FLOOD PLAIN ANALYSIS AND VULNERABILITY

ASSESSMENT OF TINAU KHOLA WATERSHED, NEPAL

A DISSERTATION

FOR THE PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE COMPLETION OF MASTER‟S DEGREE IN

ENVIRONMENTAL SCIENCE

SUBMITTED TO

CENTRAL DEPARTMENT OF ENVIRONMENTAL SCIENCE

INTITUTE OF SCIENCE AND TECHNOLOGY

TRIBHUVAN UNIVERSITY, KIRTIPUR

KATHMANDU, NEPAL

SUBMITTED BY

Uttam Paudel

T.U. Regd. No. 5-2-37-655-2003

July, 2011

ii

LETTER OF APPROVAL

Date: July, 2011

The dissertation entitled “FLOOD PLAIN ANALYSIS AND VULNERABILITY

ASSESSMENT OF TINAU KHOLA WATERSHED, NEPAL” submitted by Mr.

Uttam Paudel is accepted and duly approved as the partial fulfillment of the

requirements for the completion of Master‟s degree part II (Mountain Environment) in

Environmental Science.

____________________________

Ananta Man Singh Pradhan

Supervisor

Department of Electricity Development

(DOED)

Ministry of Energy, Government of Nepal

____________________________

Kedar Rijal, Ph.D.

Associate Prof. and Department Head

Central Department of Environmental

Science (CDES)

Tribhuvan University, Kirtipur, Nepal

___________________________

Suman Man Shrestha

Internal Examiner

Central Department of Environmental

Science (CDES)

Tribhuvan University, Kirtipur, Nepal

____________________________

Dr. Khada Nanda Dulal

External Examiner

Department Head,

Kantipur Engineering College,

Institute of Engineering (IOE)

Tribhuvan University, Nepal

______________________________

Gyan Kumar Chippi Shrestha

Co-Supervisor

Central Department of Environmental

Science (CDES)

Tribhuvan University, Kirtipur, Nepal

iii

LETTER OF RECOMMENDATION

Date: July, 2011

This is to certify that Mr. Uttam Paudel has completed this dissertation work entitled

“FLOOD PLAIN ANALYSIS AND VULNERABILITY ASSESSMENT OF

TINAU KHOLA WATERSHED, NEPAL” for the partial fulfillment of the

requirements for the completion of Master‟s degree part II (Mountain Environment) in

Environmental Science and that the work was completed under our supervision and

guidance. To my knowledge, this research reflects the researcher‟s own effort and has

not been submitted for any other degree, anywhere else. We therefore recommend the

dissertation for acceptance and approval.

_____________________________

Ananta Man Singh Pradhan

(Supervisor)

Department of Electricity Development

(DOED)

Ministry of Energy,

Government of Nepal

______________________________

Gyan Kumar Chippi Shrestha

(Co-Supervisor)

Central Department of Environmental

Science (CDES)

Tribhuvan University, Kirtipur

Kathmandu, Nepal

iv

ACKNOWLEDGEMENT

First and foremost, I offer my sincere gratitude to my Supervisor, Mr. Ananta Man

Singh Pradhan, whose propitious guidance and support alongside constant

encouragement helped me throughout my dissertation work. I owe him a million thanks

for his belief in me, for that gave me the confidence to achieve.

I express my heartiest gratitude to the head of department, Central department of

Environmental Science, Associate Prof. Kedar Rijal, Ph.D., for his incessant

encouragement and support for the research work.

Especial thanks to my co-supervisor Mr. Gyan Kumar Chippi Shrestha, for his valuable

inputs and suggestions.

I highly value the long cooperation and continuous encouragement of my friends Mr.

Aavash Paudel, Mr. Rabin Raj Niraula, and Mr. Sajan Neupane whose help and support

enabled me to accomplish my research. Thank you buddies.

I am also obliged to all the teachers and staffs of Central Department of Environmental

Science for their continuous help and support in one way or the other during the entire

course of my study.

My heartful appreciation goes to my father Mr. Pramod Kumar Paudel and my mother

Sarala Devi Paudel, my sisters Mrs. Sunita Gautam, Mrs. Sangita Tripathee and Mrs.

Ranjita Uprety. They deserve a special mentioning for their inseparable support, love

and prayers that blessed my heart and supported me all these years for what I am today.

In the end I would like to thank my wife Mrs. Neelam Niroula Paudel, for her love and

support.

Uttam Paudel

v

DECLARATION

I, Uttam Paudel, hereby declare that this thesis entitled “FLOOD PLAIN ANALYSIS

AND VULNERABILITY ASSESSMENT OF TINAU KHOLA WATERSHED,

NEPAL” is my own work except wherever acknowledged. Errors if any are the sole

responsibility of my own.

I have not submitted the study or any of part of it for an academic degree anywhere else.

Uttam Paudel

July, 2011

TABLE OF CONTENTS

LETTER OF APPROVAL ......................................................................................................................................... II

LETTER OF RECOMMENDATION .........................................................................................................................III

ACKNOWLEDGEMENT ....................................................................................................................................... IV

DECLARATION .................................................................................................................................................... V

LIST OF ABBREVIATIONS ................................................................................................................................... IX

ABSTRACT .......................................................................................................................................................... X

CHAPTER I ........................................................................................................................................................... 1

1. INTRODUCTION ......................................................................................................................................... 1

1.1 BACKGROUND ................................................................................................................................................. 1

1.2 OBJECTIVES ..................................................................................................................................................... 4

1.3 RATIONALE ..................................................................................................................................................... 5

1.4 STEADY FLOW LIMITATIONS ............................................................................................................................... 6

CHAPTER II .......................................................................................................................................................... 7

2. LITERATURE REVIEW .................................................................................................................................. 7

2.1 ASSESSMENT AND ANALYSIS OF FLOODS IN NEPAL .................................................................................................. 8

2.2 FLOOD CONTROL STRUCTURE ............................................................................................................................ 11

2.3 TINAU RIVER FLOODING .................................................................................................................................. 12

2.4 FLOOD FREQUENCY ANALYSIS ........................................................................................................................... 13

2.5 TOOLS FOR FLOODPLAIN ANALYSIS AND MAPPING ................................................................................................ 13

2.5.1 Geographical Information System (GIS)........................................................................................... 14

2.5.2 Details of Selected Flood Simulation Models ................................................................................... 14

2.6 FLOOD RISK ASSESSMENT ................................................................................................................................ 16

CHAPTER III ....................................................................................................................................................... 18

3. MATERIALS AND METHODS ..................................................................................................................... 18

3.1 HYDROLOGICAL ANALYSIS ................................................................................................................................ 18

3.1.1 Comparison BetweenFlood Frequency Methods ............................................................................. 18

3.1.2 Flood Flow Calculation ..................................................................................................................... 19

3.1.3 Selection of Model/Tools for Analysis .............................................................................................. 22

3.1.4 Methods for Steady Flow Model ...................................................................................................... 22

CHAPTER IV ....................................................................................................................................................... 28

4. STUDY AREA ............................................................................................................................................ 28

4.1 PHYSIOGRAPHY AND SUB-BASINS ...................................................................................................................... 29

4.2 SOCIO-ECONOMIC CONDITION .......................................................................................................................... 32

CHAPTER V ........................................................................................................................................................ 33

5. RESULT..................................................................................................................................................... 33

5.1 FLOOD FREQUENCY ANALYSIS ........................................................................................................................... 33

5.2 LANDUSE/LAND COVER MAP, DEM, .................................................................................................................. 34

5.3 FLOOD HAZARD ANALYSIS ................................................................................................................................ 35

5.4 FLOOD VULNERABILITY ANALYSIS ...................................................................................................................... 39

5.5 FLOOD RISK ANALYSIS ..................................................................................................................................... 41

CHAPTER VI ....................................................................................................................................................... 45

6. DISCUSSION ............................................................................................................................................. 45

6.1 FLOOD FREQUENCY ANALYSIS ........................................................................................................................... 45

6.2 FLOOD VULNERABILITY AND RISK ANALYSIS .......................................................................................................... 45

CHAPTER VII ...................................................................................................................................................... 48

7. CONCLUSION ........................................................................................................................................... 48

CHAPTER VIII ..................................................................................................................................................... 50

8. RECOMMENDATIONS .............................................................................................................................. 50

REFRENCES........................................................................................................................................................ 51

ANNEX .............................................................................................................................................................. 57

LIST OF FIGURES

FIGURE 1 ONE DIMENSIONAL FLOODPLAIN ANALYSIS USING HEC-RAS, ARCGIS AND HEC-GEORAS. .......................................... 25

FIGURE 2STUDY AREA - TINAU RIVER BASIN ....................................................................................................................... 28

FIGURE 3 LONGITUDINAL PROFILE OF TINAU RIVER ............................................................................................................. 29

FIGURE 4 TINAU WATERSHED SHOWING THE LOCATION OF THE NEAREST METEOROLOGICAL STATIONS ........................................ 31

FIGURE 5 LAND HOLDING PATTERN IN TINAU WATERSHED ................................................................................................... 32

FIGURE 6 MAXIMUM INSTANTANEOUS ANNUAL FLOOD DISCHARGE DATA AT TINAU RIVER, DHM STATION NO. 390 ....................... 33

FIGURE 7LANDUSE/ LAND COVER MAP OF THE TINAU BASIN ................................................................................................. 34

FIGURE 8PIE-CHART SHOWING AREA UNDER VARIOUS LANDUSE CLASS, ................................................................................... 35

FIGURE 9DEM OF THE STUDY AREA ................................................................................................................................. 35

FIGURE 10 RETURN PERIODS AND AREA INUNDATION RELATIONSHIP ..................................................................................... 36

FIGURE 11 FLOOD HAZARD MAP FOR THE 1 IN 5 YEARS FLOOD ............................................................................................. 36

FIGURE 12 FLOOD HAZARD MAP FOR THE 1 IN 10 YEARS FLOOD ........................................................................................... 37

FIGURE 13 FLOOD HAZARD MAP FOR THE 1 IN 50 YEARS FLOOD ........................................................................................... 37

FIGURE 14 FLOOD HAZARD MAP FOR THE 1 IN 100 YEARS FLOOD ......................................................................................... 38

FIGURE 15FLOOD HAZARD MAP FOR THE 1 IN 200 YEARS FLOOD .......................................................................................... 38

FIGURE 16VULNERABILITY CLASSIFICATION OF LANDUSE TO FLOOD HAZARD OF VARIOUS RETURN PERIODS ..................................... 39

FIGURE 17VULNERABILITY CLASSIFICATION OF IMPORTANT LANDUSE TO FLOOD HAZARD OF VARIOUS RETURN PERIODS .................... 40

FIGURE 18 CHANGE IN INUNDATED AREA WITHIN SETTLEMENT AREA ALONG VARIOUS RETURN PERIODS ........................................ 40

FIGURE 19 CHART SHOWING DEPTH OF INUNDATIONAND THE RESPECTIVE INUNDATED AREA ALONG THE DEFINED RETURN PERIOD. .... 41

FIGURE 20 RISK CLASSIFICATION OF SETTLEMENT LANDUSE TYPE ............................................................................................ 42

FIGURE 21 RISK CLASSIFICATION OF AGRICULTURAL LAND..................................................................................................... 42

FIGURE 22 FLOOD RISK MAP OF BUTWAL MUNICIPALITY ..................................................................................................... 43

LIST OF TABLES

TABLE 1FLOOD DISCHARGE ESTIMATION (LOG PEARSON TYPE III METHOD) ............................................................................ 19

TABLE 2 FLOOD DISCHARGE ESTIMATION (GUMBEL'S EV1 METHOD) .................................................................................... 20

TABLE 3 VALUES OF STANDARD NORMAL VARIATE S USED IN WECS/ DHM ............................................................................. 22

TABLE 4MANNING'S ROUGHNESS COEFFICIENT FOR DIFFERENT LANDUSE (CHOW, 1959) ......................................................... 24

