flood risk mapping of upper bagmati basin

FLOOD RISK MAPPING OF UPPER BAGMATI BASIN

A final year project submitted in partial fulfilment of the requirement for the degree of Bachelors of Engineering

Tri Dev Acharya (42292) Sunil Banjara (42310) Sudip Deuja (42311) Sushil Thapa (42314)

DEPARTMENT OF CIVIL &GEOMATICS ENGINEERING SCHOOL OF ENGINEERING KATHMANDU UNIVERSITY DHULIKHEL, NEPAL September, 2011

DECLARATION

We, Mr. Nawaraj Subedi, Senior Instructor, Land Management Training Centre, Dhulikhel and Mr. Nawaraj Shrestha, Lecturer, Kathmandu University hereby declare that the work presented herein is genuine work done originally by Mr. Tri Dev Acharya, Mr. Sunil Banjara, Mr. Sudip Deuja and Mr. Sushil Thapa and has not been published or submitted elsewhere for the requirement of a degree programme. Any literature, data, or works done by others and cited within this report has been given due acknowledgement and listed in the reference section.

_______________ Mr. Nawaraj Subedi Senior Surveyor Land Management Training Centre Dhulikhel, Nepal

_______________ Mr. Nawaraj Shrestha Lecturer Department of Civil and Geomatics Engineering, Kathmandu University Dhulikhel, Nepal

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CERTIFICATION

FINAL YEAR PROJECT REPORT On FLOOD RISK MAPPING OF UPPER BAGMATI BASIN

Submitted by: Tri Dev Acharya (42292) Sunil Banjara (42310) Sudip Deuja (42311) Sushil Thapa (42314)

Approved by: 1. Project Supervisor ___________________

____________________________

(Signature)

(Name)

_______________ (Date)

2. Project Supervisor ___________________

____________________________

(Signature)

(Name)

_____________ (Date)

3. External Examiner _________________

____________________________

(Signature)

(Name)

_______________ (Date)

4. Head of the Department ___________________ (Signature)

____________________________ (Name) ii

_______________ (Date)

DECLARATION BY STUDENTS

We, declare that this project report has not been previously accepted in substance for any degree and is not being concurrently submitted in candidature for any degree. We state that this report is the result of our own independent work/investigation, except where otherwise stated, we hereby give consent for our report, if accepted, to be available for photocopying and understand that any reference to or quotation from our project report will receive an acknowledgement.

_______________ Tri Dev Acharya Roll No. 42292

_______________ Sunil Banjara Roll No. 42310

_______________ Sudip Deuja Roll No. 42311

_______________ Sushil Thapa Roll No. 42314

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ACKNOWLEDGEMENT

We owe sincere gratitude to our Supervisors Mr. Nawaraj Subedi, Director, Land Management Training Center and Mr. Nawaraj Shrestha, Lecturer, Department of Civil and Geomatics Engineering, Kathmandu University for their continuous guidance and encouragement during project period. They have been our inspiration as we hurdle all the obstacles in the completion of this project work. We feel motivated and encouraged every time we attend their meeting. Without their encouragement and guidance this project would not have been materialized.

We would like to thank Mr. Deep Khadka, for his support as a staff person. We are also very much thankful to Ms. Aayushma Regmi and Mrs. Ambika Badal for arranging accommodation of the instruments during our field work.

We would also, like to thank Mr. Narayan Pokhrel and Mr. Shiva Poudel for their guidance during our field work.

We would like to express our thankfulness to the staffs of Survey Department, DHM and DWIDP for providing us with relevant data required for this project. We pay a deep veneration to Dr. Dilip Kumar Gautam, Senior Hydrologist, DHM for providing important information regarding flood frequency analysis to accomplish the project successfully.

As this project would not have been possible without the guidance and help of several individuals who in one way or another contributed and extended their valuable assistance in the preparation and completion of this project. Therefore we take this opportunity to express our sincere gratitude to our Head of Department, coordinators, professors, teachers, friends and all the family of Department of Civil and Geomatics Engineering, Kathmandu University and Land Management Training Center for uplifting our academic background from the very beginning of the course.

