Preparation of flood risk and vulnerability map final report ktm sept_17

Government of Nepal
Ministry of Water Resources
Department of Water Induced Disaster Prevention
Pulchowk, Lalitpur, Nepal

FINAL REPORT

THE PREPARATION OF FLOOD RISK AND VULNERABILITY MAP
OF THE KATHMANDU VALLEY

May 2009
Submitted by the Joint Venture of:

FBC

Full Bright Consultancy (Pvt.) Ltd. GEO Consult (P.) Ltd.
P. O. Box 4970, Maitidevi and Sankhamul, Kathmandu
Kathmandu, Nepal Tel: 47 82758
Tel: 44 33149 and 44 11780 E-mail: info@geoconsultnepal.com
Fax: ++ 977-1-44 13331 Homepage: www.geoconsultnepal.com
E-mail: fbc@mos.com.np

EXECUTIVE SUMMARY

The assigned work entitled “Preparation of Flood Risk and Vulnerability Map of the
Kathmandu Valley” is carried out by the joint venture of Full Bright Consultancy (Pvt.) Ltd.
and Geo Consult (P) Ltd. based on the scope of works. The study area is situated within the
Kathmandu Valley between latitude 27°32' 00" N to 27°49'16" N and longitude 85°13'28" E
to 85°31'53" E. The area has covered all the major sub-basins of Bagmati River Basin,
namely Bishnumati, Hanumante, Manohara, Dhobi Khola, Balkhu, Nakhu, Kodku, Kalimati
Khola (Manamati Khola) and Samakhushi Khola Sub-basins. These rivers flow towards
valley center to join with the Bagmati River that eventually drains out of the study area
through the Chovar gorge.

Main objective of the study is to prepare the flood risk and vulnerability map of the
Kathmandu valley by utilizing satellite image and GIS.

Geomorphologically, the Kathmandu Valley and its surrounding mountainous parts can be
subdivided into hill and hill slopes, rocky outcrop, terraces, floodplain and riverbed. Most of
the rivers originate (except Samakhusi) at the hilly areas and traverses through the terraces
developing flood plain. Geologically the Kathmandu valley and its surrounding area can be
subdivided into two major geological units as: basement rocks having the rocky areas and
the overlying basin-fill sediments.

To prepare preliminary flood maps according to scope of work the data on water-induced
disaster, flood risk and vulnerability of Kathmandu valley were acquired from aerial
photographs (1978, 1985, and 1992), recent ALOS image (spatial resolution of 2.5 m),
QuickBird image (0.6 m spatial resolution), all available hardcopy and digital maps like
topographical, geological, and engineering geological of the Kathmandu valley, available
relevant literature and reports on in the internet, local libraries, government institutions (like
Ministry of Home Affairs, Department of Mines & Geology, DWIDP, DHM etc.), ICIMOD,
Nepal Red Cross Society and NGOs etc. Similarly the Consultant had also collected the
precipitation/rainfall data from DHM as recorded at different rain gauge stations within the
Kathmandu Valley and its adjacent areas, and the hydrological data of the rivers i.e. stream
flow summary under present study as far as their availability.

All the data were spatially linked for analysis purpose in the GIS environment. Once the
DEM was generated then the river network was generated from the DEM in the GIS
environment. The HEC GeoHMS was used to evaluate the raster data of the DEM. The
stream network data were then overlaid with satellite images and were verified the accuracy
of the network that is really in the filed and that generated. Based on the calculation made
from the rainfall-runoff model and other empirical methods, the discharges of the rivers
under consideration for various return periods have been obtained at the desired points and
locations. The computed discharge has been used as input parameter while running the
GIS-based flood model using HEC-RAS and with geometric data generated from HEC
GeORAS to prepare preliminary hazard map. The hydrological analysis is carried out based
on the empirical formula and observed data. The discharge data of the Bagmati River at
Chovar gorge is used, which is available for the period between 1963 and 1980. This
observed data has been used to estimate the Bagmati River discharge at various return
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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
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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

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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.

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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

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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
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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

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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

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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
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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

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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.

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• 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.

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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

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• 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.
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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.

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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)

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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.

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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


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)

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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


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

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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.

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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


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


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


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


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


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


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


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


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


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


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


4. SOCIO-ECONOMIC CONDITION

Social survey was furnished by collecting key information from interview as well as focus
group discussion with the people residing nearby the studied river banks. The team of social
scientist carried out the study at the areas identified during the preliminary study and
technical survey along the river banks. These locations are identified as flood prone areas
where regular impact of flood and also human encroachment to the natural regime of river
has been taking place (Figure 4-1).

Figure 4-1: Location of Social Survey

Because of very dense population and complete river capturing in the downtown areas of the
Karakhusi, Tukucha and Samakhusi Kholas by residential houses and roads, it was not
possible to interact with the local people. During the survey two to three persons were
sampled in each site for interview using the pre-developed questionnaire. Likewise, focus
group discussions were made at each selected sites. Besides interacting with the local
people, the team made their own judgement on the social impact due to flood and vice
versa.

The following are some of the issues related to social survey in the Kathmandu Valley:

• difficulty to find out the key informant as the people are very busy
• people have negative impression about the studies to be carried out by different
organization and hence are unsupportive in some cases
24

Preparation of flood risk and vulnerability map final report ktm sept_17

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