materials are comprised by 1.2 to 1.75 m thick silty sand and sandy gravel. In some places banks were partly slumped owing to loose sediments at top of the bank. Very sparse riparian vegetations in the river stretch between Khatrigaon and Manohar Tirtha Ghat cover on the banks and represented mostly by bushes and grasses. Bank section is composed of Sangla Khola river deposit as well as the river deposit and Kathmandu Basin deposit. River deposit is comprised by 2 to 2.5 m thick silt, sand and sandy gravel, while the Kathmandu Basin deposit consists of 5 m thick semi-consolidated mud with lignite in some places and 4 to 5 m thick more compacted silt, sand and gravel in the sand excavation area e.g. Phutung area. The banks to downstream section from the nick point near Phutung village are highly unstable and several blocks of semi-consolidated mud have been sliding down due to toe erosion (Plate 5.29). Bank slope varies from 70 to 85 degrees and comprised of thick silt, sand and sandy gravel at top and semi-consolidated mud of Kathmandu Basin deposit at bottom. Bank height tremendously increases to the downstream area. The banks to downstream from the bridge over Bishnumati River at Ring Road near Gongabu Bass Park are also failed due to toe erosion. Flood control structure In the upper reaches of the river people use sand bags and bamboo for protection of banks in some places. Banks in Bishnumati River at upstream from Manohar Tirtha Ghat are protected in some belts by Gabion, masonry as well as RCC wall. Height of the structure varies from 1.5 to 3 m. Banks to downstream from Gongabu Bass Park are protected in most of the places by Gabion structures of 2 to 3 m height. Riparian vegetation is not seen in the densely populated area. In the Khusibu Town Planning area blacktopped road runs along the left bank of the river and gravel road along the right bank. Both roads are protected in most of the places by Gabion wall. River banks in the area around Kangleswori are also protected by 2 to 3 m high 53
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Gabion walls. The area is densely urbanized in the river sides. Around the Teku area river banks are protected by 3 m high Gabion wall. Vulnerable infrastructure Near Sangla village a concrete bridge over the Sangla Khola has wider span than upstream width of river, however, foundation of the bridge is scoured, which is protected by Gabion check dam at little downstream. Gravel road running along the left bank of river at Phutung village is partially subsided in several places, where sand excavation is massive. River bank scouring is partly protected by Gabion wall. Foundation of a concrete bridge over a tributary of Sangla Khola near its’ confluence around Phutung village is damaged due to river bed scouring. The banks of the tributary to upstream direction from the bridge are also failed due to toe erosion. Extensive sand mining from the river environment in the Phutung village has tremendously increased vulnerability surrounding the river area by increasing the bank height (Plates 5.30 and 5.31). Deepening of the river channel has made the inaccessibility of surface irrigation in the agricultural land of the river terrace (Plate 5.32). Impact of sand excavation from river channel is found large at the foundation of the concrete bridge over Bishnumati River at Ring Road near Gongabu Bass Park, where the foundation of the bridge is exposed more than the length of the piers (Plate 5.33). The bridge is in high danger condition and necessary preventive measure has been taken immediately. Similar magnitude of impact is also seen at foundation of the concrete bridge at Sohrakhutte-Balaju road, where the foundation is highly exposed (Plate 5.34). Preventive measure in this location against foundation scouring is also lacking. Vulnerability is increased by extreme river area encroachment (Plate 5.35). Foundation of concrete bridge over the river at Teku is also exposed due to river bed scouring. 5.4 HANUMANTE KHOLA The characteristics of Hanumandte Khola are described in the following sub-headings: River morphology The Hanumante Khola originates in the hilly area of Nagarkot Range and flows towards west. The river flows very closely with its tributary for certain length near the Jagatie village before they merge. Generally, the lands on both sides of the river stretch are used for agriculture purpose. As the river is extremely polluted to downstream from Bhaktapur due to haphazard accumulation of solid waste on the river channel, roughness of the river certainly increases and water level could be raised in monsoon. Point bars are made up by sand and gravel mixture. There is almost lack of well developed floodplain along the river corridor. The river has well developed natural levee with riparian vegetation in most of river segment. The river passes through the settlement area around Thimi. River shifting As Hanumante Khola has good riparian vegetation in many places, river shifting is not seen except relocation of the river channel for about 100 m near the upstream from the confluence of the river and the Manohara River. 54
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Flooding area Active channel flow width and depth in the river sector between Jagati village and the concrete bridge over the river at Shyamashyamadham vary, respectively, from 2 to 3 m and from 0.05 to 0.1 m. High flood level mark was found at a range of 1 to 1.5 m from river bed and bankfull width is about 5 to 9 m. Lower terrace height ranges in height from 2 to 3.5 m from river bed. According to farmers high discharge during monsoon overtops the natural levee of the river around the Jagati village and the surrounding cultivated lands inundate in each 3 to 4 years because of narrow river valley width (9 to 12 m). Current channel flow to downstream from the concrete bridge over the river at Shyamasyamadham has width of 4 to 6 m and depth of 0.05 to 0.15 m. Lower terrace ranges in height from 1 to 3 m, while high flood level rises up to 1 m from the river bed. Width of river bank ranges from 10 to 15 m in bankfull stage. Normally, width of river bank in flood prone stage ranges from 15 to 20 m. Only to the upstream area from concrete bridge over the river at Lohakanthali-Tikathali road connection has wide floodplain. Lower terrace is about 1 m high in the area. High flood level was marked at 0.5 m above the river bed. Hence part of lower terrace area lies in the flood prone zone too. According to villagers the area was inundated in 1981 for 3-4 hours and in 1988 for short duration. Lands on both side of the river were cultivated. Bank stability Banks of the river at upper reaches were covered by riparian trees and bushes in most of the places. Both banks of the river to upstream from concrete bridge at Shyamashyamadham are stable. Bank slope varies from 75 to 85 degrees and are almost stable in the area. Bank materials consist of sand and gravel. To downstream section riparian vegetations of trees and bushes are partly distributed only in few patches of the river corridor. Banks have generally 60 to 85 degrees slope and stable in most places. River channel to the downstream from the bridge over the river at Lohakanthali-Tikathali road is realigned according to cadastral map of government (Plate 5.36). Banks in the area have nearly vertical slope and were slided down in several places. The bank areas were barren. Flood control structure Gabion boxes are constructed to upstream and downstream of bridges in the river sector of Bhaktapur and Shyamashyamadham area. To downstream from the concrete bridge over the river at Shyamashyamadham Gabion boxes are constructed along the river for short distance. In contrast, settlement area around the Thimi is well protected from river encroachment by constructing 3 m high Gabion boxes along the river banks. 55
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Vulnerable infrastructure Margin of a gravel road along the right bank of the river near Jagati village was inundated for many times. Protection measure is not taken yet. Likewise, water supply system located close to the river in the area is also unprotected (Plate 5.37). Foundation of concrete bridge over the river at Shysmashymadham is highly scoured. Gabion check dam across the river little downstream from the bridge foundation is also damaged due to river bed erosion. Human encroachment to the river area near the location of concrete bridge over the river at Kausaltar area has augmented the vulnerability of the bridge foundation by channelising the flow in a specific narrow site of bridge foundation area (Plate 5.38). 5.5 DHOBI KHOLA The river characteristics of Dhobi Khola are described in the following sub-headings: River morphology Dhobi Khola flows down from hilly area of Shivapuri Range with high river gradient. It has wide floodplain in the upper reaches, while the floodplain becomes narrow in the valley floor area due to human encroachment to floodplain. It has straight to low meandering morphology in Bhanglatar and down to Chunikhel area. Point bars are comprised of gravel and sands. River terraces of the area are covered mostly by cultivated land and by sparse settlement. River channel depth increases at around Mandikatar area. Terrace at left side of the river around Mandikatar is sparsely populated, while building construction on the terrace of right side of the river is in increasing trend. Human encroachment in the river bank area is high to downstream. In general, the river has wide channel, however, in few places mid- channel bars seem to be obstacle for high discharge of monsoon that consequently increase inundation in upstream direction. River width between the stretches downstream of Seto Pool at Maitidevi and around the Sukumbasi Tole slums near Anamnagar is narrow, where the average width of the river was about 7 m. River shifting The river passes through highly urbanised area in the middle and lower reaches, where the channel is confined by settlement, road and river protection works. River shifting is not observed in the upper reaches. Flooding area In the river sector between Bhangal to Mandikatar area, width and depth of active river channel flow are, respectively, from 2.5 to 3.5 m and from 0.15 to 0.2 m. High flood level was marked at 0.5 to 1.7 m above the current river bed and bankfull width varies from 4 to 10 m. Lower river terrace in the area ranges from 2 to 3 m. Inundation of 2008 had affected pig farm as well as the protection wall of Gabion and masonry types of the area were also overtopped in some places (Plate 5.39). The inundated area also covered the area allocated for household planning of private company. Further downstream direction inundation was occurred for 2-3 hours and about 20 cm thick fine sand and silt were deposited on the lower terrace of the river, which is located at 2 m above the river bed. One reinforced building at left bank was partly affected. Earthen road along the right bank of the river was also affected. Because of narrow width of river channel in Mandikatar area, inundation was 56