TABLE 5 BASIN CHARACTERISTICS OF THE TINAU RIVER ........................................................................................................ 30

TABLE 6SUB-BASIN CHARACTERISTIC OF THE TINAU WATERSHED ............................................................................................ 30

TABLE 7 MAXIMUM INSTANTANEOUS ANNUAL FLOOD DISCHARGE DATA OF THE TINAU RIVER AT DHM STATION NO. 390 ............... 31

TABLE 8 ESTIMATED PEAK DISCHARGE AT TINAU RIVER ........................................................................................................ 33

TABLE 9 ESTIMATED PEAK DISCHARGE OF THE UPPER REACHES OF TINAU RIVER ......................................................................... 34

TABLE 12 VDC WISE FLOOD HAZARD AT VARIOUS RETURN PERIODS ........................................................................................ 43

TABLE 13 FLOOD IMPACT TO THE BUILDING UNITS ............................................................................................................. 44

ix

LIST OF ABBREVIATIONS

CRED Centre for Research on the Epidemiology of Disasters

DEM Digital Elevation Model

DHM Department of Hydrology and Meteorology

DoI Department of Irrigation

DPTC Disaster Prevention Technical Centre

DWIDP Department of Water-Induced Disaster Prevention

EM-DAT The OFDA/CRED International Disaster Database

ESRI Environmental System Research Institute

EV1 Extreme Value Type 1

GIS Geographic Information System

ha. Hectare

HEC-RAS Hydraulic Engineering Centre‟s River Analysis System

HFT Himalayan Frontal Thrust

HMG Then His Majesty‟s Government, Nepal

ICIMOD International Centre for Integrated Mountain Development

IPCC Intergovernmental Panel on Climate Change

JICA Japan International Co-Operation Agency

km. Kilometre

km2 Square kilometre

m Metre

m3 Cubic metre

mcum million cubic meters

MRE Mountain Risk Engineering

msl Mean sea level

MW Mega-watt

NOAA National Oceanic and Atmospheric Administration

NRCS Nepal Red Cross Society

OFDA The Office of U.S. Foreign Disaster Assistance

SAARC South Asian Association for Regional Cooperation

TIN Triangulated Irregular Network

UNDP United Nations Development Programme

VDC Village Development Committee

WECS Water and Energy Commission Secretariat

x

ABSTRACT

Floods are the most common destructive natural hazards that occur regularly across the

world. It is one of the serious common and costly natural disasters that hit the poorest

nation the hardest. Terai, the southern part of the country occupying about 17% of the

total area is most vulnerable to flooding every year. This study is focused in accessing

the risk and vulnerability of Tinau Khola watershed. The study area is located in parts of

Palpa and Rupandehi districts covering an area of approximately 562 km2

.

Flood estimates for various return periods in the current study was made by a

comparison of flood discharge calculated using; the generalised Extreme Value 1

(EV1), log Pearson Type III and the WECS/DHM method. The flood hazard analysis of

the floodplain was done using the one dimensional steady flow model HEC-RAS.

ArcGIS and HEC-GeoRAS were used for the preparation of flood hazard maps.

Vulnerability to flood hazard was accessed in consideration with landuse type, affected

VDC/ Municipality and building points. The flood risk analysis was done by crossing of

vulnerability classes with flood depth hazard class.

The analysis of the flood hazard map indicated a considerable increase in inundated area

with increase in discharge.The total inundated area of a flood with 5 year return period

is 350.31 ha.and that of the flood with 200 years return period is 430.54 ha. The flood

vulnerability analysis indicated that in each successive return period from 1 in 5 years to

1 in 200 years, the area of agricultural land inundated increased by 6.5%, 13.0%, 5.3%,

5.3% respectively while that of settlement area changed by 3.1%, 90.5%, 49.5%, 55.3%.

The flood hazard in the Settlement area showed a gradual increase in all hazard class in

all return periods. The flood risk classification of Butwal Municipality shows a gradual

lowering in the area under low hazard (<2 m) after the 1 in 10 year flood while the area

under moderate flood hazard (2-5 m) shows a continuous increase.The analysis of a 5

year and 200 year flood shows that 111 building units and up to 215 building units will

be flooded respectively.

The findings of the study may help in planning and management of floodplain area of

Tinau Khola to mitigate future probable disaster through technical approach

Keywords:

Flood, Tinau Khola, Flood Frequency, Hazard, Vulnerability, Risk, HEC-RAS, ArcGIS.

1

CHAPTER I

1. INTRODUCTION

1.1 Background

Destructive natural events occur regularly across the world, although most do not cause

enough damage to be considered natural disasters. Among those that do, floods are the

most common (Ferreira, 2011). Dilley et al. (2005) estimated that more than one-third

of the world‟s land area is flood prone affecting some 82 percent of the world‟s

population. About 196 million people in more than 90 countries are exposed to

catastrophic flooding, and that some 170,000 deaths were associated with floods

worldwide between 1980 and 2000(UNDP, 2004).Between 1985 and 2009, floods

accounted for 40 percent of the natural disasters recorded by EM-DAT (OFDA/CRED

2010), (Ferreira, 2011). Those figures show that flooding is a major concern in many

regions of the world. Flooding is one of the serious, common, and costly natural

disasters that affect the communities around the world. Globally, the economic cost of

extreme weather and flood catastrophes is severe, and if it rises owing to climate

change, it will hit poorest nations the hardest.

Nepal is situated at the center of the 2,400 km long Hindu-Kush Himalayan belt and

extends for about 800 km from the high Himalayas to the plains of Terai. The flat Terai

to the southern part of the country occupying about 17% of the area is most vulnerable

to flooding every year. As the rivers emerge into the plain from steep and narrow

mountain gorges, they spread out with an abrupt gradient decrease that has three major

consequences: deposition of the bed load, changes in river course, and frequent

floods(Jollinger, 1979). Each year, floods of varying magnitudes occur due to intense,

localized storms during the monsoon months (June to September) in Nepal‟s numerous

streams and rivers. These natural events can wipe out development gains and

accumulated wealth in hazard prone areas.

Most of the rivers in Nepal are snow and glacier fed and promotes sustained flows

during dry seasons to fulfill the water requirements of hydropower plants, irrigation

canals and water supply schemes downstream. According to Baidya et al. (2007), In the

context of recent global warming phenomena and a consequent increase in the intensity

2

of extreme precipitation events (i.e. ≥100 mm/day), the dynamics of glacial lakes in

high mountain areas and the probability of occurrence of potentially damaging floods is

likely to increase. Accelerated retreat of glaciers and increased intensity of monsoon

precipitation observed during recent years have, most probably, contributed to increased

frequency of floods (Agrawalaet al., 2003). Some of the physical features sensitive to

climate change are agriculture and livestock, regions with seasonal precipitation or

snowmelt and topography and land use patterns that promote soil erosion and flash

floods (IPCC, 2001).The encroachment of areas susceptible to floods to establish human

settlements and to carry out infrastructural development in the recent past has increased

the exposure of these areas to flood hazards (Khanalet al., 2007),and increased the risk,

the expected degree of loss, from flood hazard. In Nepal, between 1983 and 2008, flood

and landslides caused 57.89% of the total loss of properties from different types of

disasters. On an average yearly, 290 people lost their lives accounting to over 33.8% of

those who died due to different types of disasters (DWIDP, 2008). The Terai, regarded

as the granary of Nepal is the region of utmost concern. In recent years, between 1987

and 1998, three events of extreme precipitation with extensive damage have been

reported (Chalise and Khanal, 2002).

Chaulagain (2006) found that the number of rainy days in Langtang Basin was

increasing. And the rate of increase in rainy days with heavier precipitation was much

higher than that with lighter precipitation for period 1988-2000. Also, IPCC (2001)

pointed out that there was some evidence of increases in the intensity and frequency of

extreme weather events like intense rainfall, prolonged dry spells etc in Asia throughout

the 20th

century. Therefore, both the findings give an evidence of intense floods in the

years to come, particularly in Asia. For Nepal, the normal hydrological system brings an

average annual precipitation of 1600 mm (Alford, 1992). About 80% of this falls

between June and September, during the monsoon season. Within the monsoon itself,

the total amount of rain comes within just 20% of its duration and exhibits considerable

macro, meso and micro-scale variations.Global Circulation Model project a wide range

of precipitation changes within Koshi basin, especially in the monsoon: 14 less to 40%

more by the 2030s and 52 less to 135% more by the 2090s (Dixit etal., 2009).

The monsoon precipitation pattern is changing too; with fewer days of rain and more

high-intensity rainfall events. Floodplain analysis and flood risk assessment of the Babai

Khola, using GIS and numerical tools (HEC-RAS & AV-RAS) was carried out by

3

Shrestha et al., (2000). Similarly, GIS was applied for flood risk zoning in the Khando

Khola in eastern Terai of Nepal by Sharma et al. (2003). Awal etal. (2003, 2005 and

2007) used hydraulic model and GIS for floodplain analysis and risk mapping of

Lakhandei River. After the disastrous climatologic event of 1993, hazard maps were

prepared for the severely affected areas of Central Nepal (Miyajima and Thapa, 1995).

Various definitions of vulnerability have been provided in the context of natural hazards

and climate change (Varnes, 1984; Blaikieet al., 1994; Twigg, 1998; Kumar, 1999;

Kasperson, 2001). From these definitions, vulnerability can be viewed from the

perspective of the physical, spatial or locational, and socioeconomic characteristics of a

region. Physical vulnerability could be referred to as a set of physical conditions or

phenomena, such as geology, topography, climate, land use and land cover, and so

forth, which renders a place and the people living there susceptible to disaster. Spatial

vulnerability is closely related to physical vulnerability. The degree of danger or threat

and the levels of exposure and resilience to threat are closely associated with location.

Hence, spatial vulnerability is a function of location, exposure to hazards, and the

physical performance of a structure, whereas socioeconomic vulnerability refers to the

socioeconomic and political conditions in which people exposed to disaster are living.

In recent years, a number of studies have recognized the importance of estimating

people‟s vulnerability to natural hazards, rather than retaining a narrow focus on the

physical processes of the hazard itself (Hewitt, 1997; Varley, 1994; Mitchell, 1999).

Cannon (2000) argued that natural disaster is a function of both natural hazard and

vulnerable people. He emphasized the need to understand the interaction between

hazard and people‟s vulnerability. Nepal‟s vulnerability to climate-related disasters is

likely to be exacerbated by the increase in the intensity and frequency of weather

hazards induced by anthropogenic climate change (IPCC, 2007). Vulnerability to flood

hazards is likely to increase unless effective flood mitigation and management activities

are implemented. An important prerequisite for developing management strategies for

the mitigation of extreme flood events is to identify areas of potentially high risk to such

events, thus accurate information on the extent of floods is essential for flood

monitoring, and relief (Smith, 1997).

Hazard may be defined as,

a source of potential harm.

4

a threat or condition that may cause loss of life or initiate any failure to the

natural, modified or human systems.

The initiating causes of a hazard may be either an external (e.g. earthquake, flood or

human agency) or an internal (defective element of the system e.g. an embankment

breach) with the potential to initiate a failure mode. Hazards are also classified as either

of natural origin (e.g. excessive rainfalls, floods) or of man-made and technological

nature (e.g. sabotage, deforestation, industrial site of chemical waste). Concentrating on

the flood hazard, it can be supported that the capture of the natural phenomenon

requires the frequency of the flood events as well as their magnitudes (and thus their

anticipated flood damages) (Alexander, 1991). Since the magnitudes of flood events can

be modeled by a probability density function, flood hazard can be estimated by the

probability that the flood damage that occurs in any one year. In general, risk as a

concept incorporates the concepts of hazard {H} (initiating event of failure modes) and

vulnerability {V} (specific space/time conditions). It is customary to express risk (R) as

a functional relationship of hazard (H) and vulnerability (V).