September, 2011 Dhulikhel, Nepal

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ABSTRACT Flood, a common water induced disaster of monsoon season, is the recurring phenomenon in Nepal. It causes many deaths and injuries besides severe impacts on the vital infrastructures of the nation. This final report of our project develops a framework for flood risk mapping that integrates GIS and a hydrological model (HEC-RAS) successfully carried out in three stages: desk study, field study, and post study (data analysis). The Upper Bagmati Basin (catchment area of about 74 km2) in Kathmandu Valley is the domain of study. Besides the river‟s religious and cultural importance, it also includes Pashupatinath Temple and Tribhuwan International Airport, the only International Airport of Nepal.

Collection of relevant publications, reports, books and different literatures, hydrological data, VDC map, delineation of the watershed boundary were accomplished during the desk study. In the field study, spot height data were taken using total station. Collection of data and information related to floods, inundations, as well collection of the information on the past disaster events were done during field visit. Flood frequency analysis for 5, 10, 25, 50 and 100years return period was done by Gumbel‟s distribution, Log Pearson Type III distribution, Log Normal distribution and WECS/DHM method based on Maximum Instantaneous flow recorded at Sundarijal and Gaurighat station.

Based on the calculated discharge values, one dimensional hydraulic model (HEC-RAS) was used to model steady flow through the river channel network. Triangulated Irregular Network (TIN) was prepared from contour and spot elevations. Required data layers: stream centerline, banks, flow paths and cross-sections were created with HEC-GeoRAS extension then RAS to GIS import file was prepared. Hydraulic modeling was done in HEC-RAS. Finally, the floodplain delineation was done in ArcGIS. Hazard maps, landuse vulnerability maps and risk maps at a scale of 1: 50,000, for various return periods (5yrs, 10yrs, 25yrs, 50yrs and 100yrs), were prepared in ArcGIS. Similarly, VDC vulnerability map were produced at scale of 1:75,000. Various interpretation and analysis were done in GIS.

The results of this project will benefit future mapping efforts by providing a tool for hydrological forecasts of flooding. Also, the study may help in planning and management of flood plain area of Upper Bagmati Basin to mitigate future probable disaster through technical approach. While

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designed for the Upper Bagmati Basin, the output maps may be used as a prototype for various applications in other areas of the country.

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TABLE OF CONTENTS ABSTRACT .................................................................................................................................... v TABLE OF CONTENTS .............................................................................................................. vii LIST OF FIGURES ......................................................................................................................... x LIST OF TABLES ......................................................................................................................... xi LIST OF SYMBOLS ..................................................................................................................... xii LIST OF ACRONYMS ................................................................................................................ xiv CHAPTER 1 INTRODUCTION................................................................................................... 1 1.1

Project Background .......................................................................................................... 1

1.2

Rationale of the Project .................................................................................................... 2

1.3

Objectives ......................................................................................................................... 2

1.4

Limitations ........................................................................................................................ 3

CHAPTER 2 LITERATURE REVIEW........................................................................................ 4 2.1

River Flooding .................................................................................................................. 4

2.2

Flood Frequency Analysis ................................................................................................ 5

2.2.1

The Gumbel‟s Method............................................................................................... 6

2.2.2

The Log Pearson Type III Method ............................................................................ 6

2.2.3

The Log Normal Method ........................................................................................... 7

2.2.4

WECS/DHM Method ................................................................................................ 7

2.3

GIS .................................................................................................................................... 7

2.4

Google Earth ..................................................................................................................... 8

2.5

HEC-GeoRAS................................................................................................................... 8

2.6

HEC-RAS ......................................................................................................................... 9

2.6.1

Energy Equations ...................................................................................................... 9

2.6.2

Manning‟s Loss Coefficient .................................................................................... 11

2.6.3

Steady and Unsteady Flow ...................................................................................... 11

2.6.4

Boundary Condition ................................................................................................ 12

2.7

Risk, Vulnerability and Hazard ...................................................................................... 13

CHAPTER 3 METHODOLOGY ................................................................................................ 15 3.1

Project Site ...................................................................................................................... 15

3.1.1

Project Boundaries .................................................................................................. 15

3.1.2

Hydrological and Meteorological Characteristics ................................................... 16

3.1.3

Availability and the use of Water ............................................................................ 16

3.1.4

Geomorphology ....................................................................................................... 17