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report occurred due to over flow of high discharge in monsoon of 2008 up to lower terrace situated at 2 m above the river bed (Plate 5.40). Lower terrace height in the area between Mandikatar and Chabahil Ringroad is in range from 2.5 to 3 m and high flood level mark was seen up to 1 m above the river bed, while current channel flow has width and depth range from 4 to 6 m and 0.1 to 0.15 m, respectively. Downstream direction from the concrete bridge over the river at Ring Road at Chabahil high flood level mark was noted at 0.6 to 1.25 m and bankfull width has a range from 6 to 11 m. Floodplain lies at a height of about 0.6 m. Near Bishalnagar area inundation during monsoon occurs on cultivated lower terrace of 2 m high due to blockade of high river discharge at z- type of river bend (Plate 5.41). The exposure at the river bend is composed of semi- consolidated mud of Kathmandu Basin deposit. Within the sector between Rato Pool and Seto Pool settlement area and other structures can also be severely affected by flash flood. The river channel in the stretch around the Sukumbasi Tole slums at Anamnagar has shallow depth and inundation occurs in this sector in high discharge of monsoon season. Flood of 2000 had affected the residents of the area. Concrete bridge in the area has narrower opening than the river width at upstream direction, which further trigger inundation (Plate 5.42). Bank height and width of river between concrete bridge at Bijuli Bazar and confluence of Dhobi Khola and Bagmati River is relatively low and narrow. The area nearby the Royal Drugs Ltd. was also inundated. Bank stability River banks at upper reaches of the river are covered partly by riparian vegetations like trees, bushes and grasses in several places, in contrast, the banks in the urban area were mostly barren. Generally, slope of river banks varies from 40 to 70 degrees and is mostly stable. Bank materials are comprised by 0.75 to 1.5 m thick silt and sand at top and 1 to 1.25 m thick compacted sandy gravel at bottom. In some places semi-consolidated mud layer is also appeared at the banks. Around Mandikatar area Gabion walls constructed for the protection of bank were failed due to toe cutting by the current of monsoon flow in 2008. In some places the wall height is up to 3 m. To downstream area illegal sand mining from river channel increases the depth of river up to 4 m and also reduces bankfull width of the river. Bankfull width varies from 5 to 10 m. Banks in generally have slopes at a range from 65 to 80 degree and are composed of 0.25 to 1 m thick silt and sand at top and 1.5 to 2.5 m thick semi-consolidated mud of Kathmandu Basin deposit at bottom. High flood level was marked at 0.5 to 0.8 m above the river bed. Bank scouring by river current was active in few places and needs to protect by engineering structure. Flood control structure Gabion walls at upper reaches of the river are partly constructed on the banks to protect agriculture lands, buildings, roads and bridges from flood and erosion. Around Mandikatar area banks are protected by 3 m high Gabion wall. In the area downstream from Mandikatar Gabion wall and concrete walls in some places are constructed to protect the nearby households, school and road from river encroachment. However, the slope protection walls 57
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report were eroded at their toe and damaged in several places because of sand mining activities (Plate 5.43). Vulnerable infrastructure Small concrete crossovers and metal ferry between Mandikatar and Chabahil Ringroad are in good condition. Foundation of concrete bridge piers of Ring Road near Chabahil is deeply exposed due to river bed scouring. Check dam across the river at little downstream from bridge has also been constructed to protect the foundation of the concrete bridge from high speed erosion, however, bridge site is in a serious condition. To downstream of the bridge protection for river erosion is necessary for the new road aligned at left bank of the river, which also save Om Hospital and Research Centre situated close to the road. The road runs along the left bank of the river and joins Bhatkeko Pool at Hadigaun. Protection measures are also needed to mitigate river current erosion in some places of the road section. Private houses constructed close to river channel obviously put the structure in to a high vulnerable zone of flood (Plate 5.44). 5.6 SAMAKHUSI KHOLA The Samakhushi Khola originates at Tokha Sarashowti area in the northern part of the valley and flows southwards to drain into Bishnumati River near Mhaipi area. It has small watershed area. The gradient of the river is low. Though the densely urbanized area is seen after crossing the ring road around Mhaipi, Samakhusi and Ranibari, even the upstream areas including Basundhara, Dhapasi and further upstream areas are surrounded by dense settlements. No significant channel shifting observed in these rivers. Instead, the river courses has been confined to narrow channel like small irrigation channel in its headwater area and human encroachment at downstream of the river is further extreme. In many sectors of the river the channel is completely covered by concrete slab and the river is invisible (Plate 5.45). In some cases, private houses and path for pedestrian have been built over the channel by constructing concrete slab over the channel (Plate 5.46 and 5.47). Causes of flooding in the area are because of narrow channel width and bad management of drainage system for monsoon precipitation in highly urbanised area. Thus, occasional inundation occurs in the Samakhusi area during monsoon season (Plate 5.48). 5.7 BALKHU KHOLA Major findings of the Balkhu Khola are described below: River morphology Gradient of Balkhu Khola is high around the foot of hilly area. When the river traverses through the terraces of the fluvio-lacustrine sediments, river gradient decreases. Exceptionally, at few places the river encounters hard rock areas even at the valley side and steep gorges were observed at the locations when the river cuts these rocks. For example, it has cut the rocks near the Ring Road (back of UML party office) and near Tinthana village. The morphology of the river corresponds to braided and meandering at the upper reaches while it is dominantly represented by meandered type at the downstream portion. Channel bar is widespread in the braided portion of the river while point bar is dominant in the meandered portion. Point bars are comprised by gravel and little sands. Terraces of the river in the area to upstream from the Nariwalphant are mostly cultivated, while the terraces to 58
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report downstream section are mostly covered by settlement area. The floodplain is wider in upper reaches (Plate 5.49). River shifting The Balkhu khola exhibits river shifting at various locations. These locations are having low gradient and consisting of dominantly mixture of sand silt and clay particles. The shifting of river is both the natural as well as artificial. The Balkhu Khola has shifted up to around 50 m in Balkhu Chowk and 20-80 m at Kalanki. Figure 5-5 and Figure 5-6(between Balkhhu chowk and Kalanki Chowk) depict that the pressure exerted due to the increasing urbanization in the Kathmandu valley has seriously affected the natural regime of rivers by narrowing its channels through engineering constructions to occupy the flood plain of the river. In order to conduct such study, the use of high resolution satellite image plays important role. This can be understood from the information obtained from Figure 5-5 (Alos satellite image, sp. res. 2.5 m) and Figure 5-6 (Quickbird image, sp. Re. 0.6 cm). Figure 5-5: Balkhu River course on topographical map (blue area) and on Alos satellite image between Balkhu Chowk and Kalanki) The river course is not so clear in this image. 59
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 5-6: Balkhu River course on topographical map (blue area) and on Quickbird satellite image between Balkhu Chowk and Kalanki) The river course is distinct in this image showing the impact of human activities. Flooding area The width of the river channel ranges from 5-10 m and the bank height is 1.5-3 m. The average depth of water in the active channel is 0.25 to 0.50 m while the wetted perimeter is around 6 m. High flood level was marked at 0.5 to 1.5 m. The Balkhu Khola inundates 5 to 20 m distance during annual flood and 30 to100 m during the high flood events. The Balkhu Chowk and around the Dipendra Hospital near Satungal will be severely affected by occupying lateral distance of up to 50 m from the river banks during the high flood events. The other areas to be affected are the Kalanki and near UML Party office where it covers lands up to the distance of 90 and 15 m, respectively. Due to 24 hour long incessant rainfall of average 207 mm on 23rd July, 2002 over Kathmandu Valley, huge flash flood was triggered in Balkhu River. Heavy precipitation of the monsoon had triggered debris flow in the upper reaches and inundation in the lower reaches of the river. Loss of human lives, big loss of crops and agriculture lands were the worst impact of the flood in the river corridor. At Matatirtha village, 3 households were swept away and 12 people were killed by debris flow. Depth of the flood in the Balambu area was up to 4 m. Likewise, large area at upstream direction near Kalanki was also under the flood (Figure 5-7). The area near Oriental colony and Metro cinema hall located at about 500 m upstream from the concrete bridge over Balkhu Khola at Ring Road was flooded up to 2.2 m of colony’s first floor (Plate 5.50). Inundation in Balkhu area had overtopped the natural banks and extended widely. According to local people high flood level was elevated up to 5 m from the river bed (Plate 5.51). One member of the project is witness of the flood at confluence of 60
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Balkhu and Bagmati rivers. The river gets flooded once in every 10 years period in the area. In 1994, Satungal, Bishnudevi Temple area was hit by the flood occurred after heavy precipitation. Flood water was raised up to 4 m and Bishnudevi Temple was inundated. Agriculture land and crops were damaged due to the flood. Figure 5-7: Flash flood of 23 July, 2002 in Balkhu River near Kalanki (Photograph adopted from Kantipur Publication, 2002) Bank stability Grasses and bushes are the main riparian vegetation in the upper reaches of the river, while the vegetation is lacking at most of the places in lower reaches. Intense bank cutting was observed at the meandered belt of the rivers particularly observed around the Ring Road. Owing to encroachment in river to downstream from Kalanki, particularly at urbanized area, where erosion of the river at toe of bank is high. High discharge through narrow channel in the area further laterally erodes the bank of the river and hardware engineering structures are damaged (Plate 5.52). Flood control structure In the upper reaches of the river, Gabion boxes were found in localised area of settlement. In contrast, because of heavy engineering works, the rivers are trained at several locations in the lower reaches of the river, particularly in highly urbanised areas. Continuous masonry walls on both side of the river at upstream from the concrete bridge over the river at Ring Road sometime narrow down the width of the river. 61
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Vulnerable infrastructure Bridge foundation at Ring Road near Kalanki, subway blacktop road along the river near Kalanki, private teaching hospital at Khasibazar area of Kalanki and private colony near Balkhu are the vulnerable structures to flood and river erosion. 5.8 KARA KHUSHI KHOLA (MANAMATI KHOLA) The Kara Khushi originates around Bhimdhunga area in the north western part of the valley and flows towards southeast draining into Bishnumati River near Kalimati–Kuleshwor area. It is a small stream and the urban development along the river is dense, especially between Waphal and Kalimati. There is rapid unbinization around the Syuchatar area and the river is extremely encroached (Plate 5.53). The river corridor in most of the places between Waphal and Kalimati is protected by civil engineering structures, however, in some places these structures are damaged due to undermining of the structure by river current e.g. around Sai Millennium Boarding School near Kalimati. Bank height ranges from 2.5 to 3 m and the width of the river varies from 4 to 10 m. Riparian vegetations (trees, bushes, and grass) have been protecting the banks in many places of the river, nonetheless, bank failures were occurred owing to undermining in Syuchatar, Waphal, Soltimod etc (Plate 5.54). Bank materials are comprised of 1 to 1.25 m thick muddy gravel at base, sandy mud of 0.4 to 0.55 m thick at middle and 0.6 to 1 m thick mud at top. Single lain road runs along the river bank in most of the places of the river corridor. The flood of Karakhuai Khola in 2007 at Kalimati area was raised up to 3.5 m from the bottom of the Karakhusi river channel for few hours. Only the unprotected banks were inundated from the flood in the area of Kalimati, Grand Hotel and Waphal. According to local people the flood level was reached up to 2 m high from the channel in Syuchatar area and it had accumulated large amount of sediments in the agricultural lands. The main input of sediments in the river is from the active landslides and bank erosion in the upstream area of the river. 5.9 NAKHU KHOLA River characteristics of Nakhu Khola are described in the following sub-headings: River morphology The Nakhu Khola drains the hilly area of Phulchauki Range with the highest gradient. It traverses through the terraces of the fluvio-lacustrine sediments once it debouches from the mountainous parts. Exceptionally, at few places the river encounters hard rock areas even at the valley side e.g. at Itahiti, Nakhudol areas. The Nakhu River shows braided and meandered nature at the upper reaches while it is dominantly meandered at the downstream portion. Channel bar is widespread in the braided portion of the river while point bar is dominant in the meandered portion. The floodplain also becomes wider when the river exhibits braided nature (5.55). River channel aggradation is found more to the area upstream from Baniyagaon area. Channel and point bars are comprised by dominant gravels with little sands. River terraces except in few locations are cultivated. 62