(R) = (H) × (V)

Vulnerability to flood disasters is great. Nepal is a least-developed, landlocked, and

mountainous country with limited access to socioeconomic infrastructure and service

facilities. Inaccessibility, a low level of human development, and mass poverty are

prominent reasons for the poor capacity to anticipate, cope with, resist, and recover

from and adapt to different types of hazards, floods being among them (Khanalet al.,

2007).

1.2 Objectives

The main objective of this study is to integrate flood simulation model, remotely sensed

data with topographic and socio-economic data in a GIS environment for flood risk

mapping in the flood plain of Tinau River in Nepal. Identification and mapping of flood

prone areas are valuable for risk reduction. Flood risk mapping consists of modeling the

complex interaction of river flow hydraulics with the topographical and land use

characteristics of the floodplains. Integrating hydraulic models with geographic

information systems (GIS) technology is particularly effective.

5

One dimensional hydraulic model HEC-RAS, ArcGIS and HEC-GeoRAS to inference

between HEC-RAS and ArcGIS were used to analyze the flood hazard and related

vulnerabilities in the Tinau river floodplain. The specific objectives of study are as

follows:

i. To analyze the floodplain using the one dimensional steady flow model.

ii. To prepare flood hazard map of the study area at various return periods.

iii. To analyze the vulnerability and risk to flood hazard in the basin.

1.3 Rationale

Global warming will induce higher temperature differences between land and sea

surfaces, causing an increased transport of perceptible water to the continents and an

increase in frequency of intense rainfall.

Nepal‟s 83% land mass is mountainous terrain. The wide range in altitudinal variation

along its width gives rise to a steep and rugged topography and extreme relief. Steep

and unstable slopes, rugged terrain, active geodynamic processes and intense monsoon

rains make the Himalaya an active and fragile mountain range. As the nature of the

Himalaya suggests, landslides and debris flows and floods are the main types of water-

induced hazards in the region and in Nepal. These hazards wipe out entire villages,

wash out roads, bridges, canals and hydropower plants and damage hectares of valuable

agricultural land during the monsoon season. Besides substantial economic losses, more

than 320 people on average lose their lives in the Nepal Himalaya alone. Other losses

from these hazards are on a rise every year. Many factors trigger debris mass movement

or debris flows. Among the most common triggers in the Himalaya are prolonged or

heavy monsoon rains (Chhetri, 2010).

The assets at risk from flooding can be enormous and include private housing, transport

and public service infrastructure, commercial and industrial enterprises, and agricultural

land. In addition to economic and social damage, floods can have severe consequences,

where cultural sites of significant archaeological value are inundated or where protected

wetland areas are destroyed.

As a result, vulnerability to flood hazards is likely to increase unless effective flood

mitigation and management activities are implemented. An understanding of the types,

frequency, and magnitude of flood events causing harm to life and property; the extent

6

of loss and damage from such events; and their spatial concentration is necessary in

order to develop appropriate mitigation and management strategies to reduce risk and

vulnerability to flood hazards.

1.4 Steady Flow Limitations

The following assumptions are implicit in the analytical expressions used in the current

version of the program. (Source: HEC-RAS Manual)

i. Flow is steady.

ii. Flow is gradually varied. (Except at hydraulic structures such as: bridges;

culvert; and weirs. At these locations, where the flow can be rapidly varied, the

momentum equation or other empirical equations are used.)

iii. Flow is one dimensional (i.e., velocity components in directions other than the

direction of flow are not accounted for).

iv. River channels have „small‟ slopes, say less than 1:10

7

CHAPTER II

2. LITERATURE REVIEW

Freeman et al.(2003)suggested that the growth in hydrological disasters has two causes:

increased populations in flood plains and other high-risk areas and an increase in the

frequency and intensity of extreme weather events. This second development is

associated with climate change and is expected to become more pronounced over this

century. Wetherald and Manabe(2002) linked the positive growth in hydrological

disasters with climate change saying that a warmer climate, with its increased climate

variability, will increase the risk of both floods and droughts

Ferreira (2011) found that population exposure affects the number of deaths both

directly and indirectly. We obtain estimates of the population exposed to a flood event

by overlying maps of the areas affected by floods with global population maps using

GIS. Higher population exposure is associated with more deaths once the flood has

occurred. However, precisely because more people increase the potential for damage

and deaths, this increases the payoffs of investments in flood mitigation and

management, resulting in smaller floods. In developing countries more population

exposure is also associated with fewer floods.

Ferreira(2011) also found that income also has a negative impact on flood magnitude (a

one percent increase in income is associated with around 0.2 percentage points lower

flood magnitude) possibly reflecting more resources available for flood control.

Interestingly, the indices of corruption and ethnic tensions exhibit a positive sign. A

reduction in the obstacles for collective action and efficient provision of public services

associated with an increase in the magnitude of the flood.

Stevens et al.(2010) indicated that government policies intended to reduce flood losses

can increase the potential for catastrophe by stimulating development inside the

floodplain, a phenomenon referred to as the “safe development paradox.” and hence

recommended changes in New Urbanist design codes and local government floodplain

management to increasingly direct new development away from the floodplain into the

watershed.

8

The studies and views suggest the watershed approach for flood control and importance

of people participation to achieve the bigger goal of disaster risk reduction in a climate

that is changing and offering an increased frequency and intensity of extreme events.

2.1 Assessment and Analysis of Floods in Nepal

Natural hazard assessment in Nepal is still in an early stage and no serious concern on

comprehensive flood risk assessment and hazard mapping was shown until the disaster

of 1993. Flood risk assessment in Nepal is still in a very rudimentary stage. A few

relevant literatures pertaining to hazard assessment carried out in Nepal is being

reviewed here.

After the disastrous climatologic event of 1993, hazard maps were prepared for the

severely affected areas of Central Nepal (Miyajima and Thapa, 1995). Preliminary

hazard assessment for the region was carried out by delineation of areas with rock and

soil slopes. The hazard was calculated with respect to different rating. Samarakonet

al.(1996) attempted to identify the changes of river channel in the floodplain of Ratu

Khola that originates from the Siwalik Hills in Central Nepal, using satellite data

covering 20-year period. Reason for change was examined with field observation and

the present trend in the channel plain form change was established in predicting flood

prone areas in the future flood events.

Sah (2009) conducted an inundation analysis for Tinau River basin excluding as well as

including dam breach simulation; Inundation analysis of natural dam breach at 20 m

shows that Bhairahawa city will be inundated up to 3.08 m in case of dam breach.

Hazard maps have been prepared for the Sun Koshi and BhoteKoshi catchments in

Central Nepal (ITECO, 1996). The conclusion of this mapping exercise was that

development of human settlements in hazardous areas increases the risk of floods and

landslides. Measures to reduce the impact of natural disasters in these catchments have

also been suggested.

Shakyaet al.(2002) in an study for the estimation of 2002 B.S extreme flood over

Balkhu River,used NOAA Based Satellite rainfall and HEC-HMS hydrological model,

and assessment of flood education of people living near the flood risk zone of Balkhu

River. River gets flooded from the control bank at least every 10 years. Urban flood

9

disaster is not only due to extreme rainfall but equally from human activities at flood

plains and improper government policy.

Mapping and Assessing Hazard in the Ratu watershed was done by Ghimireet al.

(2007), the study begins with impact of flood disaster and resilience of the people at the

national level and then to watershed level at meso-scale and village development

committee/municipality at micro level. Hazard and risk mapping was done in watershed

level using GIS and RS and the numerical model (HEC-RAS & AV-RAS).

Karkiet al.(2011) conducted flood hazard mapping and vulnerability assessment in the

flood plain of Kankai River in Nepal by integrating flood simulation model, remotely

sensed data with topographic and socio-economic data in a GIS environment. He found

that a total of 59.3 km2 and 59.8 km

2 of the study area would be under flooding in a 25-

year return period flood and 50-year-return period flood respectively. Also, the hazard

prone area would be considerably increased from 25-yearreturn period flood to 50-

yearreturn period flood. Level of hazard showed that high hazard area would be

increased and more settlement would be under the high hazard zone. Vulnerability

assessment regarding flooding and climate change depicted that peoples' livelihood are

worsening each year.

Manandhar(2010) conducted a research on floodplain analysis and risk assessment of

Lothar Khola which lies between Makwanpur and Chitwan district. The study describes

the technical approach of probable flood risk, vulnerability and hazard analysis. He

applied Flood frequency analysis for 2, 10, 50, 100 and 200-years return period using

Gumbel, Log Pearson Type III, and Log Normal method based on maximum

instantaneous flow recorded at Lothar Khola station and also by WECS/DHM method.

Also, one dimensional hydraulic model HEC-RAS with HEC-GeoRAS interface in co-

ordination with ArcView was applied for the analysis.

Samarakonet al.(1996) attempted to identify the changes of river channel in the

floodplain of Ratu Khola that originates from the Siwalik hills in Central Nepal, using

satellite data covering 20-year period. Reason for change was examined with field

observation and the present trend in the channel plain form change was established in

predicting flood prone areas in the future flood events.

10

Hazard map covering about 665 km2 of the upper reach of the Kamala River was

prepared by Mahatoet al. (1996) based on modified Mountain Risk

Engineering/ICIMOD rating method. Ministry of Water Resources, Water Induced

Disaster Prevention Technical Center (DPTC) prepared longitudinal profiles and cross

section in the Lagdaha Khola to assess the damage condition at Sindhulimadi, and the

work was followed by a detailed study on debris flows and landslides occurred during

the disaster in July 1993 and Aug 1995 in the Kamala river watershed.

The Department of Hydrology and Meteorology, Nepal (DHM), in 1998 prepared a

preliminary flood risk of the Tinau Khola, downstream of Butwal and the Lakhandehi

Khola. One dimensional IDA's method was used to determine flood levels using just

seven river cross-section in 42.5 km river length. The study considers insufficient cross-

sections and no longitudinal section were surveyed. The result was also not verified

with past flood. Under the study on Flood Mitigation Plan for selected rivers in the

Terai plain in Nepal, prepared a flood hazard map of the Lakhandehi Khola based on

field study after 1993 flood. The study made flood flow analysis by using an unsteady

flow simulation model. The simulated results shows that in many cross-sections the

water levels go far beyond the river cross-sections that they couldn‟t represent the actual

flood water levels. These maps also don‟t show a relationship of the hazard to the return

period of flooding and to the flood water depth (JICA/DOI, 1999).

Awalet al. (2003, 2005 and 2007) used hydraulic model and GIS for floodplain analysis

and risk mapping of Lakhandei River. Most of the previous studies used steady flow

model however this study used both steady and unsteady flow model for floodplain

analysis. This study also assessed change in river course using satellite image.

Floodplain analysis and flood risk assessment of the Babai Khola, using GIS and

numerical tools (HEC-RAS & AV-RAS) was carried out by Shrestha et al.(2000).

Similarly, GIS was applied for flood risk zoning in the Khando Khola in eastern Terai

of Nepal by Shramaet al. (2003). Government of Nepal, DWIDP & Mountain Risk

Engineering (MRE) Unit 2003 prepared water induced hazard maps of part of

Rupandehi district on the basis of field study and numerical modeling.

Dangol (2008) prepared Flood Hazard Map of Balkhu Khola using GIS and Remote

Sensing, found huge area of barren land area affected by flood and few percentage of

11

settlement area indicating the damages to the human lives. Apart from such piecemeal

approaches on a limited scale, no pragmatic efforts at comprehensive flood vulnerability

assessment and hazard mapping have materialized as yet.

Ghanbarpour (2000) used the HEC-RAS model integrated with HEC-GeoRAS in GIS

for floodplain analysis to delineate flood extents and depths within an urban area of the

Neka River plain in Iran. He aimed to present a methodological framework to map flood

hazard zones for evaluating spatial urban development options for adapting to extreme

storm events and sustainable planning in this urban area. The results of the study

indicate that integration of HEC-RAS with HEC-GeoRAS in GIS provides an effective

platform for both flood hazard mapping and urban planning purposes.