3.1.5

Climate and Vegetation ........................................................................................... 17 vii

3.1.6 3.2

Socio-economic Condition ...................................................................................... 17

Data Collection ............................................................................................................... 17

3.2.1

Hydro-metrological Data Collection ....................................................................... 17

3.2.2

Topographic Map and Control Points ..................................................................... 18

3.2.3

VDC Map Collection............................................................................................... 18

3.2.4

Secondary Data Sources .......................................................................................... 18

3.2.5

Preparation of Flood Hazard Classification Criteria ............................................... 18

3.2.6

Field Survey and Other Activities ........................................................................... 19

3.2.7

Watershed Delineation ............................................................................................ 20

3.2.8

Preparation of Landuse/Landcover Map ................................................................. 20

3.2.9

Tools for Analysis ................................................................................................... 20

3.3

Floodplain Mapping........................................................................................................ 20

3.3.1

Pre-processing of Geometric Data .......................................................................... 20

3.3.2

HEC-RAS Processing.............................................................................................. 23

3.3.3

Post-processing of hydraulic results ........................................................................ 24

3.4

Flood Risk Assessment ................................................................................................... 25

CHAPTER 4 RESULTS AND DISCUSSION ........................................................................... 28 4.1

Results of Flood Frequency Analysis ............................................................................. 28

4.2

Preparation of Landuse/Landcover Map ........................................................................ 31

4.3

Preparation of TIN .......................................................................................................... 32

4.4

Steady Flow Analysis and Flood Risk Analysis ............................................................. 34

4.4.1

Steady Flow Analysis .............................................................................................. 34

4.4.2

Flood Hazard Analysis ............................................................................................ 35

4.4.3

Flood Vulnerability Analysis .................................................................................. 38

4.4.4

Flood Risk Analysis ................................................................................................ 43

4.5

Discussion ....................................................................................................................... 47

CHAPTER 5 CONCLUSION ..................................................................................................... 48 BIBLIOGRAPHY ......................................................................................................................... 49 ANNEXES .................................................................................................................................... 52 Annex A: Data ........................................................................................................................... 52 A-1 Extreme Rainfall (mm) of Sundarijal ............................................................................. 52 A-2 Extreme Discharges (m3/s) of Sundarijal Station........................................................... 52 A-3 Extreme Discharges (m3/s) of Gaurighat Station ........................................................... 54 A-4 Manning's Roughness Coefficient for Different Landuse .............................................. 55 Annex B: Profile Plots ............................................................................................................... 56 B-1 Longitudinal Plot ............................................................................................................ 56 B-2 Cross Section Plot .......................................................................................................... 57 viii

Annex C: Output Maps.............................................................................................................. 61 C-1 Hazard Maps................................................................................................................... 61 C-2 Vulnerability Maps ......................................................................................................... 66 C -3 Vulnerable VDC Maps .................................................................................................. 71 C-4 Risk Maps ....................................................................................................................... 76 Annex D: Snapshots .................................................................................................................. 81

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LIST OF FIGURES

Figure 1 : Google Earth image of project site. ................................................................................ 8 Figure 2 : Representation of the terms in the Energy Equation (USACE, 2010a) ........................ 10 Figure 3 : Risk Conceptualization (Gilard & Givone, 1996) ........................................................ 14 Figure 4 : Map of Upper Bagmati Basin ....................................................................................... 16 Figure 5 : Workflow of Floodplain Mapping (USACE, 2011) ..................................................... 26 Figure 6 : Flood Risk Mapping (Gilard & Givone, 1996)............................................................. 27 Figure 7 : Landuse map of project site .......................................................................................... 31 Figure 8 : Watershed of Upper Bagmati Basin ............................................................................. 32 Figure 9 : TIN of Project Site ........................................................................................................ 33 Figure 10 : Return Periods and Inundated Area Relationship ....................................................... 34 Figure 11 : Discharge versus Inundated Area Relationship .......................................................... 35 Figure 12 : Return Period-Flood Depth Relationship ................................................................... 37 Figure 13 : Depth-Flooded Area Relationship .............................................................................. 37 Figure 14 : Inundated Landuse for 5 yrs return period.................................................................. 39 Figure 15 : Inundated Landuse for 10 yrs return period ................................................................ 39 Figure 16 : Inundated Landuse for 25 yrs return period ................................................................ 40 Figure 17 : Inundated Landuse for 50 yrs return period ................................................................ 40 Figure 18 : Inundated Landuse for 100 yrs return period.............................................................. 41 Figure 19 : Inundation Area versus VDC-Water depth relation ................................................... 43 Figure 20 : Landuse-Flood Depth Relationship for 5yrs Return Period Flood ............................. 45 Figure 21 : Landuse-Flood Depth Relationship for 10yrs Return Period Flood ........................... 45 Figure 22 : Landuse-Flood Depth Relationship for 25yrs Return Period Flood ........................... 46 Figure 23 : Landuse-Flood Depth Relationship for 50yrs Return Period Flood ........................... 46 Figure 24 : Landuse-Flood Depth Relationship for 100yrs Return Period Flood ......................... 47