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report River shifting The Nakhu shows intense river shifting behaviour at some localities. These locations are having low gradient and consisting of dominantly mixture of gravel, sand, silt and clay particles. High shifting of river was found to be occurred to downstream area from the concrete bridge over the river at Nakhu Jail. Flooding area Channel width of Nakhu khola ranges between 9-12 m at the downstream areas while it increases up to around 50 m in the upstream areas before it enters the rocky areas. Similar situation was also observed in the river where the significant channel shifting and modification was observed near the confluence of Nakhu Khola with the Bagmati River. High flood level mark varies in height from 0.5 to 1.5 m except near the confluence of Nakhu and Bagmati River, where it raised up to 2 m. The Nakhu Khola has significant effect of flooding around the Nakhu Village where it occupies up to around 70 m distance during the high flood period and around 20 m during the annual flood. However, in the upstream areas, the flood problem is reported minimum. River terrace of Nakkhudol area in 2004 was flooded and about 3 ropani rice fields were lost thanks to sedimentation from flood. River terrace at Thapagaon in 2007 were flooded and rice fields along the river were lost. The same flood around the area near the confluence of Nakhu Khola and Bagmati River had inundated the terrace level located at an elevation of 2.5 to 3 above the river bed (Plate 5.56). Additional cause of the flood was the blockage of river at the confluence. The flood had killed domestic animals and also damaged rice fields and pig farms of the area. Effect on human lives, domestic animals and agricultural lands were already occurred badly in the area situated little upstream from the confluence by the flood of 1991. Bank stability Bank materials are comprised of 1 to 1.25 m thick gravel and 0.5 to 1 m thick mud layers at top. In some places mud of Kathmandu basin fill sediment is exposed at base of the banks. Bank height ranges from 1.5 to 3 m from the river bed. Intense bank cutting was observed at the meandered belt of the rivers particularly, around the area of Chunikhel, Chhayasikot, Okhthali, Nakhodol and Durgabasti area of Nakhu village as well as near confluence of Nakhu and the Bagmati River. Flood control structure Flood protection walls were constructed for short river stretches at upstream and downstream from the Nakhu Bridge near Nakhu Jail. Undermining action of the river current during monsoon flow has damaged floodwall at the outer bend of the river (Plate 5.57). Vulnerable infrastructure River erosion and flooding phenomena have little impact on the foundation of bridges at Nakhu and Nakhudol. Public school building and its playground located near to the floodplain of the river at Nakhu Jail area is vulnerable to flood (Plate 5.58). 63
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 5.10 KODKU KHOLA Field characteristics of Kodku Khola are given below: River morphology Kodku Khola drains the lower hill slope of the Phulchauki Range. It has high gradient in the upper reaches and gradually the gradient decreases when it arrives in the valley floor area. The Kodku Khola has straight river pattern at upstream from Tasinchok village and converts to braided and meandering type up Thangu village at downstream. Again it shows more or less straight river type close to Thaiba then it has braided and meandering morphology in most of the downstream sides. Channel materials are comprised of small boulder, cobble, gravel and little sand. Floodplains are very narrow. To downstream direction channel bar is widespread in the braided portion of the river while point bar is dominant in the meandered portion. The floodplain also becomes wider when the river exhibits braided nature. For example, in the river section, point bar deposits are frequent between Imadol and Harisiddhi. Upstream of Harisiddhi, point bar and channel bar is predominant. Small oxbow lakes are observed at the Kodku khola at some locations, e.g. Harisidhi area and Hattiban area (Plate 5.59). These portions represent the wetland areas very suitable for the agriculture purpose. The intense meandering of Kodku river was observed at Imadol, Gwarko, Hattiban, and Harisiddhi area (Plate 5.60). The width of the river channel ranges from 5-7 m near the confluence of Kodku and Manohara. The bank height is 2-3 m in these localities. The average depth of water is 0.25 to 0.50 m while the wetted perimeter is around 6 m. The width of Kodku Khola becomes more in the upstream portion reaching up to around 40 m. River shifting The Kodku Khola shows intense river shifting behavior at some localities. These locations are having low gradient and consisting of dominantly mixture of gravel, sand, silt and clay particles. The shifting of river is both the natural and artificial. These locations were marked on the based map (Satellite Image) used in the field and later necessary analysis was carried out during the post field stage. The channel of Kodku Khola has experienced intense human interference. There has been shifting of river ranging from 19 to 60 m at Balkumari, 16-98 at Gwarko (near Cinema Hall), 25 to 82 m between Gwarko and Hattiban and around housing complex (Harisiddhi). At Balkumari (Figure 5-8 and Figure 5-9), there has been quite change in river course in comparison to that of the aerial photo (1993). Still the river is confined within narrow channel due to engineering constructions. Similar phenomena have been observed around Hattiban and Gwarko Cinema Hall (Figure 5-10). Three different channels of the Kodku Khola are observed on aerial photo, on Satellite image and the presently observed in the field. It is a good example how the meandered river is shifting its channel naturally and also it is confined to very narrow channel through engineering constructions. The private housing company has diverted the Kodku khola channel (as seen on aerial photo) by constructing protection wall. The emerging housing complex in the Kathmandu valley are the prime reasons for the artificial river shifting – a prime example being the one at Harisiddhi. 64
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 5-8: Kodku River confined to narrow channel due to River training structures (Information is not clear on Alos image). Figure 5-9: Kodku River confined to narrow channel due to River training structures (Information is clear on Quickbird image). 65
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 5-10: Comparison of the information obtained to study the impact of human encroachment as well as natural process on the Kodku River through satellite images of different resolution (Alos on the right and Quickbird on the left).The blue area is river course as of topographical map. Flooding area The Kodku Khola has normal flooding with the lateral inundation ranging from 24 m (Balkumari) to around 66 m (Harisiddhi). Likewise, in these locations, the distance occupied from the river bank is 55 and 70 m respectively. High flood level was noted at 0.5 to 1 m in the banks of agriculture land area and the lower terrace height ranges from 1.5 to 2.5 m. But the flood level is raised from 2 to 2.5 m in the densely urbanised area e.g. Gwarko, where river channel is narrowed down (Plate 5.61). About 100 ropani of agricultural land and four houses were lost by flood of 1979 in Dhapakhel area. About 500 m river stretch to upstream from the bridge at Gwarko, the river channel is narrowed down due to intense river encroachment so that the upstream part is often inundated in monsoon season. According to villagers, flood of 1981 had damaged about 150 ropani of agricultural land and few houses in Imadol-Gwarko area. One person was killed by the flood. In 1993, a devastating flood had washed out about 200 ropani agriculture lands and the crops of about 50 ropani lands was damaged in the same area. A garment company was also damaged. Three workers of the garment company were killed by the event. River water was raised up to about 6 m. The flood of 2007 in Imadol area had swept about 0.5 ropani lands and also had damaged the crop. Banks were severely failed. Water level was raised up to about 4 m. The same flood had also affected the Thaiba and 66
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Badikhel areas. Crop fields were inundated and small amount of sediments were accumulated on the fields. The Gwarko area was also affected by the flood of 2002. Bank stability At the upper reaches of the river riparian trees, bushes and grasses sparsely cover the river banks. Intense bank cutting was observed at the meandered belt of the rivers, particularly, around the Balkumari, Imadol, Gwarko and Hattiban areas (Plate 5.62). If the river is trained without considering the proper width of waterway, the protective structures could not remain intact (Plate 5.63). Flood control structure The private housing company has constructed flood control wall near Saphaltol area, where the channel width is highly reduced. The walls have been damaged in several places since the natural flow regime of the Kodku Khola is disturbed by constructing flood wall without considering river dynamics (Plate 5.64). Banks around the Little Angelus School was also protected by Gabion boxes and masonry walls. Similarly, banks were partly protected in the Gwarko and Imadol area. Vulnerable infrastructure Since the school buildings at Imadol, Gwarko and Hattiban area are located close to the river banks, these structures are vulnerable to flood. Similarly colony buildings are also constructed close to the river in Hattiban, Gwrko and Imadol areas, which are not safe from flood. Bridge over the river at Gwarko has damaged due to river erosion on foundation of the bridge. As the river encroachment trend in the urban area is high, most of the buildings, commercial places, etc. located near the banks are not safe from flood (Plate 5.65). 5.11 GODAVARI KHOLA Major characteristics of the Godavari Khola are described below: River morphology Godavari khola flows down from the hills of Phulchauki Range toward north direction in the valley floor area. The river has a steep gradient in its upper reaches, which gradually decreases downstream. River shows gravely meandering morphology with occasional straight channel. Channel width is narrower near foot of hills and Damaitar area. Floodplain becomes wider to the valley floor area (Plate 5.66). There are only cultivated lands along the river corridor. Terraces on both sides of river are used for cultivation with insignificant human settlements. In most of the places there is danger only to agricultural land during high flow. Toe cutting of high level terrace by river current has triggered slope failures in some places (Plate 5.67). River around Borcha village and downstream shows meandering and partly straight morphology. Points bar deposits were mostly confined in meander sector of the river and small mid-channel bars were also developed in the straight stream sector. Point bars and middle channel bars are comprised of dominantly gravels with very little sands. River channel aggradation in some area reduces the height of bank. Floodplains are wider than in the upstream sector. 67