Patel (2009) applied GIS in watershed modeling with fast processors and interfaces such

as ArcHydro, HEC-GeoHMS, and HEC-GeoRAS linking hydrologic and hydraulic

models to the ArcGIS environment in Wreck Pond Brook Watershed, a new coastal

New Jersey area.

2.2 Flood control structure

A Regional study on the causes and consequences of natural disasters and the protection

and preservation of the environment by, SAARC in 1992 highlighted the importance of

reservoir type dams to check the flooding in the region (Ganges floodplain). Nepalwith

a net storage potential of 61,000 mcum(million cubic meters), except for a small

reservoir of 73 mcum net storage on the Kulekhani, none of the other reservoirs have

been built primarily due to the lack of funding. (SAARC, 1992)

A small hydroelectric dam 65 m long (across the river) 8 m in height and 4 meter wide

is located in the waterway between Dobhan (confluence of Tinau river with its

Tributary Dobhankhola) and Butwal Municipality. One tenth of the river flow is

diverted into the tunnel for Tinau Hydropower Project (1 MW), Butwal, which is true

for most part of the year. There is a small impoundment area the average width and

depth were 30.0 m and 0.54m (max. depth 1.0 m ) with average velocity of 0.67 m/s

(max. 1.4 m/s) (Sharma, 2003). The volume of the impoundment is hence negligible

primarily due to the siltation of the reservoir in a watershed with large mass wasting

phenomenon. A study by Paudel, 2004, shows that 54.3% of the watershed falls in the

class of moderate to high landslide hazard zone. The reduced impoundment volume

12

hence cannot be considered substantial to check the flooding in the region as suggested

by SAARC (1992).

SAARC (1992) highlighted that selective treatment of watersheds undertaken for Tinau

and Kulekhani has been implemented as topsoil conservation measure than as a flood

protection measure. It implied the importance of hazard maps to improve flood

forecasting systems. The study concluded that though there are separate agencies

working on the different aspects of floods ranging from hydrology to relief and

rehabilitation, there is still a conspicuous absence of a national flood management plan

DPTC (1993) prepared a flood hazard map of the Bagmati River in the Sarlahi and

Rautahat districts. thenHMG Nepal, UNDP, and ICIMOD (2001) carried out flood

hazard mapping in two VDCs of the Chitwan and two VDCs of the Bardiya districts

using geographic information system and remote sensing techniques coupled with field

verifications

2.3 Tinau River Flooding

1981

A huge flood in Tinau River in 1981 destroyed parts of the intake, 2 suspension bridges

& powerhouse shaft (Himal Hydro, 2011)

2007

Rupendehi district witnessed worst disaster hit after decades as locals said. At least 500

households in ward numbers 8, 11 and 13 of the Butwal municipality were displaced

due to the flooding in Tinau River. Incessant rainfall disrupted life at Sundarnagar,

Pabitranagar, Durganagar, Pragatinagar, Ekatanagar, Hattisud, Budhhanagar and

Majhauwa areas of Butwal. The flooding is assumed to be the worst disaster to hit the

area after devastating flood of 1981. The flood was of the similar scale flooding as

experienced in 1981 when the whole of DaureTole, now a bus park, was swept away.

River water flooded into the settlements because of weak embankment. The flood has

also destroyed the embankment built by the people. Almost half of 600 squatters sheds

at Sundarnagar of Khayardhari area at ward no. 8 were flooded. The displaced sheltered

at nearby bus stops, temples, sports hall and the building of Chamber of Commerce and

Industries. Others have been forced to stay under the open sky.(www.nepaldisaster.org,

2007)

13

2008

The year 2008 is unforgettable in the history of disaster of Nepal. Especially the

outburst of Koshi embankment and humanitarian crisis. As per the preliminary

assessment of the Home Ministry, a total of 40,378 people and 7,102 households were

completely displaced because of the collapse of Koshi embankment. (Gorkhapatra,

2008).

A flood event at Tinau Kholaon 19th

August, 2008, affected 200 to 250 households with

500 to 600 individuals of Hattisud, Butwal Municipality. Two people were reported

missing in the event. (NRCS, 2008).

2.4 Flood Frequency Analysis

The flood frequency analysis is one of the important studies of river hydrology. It is

essential to interpret the past record of flood events in order to evaluate future

possibilities of such occurrences. The estimation of the frequencies of flood is essential

for the quantitative assessment of the flood problem. However, for reliable estimates for

extreme floods, long data series is required; the use of historical data in the estimation

of large flood events has increased in recent years (Archer, 1999; Black & Burns, 2002;

Williams & Archer, 2002).

Various empirical approaches such as Creager's formula, WECS/DHM Method,

Modified Dicken's Method, B.D. Richard's Method, Synder' Method, etc. are also used

for determining discharge for un-gauged basin. In WECS/DHM Method, the most

significant independent variable is the area of the basin below 3000m elevations. In

most of the flood analysis cases in Nepal, the WECS/DHM Method seems to be

reasonable (Ranjit, 2006)

2.5 Tools for Floodplain Analysis and Mapping

Various studies have indicated that GIS is an effective environment for floodplain

mapping and analysis that can be included in plans to reduce the vulnerability to flood

and associated risk.

There are a number of commercial and non-commercial software, tools available for

numerical modeling and analysis in GIS. Based on information on the lateral

14

distribution of flow across a cross section the models can be further divided into one-

dimensional and two-dimensional model.

Descriptions of some of the software tools are presented below.

2.5.1 Geographical Information System (GIS)

A geographic information system (GIS), geographical information system, or geospatial

information system is a system designed to capture, store, manipulate, analyze, manage

and present all types of geographically referenced data(Redlands, 1990).

In context of flood hazard management, GIS can be used to create interactive map

overlays, which clearly and quickly illustrate which areas of a community are in danger

of flooding. Such maps can then be used to coordinate mitigation efforts before an event

and recovery after (Raford, 1999). GIS, thus, provides a powerful and versatile tool to

facilitate a fast and transparent decision-making.

There are number of GIS software;ArcGIS is one of the most recommended. ArcGIS is

a suite consisting of a group of geographic information system (GIS) software products

produced by ESRI.It has easy to use, point and click graphical user interface that makes

easy loading of spatial and tabular data so that it can be display the data layers as maps,

tables and charts. ArcGIS ver.9.3 was used in the present study.

2.5.2 Details of Selected Flood Simulation Models

HEC-RAS

HEC-RAS is a computer program that models the hydraulics of water flow through

natural rivers and other channels. The program is one-dimensional, meaning that there is

no direct modeling of the hydraulic effect of cross section shape changes, bends, and

other two- and three-dimensional aspects of flow. The program was developed by the

US Department of Defense, Army Corps of Engineers in order to manage the rivers,

harbors, and other public works under their jurisdiction; it has found wide acceptance by

many others since its public release in 1995.

The current version of HEC-RAS supports Steady and Unsteady Flow Water Surface

Profile calculations, perform sediment transport simulation and perform water quality

simulation.

15

Steady Flow Surface Profiles: This component of the modeling system is used

for calculation of water surface profiles for steady gradually varied flow. The

system can handle a single river reach, a dendritic system, or a full network of

channels.

Unsteady Flow Simulation: This component of the HEC-RAS modeling

system is capable of simulating one-dimensional unsteady flow through a full

network of open channels. The unsteady flow component was developed

primarily for subcritical flow regime calculations.

Awal (2003) made comparison between steady and unsteady flow analysis using

HECRAS.

Dangol (2008) assessed the flood inundation problem in Balkhu Khola using Steady

flow analysis which shows barren area near the river is susceptible to flood hazard,

which indicates future human lives are more prone to disasters as those lands have gone

through planning for future settlement.

HEC-GeoRAS

HEC-GeoRAS is an ArcGIS extension specifically designed to process geo-spatial data

for use with the Hydrologic Engineering Center River's Analysis System (HEC-RAS).

The extension allows users to create an HEC-RAS import file containing geometric

attribute data form an existing Digital Terrain Model (DTM) and complementary data

sets. Water surface profile results may also be processed to visualize inundation depths

and boundaries. HEC-GeoRAS extension for ArcGIS used an interface method to

provide a direct link to transfer information between the ArGIS and the HEC-RAS.

Pre-processing to Develop the RAS GIS Import File

To create the import file, the digital terrain model (DTM) of the river system in a

TIN format is required. The other data required for the re-processing includes series

of line themes; Stream Centerlines, Flow Path Centerlines, Main Channel Banks,

and Cross Section Cut Lines referred as the RAS Themes. Using 2D RAS Themes

and TIN, the 3D Streamline and the 3D Cut Lines themes can be generated. The

RAS GIS Import File consists of geometric attribute data necessary to perform

hydraulic computations in HECRAS. The cross-sectional geometric data is

16

developed from DTM of the channel and surrounding land surface, while the cross-

sectional attributes are derived from points of inter-section of RAS Themes.

Additional RAS Themes may be created / used to extract additional geometric data

for import in HEC-RAS. These themes include Land Use, Levee Alignment,

Ineffective Flow Areas, and Storage Areas. Expansion/contraction coefficients,

hydraulic structure data such as bridges and culverts are not written to the RAS GIS

Import File and need to be added to the model through the RAS interface.

Post-Processing to Generate GIS Data form HEC-RAS Results

Post-Processing (postRAS) facilitates the automated floodplain delineation based

on the data contained in the RAS GIS output file and the original terrain TIN.

Based on the RASGIS export file, cross-sections theme (with water levels for each

modeled profile as attributes) and bounding polygon (to the edge of the modeled

cross-sections) can be generated. The water surface Tin is generated using these

cross-sections and bounding polygon themes. With the water surface TIN and the

original terrain TIN, inundated depth grids and floodplain polygons can be

automatically generated. Apart from this, HEC-GeoRAS can also generate the

velocity TIN and grid (ESRI, 1999).

2.6 Flood Risk Assessment

Flood risk is a complex interaction of hydrology and hydraulics of the river flow with

the potential of damage to the surrounding floodplains. The element of risk has both the

spatial and the temporal domain and is also, a function of the level of human

intervention of the surrounding floodplains.

Plate (2000) described the flood risk assessment requires a clear understanding of the

causes of a potential disaster, which includes both the natural hazard of a flood, and the

vulnerability of the elements at risk, which are people and their properties. Flood risk

assessment therefore consists of understanding and quantifying this complex

phenomenon.

Correia, F.N. et al. (1998) submitted report on flood hazard assessment and

management: Interface with the Public. This focus on the understanding of how people

evaluate and respond to natural hazards in an urban area, and how this knowledge can

17

be integrated in the planning and management process, are becoming very important

elements of a comprehensive and participatory approach to flood hazard management.

Such an approach demands a clear comprehension of the processes of the risks

perception, causal attribution, possible solutions for the problem and patterns of

behavior developed during hazard situations. The willingness of the public to participate

in flood management, and the attitudes to previous initiatives also need to be addressed.

18

CHAPTER III

3. MATERIALS AND METHODS

The study is primarily based on data obtained from secondary sources. The

methodologies outlined below are the approaches used for their analysis.

3.1 Hydrological Analysis

Comparative maximum instantaneous annual flood discharge data, calculated using

various distribution methods (Log-Pearson III , Gumbel‟s Extreme Value Type I,

WECS/ DHM) were used in the flood mapping of gauged river (Tinau river from

Dobhan to Butwal Municipality), while methodology suggested by Water and Energy

Commission Secretariat/ Department of Hydrology and Meteorology (WECS/DHM) of

Nepal, was used to estimate instantaneous annual flood discharge of the un-gauged

tributaries in the upper reach.

