Flood Hazard Modeling and Flood Risk Zoning for a River Basin
RAKESH KUMAR
National Institute of Hydrology, Roorkee-247667, Uttarakhand, India
rk18p@yahoo.co.in and rakesh@nih.ernet.in
Abstract In recent years the number and scale of water related disasters, flood in particular
has been increasing. The losses from floods often offset years of hard-won social andeconomic development. The problem is further expected to be aggravated with thephenomena of climate change. Therefore, mitigation and managing of flood hazards has
become a priority for alleviating poverty, ensuring socio economic progress, preserving our
eco-systems and ensuring the gains of development. The paper presents procedure for flood
hazard modeling and flood risk zoning for a river basin. Floods of various return periods havebeen estimated using the L-moments approach. Rating curves have been developed
employing the Artificial Neural Network and the least squares techniques. Depth of flooding,inundation for various return periods and risk associated with the flooding have been
simulated using the HEC-RAS package. The flood inundation simulated by the HEC-RAS
has been compared with the flood inundation mapped using the satellite data.
Key Words Flood hazard, Hydrologic Modeling, L-moments, Flood risk zoning.
INTRODUCTION
Flood is the most frequent natural disaster claiming loss of life and property compared to any
other natural disaster. About one-third of all losses due to nature’s fury are attributed tofloods. On an average floods claim a loss of more than 50 billion US dollars per year and
40000 victims per year in the last decades of the twentieth century in the world (Berga,
2000). In India also, floods are the most frequently faced natural disasters. As reported byCentral Water Commission (CWC) under Ministry of Water Resources, Government ofIndia, the annual average area affected by floods is 7.563 million ha. This observation isbased on the data for the period 1953 to 2000 published in Indian Water Resources Society(IWRS, 2001) with variability ranging from 1.26 million ha in 1965 to 17.5 million ha in
1978. On an average floods have affected about 33 million persons during 1953 to 2000 andaverage annual damage due to floods is about IRs. 46 billion. There is every possibility that
this figure may increase in future due to rapid growth of population and increased
encroachments of the flood plains for habitation, cultivation and other activities (Kumar etal., 2005).
Some of the important policies on flood management have been described by Kumar etal. (2005). Various types of structural as well as non-structural measures have been envisaged
to reduce the damages in the flood plains in India. Construction of embankments, levees,
spurs, etc. have been implemented in some of the states. The total length of constructedembankments is 16800 km and drainage channels are of 32500 km. A total of 1040 towns
and 4760 villages are currently protected against flood. Barring occasional breaches inembankments, these have provided reasonable protection to an area of about 15.07 M ha. A
large number of reservoirs have been constructed and these reservoirs have resulted in
reduction of intensity of floods. The non-structural measures such as flood forecasting andwarning are also being adopted. The flood forecasting and flood warning in India commenced
in 1958, for the Yamuna river in Delhi. It has evolved to cover most of the flood proneinterstate river basins in India. The Central Water Commission has established a flood
forecasting network for 70 rivers basins covering 18 States/Union Territories. The forecasts

are issued at about 175 stations. Out of these 145 stations are for river stage forecasts and 28
for inflow forecasts to the reservoirs. A Working Group of National Natural Resources
Management System (NNRMS, 2002) standing committee on water resources for flood riskzoning of major flood prone rivers considering remote sensing input was constituted by the
Ministry of Water Resources in 1999. The working group recommended flood risk zoning
using satellite based remote sensing with a view to give thrust towards implementation offlood plain zoning measures. There is a need for taking more effective structural and nonstructural measures of flood management and flood damage reduction based on long term
reliable data, advance analyses and modelling procedures and antecedent rainfall forecasting
using information based on radar, satellite based instrumentation and high resolution
Numerical Weather Prediction (NWP) models. It also necessitates capacity building for
implementation of these measures and bridging the gaps between the developed advance androbust procedures and their field applications.
Definitions of Terms
The definitions of terms (i) flood inundation map, (ii) flood hazard map, (iii) flood risk zone
map and (iv) flood plain zoning map used in the study are described below.
Flood inundation map
A flood inundation map provides information about the areal extent of inundation for a reachof a river during a flood event when the flood water in the river overtops its banks and leads
to the flooding of adjoining areas or flood plains. The flood inundation map for a reach ofriver may be prepared by demarcation with physical inputs i.e. by demarcating the various
locations of the flood plains which get inundated during a particular flood, by
hydraulic/hydrologic modelling and using the satellite data.
Flood hazard map
A flood hazard map provides information about the return period associated with the areal
extent of inundation for a reach of a river. The flood hazard maps are prepared delineatingareas subjected to inundation by floods of various magnitudes and frequencies. These maps
may serve as important tools in proper flood plain management. It is necessary that the area
flooded by a particular flood is shown by suitable colour scheme or designs on the map. In
addition, explanatory notes, tables and graphs may also be provided with flood hazard maps
to facilitate their use.
Flood risk zone map
A flood risk zone map provides information about the risk associated with the damages
caused or losses resulting from a flood event in a particular area or flood risk zone.
Preparation of flood risk zone map incorporates the financial aspects and provides theactuarial inputs for flood insurance plans and for other purposes.
Flood plain zoning map
A flood plain zoning map categorizes various zones based on administrative legislations forplanning and development of the flood plains for various purposes such as agriculturalactivities, play fields, industrial areas and residential areas etc. Preparation of flood plain
zoning maps takes into consideration the inputs from flood inundation, flood hazard andflood risk zone maps.