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report River shifting Minor shifting in river channel was observed in Dharmeswor, Tikathali, Khdkatol and Bisnudol areas. Flooding area Riverbed materials are made up of small boulders, cobbles and gravels with little sand in the upper reaches and the size of the bed load material is dominated by gravels with few sands in the lower reaches. Around the Charghare village area patches of floodplain were observed along the river strip at an average elevation of 1 m above the river bed. High flood has marked the level in a range of 1 to 1.5 m above the river bed. Lower terrace height ranges from 2 to 2.6 m from river bed. Width of flood prone stage and bank full stages were, respectively, up to 10 and 6 m. High flood level downstream from Charghare, around Borcha and Tikathali village area was found at a range of 0.5 to 0.8 m above the river bed. Floodplain height is about 0.75 m in average. Lower terrace is 1 to 2.6 m high from the river bed. Width of flood prone stage and bank full stage are increases up to 60 and 20 m, respectively. The river has relatively narrow channel width and wide floodplains that cause considerable water level rise during floods e.g. in Tikathali area (Plate 5.66). About 300 m upstream of river stretch from the confluence of Hanumante and Godavari Khola high flood level mark was observed at 2 m above the river bed. According to villagers the area to the Balkot and Dharmeswar village sides were inundated for one to three hours in 2006. River level was upraised up to 6 m above from the river bed due to blockade of stream flow at the confluence of Hanumante and Godavari rivers as well as due to narrow river valley width near the outlet of the river (Plate 5.68). Two to three houses at the right bank of the river i.e. to the Balkot village side were affected from inundation. Bank stability At the mountainous area riparian vegetations are dense and represented by trees and bushes, while in valley floor areas riparian vegetations are sparse and represented mostly by grasses. Weeds and grasses grow in the river banks and also in the river bed in dry season. The river banks of upstream direction from Charghare village area are mostly protected by natural riparian vegetation. However, at the bend of river in some places banks are partly eroded (Plate 5.69). Bank materials are composed of 1 m thick alternating silt, sand and gravel, and 0.5 m thick gravel that are underlained by 0.5 m thick semi-consolidated mud of Kathmandu Basin deposit. Bank materials downstream from Charghare village area are composed of silt, sand and sandy gravels, which are tightly packed. Therefore river banks are mostly stable. Bank materials around Borcha village and downstream are also comprised by silt, sand and sandy gravel, which are tightly packed. Bank erosion was seen in some areas, where protection measures were taken by Gabion boxes. Grasses and sparse bushes are the main riparian vegetation in the area, however in some places sparse trees were also found. Riparian vegetations are very good elements to protect bank erosion of water currents. Unstable bank of river at meander outer bend in Dharmeshwar village area is partly protected by 3 m high Gabion wall. 68
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Flood control structure Flood control structure is very sparsely found in the whole river corridor. Gabion wall is constructed on the floodplain near the bank of river at Tikathali village to reclaim the land that occupied by river during monsoon. Vulnerable infrastructure Some private buildings constructed on the floodplain of the river are lying in danger zone of monsoon high flood (Plate 5.70). A three phase electrical pole located at the left bank of the river near Tikathali village is protected by Gabion structure, however, bank erosion at toe of the structure could damage the foundation of the pole (Plate 5.71). 5.12 WATER LEVEL AT CONCRETE BRIDGES FROM BENCHMARK SURVEY Benchmark surveys across the river section close to the foundation of concrete bridges in the Ring Road e.g. the bridges at Tilganga, Chabahil, Gongabu, Balkumari, Balkhu etc and the other major concrete bridges located at Subidhanagar, Sinamangal, Bijulibazar, Teku, Balaju etc. in the study area show that the water level of the rivers during the survey time was appeared only near the bottom of the foundation of all the bridges. The water level derived from flood modelling for 50 years return period also lies below the bridge level. 69
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 6. HAZARD, VULNERABILITY AND RISK ASSESSMENT The main aim of the project is to prepare flood vulnerability and risk map of the proposed river corridor of the Kathmandu Valley. To achieve the goal the proposed rivers namely Bagmati, Manohara, Bishnumati, Hanumante, Dhobi Khola, Mahadev Khola, Samakhusi Khola, Manamati Khola, Balkhu Khola, Nakhu Khola, Kodku Khola and Godavari Khola were studied in field. During the field survey necessary data for the hazard, vulnerability and risk assessment were collected. Here, hazard is simply defined as a probability of occurrence of extreme natural events that can cause loss of life, and/or damage to property within a certain period of time in any space. In case of the Kathmandu valley such hazards can be induced or aggravated by a combination of extreme natural events as well as human interventions in the natural flow regime of the rivers. Likewise, the intensity and probability of occurrence can differ by place to place, which in turn vary the influence level of possible damage owing to variation in magnitude of triggering mechanism. The flood hazard map has prepared in the field and will be discussed in below. Vulnerability means the degree to which an area, people, or physical structures or economic assets are exposed to loss, injury or damage caused by the impact of a hazard. It indicates the inadequate means or ability to protect oneself against the adverse impacts of natural events and, to recover quickly from their effects. It is the degree of damage with a function of magnitude of event, and type of element at risk. By considering the different element at risk and the level of hazard, the flood vulnerability map is obtained. Details about the vulnerability map will be discussed in below. Disaster risks of a region, a family, or a person designate the extent of the damage and loss of properties and life by a natural event that expected to cause. It is the product of hazard and vulnerability. A risk exists only if there is vulnerability to the hazard posed by a natural event. When a building, for example, is constructed in an area located far from flood prone zone, the hazard approaches zero because it will be in a safer place and would only be vulnerable to very extreme events. Thus, risk signifies the potential for disaster and it is the product of hazard, vulnerability and management. Disaster means an event, natural or man- made, sudden or progressive, which impacts with such severity that the affected community has to respond with exceptional measures. By considering the aspect of vulnerability, the flood risk map is obtained. The areas along the river corridor as defined in ToR were studied in field for the preparation of flood risk and vulnerability map. In this context field based tasks are described in the following sub-topics. 6.1 FIELD BASED HAZARD ASSESSMENT CRITERIA All the information like location of bank failure along the river course, existing protective measures, sites of sand mining from the river channel and its surroundings, former and new channel courses, area of river bank encroachment etc. were gathered on the hard copy of the QuickBird image during the field survey at 1:2000 scales. Further verification of the image is done by using topographical map of 1:25,000 scale and GPS measurement. In the densely urbanized valley floor area natural morphology of the rivers are largely modified and channel flow velocity is also retarded by garbage disposed in the channel area. Work of hazard assessment is performed by applying the following methods. 70
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Parameters for field assessment Data were collected from field surveys along the defined river corridors in the Kathmandu valley. The priority information needed in defining the hazard level along the river valleys was concentrated on following major parameters: • River morphology (straight, braided and meandering river types, floodplain, lower terrace, paleo-channels and ox-bow lakes, Sinuosity of river, etc.) • Relief of river terraces and floodplain • Distribution pattern of fluvial sediment • Width of river bank • Bank height from active channel • Bank instabilities due to bank toe erosion, lithology and bank slope • Condition of Bankfull stage • Type and density of riparian vegetation • Flood history of the surveyed areas • Flood level information (flood in terms of return periods, duration and inundation level relative to river banks, level of danger, etc.) • Historical flood data collected from other sources like news paper, documents, etc. • River bank protection work like floodwall • Floodplain and river channel encroachment Rating for hazard zones Certain causative factors of flood hazard in each studied river corridor were determined for rating to get hazard level and their significance was indicated by weighting certain values. The causative factors taken into account for the study are: • River morphology and bank stability • Bankfull stage, bank height and bank width, and • Infrastructure that influences drainage of the study area According to the significance of each factor for causing flood and lateral erosion hazard, the following weight is assigned for each factor: • River morphology, bank stability and its lithology = 4 • Bankfull stage and bank height = 3 • Infrastructure and river encroachment = 2 Again each factor is divided into a number of classes that allocated certain weight depending on the estimated significance for flood. The maximum value of weight ranges from 4 to 2 in each class. Division of classes: Divisions of factor classes are given below: River morphology and bank stability Degree of channel flow obstruction depends on the types of river morphology (i.e. straight, meandering and braided) and fluvial sediment accumulation that promotes flooding in an area. Lithological composition on river bank i.e. proportion of sand vs. mud (silt and clay) play significant role in the stability of bank. River banks are generally found to be stable in the area covered by dense riparian vegetation of trees and bushes. The unstable banks in 71
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report most places are protected by civil engineering structures. However, the banks are unprotected in most of the river section at their upper reaches. The weight of each class is estimated as follows (Table 6-1): Table 6-1: Ranking value for river morphology and bank instability Class River morphology and bank instability Weight 1 Meandering or braided rivers with unstable banks 4 having loose sand and gravels 2 Meandering or braided or straight rivers with bank 3 protection work height ≤ bank height and/or semi- consolidated bank lithology 3 Meandering or braided or straight rivers with 2 floodwall protection height > bank height or stable banks Bankfull level and relief of surrounding area with respect to active channel Low terrace is an abandoned floodplain. A clear distinction between the elevations of the low terrace and the active floodplain was made in field and the active floodplain was used as the indicator of bankfull stage. Historical food information in the field matches with the flood prone height, which is the two fold of bankfull height (Rosgen 1994). In many places such flood prone area covers the low terrace of river. Hence, the flows necessary to over-top the low terrace bank must be associated with a flood of large magnitude, much larger than the actual bankfull discharge. Validation of flood prone area is done in field from interview of older age group of people concerning flood history of the area. According to the villagers the flood prone area corresponds to the area under 10 to 30 years historical flood. The rating value is given in Table 6-2. Table 6-2: Ranking value for bankfull level and relief of surrounding area Class Bankfull level and relief Weight 1 a) Bankfull level (BFL) >1.5 m and Relief (R) ≤ 2 m; 4 b) BFL = 1 to 1.5 m and R ≤ 1.5 m; c) BFL ≤ 1 m and R ≤ 1 m 2 a) BFL >1.5 m and R = 2 to 2.5 m; 3 b) BFL = 1 to 1.5 m and R >1.5 to 2.5 m; c) BFL ≤ 1 m and R > 1 to 2 m 3 a) BFL >1.5 m and R > 2.5 to 4 m; 2 b) BFL = 1 to 1.5 m and R >2.5 to 3 m; c) BFL ≤ 1 m and R > 1 to 2 m Infrastructure and river encroachment Highways, roads, bridges, tracks and buildings that are causing narrow down the channel width and obstructing the flow of the river, which consequently trigger flooding in an area have allocated weight according to their significance as follows (Table 6-3): 72