3.1.1 Comparison BetweenFlood Frequency Methods

Various studies for the selection of a suitable method of flood frequency analysis that

included various two parameter; log normal,gamma, log gamma and Gumbel (EV1),

and three-parameter distributions; Pearson Type III, log Pearson III, Hazen, GEV, log

Gumbel (EV2), pointed out that the three-parameter log Pearson III method is far better

than the other two-parameter methods but that the latter should not be completely

discarded (Ponce, 1989). In another study, Patrick, 2011, points out that Hydrologic

events such as flood peak and flood volume usually are positively skewed and may

follow the lognormal distribution and since lognormal is a special case of the log

Pearson III, the latter is the most appropriate for flood frequency analysis.

Flood estimates for various return periods in the current study was made by a

comparison of flood discharge calculated using the three best suited methods; the

generalised Extreme Value 1 (EV1), log Pearson Type III and the WECS/DHM method.

Maximum instantaneous annual flood discharge data at Tinau River, DHM station no.

390, was used for the analysis. The maximum value of flood discharge predicted in the

respective return period, by any of the above listed methods, was adopted for flood

hazard mapping.

19

3.1.2 Flood Flow Calculation

The Log Pearson III Method

In this method the variate is first transformed into logarithmic form (base 10) and the

transformed data is then analyzed (Subramanya, 1994). If X is the variate of a random

hydrological or meteorological series, then the series of Z variate where,

𝑍 = 𝑙𝑜𝑔 𝑋 …………………….(a)

Are first obtained for this Z series, for any recurrence interval T.

𝑍𝑇 = 𝑍𝑎𝑣𝑔 + 𝐾𝑍𝑆𝑍 …………...(b)

Where Zavg = arithmetic mean of Z values KZ is a frequency factor which is a function

of recurrence interval T and the coefficient of skew CS, For N = number of sample = n

number of years of record.

SZ = Standard deviation of Z variate sample 𝑧−𝑧𝑎𝑣𝑔

2

𝑁−1

CS = Coefficient of skew of variate Z = 𝑁 𝑧−𝑧𝑎𝑣𝑔

3

𝑁−1 𝑁−2 𝜎𝑠 3

Corresponding value of X= antilog (ZT)

Table 1Flood Discharge Estimation (Log Pearson Type III Method)

S.No. Return period T

(yr)

Probability P

(percent)

Frequency

factor K

y = log (Q) Flood

discharge Q

(m3/s)

1 2 50 -0.049 2.749 561

2 5 20 0.823 3.057 1141

3 10 10 1.309 3.229 1693

4 25 4 1.849 3.419 2627

5 50 2 2.21 3.547 3523

6 100 1 2.543 3.665 4620

7 200 0.5 2.856 3.775 5957

Skew coefficient of the logarithms Cs = 0.3

Gumbel’s Extreme Value Type I Method

The Gumbel distribution is one of most frequently adopted distribution types for

modeling hydrological extreme events such as floods and storms widely used

throughout the world. (Gumbel 1958; Todorovic 1978; Castillo 1988; Stedingeret al.,

1993). Ponce(1989), presented the methodology as follows,

20

The cumulative density function F(x) of the Gumbel method is the double exponential

function (F(x),

Where, 𝐹 𝑥 = ─ 𝑒−𝑒−𝑦 ………...…….(a)

in which F(x) is the probability of nonexceedence. In flood frequency analysis, the

probability of interest is the probability of exceedence, i.e. the complementary

probability to F(x):

𝐺(𝑥) = 1 ─ 𝐹(𝑥)…….………… (b)

The return period T is the reciprocal of the probability of exceedence. Therefore,

1

𝑇= 1 ─ 𝑒−𝑒−𝑦

………………(c)

𝑦 = 𝑙𝑛. 𝑙𝑛𝑇

𝑇−1 ……………….….(d)

In the Gumbel method, value of flood discharge are obtained from the frequency

formula,

𝑥 = 𝑥 + 𝐾𝑠 ………………….….(e)

The frequency factor K is evaluated with the frequency formula:

𝑦 = 𝑦 𝑛 + 𝐾𝜎𝑛 ………………….. (f)

In which y- Gumbel (reduced variate, a function of return period; and 𝑦 𝑛 and 𝜎𝑛are the

mean and standard deviation of the Gumbel variate, respectively. These values are a

function of record length n.

From above equations (e) and (f);

𝑥 = 𝑥 +𝑦−𝑦 𝑛

𝜎𝑛𝑠 ………………… (g)

Combining equations (d) and (g)

𝑥 = 𝑥 +𝑙𝑛 .𝑙𝑛

𝑇

𝑇−1−𝑦 𝑛

𝜎𝑛𝑠

Table 2 Flood Discharge Estimation (Gumbel's EV1 Method)

21

S.No. Return period T

(yr)

Probability P

(percent)

Gumbel variate

y

Flood discharge Q

(m3/s)

1 2 50 0.367 701

2 5 20 1.5 1472

3 10 10 2.25 1983

4 25 4 3.199 2627

5 50 2 3.902 3106

6 100 1 4.6 3581

7 200 0.5 5.296 4054

Water and Energy Commission Secretariat/ Department of Hydrology and

Meteorology (WECS/DHM) Method

As per the recommendation of the Water and Energy Commission Secretariat/

Department of Hydrology and Meteorology (WECS/DHM) of Nepal, the flood flow of

any river of catchment area A km2 lying below 3000 m elevation are given by the

equation developed by WECS and DHM (Sharma, et al., 2003) for 2‐year and 100‐year

floods is adopted for the study.

Here,

Q2 = 2.29(A<3K) 0.86

Q100 = 20.7(A<3K) 0.72

Where,

Q is the flood discharge in m3/sec and A is basin area in km2.

Subscript 2 and 100 indicate 2‐year and 100 year flood respectively

Similarly, subscript 3k indicates area below 3000m altitude.

Further following relationship (WECS and DHM, 1990) is used to estimate floods at

other return periods.

QT = exp(lnQ2+sσ),

Where, σ = ln(Q100/Q2)/2.326

22

s = standard normal variant for particular return period (T) given in table

below.

Table 3Values of standard normal variate s used in WECS/ DHM

S.No. Return period (T) in years Standard normal variate (s)

1 2 0

2 5 0.842

3 10 1.282

4 20 1.645

5 50 2.054

6 100 2.326

7 200 2.576

8 500 2.878

9 1000 3.09

3.1.3 Selection of Model/Tools for Analysis

In this study, HEC-RAS version 4.0 was used to calculate water surface profiles and

ArcGIS ver. 9.3 was used for the GIS data processing. HEC-GeoRAS 4.3.93 for ArcGIS

is used to provide the interface between the systems. These software tools HEC-RAS,

and HEC-GeorRAS were used in this research mainly because of the free availability of

the systems. ArcGIS was used as it is most commonly used and recommended package

for GIS data processing.

3.1.4 Methods for Steady Flow Model

Application Procedure for Steady Flow Analysis

The general method adopted for floodplain analysis and flood risk assessment in this

study basically consists of five steps:

i. Preparation of TIN in ArcGIS.

ii. HEC-GeoRAS Pre-processing to generate HEC-RAS Import file.

iii. Running of HEC-RAS to calculate water surface profiles.

iv. Post-processing of HEC-RAS results and floodplain mapping.

v. Flood risk assessment.

The approach used for floodplain analysis and risk assessment using one-dimensional

model, HEC-RAS, ArcGIS and HEC-GeoRAS is depicted in the flow chart below.

23

Preparation of TIN

The topographic data from Department of Survey, Government of Nepal, spot elevation

generated from Google Earth using KMLer in ArcGIS environment, Survey data

obtained from DWIDP, was used for Triangulated Irregular Network (TIN) generation.

ArcGIS ver. 9.3 was used to generate TIN which was used as Digital Elevation Model

(DEM) required in HEC-GeoRAS environment in order to prepare data sets required as

input to the HEC-RAS simulation.

Preparation Landuse/ Land Cover Map

The landuse/land cover map of the Tinau Khola was derived from the 1992 topo-sheet

along with field verification.

Model Application

PreRAS, postRAS and GeoRAS_Util menus of HEC-GeoRAS extension in ArcGIS

environment were used to create data sets, making import file for model simulation in

HEC-RAS.

Pre GeoRAS Application

The preRAS menu option was used for creating required data sets for creating import

file to HEC-RAS. Stream centerline, main channel banks (left and right), flow paths,

and cross sections were created. 3D layer of stream centerline and cross section was

also created. Land use manning table containing land use type of the study area and

Manning roughness coefficient, „n‟ value was created from GeoRAS_Util menu for

different land uses. Thus, Manning‟s „n‟ value was assigned as taken from HEC-RAS

hydraulic reference manual (2002) for different land use types within the study area.

Thus, after creating and editing required themes, RAS GIS import file was created.

HEC RAS Application

This is the major part of the model where simulation is done. The import file created by

HEC-GeoRAS was imported in Geometric Data Editor interface within HEC-RAS. All

the required modification, editing was done at this stage. The flood discharge for

different return periods were entered in steady flow data. Reach boundary conditions

were also entered in this window. Then, water surface profiles were calculated in steady

24

flow analysis window. After the simulation, RAS GIS export file was created. Water

surface profiles were computed from one cross section to the next by solving the energy

equations with an iterative procedure. The flow data were entered in the steady flow

data editor for five return periods as 2-year 10-year, 50-year, 100-year and 200-year.

Similarly, upper most cross section was taken as upper stream boundary. Boundary

condition was defined as critical depth for both upstream and downstream. Sub critical

analysis was done in steady flow analysis. Then after, water surface profiles were

computed. The resulted was exported creating the RAS GIS export file.

Table 4Manning's Roughness Coefficient for Different Landuse (Chow, 1959)

Land use type Manning’s n Value

Barren Land 0.030

Bush 0.050

Cultivation Area 0.035

Cutting Area 0.040

Forest 0.100

Grass land 0.035

Orchard 0.055

River 0.040

Sand 0.030

Post-processing of HEC-RAS Results and Floodplain Mapping

After the development of a GIS import file form HEC-RAS, post-processing steps

starts. Different steps involved in this process are:

i. Stream Network, Cross Section and Bounding Polygon Generation:

After completing "Theme Setup" and "Read RAS GIS Export File", this will

read the results from the export file and create initial data sets. The stream

network, cross section data, bank station data and bounding polygon data will be

read and shape files will automatically be generated.

ii. Water Surface TIN Generation:

Based on water surface elevations of the cross-sectional cut lines and bounding

polygon theme, water surface TIN was generated for each water surface profiles.

25

iii. Floodplain Delineation:

After the generation of Water surface TIN, the next step is the delineation of the

floodplain. The floodplain delineation will create a poly-line theme identifying

the floodplain and a depth grid. The water depth grid is created by the

subtraction of the rasterized water surface TIN from the Terrain TIN.

Figure 1 One dimensional floodplain analysis using HEC-RAS, ArcGIS and HEC-

GeoRAS.

26

Flood Risk Assessment

The methodology adopted for flood risk assessment follows the approach developed by

Gilard (1996). The flood risk is divided into the hazard component and the vulnerability

component. The vulnerability assessment is facilitated by the use of the binary model,

based on the presence or absence of a flood of particular intensity in a particular land

use type. The spatial coexistence model is used for the hazard assessment, reclassifying

the floodwater depth. The results of these two analyses are combined for the flood risk

assessment. This risk assessment process is automated by the use of customized

graphical user interface in the ArcGIS.

Flood Hazard Analysis, Gilard (1996)

The hazard aspect of the flood risk is related to the hydraulic and the hydrological

parameters. This implies that the same flood will affect a particular area with the same

hydraulic properties regardless of the land use. Hazard level may be defined by the

parameters like flood depth and exceedence probability of a particular flood event. For

the quantification of the flood hazard and potential of damage, water depth is a

determining parameter. For this the weighted spatial coexistence model facilitates the

analysis by ranking the hazard level in terms of water depth. In this study, the hazard

level is determined by reclassifying the flood grids flood depths polygons bounding the

water depth at the intervals of <2 , 2-5, 5-10 and >10. The areas bounded by the flood

polygons were calculated to make an assessment of the flood hazard level.

