1. 120
Thesis Title: FLOOD RISK MAPPING OF KATHMANDU VALLEY
Submitted by: Shuvra Bijukshe
Supervisor: Prof. Dr. Narendra Man Shakya
ABSTRACT
Floods are probably the most recurring, widespread, disastrous and frequent natural hazard of
the world. Nepal is one of the most flood affected countries. Flood mitigation measures
generally consists two approaches, one is structural method and another one is non-structural
method. Among them flood risk mapping is one of the prominent non-structural methods for
flood control and mitigation purposes. The non-structural measure for risk reduction strategy
is the delineation of flood prone areas. This study attempts the application of GIS and steady
flow model for the preparation of hazard, vulnerability, exposure and risk maps of
Kathmandu valley. The major tools used in this study are ArcGIS 9.3 and one-dimensional
numerical model HEC-RAS 4.0 with HEC-GeoRAS 4.3 for interfacing between HEC-RAS
and ArcGIS.
The study describes the technical approach of probable flood hazard; vulnerability, exposure
and risk analysis of Kathmandu valley. The Kathmandu valley is an intermountain valley
with an approximate area 626 sq.km. lies between latitude 27° 32' 00" N to 27° 49'16" N and
longitude 85°13'28M
E to 85°3r53" E. The average annual rainfall in the area is 1600 mm.
Flood frequency analysis for 2, 5, 10, 20, 50 and 100 years return period was done by WECS,
Modified Dicken's and Snyder's method based on extreme average annual precipitation
recorded at available fifteen stations around the valley.
Triangulated Irregular Network (TIN) was prepared from Digital Elevation Model (DEM) of
10 m*10 m spatial resolution in ArcGIS. Required data sets as stream, banks, flow paths and
cross sections were prepared by HEC-GeoRAS thus, creating import file and imported in
HEC-RAS. Similarly, maximum flood discharges among all the methods of different return
periods were also entered in HEC-RAS. Steady flow analysis was done in HEC-RAS with
Manning's roughness coefficient 0.035 for bed and 0.04 for flood plain and normal depth as
boundaiy condition. The simulated flood is imported to GIS using HEC-GeoRAS and flood
inundation map is prepared.

2. 121
Hazard map is prepared based on the depth of flood. The vulnerability map is prepared based
on the land use that is affected by flood and the exposure map is prepared based on the
population affected by the flood. The results of hazard, vulnerability and exposure are
combined to prepare the risk map of the study area. The assessment of the flood water depth
shows that most of the areas under flooding have water depth less than 1.0 m. The assessment
of flood vulnerability indicates that a large percentage of vulnerable area (more than 70%) is
cultivation area followed by the water body and sand, comprising 18% and 7% respectively.
The exposure assessment of the study area indicates that Kathmandu M.N.P., Lalitpur N.P.,
Jorpati and Madhyapur Thimi N.P. are highly susceptible of exposure; Sundarijal and Suntol
are not likely to be affected. From the study it is found that, most of the areas are under
moderate risk. Cultivation is the major affected land use pattern for all return year flood and
are under moderate and high risk area.
The automated risk mapping and analysis using these tools provide more efficient, effective
and standardized results and saves time and resources. The presentation of results in GIS
provides a new perspective to the modeled data for better visualization. The visualization and
the quantification of the flood risks as facilitated by this approach can help the decision
makers better understand the problem.