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 6-3: Ranking value for infrastructure and buildings Class River encroachment by infrastructure and Weight buildings 1 Infrastructure and buildings obstruct river flow at 4 more than 10 locations at 500 m distance 2 Infrastructure and buildings obstruct river flow at 3 more than 5 to 10 locations at 500 m distance 3 Infrastructure and buildings obstruct river flow at 2 less than 5 locations at 500 m distance For flood hazard map preparation various layers of the above factors are prepared in the GIS environment. Maximum and minimum total weight of the factors for rainfall; size of watershed; River morphology, bank stability and its lithology; Bankfull stage, bank height and bank width; and infrastructure were calculated. Then the weight of each hazard level is established from half sigma standard deviation of the maximum and minimum values as: • High hazard = 32-36 • Moderate hazard = 23-32 • Low hazard = 18-23 The given ranges of the values are arranged according to the rating criteria. In summary, flood hazard zonation into high, medium and low is performed by integration and analysis of all the data with aid of remote sensing information in GIS tool and field visit was performed to verify the map (Figure 6-1). Riparian vegetation River protection work River morphology Bankfull stage Sinuosity of river Bank height Hazard Map Relief of floodplain Bank instability & river terrace Sediment distribution Width of river bank River encroachment Past flood information Figure 6-1: Flow chart showing the collection of field level information for hazard mapping. Considering all the above information and field condition the hazard level is corrected as: • High hazard = 28-36 • Moderate hazard = 23-28 • Low hazard = 18-23 73
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 6.2 FIELD BASED FLOOD HAZARD MAP Field based hazard map is shown in Annex 2. In the Bagmati River corridor, cultivated land of lower terrace in the area downstream from Sundarijal falls in moderate to low hazard zone. High hazard areas are mostly confined to the floodplain and point bar at the meandering loop, which covers very little area. In such areas riparian trees are lacking. Upstream to Ruksetar wider area is covered by low hazard zone. Likewise, very wide area located at upstream from Uttarbahini is covered by low hazard zone since the river enters in to a narrow passage to downstream of Uttarbahini. The area downstream from Gokarneswor has very narrow strip of moderate to low hazard zone, which is justifiable because of good protection of river banks by floodwall. Similarly, the area downstream from Pashupatinath is characterized by narrow strip of moderate to low hazard zone, where bank height is higher than in the upstream area of upper reaches. The river corridor to downstream from the confluence of Manohara River reveals that wide area is covered by moderate to low hazard zone with some disconnected small strip of high hazard area. Height of river bank is high and width of the river is wider in most of the places of the area. To upstream of Bagmati River from the confluence of Bishnumati River, a considerable area is occupied by high hazard zone. It is realistic since the blockage of the river at the confluence as well as having the lower bank height in the area. Likewise, in the Bagmati river section upstream from the confluence of Balkhu Khola falls under three categories of hazard level as high, moderate to low. Similarly, wide area to downstream from Tribhuvan University Gate is covered by high, moderate to low hazard, particularly, in the area of wide meandering loop. In the Manohara River corridor, wider low hazardous areas are found in Khulaltar, Phuyalgaon, Gothatar and Balkumari area, while the moderate to high hazard zones are appeared in considerable proportion in Khulaltar, area close to Nepal Engineering College, Godar and Gothatar. Further, the river in the areas has reviled high meandering morphology with high degree of river bank erosion since the banks are consisting of loose sediments. In the Bishnumati River corridor, low and high hazardous zones are distributed in very limited area with little proportion of overbank coverage since the river is protected in many places. Protection work is particularly high when the river enters into urban area. Moreover, heights of river bank are very high mainly in the location of sand excavation. However, wider area of moderate hazard zone is obtained at upstream of Karkigaon, Gongabu, Nayabazar, Dhalko and Teku Bridge. In the Dhobi Khola river corridor, wider area of low hazard zone is found at downstream of Chunikhel area, Galphutar and the area downstream to Om Hospital. Nonetheless, high and moderate hazard zones are confined in limited area of Mandikhatar, downstream of Om Hospital and Anamnagar. The upper reaches of the Balkhu Khola lie mostly in high and moderate hazardous zone e.g. in Banasthali, Balkumari and Satungal, while the lower reaches have wide area of low hazard zone e.g. in Nariwalphat, Kalanki and Balkhu as the area has in most places protected by floodwall structure. 74
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Wider area of low to moderate hazard zones can be found at lower reaches of Nakhu Khola i.e. area of Nakhodol to the confluence of Nakhu and Bagmati rivers. Small strip of high to moderate hazardous zones are also located to upstream from Nakhodol since the area is characterized by wider channel with braided river pattern, where river aggradation is high as well as the bank heights are low. Dhapakhel of the Kodku Khola shows wider area of high hazardous zone. Considerable area to downstream of Hattiban and upstream of Gwarko falls in high hazard level. Wider area of moderate hazard zone is found in Dhapakhel area, while the area surrounding Gorkhu situated in low hazard. The area of Godavari Khola near Charghare is characterized by mostly moderate to low hazard zone, while the area near Khadkagaon shows small area of high hazard and larger area of low hazard. Similarly, the large area near Tikathali illustrates low hazard level. 6.3 FIELD BASED VULNERABILITY ASSESSMENT CRITERIA Kathmandu valley has high population density and it is further increasing along river sides; nonetheless, there are still some agricultural lands along some river corridors, which are one of the important bases for agriculture product. Urban densities along the river sides are rapidly increasing, particularly within the Ring Road area. Socio-economically river valley sides are more important as the large population has been residing on the periphery of the rivers. Moreover, other infrastructures such as deep tube wells for the purpose of water supply to the capital are also found nearby the rivers. First approach for the preparation of the vulnerability map of the study area is done by preparing flood hazard map. Identification of vulnerable area in the field was performed. Such areas were also surveyed from the aspect of sociology. Then all the data were compiled in GIS tool including the extracted necessary information from the satellite image e.g. footprint of some new buildings etc. Vulnerability is the degree of damage to a given element at risk or set of such elements resulting from the occurrence of a natural phenomenon of a given magnitude. It is the function of magnitude of event and the type of elements at risk which have or may have certain degree of damage. To prepare vulnerability map of the river corridors, field assessments were concentrated on the following elements that can have direct impact of the hazard: • Infrastructure: bridge, road and buildings • Land use: settlement area, agriculture area and barren area, and • Population: above 65 and children, female and male Rating of flood vulnerable area applied here is considered from the following factors: • Degree of probability of flooding in flood hazard zone • Vulnerable infrastructure to flood hazard zone • Vulnerability of land use to flood hazard zone • Vulnerable population to flood hazard zone Vulnerable data regarding the above mentioned aspects were acquired during the field survey. The ranking value at a range of 0 to 1 allocated for the factor class at risk in accordance with their importance is as follows as in other city areas (Du and Shi, 2006): 75
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report • Infrastructure: 0.3 • Land use: 0.16 • Population: 0.54 Ratings of each factor class are described in following sub-headings. Infrastructure The major elements considered in this class are bridge, road and buildings. For hazard level of bridge the considered factors are the degree of bridge foundation exposure owing to river bed erosion, scouring on abutment, sedimentation around the piers of bridge and reduction in width of active channel flow, which consequently concentrate erosion of river bed in a particular space for long time. For other elements, above described hazard map is used. According to the significance of each element in the factor class of infrastructure for high, medium and low flood hazard level, the weight is assigned in the following Table 6-4: Table 6-4: Ranking vulnerability value for infrastructure and buildings Element class Value Hazard level Value High 0.28 Bridge 0.28 Medium 0.18 Low 0.09 High 0.01 Road 0. 01 Medium 0.006 Low 0.003 High 0.01 Building 0.01 Medium 0.006 Low 0.003 Land use Land use pattern is separated into settlement area, agriculture area and barren area. According to the significance of each element in the land use class for high, medium and low flood hazard level, the following weight is assigned (Table 6-5): Table 6-5: Ranking vulnerability value for land use element class Element class Value Hazard level Value High 0.09 Settlement area 0.09 Medium 0.06 Low 0.03 High 0.06 Agriculture area 0.06 Medium 0.04 Low 0.02 High 0.02 Medium 0.012 Barren area 0.02 Low 0.006 Medium 0.06 Low 0.02 Population The population of the study area is divided according to ToR into three groups as above 65 and children, female and male. In order to get the number of population of each group footprint of the number of household located in the flood hazard zone is captured. From statistical analysis of the surveyed data as well as the data of National Population Census, 76
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 2001, the number of population of each group for a household lying on the flood hazard area is estimated (Figure 6-2, Figure 6-3 and Figure 6-4). According to the significance of each element in the factor class of population density for high, medium and low flood hazard level, the following weight is assigned (Table 6-6): Table 6-6: Ranking vulnerability value for population element class Element class Value Hazard level Value High 0.27 Above 65 & 0.27 Medium 0.18 children Low 0.09 High 0.19 Female 0.02 Medium 0.12 Low 0.06 High 0.08 Male 0.08 Medium 0.06 Low 0.02 Figure 6-2: Flow Ward-wise population distribution of females (data from NPC, 2001 and present study). 77
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 6-3: Ward-wise population distribution of male (data from NPC, 2001 and present study). Figure 6-4: Ward-wise population distribution (data from NPC, 2001 and present study) 78