Flood Vulnerability Analysis, Gilard (1996)

The flood vulnerability is affected by the land use characteristics of the areas under the

influence of flood. That is to say, a flood of same exceedence probability will have

different levels of vulnerability according to the landuse characteristics and potential for

damage. The vulnerability analysis, therefore, consists of identifying the land use areas

under the potential influence of a flood of particular return period. For this, vulnerability

maps are prepared by clipping the land use themes of the floodplains with the flood area

polygons for each of the flood events being modeled. This depicts the vulnerability

aspect of the flood risk in the particular area in terms of the presence or the absence of

flooding of a particular return period as a binary model. The land use areas under the

27

influence of each of flooding events are reclassified for the calculation of the total

vulnerable areas.

Flood Risk Analysis, Gilard (1996)

The flood risk analysis includes the combination of the results of the both the

vulnerability analysis and the hazard analysis. This is defined by the relationship

between the land use vulnerability classes and the flood depth hazard classes in a

particular area. For this, the flood risk maps are prepared by overlaying the flood depth

grids with the land use map. The flood depth polygons prepared during the hazard

analysis are intersected with the land use vulnerability polygons. The resulting attribute

tables are reclassified to develop the land use - flood depth relationship. This hence

depicts potential flood areas in terms of both the land use vulnerability classes and water

depth hazard classes (Shrestha, et al., 2002).

Data analysis/ Applications and Platforms used

The primary data analysis and presentation was carried out using Microsoft Office

2010. Topographic maps, survey points and spot elevation generated from Google Earth

using KMLer as an extension of ArcGIS was used for the preparation of the terrain

profile (TIN). KMLer returns Z values for current feature class from Google Earth

Terrain. HEC-GeoRAS was used in ArcGIS ver. 9.3 environment to generate various

data sets which were then fed to HECRAS 4.0 for the one dimensional steady flow

modeling. Floodplain delineation is the postRAS application of ArcGIS using the RAS

import file. Remote Sensing was primarily limited to floodplain delineation and

verification of landuse. ENVI was used for the re-classification of LANSAT images.

28

CHAPTER IV

4. STUDY AREA

The Project area is the Tinau River basin up to the East–West Highway. The watershed

spans over two districts from mountains and hills of Mahabharat and Siwaliks at Palpa

to plains of Terai at Rupandehi. Those included are 27 VDCs and Tansen Municipality

of Palpa district and Butwal Municipality of Rupandehi district.

The total area of the Tinau watershed is about562 km2. The Tinau watershed lies in the

Terai, Siwaliks, and the Mahabharat Range of western Nepal as shown in Figure 2. The

area exhibits diverse physiographic and climatic conditions due to the elevation

differences from the East–West Highway in the Terai to the mountain summits of the

Mahabharat Range.

The upper part of the watershed lies mainly in the Mahabharat Range whereas the

middle portion is the Madi valley of the Tinau River, one of the most fertile lands in the

watershed. The Lesser Himalayan rocks comprise most of the watershed whereas the

Siwaliks are found only in its south belt. The project area covers the following nine

digital topographic maps: 2783 03A, 03C, 03D, 02C, 02D, 06A, 06B, 07A, and 07B.

Figure 2Study area - Tinau River basin

29

4.1 Physiography and Sub-Basins

The project area of the Tinau watershed lies essentially in two major physiographic

regions, namely the Siwaliks and the Mahabharat Range. The watershed is elongated in

the east–west direction and acquires an oval shape. The total area of the Tinau basin is

about 562 km 2. The Tinau River is the main watercourse and the Dobhan Khola and

Jhumsa Khola are other two main tributaries. The Tinau River is a rain-fed stream and

its discharge depends on the groundwater and surface runoff. The Dobhan Khola starts

from the western part of the watershed and the Jhumsa Khola originates from the east

end. Both of them join with the Tinau River near the Dobhan village and finally, the

Tinau River enters into the Terai passing through a Siwalik gorge. Most of seasonal

streams originating from the north slopes of the Siwalik Hills flow towards the north

and join with the main streams. The longitudinal profile of the Tinau River is given in

Figure 3 and its physiographic characteristics are summarized in Table 5. The drainage

pattern in the watershed is controlled primarily by faults, joints, and rock types. The

main drainage patterns observed are dendritic, trellis, centripetal, sub-parallel, and

parallel. The upper reach of the Tinau River in the Madi valley and the upper reach of

the Jhumsa Khola at the Godadi villages exhibit centripetal drainage patterns. A

dendritic drainage pattern is observed in the Kusum Khola, at Anpchaur, at the Porakni

village, whereas trellis and sub-parallel to parallel patterns are noticed mostly in the

Siwalik region.

Figure 3 Longitudinal Profile of Tinau River

30

Table 5Basin characteristics of the Tinau River

1 Gauging Station No. 390

2 Name of the stream Tinau River

3 Relevant rainfall and climatological Stations 71185, 70285, 70385, 81085

4 Location details

Name of the place Butwal

Latitude, longitude, elevation 27o42'10" E; 83

o27'50" N, 195 m

5

Physiographic details

Elevation (m)

Lowest 165 m

Highest 1893 m

Area details

Altitude (m) Area below the given altitude (km2)

1,000 300.76

3,000 561.42

Total area of basin 561.42

Perimeter (km) 135.32

Basin shape factor 1.599

Main Channel profile data

Total length of mainstream channel L=43.995 km

Total elevation difference (H) =1728 m

Average channel slope= 2.580%

Time of concentration tc= 5 hrs

Basin shape information

Length 38 km

Maximum width 19 km

Table 6Sub-basin characteristic of the Tinau watershed

Attributes Tinau Dobhan sub basin Jhumsa sub basin Butwal-Tansen Road

Main river Tinau Dobhan Jhumsa Tinau, Jhumsa and

Dobhan

Origin of river Lesser Himalaya Lesser Himalaya Lesser Himalaya Lesser Himalaya

Districts traversed Palpa and

Rupandehi

Palpa Palpa Palpa and Rupandehi

Geographical

co-ordinates

Easting

(m)

431284–447727 443655–469922 451341–469667 445640–462541

Northing

(m)

3068341–3084325 3065693–3084121 3066661–3074398 30.63759–3083918

Area of sub-basin (km2) 561.41 176.31 96.42 (not applicable)

Predominant rocks limestone, slate,

phyllite, quartzite,

shale

mudstones,

sandstone, siltstone

mudstones,

sandstone, siltstone,

limestone

mudstones, sandstone,

siltstone, limestone,

phyllite, quartzite

Hydro-Meteorological Characteristics

The rainfall, temperature and data at hydrological and meteorological stations in the

vicinity of the Tinau watershed, collected from the Department of Hydrology and

Meteorology. The discharge data of the Tinau River at DHM station No. 390 (latitude:

2742'10"; longitude: 8327'50") were used for estimating different return periods of

31

flood. The annual maximum instantaneous flood discharges of the Tinau River for the

period from 1964 to 1969 and from 1984 to 1992 were available from the Department

of Hydrology and Meteorology (DHM), and they are presented below. The river flow

data after 1992 was not available.

Table 7 Maximum instantaneous annual flood discharge data of the Tinau River at

DHM Station No. 390

Year Discharge (m3/s) Year Discharge (m

3/s)

1964 417 1986 644

1965 2220 1987 580

1966 1180 1988 565

1967 1950 1989 457

1968 2000 1990 260

1969 600 1991 288

1984 390 1992 134

1985 325

Source: DHM (2011)

Figure 4 Tinau Watershed Showing the Location of the Nearest Meteorological

Stations

32

4.2 Socio-economic condition

The main source of income of the local people is agriculture. About 92% households are

engaged in agriculture as a main source of income. However the landholding pattern is

not even. About 6% households are landless and 24% households are marginal having

up to 0.25ha land (Paudel, 2004).

Landless, 6%

0-0.25 ha, 24%

0.26-0.5 ha, 26%

0.51-1 ha, 27%

1-2 ha, 13%

>2 ha, 4%

Figure 5 Land Holding pattern in Tinau Watershed

33

CHAPTER V

5. RESULT

5.1 Flood Frequency Analysis

Flood estimates for various return periods in the current study was made by a

comparison of flood discharge calculated using, the generalised Extreme Value 1

(EV1), log Pearson Type III and the WECS/DHM method. Maximum instantaneous

annual flood discharge data at Tinau River, DHM station no. 390, was used for the

analysis.

Figure 6 Maximum instantaneous annual flood discharge data at Tinau River, DHM

station no. 390

The maximum value of flood discharge predicted in the respective return period, by any

of the above listed methods, was adopted for flood hazard mapping.

Table 8Estimated peak discharge at Tinau River

Return

Period

(yrs.)

Flo

od

flo

w (

m3/s

) Generalized EV1

Log

Pearson

Type III

WECS/

DHM Adopted

5 1472 1141 738.175 1472

10 1983 1693 916.017 1983

50 3106 3523 1337.77 3523

100 3581 4620 1528.74 4620

200 4054 5957 1728.21 5957

0

500

1000

1500

2000

2500

Ma

xim

um

inst

an

tan

eou

sa

nn

ua

l

flo

od

dis

cha

rge

da

tao

fth

eT

ina

u

Riv

era

tD

HM

Sta

tio

nN

o.

39

(m3/s

)

Year

34

The maximum value amongst the discharge calculated using above mentioned methods

was used for the hazard mapping.The flood discharge with return period of 5 years and

10 years, 1472 m3/s and 1983m

3/s respectively was calculated using Generalized EV1

and that of the return period of 50 years, 100 years and 200 years was calculated using

Log Pearson Type III.

In case of upper un-gauged reaches, WECS/ DHM methodology was used to obtain the

maximum instantaneous discharge as tabulated below.

Table 9 Estimated peak discharge of the upper reaches of Tinau river

Flo

od

flo

w (

m3/s

)

Return Period Upper un-gauged Reaches

Dobhan Tinau Upper

5 284.446 541.012

10 364.259 678.272

50 562.167 1008.546

100 655.035 1159.843

200 753.863 1318.843

5.2 Landuse/land cover Map, DEM,

The landuse/ land cover map of the Tinau Khola watershed was prepared using the

topographic map (1992) from Department of Survey, Government of Nepal. The

attributes of the landuse map were analyzed, summarized below. The TIN was created

using 3D Analyst in ArcGIS environment using various spatial details

Figure 7Landuse/ Land cover map of the Tinau basin

35

Figure 8Pie-Chart showing area under various landuse class,

Analysis of the landuse map showed that the forest landuse class has the highest value

of 61% followed by Agriculture 33% and Bush 5%, the others (Settlement, Barren,

Water bodies, Sand) occupying 1% of the total area.

Digital Elevation Model (DEM) generated from TIN (Triangulated Irregular Network)

using elevation data is presented as Figure 9. The vertical profile of the watershed

ranges 165m to 1940m asl.

Figure 9DEM of the Study Area

5.3 Flood Hazard Analysis

The hazard aspect of the flooding is related to the hydraulic and the hydrological

parameters. The analysis of the flood hazard map indicated a considerable increase in

Settlement0%

Agriculture33%

Forest61%

Bush5%

Swamp0%

Sand1%

Barren0%

River/ Stream

0%Pond/ Lake0%Other

2%

36

Figure 11 Flood Hazard Map for the 1 in 5 years flood

inundated area with increasing discharge from a flood with 5 year to 50 year return

period. The change was less visual in subsequent return period after 50-year return

period. The total inundated area of the flood with 5 years return period is 350.31 ha. and

that of the flood with 200 years return period is 430.535 ha. The percentile increase

amongst the successive return period was 5.05%, 8.71%, 3.55%, 3.92%.

Figure 10 Return Periods and Area Inundation Relationship

Figure 11, 12, 13, 14, 15, represent Flood hazard Maps with the probable occurrence

return period of 1 in 5 years, 1 in 10 years, 1 in 50 years, 1 in 100 years, 1 in 200 years,

respectively.