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LIST OF TABLES Table 1 : Hydrometric Details ....................................................................................................... 18 Table 2 : Metrological Details ....................................................................................................... 18 Table 3 : Classification of Flood Depth ........................................................................................ 19 Table 4 : Flood Frequency of Sundarijal Station .......................................................................... 28 Table 5 : Flood Frequency of Gokarneswor Station (Interpolated) .............................................. 28 Table 6 : Flood Frequency of Gaurighat Station ........................................................................... 29 Table 7 : Flood Frequency using WECS method .......................................................................... 29 Table 8 : Calculation of Flood Area according to Flood Hazard .................................................. 36 Table 9 : Classification of Flood Area according to Landuse Vulnerability................................. 38 Table 10 : Inundation Area versus VDC-water depth relationship ............................................... 42 Table 11 : Flood Risk Classification ............................................................................................. 44

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LIST OF SYMBOLS °

Degree

„

Minute

ˮ

Second

%

Percentage

≤

Less than and equals to

>

Greater than

∑

Summation

°C

Degree centigrade

avg

Average

Cs

Coefficient of skew

exp

Exponential

Km

Kilometer

ln

Logarithmic base exponential

log

Logarithmic base 10

lbd

Left bank bottom

lbt

Left bank top

lr1

Left road edge1

lr2

Left road edge2

lwd

Left water depth

lwl

Left water level

xii

M

Meter

M3/s

Cubic meter per second

Mm

Millimeter

rbd

Right bank bottom

rbt

Right bank top

rr1

Right road edge1

rr2

Right road edge2

rwd

Right water depth

rwl

Right water level

s

Second

Sq. Km.

Square Kilometer

σ

Population Standard Deviation

SN

Sample Standard Deviation of sample size N

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LIST OF ACRONYMS 2D

Two Dimensions

3D

Three Dimensions

ASCII

American Standard Code for Information Interchange

DHM

Department of Hydrology and Meteorology

DTM

Digital Terrain Model

DWIDP

Department of Water Induced Disaster Prevention

EU

European Union

ESRI

Environmental Systems Research Institute

GE

Google Earth

GIS

Geographic Information System

GRASS

Geographic Resources Analysis Support System

HEC

Hydrologic Engineering Center

HMS

Hydraulic Modeling System

ICIMOD

International Centre for Integrated Mountain Development

ILWIS

Integrated Land and Water Information System

LN

Log Normal

LP III

Log Pearson TYPE-III

NP

Nagar Palika (Municipality)

RAS

River Analysis System

RC

River Center

TIN

Triangulated Irregular Network xiv

UNDRO

United Nations Disaster Relief Organization

USACE

United States Army Corps of Engineers

VDC

Village Development Committee

WECS

Water and Energy Commission Secretariat

WGS84

World Geodetic System 1984

XML

Extensible Markup Language

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CHAPTER 1 INTRODUCTION 1.1 Project Background Disaster is a sudden and accidental event that causes many deaths and injuries. According to the Disaster Review, 2009 of Department of Water Induced Disaster Prevention, the total human casualty due to floods and landslide disasters from year 1983 to 2009 was 7,569 and the average death per year is 290 which are in increasing order. Similarly, the estimated total loss of property by these types of disaster in the year 2009 is about 34.248 million. Hence, a flood is major water induced disaster in Nepal which occurs repeatedly and cause tremendous losses in terms of property and lives, particularly in the lowland areas of the country.