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 6.4 FIELD BASED FLOOD VULNERABILITY MAP A final flood hazard vulnerability map is obtained by combination of the hazard data with the vulnerability data. In this regard total weight for each factor class is obtained by multiplication of the value of factor class, value separated for the element class depending on the position in hazard level and the worth of the element given to this study. The worth of elements is expressed in unit area for bridge, road, building, settlement, agriculture and barren area, while population is expressed in population density. All individual thematic layers of the elements were prepared in GIS environment and compiled into a single layer to get the total weight of the elements in consideration of area of interest. By using statistical value, vulnerability level is separated into high, medium and low. The vulnerability map is shown in annex 2. Most of the area at upper reaches of the Bagmati River shows moderate in vulnerability scale since the area is occupied by complete agriculture lands e.g. the area upstream of uttarbahini. As the area to the downstream of Gokarneswor gorge is represented by barren land in the hazard map, the vulnerability depicts the range mostly to low and very little to moderate. Large area of moderate vulnerability is found at downstream from the confluence of Manohara, while small strip of area lies in high vulnerable area e.g. in Teku. Upper reaches of Manohara River shows large area of moderate vulnerability with little area of low vulnerability e.g. in Khulaltar and the area near Nepal Engineering College. Large area in Gothatar village lies in low vulnerability zone, where the area is mostly represented by barren lands. Again large area of moderate vulnerability is found in the river strip between the confluence of Hanumante and the confluence of Bagmati. Majority of the area in the Bishnumati River corridor lies in the moderate vulnerability grade, particularly in Phutung and Manmaiju since the area lies in the sand excavation site converting to barren land. In the urban area, thin strip of low to moderate scale of vulnerability is obtained e.g. Gongabu, Balaju, Dhalko, and Teku areas. Patches of lands are found in moderate to low vulnerability range in the river corridor of Hanumante Khola e.g. Jagati, Sllaghari, Thimitar, and Balkot. Remarkable moderate vulnerability area in the Dhobi Khola river corridor is located at downstream from Chunikhel area in Galphutar and downstream to Om Hospital. Distribution of high and low vulnerable area is very little in the river corridor as the area has mostly protected by floodwall. Low vulnerable area in the river corridor of Balkhu Khola is confined in Kalanki and Teku while the area in upper reaches of the river is found in moderate to low vulnerability grade scale e.g. in Balambu, Banasthali and Satungal area. In Nakhu Khola large area of moderate to low vulnerability is found in Nakhu village and Nakhodol. The area in upper and middle reaches has thin strip of moderate to low vulnerable zones. In the Kodku Khola large area of moderate vulnerability is found in Hattiban, Gwarko and Gorkhu. Small area in Gwarko lies in high vulnerable zone. The area in the Godavari Khola is dominated only by moderate vulnerable zone in Tikathali and Charghare. 79
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 6.5 FIELD BASED RISK ASSESSMENT CRITERIA Field based risk assessment is performed with regard to both the field based flood vulnerability data and the field based flood hazard data. In fact in this study flood risk level is determined by the matrix of vulnerability grades and the flood hazardous grades. Multiplication of the vulnerability level obtained from field based data and the flood hazard level produces the risk level of the study area. Thus, the risk level is classified with regard to both the flood vulnerability grades and the flood hazardous grades. 6.6 FIELD BASED FLOOD RISK MAP The risk map is shown in Annex 2. In the Bagmati River corridor at its upper reaches risk of low value is found. The area includes the places between Uttarbahini and downstream portion of Sundarijal. Very small area in discontinuous patches belongs to the moderate risk level. The river stretch between the Gokarneswor and the confluence of the river and Manohara is characterised by thin belt of moderate to low risk level. The area downstream from the confluence of Manohar has wider area of moderate to low risk level with occasional strip of high risk area, particularly in the slum zone near Thapathali, Teku, and Tirbhuvan university Gate area. Large part of area represented by low risk level in the Manohara River corridor is located in Khulaltar, Phuyalgaon, Gothatar and Balkumari. The area of moderate risk level is found in small prportion in Khulaltar, Godar, Gothatar and Imadol. Very small area of high risk level is lying in Khulaltar, Phuyalgaon, Godar and Jadibuti areas. Bishnumati area is mostly represented by moderate level of risk. High risk area are located at the vulnerable bridge sites e.g. Gongabu Bass Park, Balaju etc. Moderate risk level of Hanumante Khola is observed at Jagati, Sallaghari, Thimitar and Balkot in the map. Low risk level is got in the area downstream of Chunikhel, Galphutar in the river corridor of Dhobi Khola. Likewise, the area downstream to Om Hospital also resides in the low risk zone. Small trip of lands in the urban sector are represented by moderate risk level, while the high risk level is confined mostly in the vulnerable bridge sites. 6.7 RAINFALL RUNOFF AND HYDROLOGICAL MODEL BASED FLOOD HAZARD MAP Hydrological Analysis The hydrological analysis was carried out based on the empirical formula and observed data. The discharge data of the Bagmati River at Chovar gorge is available for the period between 1963 and 1980. This observed data has been used to estimate the Bagmati River discharge at various return periods. The values obtained from the frequency analysis using the observed data has been compared with the one obtained from the empirical methods. As the other rivers are ungauged, the regional and empirical formulae have been adopted in estimating the flood magnitudes. The methods adopted for estimating the high flow analysis for the proposed study of the rivers are Snyder Method, modified Dickens method, and Sharma and Adhikari Method (2004). The flood estimation at various locations for the return 80
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report periods has been calculated by adapting the best fit method for the Kathmandu Valley. The flood estimation by using the above methods is presented as follows: Sharma and Adhikari Method (2004) This method has been derived using the hydrometric data up to 1995 and hence can be considered an updated version of WECS Method. The following relationship has been used for computing the flood discharge by this method: Q2 = 2.29(A3000)0.86 Q100 = 20.7(A3000)0.72 The flows for any other return period ‘R” is then given by: Qf = exp (1n Q2 + S σ) Where, σ = ln (Q100 / Q2) / 2.326 S = Standard Normal Variate (given in the Table 6-7) Table 6-7: Values for Standard Normal Variate for various return periods Return Period Standard Normal Return Period Standard Normal (T) years Variate (S) (T) years Variate (S) 2 0 20 1.645 5 0.842 50 2.054 10 1.282 100 2.326 Modified Dickens Method The Modified Dicken’s Method is widely used in Nepal and north India for the estimation of peak discharge. The peak discharge can be calculated by using the following relationship: QT = CT A 3/4 Where CT is for the return period T is given by CT = 2.342 log (0.6T) log (1185/p) + 4 With p = 100(As+ 6)/A ‘As’ is the snow covered area out of total Catchment area A. Here, As= 0, therefore P=600/A Snyder’s Method (Rainfall-Runoff Model) For an ungauged river, the flood flow estimation by deriving a synthetic unit hydrographs based on known physical characteristics of the basin is used. The method is based on the analysis of a large number of hydrographs from drainage basins ranging in area from 25 km2 to 25,000 km2. The following relationships were used to estimate the flood discharge in Nepalese rivers. tpr = 0.75 Ct(L Lc)0.3 tpr = tpr /5.5 81
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report tpR = tpr + 0.25 (tR - tr) qpR = 2.78 Cp/ tpR QpR = qpR * (CA * A) * R where, tpr = lag time from mid-point of effective rainfall of duration tr to peak of unit hydrograph in hours. tr = Standard duration of effective rainfall in hours. From the measured discharge data at Chobhar is used for the computation of discharge at various return periods (Table 6-8, Table 6-9 and Table 6-10). At the same location the values calculated for various return periods by applying the above mentioned methods illustrate that the values obtained from Snyder’s method resembles close to the observed data. Hence Snyder’s method has been used to calculate the discharges at various river stretches in the valley. Table 6-8: Measured discharge data of Bagmati at Chovar Year Discharge (cumecs) 1963 206.00 1964 251.00 1965 420.00 1966 633.00 1967 680.00 1968 497.00 1969 431.00 1970 582.00 1971 617.00 1972 856.00 1973 335.00 1974 350.00 1975 591.00 1976 245.00 1977 299.00 1978 407.00 1979 416.00 1980 254.00 Table 6-9: Flood flow estimation by various methods Return Periods S. Computation Method 2 5 10 20 50 100 200 No. years years years years years years years Sharma and Adhikari 1 Method 549 882 1130 1386 1736 2034 2341 3 Modified Dickens Method 543 883 1140 1397 1736 1993 2250 4 Snyder Method 686 908 1055 1196 1378 1515 1651 Gumbel Distribution from 5 observed data 419 578 683 783 914 1012 1109 82
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 6-10: Calculation of extreme rainfall for different return period Extreme Rainfall, Return Period (mm) Station Station % area No. Name covered 100 200 2 yrs 5 yrs 10 yrs 20 yrs 50 yrs yrs yrs 1015 Thankot 7.9 92.1 137.0 166.8 195.3 232.2 259.9 287.4 1022 Godavari 9.1 100.3 135.9 159.5 182.1 211.4 233.3 255.2 1029 Khumaltar 11.7 65.7 87.4 101.7 115.4 133.2 146.5 159.8 Kathmandu 1030 Airport 7.4 77.7 103.0 119.7 135.7 156.5 172.0 187.5 1035 Sanku 6.6 109.5 131.0 145.2 158.9 176.5 189.8 203.0 1039 Panipokhari 16.4 81.6 105.7 121.6 136.8 156.6 171.4 186.2 1043 Nagarkot 8.5 90.6 115.0 131.2 146.7 166.8 181.8 196.8 1052 Bhaktapur 8.0 76.5 117.9 145.3 171.6 205.6 231.1 256.5 Chapa 1060 Gaon 13.6 96.6 126.7 146.6 165.8 190.5 209.0 227.5 1074 Sundarijal 10.9 106.6 132.0 148.8 165.0 185.9 201.5 217.1 Weighted average 100.00 88.9 117.7 136.8 155.0 178.7 196.4 214.0 Average rainfall, cm 8.9 11.8 13.7 15.5 17.9 19.6 21.4 Reach-wise discharge computed from Snyder’s method for various return periods have been used for the preparation of the flood map in present study (Table 6-11). Moreover, Snyder’s method has been used for the computation of design discharge and for proposing the waterway and hydraulic modelling. Table 6-11: Reach-wise discharge distribution for various return periods Discharge in m3/sec for Return Period S. 100 200 No. River Station 2 yrs 5 yrs 10 yrs 20 yrs 50 yrs yrs yrs 1 Manohara Khulaltar 24.4 39.2 50.2 61.6 77.2 90.4 104.0 2 Manohara Phidol 201.7 324.0 415.1 509.3 637.9 747.2 860.2 3 Manohara Imadol 237.8 382.0 489.4 600.4 752.1 881.0 1014.1 4 Bagmati Rupsetar 36.7 59.0 75.6 92.7 116.2 136.1 156.6 5 Bagmati Gokarna 51.0 81.9 104.9 128.7 161.2 188.8 217.4 6 Bagmati Sankhamul 309.3 496.8 636.5 780.8 978.0 1145.6 1318.8 7 Bagmati Babarmahal 339.4 545.2 698.5 856.9 1073.3 1257.2 1447.3 8 Bagmati Teku Dovan 448.5 720.5 923.0 1132.3 1418.3 1661.4 1912.6 Balkhu 9 Bagmati Dovan 490.2 787.5 1008.9 1237.7 1550.3 1816.1 2090.6 Nakhu 10 Bagmati Dovan 505.6 812.2 1040.6 1276.5 1599.0 0.0 2156.1 11 Bagmati Chobhar 590.3 948.3 1214.9 1490.4 1866.9 2186.8 2517.4 12 Dhobikhola Bhangaltar 13.4 21.4 27.5 33.7 42.2 49.4 56.9 13 Sanglakhola Phutung 15.6 25.0 32.1 39.4 49.3 57.8 66.5 14 Bishnumati Okhaltar 11.1 17.8 22.8 28.0 35.1 41.1 47.3 15 Bishnumati Hilledol 32.5 52.2 66.9 82.1 102.8 120.4 138.6 16 Bishnumati Sidhitol 49.4 79.3 101.6 124.6 156.1 182.9 210.5 17 Bishnumati Kuleswor 95.5 153.5 196.6 241.2 302.1 353.9 407.4 18 Mahadevkhola Rawaltar 3.4 5.4 7.0 8.6 10.7 12.6 14.5 83
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Discharge in m3/sec for Return Period S. 100 200 No. River Station 2 yrs 5 yrs 10 yrs 20 yrs 50 yrs yrs yrs 19 Mahadevkhola Tusalgaon 8.1 13.0 16.7 20.5 25.6 30.0 34.6 20 Mahadevkhola Panyatar 13.6 21.8 27.9 34.3 42.9 50.3 57.9 21 Karakhusi Waphal 12.6 20.3 26.0 31.9 39.9 46.8 53.8 22 Balkhu Salyanthan 34.6 55.6 71.2 87.3 109.4 128.1 147.5 23 Nakhu Thapagaon 44.0 70.7 90.5 111.0 139.1 162.9 187.5 24 Kodku Ganglethok 15.7 25.3 32.4 39.7 49.7 58.2 67.0 25 Godavari Tikathali 36.9 59.3 76.0 93.2 116.7 136.7 157.4 26 Godavari Bishnudol 30.6 49.2 63.1 77.4 96.9 113.5 130.7 27 Godavari Dhansar 2.5 4.0 5.1 6.3 7.9 9.2 10.6 28 Hanumante Thimi 85.7 137.7 176.4 216.4 271.0 317.5 365.4 Hydraulic Analysis and Modelling HEC-RAS (Hydraulic Engineering Center’s River Analysis System) has been used for the hydraulic computations considering the steady state flow condition. HEC-RAS is the software most widely used throughout the world for the computation in hydraulic modelling for river channels and is developed by U. S. Army Corps of Engineers. This software contains one dimensional hydraulic component both for the steady flow water surface profile computations and unsteady flow simulations. This software is comprised of a graphical user interface (GUI), separate hydraulic analysis components, data storage and management, graphics and reporting facilities. In the case of unsteady flow simulation, the system can simulate one dimensional unsteady flow through a full network of channels. It can simulate all sub-critical, supercritical, hydraulic jumps and draw downs computations. The model can handle full network of channels. This model is capable of modelling sub- critical, supercritical and mixed flow regime water surface profiles. The computational procedure is based on the one dimensional energy equation using standard step method. Energy losses are calculated by friction (Manning’s equation) and contraction/expansion (coefficient multiplied by the change in velocity head). The momentum equation is utilized in situations where the water surface profile is rapidly varied. HEC-GeoRAS is an ARC View GIS extension specifically designed to process geospatial data for use with HEC-RAS. The extension allows users to create an HEC-RAS import file containing geometric attribute data from an existing digital terrain model DTM and complementary data set. This creates an import file with the river, reach, cross sections, overbanks, main channel, and roughness coefficients along with additional geometric data such as levee alignments, ineffective flow areas, and storage areas. The hydraulic computation is done in HEC-RAS importing this input file and the result can be exported to ARC View GIS containing water surface profile data and velocity data which can be displayed in GIS. Hydraulic analysis has been done for the various rivers considered for the present study. The above mentioned discharge data has been used as input at various reaches of the rivers in HEC-RAS. Cross-section lines were cut on each river in order to prepare data for running the model (Figure 6-5). 84