325

350

375

400

425

450

0 50 100 150 200 250

Inu

nd

ate

d A

rea

( H

ect

are

s)

Return Period (Years)

37

Figure 12 Flood Hazard Map for the 1 in 10 years flood

Figure 13 Flood Hazard Map for the 1 in 50 years flood

38

Figure 14 Flood Hazard Map for the 1 in 100 years flood

Figure 15Flood Hazard Map for the 1 in 200 years flood

39

5.4 Flood Vulnerability Analysis

The vulnerability maps for the flood areas were prepared by intersecting the land use

map of the floodplains with the flood area polygon for each of the flood event being

modeled. This depicts the vulnerability aspect of the flood risk in the particular area in

terms of the presence or the absence of flooding of a particular return period as a binary

model.

Figure 16Vulnerability classification of landuse to flood hazard of various return periods

The analysis indicated that there will only be negligible change limited under 6% in the

inundated area in successive return periods in sand type land use. The flood hazard

mapping indicated that in each successive return period from 1 in 5 years to 1 in 200

years, the area of agricultural land inundated increased by 6.5%, 13.0%, 5.3%, 5.3%

respectively.

Cliff BarrenSettlemen

tsBush Forest Stream

Agriculture

Sand

200 yrs. 0.92 2.41 14.96 49.36 67.16 69.48 91.8 134.45

100 yrs. 0.92 2.31 9.63 47.81 64.04 68.95 87.21 133.42

50 yrs. 0.9 2.17 6.44 46.98 60.62 68.49 82.79 131.68

10 yrs. 0.88 1.88 3.38 43.66 53.93 65.91 73.26 125.11

5 yrs. 0.87 1.72 2.54 40.97 50.48 64.04 68.77 120.94

0

100

200

300

400

500

600

700

Are

a in

He

ctar

es

40

The results revealed only slight increase in the vulnerability due to inundation on fields

other than settlement. However, the results were striking in the case of Vulnerability

due to inundation in settlement areas. In each successive return period from 1 in 5 years

to 1 in 200 years, the area of settlement inundated increased by 33.1%, 90.5%, 49.5%,

55.3% of the previous return period respectively (Figure 17, 18).

Figure 17Vulnerability classification of important landuse to flood hazard of

various return periods

The settlement area inundated for a 1 in 5 years flood is 2.54 ha, similarly the inundated

area gradually increases over successive return periods and the total inundated area for a

1 in 200 year flood is 14.96 ha.

Figure 18 Change in inundated area within settlement area along

various return periods

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Settlements 2.54 3.38 6.44 9.63 14.96

Forest 50.48 53.93 60.62 64.04 67.16

Agriculture 68.77 73.26 82.79 87.21 91.8

Sand 120.94 125.11 131.68 133.42 134.45

0

50

100

150

200

250

300

350

Are

a in

Hec

tare

s

2.543.38

6.44

9.63

14.96

0

2

4

6

8

10

12

14

16

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Are

a in

He

ctar

es

41

5.5 Flood Risk Analysis

The flood risk analysis includes the combination of the results of the both the

vulnerability assessment and the hazard assessment. This is defined by the relationship

between the vulnerability classes and the flood depth hazard classes in any particular

area. For this, the flood risk maps are prepared by overlaying the flood depth grids with

the land use map, of different year return period flood. The flood depth polygons

prepared during the hazard analysis were intersected with the land use vulnerability

polygons. The resulting attribute tables were reclassified to develop the land use-flood

depth relationship. This, hence, depicts potential flood areas in terms of both the land

use vulnerability classes and water depth hazard classes.

The analysis of the flood hazard map with the class value allocated for the study

indicated a gradual decrease in the area under low hazard (<2 m), while there was a

positive increase in all other hazard class (Figure 11).

Figure 19 Chart showing depth of inundationand the respective inundated area

along the defined return period.

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Very High (> 10 m) 1.39 3.11 10.6 15.98 21.6

High (5 - 10 m) 32.18 43.17 56.41 62.75 70.56

Moderate (2 - 5 m) 117.5 123.54 164.17 185.88 207.89

Low (<2 m) 199.29 198.19 168.89 149.72 130.54

0

50

100

150

200

250

300

350

400

450

500

Inu

nd

ate

d A

rea

in H

ect

are

s

42

The analysis of the relationship between the Flood hazard and Settlement area indicated

a gradual increase in all hazard class in all return periods. The settlement area under

Low hazard class (<2 m) is 1.67, 2.29, 4.45, 6.66, 10.28 hectares for return periods 5 to

200 years and that of the High hazard class (>10 m) is 0, 0, 0.08, 0.49, 0.66 hectares for

return periods 5 to 200 years.

Figure 20 Risk classification of settlement Landuse type

The analysis of the relationship between the Flood hazard and agricultural area show

that the area under low hazard class (<2 m) is in a decreasing trend over the successive

return periods while agricultural area in all other hazard class show a gradual increase in

values. The agricultural area under Low hazard class (<2 m) is 42.34, 42.94, 41.5,

40.35, 38.24 hectares for return periods 5 to 200 years and that of the High hazard class

(>10 m) is 0, 0, 0.08, 0.49, 0.66 hectares for return periods 5 to 200 years.

Figure 21 Risk classification of Agricultural land

0

2

4

6

8

10

12

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Return Period

Are

a in

Hec

tare

s

Settlement

Low (<2 m) Moderate (2 - 5 m) High (5 - 10 m) Very High (> 10 m)

0

10

20

30

40

50

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Return Period

Are

a in

Hec

tare

s

Agriculture

Low (<2 m) Moderate (2 - 5 m) High (5 - 10 m) Very High (> 10 m)

43

A detailed flood risk classification table for various landuse types is attached as Annex-

5.

The VDCs and Municipality affected due to the flooding were selected and a risk

classification in terms of depth of inundation was carried out, the resulting table 12 is

presented below.

Figure 22 Flood Risk Map of Butwal Municipality

The flood risk classification of Butwal Municipality shows a gradual lowering in the

area under low hazard (<2 m) after the 1 in 10 year flood while the area under moderate

flood hazard (2-5 m) shows a continued increase. The area inundated in an event of a

flood with reoccurrence period of 5 years will be 141.1 ha, similarly area inundated in

an event of a flood with reoccurrence period of 200 years will be 182.11ha.

Table 10 VDC wise flood hazard at various return periods

Affected

Area Flood Hazard

Are

a in

Hec

tare

s

Return Period

5 yrs.

10

yrs.

50

yrs.

100

yrs.

200

yrs.

Butwal

Municipality

Low (<2 m) 109.46 112.40 97.49 83.19 69.48

Moderate (2- 5 m ) 21.34 23.91 48.99 67.09 87.73

High (5 - 10 m ) 10.30 12.87 15.62 16.34 16.67

very High ( >10 m) 0.00 0.36 3.15 5.54 8.24

Total 141.10 149.53 165.25 172.14 182.11

Dobhan

Low (<2 m) 62.69 59.66 48.43 44.33 40.11

Moderate (2- 5 m ) 67.42 68.22 79.88 82.22 83.12

High (5 - 10 m ) 19.12 26.56 33.50 37.14 42.07

very High ( >10 m) 1.39 2.74 7.46 10.43 13.36

0.00

20.00

40.00

60.00

80.00

100.00

120.00

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Are

a in

He

ctar

es

Butwal Municipality Flood Risk Classification

Low (<2 m)

Moderate (2- 5 m )

High (5 -10 m )

very High ( >10 m)

44

Total 150.61 157.18 169.27 174.12 178.65

Juthapauwa

Low (<2 m) 1.07 1.08 1.05 1.04 1.04

Moderate (2- 5 m ) 0.93 1.10 1.40 1.50 1.57

High (5 - 10 m ) 0.00 0.00 0.02 0.04 0.10

very High ( >10 m) 0.00 0.00 0.00 0.00 0.00

Total 2.00 2.18 2.46 2.57 2.70

Kachal

Low (<2 m) 26.06 25.06 22.51 21.17 19.91

Moderate (2- 5 m ) 27.81 30.30 33.86 35.07 35.46

High (5 - 10 m ) 2.74 3.75 7.29 9.21 11.71

very High ( >10 m) 0.00 0.00 0.00 0.00 0.00

Total 56.61 59.11 63.66 65.45 67.08

The flood risk in context of the building units provided by the topographic map of 1992

was analyzed; the analysis is tabulated as table 13. The classification in terms of flood

hazard class grouped according to the depth of flood was made and numbers of building

points hence affected were calculated using GIS.

The analysis showed that in an event of a 5 year flood, 111 building units will be

inundated. The number gradually increases to 215 building units in an event of a 200

year flood. A temple was also found inundated in an event of 1 in 10 year flood and

thereafter.

Table 11 Flood Impact to the Building Units

Affected

Units Flood Hazard

Return Period

5

yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Building

Low (<2 m) 63 61 74 89 104

Moderate (2- 5 m ) 37 46 61 67 79

High (5 - 10 m ) 11 13 18 24 30

very High ( >10 m) 2 2

Total 111 120 153 182 215

Temple

Low (<2 m) 1 1

Moderate (2- 5 m ) 1 1

High (5 - 10 m )

very High ( >10 m)

Total 0 1 1 1 1

Unit in numbers

45

CHAPTER VI

6. DISCUSSION

The applications of hydraulic model and GIS for floodplain analysis and risk analysis

have been limited in countries like Nepal, where the availability of the river geometric,

topographic and hydrological data are also very limited. The situation of river flooding

in Nepal is also completely different, as there is much higher variation in the river flows

and rivers are completely unregulated. There are very few flood control structures like

spurs and dikes and the river banks and boundary lines are not clearly defined. Hence,

the floodplain analysis and modeling are subjected to number of new sets of constraints.

This study presents an approach of conducting a similar study, within these constraints.

6.1 Flood Frequency Analysis

Flood mapping of gauged river was made using Log-Pearson III, Gumbel‟s Extreme

Value Type I and WECS/ DHM, while methodology suggested by Water and Energy

Commission Secretariat/ Department of Hydrology and Meteorology (WECS/DHM) of

Nepal, was used to estimate instantaneous annual flood discharge of the un-gauged

tributaries in the upper section of the watershed.

The adopted flood discharge values with return period of 5 years and 10 yearscalculated

using Generalized EV1 was found to be 1472 m3/s and 1983m

3/s respectively. Similarly

that of the return period of 50 years, 100 years and 200 years calculated using Log

Pearson Type III3523m3/s, 4620m

3/s, 5957m

3/s.

The discharge values calculated in this studyare comparable to several other studies

with respect to the total watershed area. Manandhar (2010) used similar technique for

flood frequency analysis of watershed of 170 km2and found the maximum discharge

values for 2, 10 and 50 years return period with Generalized EV1 and for 100 and 200

years using Log Pearson Type III.

6.2 Flood vulnerability and Risk analysis

Flood simulation was conducted for the computed discharge values using HEC-RAS

and ArcView GIS. HEC-GeoRAS extension facilitated the exchange of data between

ArcView GIS and HEC-RAS.The model results suggest that there is considerable

46

flooding in the area even at flood of 5-year return period. This implies that the channel

capacity is small to carry the flood water discharge.

The analysis of the relationship between the Flood hazard and agricultural area show

that the area under low hazard class (<2 m) is in a decreasing trend over the successive

return periods while agricultural area in all other hazard class show a gradual increase in

values. This might be because agricultural terraces along the river terraces are primarily

flat plains along the river. In any case floodwaters of more than 1m are extensive for

damage to agricultural land.

The flood risk classification of Butwal Municipality shows a gradual lowering in the

area under low hazard (<2 m) after the 1 in 10 year flood while the area under moderate

flood hazard (2-5 m) shows a continued increase. The area inundated in an event of a

flood with reoccurrence period of 5 years will be 141.1 ha.Similarly, area inundated in

an event of a flood with re-occurrence period of 200 years will be 182.11ha. This shows

that about 2% of the Municipal area will be inundated every 5 years including the flood

plain.