River flood has been defined as "a general and temporary condition of partial or complete inundation of normally dry land areas from the usual and rapid runoff of surface waters from rainfall" (UNDRO, 1991). Flood vulnerability represents the sensitivity of land use to the flood phenomenon. Flood hazard depends only on the flow regime of the river, independent of the land use of the flood plain. A flood risk level requires a certain level of hazard, and for the same location, a certain level of vulnerability (Gilard & Givone, 1996).

Estimation of the extent to which the water induced disaster would cause problem in the given geographic areas can be done with the use of technology such as Geographic Information System and Remote Sensing. The proper utilization of these techniques has been done as far as possible through the accomplishment of a project work entitled “Flood Risk Mapping of Upper Bagmati Basin” in Kathmandu Valley by the study team of four members as a part of the Final Independent Project GEOM-410in the time period June-August. Regarding to the serious damages due to flood, in relevance to the previous records; the risk mapping of a specific area has been performed within the available time.

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1.2 Rationale of the Project The challenges that Upper Bagmati River confronts are rapid urban expansion, industrialization, uncontrolled migration from remote villages, unregulated land use and development, increased population pressure, expansion of the squatter settlements along the river banks etc. Due to these human induced factors, this river has lost its original appeal, good looks and life. The river at present has become a place for disposing off the wastes. The disposed solid wastes such as garbage can even obstruct the natural flow of water. Therefore, it can be considered as a major cause for the flood hazard consequence.

The Bagmati River that flows through the central Nepal has a religious importance. The river is worshipped by millions of Hindus in Nepal and India. Also the sand mining works along the river corridor and surrounding areas has affected the natural path of river by altering the river channels and diverting the flow. Though there seems to be nonexistent of the flooding problem during the low flow period in pre-monsoon season, the flooding situation in the monsoon period has threatened the lives and properties. Therefore, it was felt a necessity to carry out a detailed study and prepare the flood risk and vulnerability map of the Bagmati River.

1.3 Objectives The primary objective of the project is to prepare a Flood Risk Map of the Upper Bagmati River, flooding from Gokarneswor (Start point) to Sankhamul (End point) of Kathmandu using one dimensional steady flow model HEC-RAS, ArcGIS and HEC-GeoRAS (an extension to inference between HEC-RAS and ArcGIS).

Followings are the secondary objectives of the project:  Assess the Flood Frequency of the project site.  Preparation of flood plain map using one dimensional steady flow model for various return periods.  Preparation of Hazard and Vulnerability map.

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1.4 Limitations The limitations during the project duration are enlisted as follow:  The centerline of the river could not be surveyed at some of the places because of flooding problem during the field survey.  Although TIN was created from the scanned topographic map through the digitization of contour line of Department of Survey and Spot Heights collected from field, which may not represent actual river topography.  The HEC-RAS model only assumes fixed boundary of the river geometry and one dimensional steady flow pattern for this project. The assumption of one-dimensional flow may not be always a valid assumption.  Due to the unavailability of current satellite imagery, landuse map was extracted from Google Earth.

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CHAPTER 2 LITERATURE REVIEW 2.1 River Flooding The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water (EuropeanUnion, 2007).

Problems related to flooding and vulnerability of the population have greatly increased in recent decades due to several factors including changes in land-use in the hinterlands, urbanization of flood-prone sites, squatter settlements and sub-standard constructions, and increased household density (Munich, 2002). The effects due to flood are associated with a number of characteristics of the flood as for example depth of the river, decrease in gradient, deposition of bed load, changes in river course, and frequency of occurrence . 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, A Brief View of the Features of Nepal, 2010). The proper planning and flood risk management is therefore necessary. For prevention, mitigation, preparedness, response and recovery from the flooding problem, modeling of the flood is inevitable. The model can be simulated to analyze the future consequences of the flood. It helps in the planning and mitigation process.