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 6-5: Cross-section lines on the studied rivers as part of data preparation for HEC- RAS The inundation maps have been prepared in all of these rivers in the selected reach with the combination of HEC-RAS, HEC-GeoRAS and Arc View GIS software. The database has been prepared by the input of geometric data in Arc View with the help of HEC-GeoRAS extension, carrying out the hydraulic analysis in HEC-RAS and transporting the result to Arc View to prepare the inundation map with the help of HEC-Geo-RAS. The discharge obtained from Snyder’s method for the return period of 50 years has been used for the computation of flood maps in the studied rivers. The analysis gives the water surface elevation of flood discharge for the different return periods. This analysis has been done to find out the water surface elevation during flood and to prepare the inundation map in the selected reach of the rivers. At present study, the inundation maps (Volume II) for the 50 years return period flood for the rivers under present study gives an idea of the possible inundated areas if there are no river training structures along the river banks. However, it should be noted that these maps are generated without considering the effect of the existing river training structures and hence are purely the case when the river banks are having no structures. The condition could be quite different when the river is extensively trained. By making necessary input, the model was run and the results for 50 year return period flood are obtained (Figure 6-6). The detail map of the model based flood hazard is presented in Annex 3. 85
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Figure 6-6: model based flood hazard map of the studied rivers (50 yr. return period). Output of Model Based Modelling The following output has been obtained: The flood hazard classification on the model based approach has been carried out based on the water depth, as below: • Low hazard: < 1 m • Moderate hazard: 1 to 3 m • High hazard: >3 m The areas around Gokarneshwar and Guheswori along the Bagmati River is having moderate flood hazard, while the downstream from the confluence of Manohara and Bagmati, the main water course increases to attain deep water level. Hence the small area in this river stretch falls within the moderate to high hazard level. It is consistent with the field based hazard map, although the return period of the field based hazard could be smaller than the model based. The high hazard area increases to the downstream from the Bishnumati confluence, where the aerial coverage of deep water depth increases e.g. near Nakhu dovan. The upper reach of Manohara River is dominated by low flood hazard, however, aerial coverage of the area is higher then in the downstream area. Moderately hazardous area gradually increases towards downstream and the area near the confluence of Manohara and Kodku falls remarkably in high hazard level. The area is also consistent with field condition. For the case of the upper reaches of the Bishnumati River, dominant area lies in shallow water depth with little area having moderate water depth. To the lower reaches of the Bishnumati River, moderate water depth gradually increases towards downstream and moderate hazard level covers higher area than its upstream. Distribution of moderate hazard level in the little area to upstream from the Ring Road bridge at Chabahil and in the area around the anamnagar of Dhobi Khola is consistent with the field 86
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report observation. Deeper water level is also found in the the upstream area of Godavari Khola around the Dharmeshwar village, Gwarko area of Kodku Khola, upstream of Nakhu Dovan in Nakhu Khoa and Balkhu area of Balkhu Khola indicating the high flood hazard probability. Most of the area along the river corridors of the rivers are covered by low hazard zone, however, parts of the river corridors with small area fall in moderate hazard zone e.g. in Thimi area of Hanumante Khola, Kalimati area of Karakhusi Khola etc. 6.8 MODEL BASED FLOOD VULNERABILITY MAP Model based vulnerability map is prepared on the basis of flood hazard map developed from runoff and hydrological model, and the parameters established for the vulnerability analysis of field based map. According to the model based vulnerability map about 10%, 33% and 57% of the surveyed area (15.26 sq. km.) is occupied, respectively by high, medium and low vulnerable area. The detail map of the model based vulnerability hazard is presented in Annex 3. High vulnerable area along the river corridor of Bagmati are situated at downstream from Jorpati, upstream from Gaurighat, upstream from Tilganga, downstream from Subidhanagar, upstream from Thapathali, around the confluence of Bishnumati, and around the confluence of Balkhu. Likewise, the high vulnerable area in the Manohara River corridor is found at upstream and downstream of the confluence of Manohara and Hanumante rivers. In Dhobi Khola corridor the high vulnerable area is located downstream from Katubahal and some area in Anamnagar. Similarly, the area around the confluence of Mahadev Khola and Bishnumati, downstream from Mhaipi and Khusibu of Bishumati River corridor lie in the high vulnerable area. Small area in Soltimod of Karakhusi Khola also located in high vulnerable area. Likewise, the area downstream from Kalanki in the river corridor of Bulkhu also falls in the high vulnerable area. The major vulnerable elements are the settlement area, road segments, population and buildings. The concrete bridges located at Tilganga, Sinamangal, Subidhanagar of Bagmati River, bridge at Jadibuti of Manohara River, at Bijulibazar of Dhobikhola and bridges at Gongabu and Balaju are highly vulnerable because of extreme exposure of piers foundation due to intense river incision. However, these bridges are situated at higher water level during the flood of 50 years return period. Low vulnerable areas are noted at upstream from Gothatar of Manohara River, upstream from Jorpati of Bagmati River, upstream from Katubahal of Dhobi Khola, upstream of Mahadev and Bishnumati rivers, upstream from Kalanki of Balkhu Khola, and the river corridors of Nakhu and Godavari. Wide coverage of moderate vulnerable areas is noted in Guheswori-Gokarna segment and area around Sankhamul of Bagmati River, river sector between Magargaon to Chhapro of Manohara River, around Maitidevi in the Dhobi Khola sector, around Balaju, Banasthali and downstream from Dhalko of Bishnumati and Thimi to Lohakinthali of Hanumante Khola. Normally agriculture areas are the dominant element in the zone of low and moderate vulnerability. 6.9 MODEL BASED FLOOD RISK MAP The risk map shows about 90%, 6% and 4% of the total surveyed area (14.95 sq. km.), respectively, are located in the low, moderate and high risk level. The high risk area are situated at Thapathali, at upstream and downstream of Bishnumati confluence and Balkhu along the river corridor of Bagmati River, small strip at downstream from confluence of Manohara and Hanumante, downstream from confluence of Karakhusi and Bisnhumati, 87
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report while the moderately risk areas occur at Gaurighat, Tilganga, Shankhmul, Thapathali along the sector of Bagmati River, around and doenstream from confluence of Manohara and Hanumante, around Anamnagar of Dhobi Khola, downstream of Mhaipi and around Khusibu of Bishnumati, around Balkhu of Balkhu Khola and Thimi to the confluence of Godavari Khola along the Hanumante. The model based detail risk map is presented in Annex 3. 6.10 MINIMUM WIDTH REQUIRED FOR NATURAL WATERWAY OF RIVERS Since most of the areas under this study are located in heavily developed city area having high commercial value of land and buildings, the return period of discharge to be considered for ascertaining the waterway is an important issue and should be considered with utmost care and consideration. On the other hand the natural waterway of the river should be left undisturbed with the hydraulic consideration for the river to allow its flood to pass unhindered with ample space for the meandering. It has often produced disastrous consequences in case of failure to consider this fact. Taking into account on these facts a compromise has to be made for fixing the design discharge to get the waterway of the rivers. Considering all these arguments, different waterway with respect to the position of the river in the anticipated floods at 50 years return period as calculated from Snyder’s Method have been suggested. This indicates that if the flood of 50 years return period occurs, the area behind the respective waterway will get flooded. The present rivers in city area of the Kathmandu are in the condition of scouring river bed due to limiting the flood water in confined width. Under this scenario construction of engineering structure for the protection of concrete bridges in several places is most essential. Lacey's formula can be used to determine the width of waterway. This formula is as follows: P = 4.75 Q where, P = Required waterway for the river (m) Q = Discharge (m3/s) The waterway for the different rivers for a flood discharge of 50 years return periods has been computed and presented in the table below by using the discharge value obtained from Snyder’s Method (Table 6-12). 88
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Table 6-12: Waterway for different return periods at various locations of the rivers. Proportional Waterway in meter for Return Period S. 10 20 50 100 200 No. River Station @ 2 yrs 5 yrs yrs yrs yrs yrs yrs 1 Manohara Khulaltar 23.5 29.7 33.7 37.3 41.7 45.2 48.4 2 Manohara Phidol 67.5 85.5 96.8 107.2 120.0 129.8 139.3 3 Manohara Imadol 73.3 92.8 105.1 116.4 130.3 141.0 151.3 4 Bagmati Rupsetar 28.8 36.5 41.3 45.7 51.2 55.4 59.4 5 Bagmati Gokarna 33.9 43.0 48.6 53.9 60.3 65.3 70.0 6 Bagmati Sankhamul 83.5 105.9 119.8 132.7 148.5 160.8 172.5 7 Bagmati Babarmahal 87.5 110.9 125.5 139.0 155.6 168.4 180.7 8 Bagmati Teku Dovan 100.6 127.5 144.3 159.8 178.9 193.6 207.7 Balkhu 9 Bagmati Dovan 105.2 133.3 150.9 167.1 187.0 202.4 217.2 Nakhu 10 Bagmati Dovan 106.8 135.4 153.2 169.7 189.9 0.0 220.6 11 Bagmati Chobhar 115.4 146.3 165.6 183.4 205.2 222.1 238.3 12 Dhobikhola Bhangaltar 17.4 22.0 24.9 27.6 30.9 33.4 35.8 13 Sanglakhola Phutung 18.8 23.8 26.9 29.8 33.4 36.1 38.7 14 Bishnumati Okhaltar 15.8 20.1 22.7 25.1 28.1 30.4 32.7 15 Bishnumati Hilledol 27.1 34.3 38.9 43.0 48.2 52.1 55.9 16 Bishnumati Sidhitol 33.4 42.3 47.9 53.0 59.3 64.2 68.9 17 Bishnumati Kuleswor 46.4 58.8 66.6 73.8 82.6 89.4 95.9 18 Mahadevkhola Rawaltar 8.7 11.1 12.5 13.9 15.5 16.8 18.1 19 Mahadevkhola Tusalgaon 13.5 17.1 19.4 21.5 24.1 26.0 27.9 20 Mahadevkhola Panyatar 17.5 22.2 25.1 27.8 31.1 33.7 36.1 21 Karakhusi Waphal 16.9 21.4 24.2 26.8 30.0 32.5 34.8 22 Balkhu Salyanthan 27.9 35.4 40.1 44.4 49.7 53.8 57.7 23 Nakhu Thapagaon 31.5 39.9 45.2 50.1 56.0 60.6 65.0 24 Kodku Ganglethok 18.8 23.9 27.0 29.9 33.5 36.2 38.9 25 Godavari Tikathali 28.9 36.6 41.4 45.9 51.3 55.5 59.6 26 Godavari Bishnudol 26.3 33.3 37.7 41.8 46.8 50.6 54.3 27 Godavari Dhansar 7.5 9.5 10.7 11.9 13.3 14.4 15.5 28 Hanumante Thimi 44.0 55.7 63.1 69.9 78.2 84.6 90.8 The total waterway for a particular discharge consists of the natural river channel width and the floodway over banks at the time of flood. So the waterway to pass the flood discharge for respective return periods should be left without any construction. It is required to set this width for the river to avoid the flooding. In other cases extensive river training structures and flood embankments should be constructed to confine the flood discharge within narrower width. 6.11 VULNERABILITY AND RISK OF MAJOR CONCRETE BRIDGES As explained in the earlier text, most of the bridges in the surveyed area are suffered from strong river bed erosion. Foundations of all most all major concrete bridges have been exposed and the situation is highly vulnerable in some bridges like at Gongabu, Balaju of Bishnumati, at Tilganga, Sinamangal and Subidhanagar of Bagmati, at Jdibuti of Manohara etc. Hence these bridges are found in high risk level from the point of riverbed incision at the 89