The analysis also shows that in an event of a 5 year flood, 111 building units will be

inundated. The number gradually increases to 215 building units in an event of a 200

year flood.A temple was also found inundated in an event of 1 in 10 year flood and

thereafter. These records are based on the 1992 topographic map prepared by the

Department of survey, Nepal. Municipal Records at present reveal that there are 20281

houses with a population of 150,000 at a growth rate of 6.1 %.At the same growth rate

of 6.1 % per annum 342 households seem to be affected in case of a flood of five years

return period and 662 households seem to be affected in case of a 1 in 200 year

flood.The same view has been depicted in the map attached as annex prepared by

overlaying the flood hazard map along with the 1992 building points over the current

Google Earth Terrain.

Several studies including Dhital et al. (2005) suggest that more than 56 % of the Tinau

watershed area is in moderate (26%) to high (30%) landslide hazard category. Hence

there is equally high chances of LDOF (Landslide DammingOutburst Flood). Also Sah

(2009) conducted an inundation analysis for Tinau River basin excluding as well as

including dam breach simulation; Inundation analysis of natural dam breach at 20 m

shows that Bhairahawa city will be inundated up to 3.08 m in case of dam breach.

47

Mass poverty and low level of offfarm activities,illiteracy, and poor service facilities

have contributedto the low response and recovery capacity to dealwith disasters. Unless

the response and recoverycapacity of the local people is improved, the loss anddamage

are likely to increase. Therefore, disasterreduction and preparedness strategies should

includecomponents such as poverty reduction and women empowerment and that of

other disadvantaged groups inthe community and overall development activities in a

sustainable way.

48

CHAPTER VII

7. CONCLUSION

The flood hazard analysis of the floodplain was done using the one dimensional steady

flow model HEC-RAS. ArcGIS and HEC-GeoRAS were used for the preparation of

flood hazard maps. Flood hazard maps for floods with probable return period of 1 in 5

years, 1 in 10 years, 1 in 50 years, 1 in 100 years, and 1 in 200 years were prepared.

Vulnerability to flood hazard was accessed in consideration with landuse type, affected

VDC/ Municipality and building points. The flood risk analysis was done by crossing of

vulnerability classes with flood depth hazard class. The flood depth analysis class used

in this research were low hazard (<2 m), moderate hazard (2-5 m), high hazard (5-10

m) and very high hazard (>10 m).

i. The analysis of the flood hazard map indicated a considerable increase in

inundated area with increase in discharge from a flood with 5 year return period

to 50 year return period, while the increase was less pronounced in subsequent

years.

ii. The total inundated area of the flood with 5 year return period is 350.31 hectare

and that of the flood with 200 years return period is 430.54 ha.

iii. The flood vulnerability analysis indicated that in each successive return period

from 1 in 5 years to 1 in 200 years, the area of agricultural land inundated

increased by 6.5%, 13.0%, 5.3%, 5.3% respectively while that of settlement

significantly changed by 3.1%, 90.5%, 49.5%, 55.3%.

iv. The flood risk analysis revealed a gradual decrease in the area under low hazard

(<2 m), while there was a positive increase in all other hazard class. The

analysis of the relationship between the Flood hazard and Settlement area

indicated a gradual increase in all hazard class in all return periods.

v. The flood risk classification of Butwal Municipality shows a gradual lowering in

the area under low hazard (<2 m) after the 1 in 10 year flood while the area

under moderate flood hazard (2-5 m) shows a continued increase.

49

vi. The analysis showed that in an event of a 5 year flood, 111 building units will be

inundated. The number gradually increases to 215 building units in an event of a

200 year flood. A temple was also found inundated in an event of 1 in 10 year

flood and thereafter.

50

CHAPTER VIII

8. RECOMMENDATIONS

Limited availability of data was the primary constraint of the study hence the results

presented in this research reflects the data they represent. Therefore, the following

recommendations are hence made for the further studies in the future.

i. The accuracy of a flood hazard map largely depends on the topographical data

used for the preparation of the hazard map. For the modeling of overbank flow,

topographic data should be of high resolution so that the topography of the

floodplains could be properly represented

ii. Use of new technologies such as LIDAR (Light Detection and Ranging), which

improves the quality of the digital terrain representations can hence be used for

further study.

iii. The major hydrologic parameter, the maximum instantaneous discharge

recorded for a long duration is necessary for the estimation of flood flow inputs

of various return periods calculated using statistical approach.

iv. Turbulent flows are unsteady and natural river course are turbulent, hence

unsteady flow simulation should also be considered for flood hazard mapping.

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pp. 61 66

ANNEX

Annex 1

Percentage of land vulnerable to flooding along various return periods

Percentage of land vulnerable to flooding along various return periods

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Settlements 1.29% 1.72% 3.28% 4.90% 7.61%

Agriculture 0.38% 0.40% 0.45% 0.48% 0.50%

Forest 0.15% 0.16% 0.18% 0.19% 0.20%

Bush 1.49% 1.59% 1.71% 1.74% 1.79%

Sand 43.81% 45.32% 47.70% 48.33% 48.71%

Barren 0.97% 1.06% 1.22% 1.30% 1.36%

Stream 37.47% 38.56% 40.07% 40.34% 40.65%

Annex 2

Inundated Area with successive return periods

Inundated Area with successive return periods

Return

Period Sand Change Agriculture

Change Forest Change Settlements

Change

5 yrs. 120.94 0 68.77 0 50.48 0 2.54 0

10 yrs. 125.11 3.4% 73.26 6.5% 53.93 6.8% 3.38 33.1%

50 yrs. 131.68 5.3% 82.79 13.0% 60.62 12.4% 6.44 90.5%

100 yrs. 133.42 1.3% 87.21 5.3% 64.04 5.6% 9.63 49.5%

200 yrs. 134.45 0.8% 91.8 5.3% 67.16 4.9% 14.96 55.3%

Annex 3

Area under various Landuse type in the Tinau watershed

S.No. Landuse type Area in Hectares

1 Settlement 196.5374

2 Cutting 135.2362

3 Agriculture 18329.4147

4 Forest 34191.9519

5 Bush 2753.7455

6 Swamp 8.8201

7 Sand 276.0464

8 Barren 177.508

9 River/ Stream 170.9136

10 Pond/ Lake 5.6727

Annex 4

List of VDCs and Municipality in the Tinau watershed

S.No. Name District

1 Baldengadhi

PA

LP

A

2 Bandipokhara

3 Bhairabsthan

4 Chhahara

5 Chidipani

6 Chirtungdhara

7 Devinagar

8 Dobhan

9 Gothadi

10 Humin

11 Jhadewa

12 Juthapauwa

13 Kachal

14 Kaseni

15 Khasyauli

16 Koldanda

17 Kusumkhola

18 Madanpokhara

19 Masyam

20 Nayarnamtales

21 Palungmainadi

22 Phek

23 Pokharathok

24 Rahabas

25 Rupse

26 Tansen Municipality

27 Telgha

28 Timure

29 Butwal municipality RUPANDEHI

Annex 5

Flood Risk Classification table for Various Landuse type

Inundated Area in Hectares

Landuse Flood Hazard

Return Period

5 yrs. 10 yrs. 50 yrs. 100 yrs. 200 yrs.

Settlements Low (<2 m) 1.67 2.29 4.45 6.66 10.28

Moderate (2 - 5 m) 0.41 0.50 1.07 1.77 2.87

High (5 - 10 m) 0.46 0.59 0.84 0.72 1.16

Very High (> 10 m) 0.00 0.00 0.08 0.49 0.66

Total 2.54 3.38 6.44 9.63 14.96

Cliff Low (<2 m) 0.09 0.08 0.07 0.07 0.06

Moderate (2 - 5 m) 0.73 0.75 0.62 0.57 0.50

High (5 - 10 m) 0.05 0.05 0.21 0.27 0.36

Very High (> 10 m) 0.00 0.00 0.00 0.00 0.00

Total 0.87 0.88 0.90 0.92 0.92

Agriculture Low (<2 m) 42.34 42.94 41.50 40.35 38.24

Moderate (2 - 5 m) 23.12 25.04 33.91 37.81 42.22

High (5 - 10 m) 3.32 5.27 7.13 8.70 10.79

Very High (> 10 m) 0.00 0.01 0.24 0.37 0.56

Total 68.77 73.26 82.79 87.21 91.80

Forest Low (<2 m) 21.24 21.29 18.90 18.43 18.07

Moderate (2 - 5 m) 22.79 23.55 27.63 27.96 26.39

High (5 - 10 m) 5.88 8.09 10.54 12.61 16.23

Very High (> 10 m) 0.56 1.00 3.55 5.04 6.48

Total 50.48 53.93 60.62 64.04 67.16

Bush Low (<2 m) 36.12 37.23 31.74 25.87 19.47

Moderate (2 - 5 m) 4.72 6.23 14.88 21.53 29.43

High (5 - 10 m) 0.14 0.21 0.36 0.41 0.46

Very High (> 10 m) 0.00 0.00 0.00 0.00 0.00

Total 40.97 43.66 46.98 47.81 49.36

Sand Low (<2 m) 70.54 68.50 52.69 42.22 31.43

Moderate (2 - 5 m) 40.01 41.98 56.65 65.37 73.96

High (5 - 10 m) 10.12 14.10 20.57 22.76 24.47

Very High (> 10 m) 0.27 0.53 1.77 3.08 4.59

Total 120.94 125.11 131.68 133.42 134.45

Barren Low (<2 m) 1.06 1.03 0.85 0.85 0.82

Moderate (2 - 5 m) 0.62 0.78 1.23 1.33 1.42

High (5 - 10 m) 0.05 0.06 0.10 0.14 0.18

Very High (> 10 m) 0.00 0.00 0.00 0.00 0.00

Total 1.72 1.88 2.17 2.31 2.41

Stream Low (<2 m) 26.23 24.83 18.69 15.27 12.17

Moderate (2 - 5 m) 25.10 24.71 28.18 29.54 31.10

High (5 - 10 m) 12.16 14.80 16.66 17.14 16.91

Very High (> 10 m) 0.56 1.57 4.96 7.00 9.31

Total 64.04 65.91 68.49 68.95 69.48

Grand Total 350.32 368.00 400.07 414.28 430.54

Annex 6

Primary Examination Report of Flood victims Nepal Red Cross Society,

Rupandehi referring to the flood incident at Tinau on 3rd

Bhadra, 2065 (August 19,

2008).

Annex 7

Cross section at river station 519.6874 showing plot profile of a 1 in 200 years flood

at Dobhankhola near the confluence with Tinau River.

Annex 8

Cross section at river station 7474.066 showing plot profile of 1 in 200 year flood at

Upper reach of Tinau river near the confluence with Dobhan Khola.

Annex 9

Cross section at river station 1140.882 showing water surface of 1 in 200 year flood

Annex 11

Tinau river profile at 1 in 5 years flood

Annex 10

Tinau river profile at 1 in 200 years flood

Annex 12

Geometric data of the floodplain with 1 in 200 year flood and river stations

Annex 14

Present scenario of the Flood risk at Tinau Watershed (Focused over Butwal

Municipality)

Figure : 1 in 5 years Flood

High quality image available at http://tiny.cc/u346o

Figure : 1 in 10 years Flood

High quality image available at http://tiny.cc/ljwvx

Figure : 1 in 50 years Flood

High quality image available at http://tiny.cc/j7gll

Figure : 1 in 100 years Flood

High quality image available at http://tiny.cc/z19zm

Figure : 1 in 200 years Flood

High quality image available at http://tiny.cc/fv0mq

Annex 13

Tinau River from East-West highway viewing upstream, the point where the gradient

breaks from the Siwaliks into the flat Terai.

Photo courtesy: Witlox, 2007

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