Many methods have been used around the world in order to predict the flood in the river .Among them few are as follows:  ArcGIS / Arc View + HEC-RAS method  ArcGIS / Arc View + HEC-RAS + HEC HMS method  ArcGIS / Arc View + HEC HMS +MIIKE11 method Among the above mentioned methods, „ArcGIS / Arc View + HEC-RAS method‟ was used for this project. This method has been used successfully worldwide for flood modeling and analysis. Various studies have been made in Nepal using the ArcGIS / Arc View + HEC-RAS method.  Awal et. al. (2003, 2005 and 2007) used hydraulic model and GIS for floodplain analysis and risk mapping of Lakhandei River.

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 The Department of Hydrology and Meteorology (1998) prepared a preliminary flood risk of the Tinau Khola, downstream of Butwal and the Lakhandehi Khola.  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 settlement area indicating the damages to the human lives.  Mapping and Assessing Hazard in the Ratu watershed was done by Ghimire, Shrestha, & Khanal( 2007).  GIS was applied for flood risk zoning in the Khando Khola in eastern Terai of Nepal by Sharma, Adhikari, & Chapagain(2003).  DWIDP conducted “The Preparation of Flood Risk and Vulnerability Map of the Kathmandu Valley” in joint venture of Full Bright Consultancy (Pvt.) Ltd., GEO Consult (P.) Ltd. (2009).  The flood hazard mapping of Bagmati River in Kathmandu Valley was done by Dilip K. Gautam of Department of Hydrology and Meteorology (Gautam & Kharbuja, 2006).

2.2 Flood Frequency Analysis We can extrapolate the future possibilities floods using the past record of flood events. The estimation of the frequencies of flood is essential for the quantitative assessment of the flood problem. After a detailed study of the gauge data and its descriptive parameters such as mean and standard deviation, etc. and applying probability theory, one can reasonably predict the probability of occurrence of any major flood events in terms of discharge or water level for a specified return period (Singh, 2004). 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 (Manandhar, 2010).

There is no exact method which can compute the true value of discharge or maximum rainfall which is responsible for the flooding. Various methods available are either based on probability or empirical. Gumbel‟s, Log Normal and Log Pearson III type, etc. are the methods which are based on probability theory and Creager's formula, WECS/DHM Method, Modified Dickens‟s Method, B.D. Richard‟s Method, Snyder‟s Method, etc are also used for determining discharge for un-gauged basin based on empirical method. In WECS/DHM Method, the most significant independent variable is the area of the basin below 3000m elevation.

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2.2.1 The Gumbel’s Method The Gumbel‟s method (Gumbel Distribution) is the most widely used probability distribution function for extreme values in hydrologic and meteorological studies for prediction of flood peaks and maximum rainfalls (Patra, 2008). In this method the variate X (maximum rainfall or flood peak discharge) with a recurrence interval T is given by: XT = Xavg + K Sn-1……………………………………………. (i) where, XT

=maximum rainfall or flood peak discharge

Xavg

=average value of X

Sn-1

= Standard deviation of sample size n = √((X-Xavg)2/(n-1))

K

= frequency factor expressed as K = (YT–Yn)/Sn

YT

= reduced variate, a function of T and is given by YT= -ln.ln [T/(T-1)]

Yn

= reduced mean, a function of sample size n

n

= Sample Size, and

Sn

= reduced standard deviation, a function of sample size n.

2.2.2 The Log Pearson Type III Method In this method the variate is first transformed into logarithmic form (base 10) and the transformed data is then analyzed (Patra, 2008). If X is the variate of a random hydrologic or meteorological series, then the series of Z variate where,

Z = log X, are first obtained for this z

series, for any recurrence interval T. ZT = Zavg + KzSz………………………………………………. (ii) 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) , and

Corresponding value of X = antilog (ZT) 6

2.2.3 The Log Normal Method The method is basically same as the Log Pearson Type III method except the skewness coefficient (Cs) is taken as zero. The log normal distribution plots as a straight line on logarithmic probability paper (Patra, 2008).

2.2.4 WECS/DHM Method It is the most widely used method in Nepal for the analysis of flood frequency when the criteria for elevation is below 3000 meters and catchment area greater than 100 Sq.km (Adhikari & Sharma, 2004).If Qf is the flood discharge in m3/s, then the formula for flood discharge calculation is:

Qf = exp (ln Q2 + Sσ1).................................................................... (iii)

where, σ1

=ln(Q100/Q2)/2.326

S

= standard normal variate

Q2

= 2.29(A