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report foundation of the bridges. In contrast, according to benchmark data that obtained across the river near the foundation of the bridges the water level during the flood of 50 years return period remains at lower level near the bottom of piers of the bridges. In other words there is no sign of overtopping the bridges during the flood of 50 years return period. Thus the bridges are situated at low risk for 50 year flood. 6.12 RESCUE ROUTE As the urbanization in the Kathmandu valley is rapidly increasing, route for access to such area is also increasing in a similar trend although such access routes have generally small width. These routes are marked in the map for rescue route during the flooding events. The major road like Ring Road can be used for other facilities during the flood calamities. Nearest school buildings of the possible flooding sites are marked in the map with levels of flood hazard for the use of temporary shelter. The detail map of the rescue route for flood hazard is presented in Annex 4. 90
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 7. LIMITATION OF THE PRESENT STUDY Followings are the limitations of the present study: • Damage on infrastructure basically depends on the speed of water flow. The speed of water flow of the entire surveyed river is unavailable due to lack of hydrological station. • Peak flow discharges of the surveyed rivers are not available. Hence, it is not possible to establish relation between the opening of the bridges and the peak discharge to understand the erosion behaviour as well as blockage of river discharge due to problem of space under the bridge for water to pass. • Buildings considered in this study are reinforced but the quality of construction can be different. Consideration of degradation for all buildings during inundation can be different and the degree of damage due to speed of water can be different for those buildings located within the area of same magnitude of flooding phenomenon. • Expansion of clay, degree of erosion etc. in the foundation area of buildings during inundation and flooding are important for estimation of degree of damage, which is missing due to lack of lithological data in the foundation area. • The present satellite image is recommended for the preparation of 1:25,000 scale topographic map and the vertical accuracy is suggested as 7 m. A high resolution and precise DEM is required for preparation of river hydro-geometry and terrain topography for accurate estimation of inundation aerial extent and flood depth. • Information regarding past flood events in some river corridor through interviews in the field, unfortunately, was not possible to collect due to settlement of the new citizen from outside as they have no longer experience of the area for any specific details. 91
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report 8. CONCLUSIONS AND RECOMMENDATIONS 8.1 CONCLUSIONS The present study has revealed that the rivers in the Kathmandu valley have varied degree of problems depending upon the watershed area, discharge, river morphology and human interference. River morphology of the Kathmandu valley has been rapidly modifying since few years particularly from the immediate consequences of human attack in the river channel area and its surroundings. Extensive sand mining activity from the river channel and its surrounding areas is doing very fast particularly from Manohara and Bishnumati rivers, while human encroachment by constructing infrastructures and private houses close to the natural regime of the rivers are pronounced when the rivers inter into the sub-urban to urban areas. In addition to natural process, both government agency and local people are the major actors for the modification of river morphology. In this regards, the Bagmati, Manohara, Dhobi Khola, Bishumati, Balkhu and Kodku Kholas should be dealt with special attention. River widths are highly increased e.g. in the case of Monohara at Gothatar and Sangla Khola (tributary of Bishnumati River) at Baluwapati-Phutung area, while channel incision and width of channel reduction is found very high in the rivers when arrive to urban areas e.g. Monohara at Koteswor-Balkumari area, Bagmati from Tilganga to Balkhu, Bishumati at Gongabu-Balaju, etc. Major concrete bridges located over these rivers e.g. bridge at Gongabu and Balaju of Bishnumati River, etc. in such areas are highly vulnerable condition because of deep exposure of foundation of the bridge piers. There is tremendous pressure exerted on the rivers by channelizing the river course through the desired location. Thus, shifting of rivers is pronounced in Manohara, Balkhu and Kodku rivers which are resulted both from the natural as well as artificial reasons. Field evidence and historical data illustrate that flooding of these rivers is also inundating considerable distance from the river course because of narrow passage of river channel, strong river encroachment by private building etc. and highly meandered river morphology with low bank height e.g. in Balkhu and Kodku Kholas. The other rivers especially Karakhusi, Hanumante, and Godavari Kholas are less problematic ones as they have low catchment area in comparison to the previous ones. Likewise, the flood information obtained from the field also shows less severe than those. The river shifting is not so pronounced in these rivers. The Samakhusi and Tukucha are further less problematic. Since the catchment area of these kholas are less than 3 square kilometer and both originates from the valley itself, the discharge and flood problem is less severe. However, the natural regime of these kholas are completely destroyed by constructing river training structures, houses very close to the river bank and in some cases over the river. Bagmati and Bishnumati Rivers are also affected by human encroachment mainly because of narrowed river width and also uncontrolled sand mining. The flood hazard map based on field shows more consistency with the model based hazard map. However, some exceptional dissimilarity was observed in these maps probably as a consequence of river protection works that can not be considered in the model based map. The flood risk mapping shows that in the Bagmati River corridor, the upper reaches is having low risk areas with very small area in discontinuous patches belonging to moderate risk level. The river stretch between the Gokarneswor and the confluence of the river and 92
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report Manohara is characterised by thin belt of moderate to low risk level. The area downstream from the confluence of Manohar has wider area of moderate to low risk level. Large area represented by low risk level in the Manohara River is located in Khulaltar, Phuyalgaon, Gothatar and Balkumari. The area of moderate risk level is found in smaller area in Khulaltar, Godar, Gothatar and Imadol. Bishnumati area is mostly represented by moderate level of risk. High risk area are located at the vulnerable bridge sites e.g. Gongabu Bass Park, Balaju etc. Moderate risk level of Hanumante Khola is observed at Jagati, Sallaghari, Thimitar and Balkot in the map. Low risk level in the river corridor of Dhobi Khola is got in the area downstream of Chunikhel, Galphutar and downstream to Om Hospital. Small trip of area in the urban sector are represented by moderate risk level. Most of the bridges within the Ring Road are in vulnerable condition, particularly the bridges over the Bagmati and Bishnumati Rivers. 8.2 RECOMMENDATIONS • Awareness generation activities should be carried out so as to make the people conscious about the possible impact to the society due to the floods. Information should also be disseminated about the alternatives to be adapted to mitigate such problems. The residents who are aware of the flood vulnerability and risk should be mobilized during the awareness campaign. • There is a need of construction of river training structures and flood embankments in a planned way so as to avoid the possible effect to adjacent lands. • The other notable aspects are that in view of rapid urbanization in the adjoining areas of the rivers, there is a need to develop the areas in a planned way. The meandering portions of the rivers should be properly protected by constructing levees on both sides of the rivers with riparian vegetation. In such case the existing agriculture land and the settlement area utilized for the further urban development. Instead, at such localities, there is a need to maintain open space. • River encroachment and narrowing down of waterway should not be permitted, particularly in the bridge site area. • Relocation of slum area from the river bank and floodplain should be carried out to reduce the risk level. This would eventually help to reduce the intensity of illegal sand mining. • Extensive sand mining from a particular area should not be allowed since the river hydrodynamic equilibrium at upstream and downstream could be highly disturbed. Concerned organization should regularly monitor and prompt action should be taken against the river bank encroachment. • The existing riparian vegetation along the Hanumante River is a good example of good ecosystem. An approach should be made to follow this example even along other river corridors. • High resolution stereo-pair satellite image (e.g. Quickbird image) is suggested to prepare DEM that would require less effort for editing along the river corridor. Likewise, such image would also provide better information towards risk analysis. 93
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report • It is better to have satellite images during the monsoon period for the estimation of actual flooded area. 94
  • Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley Final Report REFERENCES Department of Mines and Geology (DMG), 1998. Engineering and environmental geological map of Kathmandu Valley, Scale 1:50,000. Department of Water Induced Disaster Prevention (DWIDP)/Masina Continental Assocaites, RIDA and DE Consultancy JV, 2006. Detail feasibility study for the implementation of River Training Works in Manahara River of Kathmandu Valley. Department of Water Induced Disaster Prevention (DWIDP/SILT Consultants, ERMC and TECHDA JV, 2006. Preparation of Water Induced Hazard Map of Bagmati River Basin. Du, Juan, and Shi, Peijun, 2006, Flood Risk Assessment of Xiang River Basin in China. The Sixth Annual DPRI-IIASA Forum, Istanbul, 13-17 August, 2006 Dunne, T. and Leopold, L.B. 1978, Water in environetal planning. Freeman, San Francisco, CA, pp. 818. HMG/UNDP Mineral Exploration Project, 1978. Geological sketch map of Kathmandu area (72 E/6), Scale 1: 63.360. Rosgen, D.L., 1994, A classification of natural rivers. Catena, v. 22, pp. 169-199. Sakai, H., 2001. The Kathmandu Basin: an archive of Himalayan uplift and past monsoon climate. Jour. Nep. Geol. Soc., Vol. 25, pp 1-8. Sakai, H., Fujii, R. and Kuwahara, Y., 2002, Changes in the depositional system of the Paleo-Kathmandu Lake caused by uplift of the Nepal Lesser Himalayas. Journal of Asina Earth Sciences, v. 20, pp. 287-276. Stocklin, J. and Bhattarai, K.D., 1981. Geological map of Kathmandu area and Central Mahabharat Range (1:250,000). Dept. of Mines and Geology, Govt. of Nepal 95
  • ANNEX-1 FIELD PHOTOGRAPHS
  • ANNEX-2 FIELD BASED FLOOD HAZARD, VULNERABILITY AND RISK MAP
  • ANNEX-3 MODEL BASED FLOOD HAZARD, VULNERABILITY AND RISK MAP
  • ANNEX-4 SAMPLE RESCUE ROUTE MAP FOR FLOOD RISK AREA