Government of Nepal
                                     Ministry of Water Resources
           Department of Water Induce...
EXECUTIVE SUMMARY

The assigned work entitled “Preparation of Flood Risk and Vulnerability Map of the
Kathmandu Valley” is...
periods. The values obtained from the frequency analysis using the observed data has been
compared with the one obtained f...
of Manohara and Kodku falls remarkably in high hazard level. The area is also consistent
with field condition. For the cas...
Khola, upstream of Mahadev and Bishnumati rivers, upstream from Kalanki of Balkhu Khola,
and the river corridors of Nakhu ...
ACKNOWLEDGEMENT


We are thankful to the Department of Water Induced Disaster Prevention for giving us an
opportunity to u...
TABLE OF CONTENTS

EXECUTIVE SUMMARY ........................................................................................
5.8       KARA KHUSHI KHOLA (MANAMATI KHOLA) ................................................................... 62 
   5....
LIST OF FIGURES

Figure 1-1: Bagmati River catchment with its major tributaries in the Kathmandu Valley ....... 3 
Figure ...
different resolution (Alos on the right and Quickbird on the left).The blue area is river
     course as of topographical ...
Table 4-19: Samakhusi Khola (Flood Hazard data) .............................................................. 35 
Table 4...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley               Final Report


1.        INTRODUCTIO...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley             Final Report

      •   The vulnerabil...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley              Final Report

1.4.2    Flooding Situa...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley               Final Report

   • By 2007, availabi...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley           Final Report




Environmental issues:
•...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley            Final Report




1.5.2    Satellite Ima...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley               Final Report

Table 1-1: List of DoS...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley              Final Report




                   F...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley        Final Report




Figure 1-3: Geological sec...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley             Final Report


2.       METHODOLOGY
Th...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley                               Final Report


     ...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley                         Final Report




In additi...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley                         Final Report

Table 2-2: L...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley                Final Report

NGOs. Such data were ...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley            Final Report

contained the preliminary...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley           Final Report

verified and marked on the...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley           Final Report

flooding, inundation etc.)...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley               Final Report

Based on the approache...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley              Final Report

within acceptable limit...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley             Final Report



         Starting a Ne...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley                                   Final Report



...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley           Final Report


3.        DEM PREPARATION...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley              Final Report


    ­    Add spot heig...
Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley          Final Report


4.       SOCIO-ECONOMIC CO...
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
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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
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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
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
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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
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
Preparation of flood risk and vulnerability map final report ktm sept_17
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Preparation of flood risk and vulnerability map final report ktm sept_17

  1. 1. 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
  2. 2. 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
  3. 3. 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
  4. 4. 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
  5. 5. 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
  6. 6. 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
  7. 7. 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
  8. 8. 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
  9. 9. 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
  10. 10. 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
  11. 11. 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
  12. 12. 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
  13. 13. 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
  14. 14. 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
  15. 15. 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
  16. 16. 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
  17. 17. 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
  18. 18. 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
  19. 19. 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
  20. 20. 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
  21. 21. 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
  22. 22. 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
  23. 23. 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
  24. 24. 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
  25. 25. 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
  26. 26. 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
  27. 27. 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
  28. 28. 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
  29. 29. 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
  30. 30. 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
  31. 31. 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
  32. 32. 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
  33. 33. 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
  34. 34. 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
  35. 35. 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

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