Methodology
The methodology for flood hazard modeling and flood risk zoning for a reach of a river basin
is briefly described as follows. The objective of the study are: (i) to develop L-moments
based flood frequency relationships using the annual maximum peak flood series and thepartial duration series for the gauging sites of the river reach, (ii) to develop rating curves forthe gauging sites of the river reach, (iii) to prepare flood inundation maps for the river reach,(iv) to prepare flood hazard maps for the river reach, (v) to prepare flood risk zone maps forthe river reach and (vi) to predict flood inundation and flood hazard areas for various stagesof river flow.
In the study, risk is defined as the probability of occurrence of a flood at least once
during successive years of design life. The return period for which a structure should bedesigned is computed based on the risk acceptable. Risk acceptable depends upon economic
and policy considerations. If for a time invariant hydrologic system the probability ofoccurrence of an event, x, greater than the design event, xo, during a period of n years is P,
then the probability of non-occurrence, Q is 1-P. The probability that x will occur at least
once in the n years i.e. the risk of failure, R is: R = 1-(1-1/T)n ; where, R is the risk, T is thereturn period for which the structure should be designed, and n is the design life of thestructure (Robson & Read, 1999). Using the above formula, the return period T can be
determined for a given R (acceptable risk) during a period of time in years (n). The floodcorresponding to this T is estimated and routed through the river reach for estimation of thewater surface profile and inundation mapping. The expression of risk mentioned above may
be extended to incorporate other variables related to losses due to floods, which can be
defined as functions of return period (T). The flow chart illustrating the general terminologyof flood inundation mapping, flood hazard mapping, flood risk zone mapping and flood plain
zoning is shown in Fig. 1.
Analysis and results
In the study, flood frequency analysis has been carried out using the L-moments approach(Hosking & Wallis, 1997; Kumar & Chatterjee, 2005; Kumar et al., 2003). For identifying
the robust frequency distribution for the respective gauging sites, L-moment ratio diagramand Zdist statistic criteria have been adopted. Table 1 gives the Zdist statistic values for sevensites of the study area. The values of the parameters of the robust identified distributions forthe stream flow gauging sites are given in Table 2. Floods of various return periods viz. 2, 10,
20, 25, 50, 100, 200, 500 and 1000 return periods have been estimated for the seven stream
flow gauging sites of study area, using the robust identified frequency distribution for each of
the stream flow gauging sites. The Digital Elevation Model (DEM) of the study area has been
prepared employing the GIS package Arc View 3.2 (Fig. 2). Rating curves have beendeveloped using the least squares approach and the Artificial Neural (ANN) technique(Kumar et al., 2004). The mosaic of satellite data for the study area covered in three satellite
scenes of IRS 1C and 1D has been prepared and flooding for some of the previous years
simulated by the hydraulic modelling have been compared with the spread of flooding overthe flood plain estimated from the analyses of the remote sensing data using the GIS package
ERDAS. Hydraulic modeling of the river reach has been carried out using the HEC-GeoRas
3.1 and HEC-RAS 3.1.2 (Hydraulic Engineering Centre-River Analysis System) packages
developed by the U.S. Army Corps of Engineers. Fig 3 shows flood plain area inundated anddepth of flooding for 1000 year return period flood for the study area. Fig 4 illustrates flood
plain area inundated for various return periods for the study area. Flood risk zone maps havebeen prepared for various risk levels and for different durations of flooding. Fig 5 shows
flood risk zone map for a risk (R) of 25% over a period (n) of 25 years.

Fig. 1 Flow chart illustrating the general terminology of floodinundation mapping flood hazard mapping, flood riskzone mapping and flood plain zoning
dist
Table 1 Z i -statistic for various frequency distributions
Sl. No.
12345Distribution
GLO
GEV
GNO
PE(3)
GPA

Site-1-0.04--0.66-0.50-0.50--1.77Site-2
0.37
-0.01
-0.08
-0.24
-0.88

Site-3
0.35
-0.62
-0.26
-0.33
-2.19

Site-4
1.91
1.51
1.38
1.12
0.57

Site-5
0.72
-0.26
0.03
0.02
-1.97

Site-6
0.10
-0.12
-0.33
-0.69
-0.76

Site-7
1.03
0.42
0.36
0.17
0.90

Table 2 Values of parameters for various distributions for seven gauging sites
Site
Site-1Site-2Site-3Site-4Site-5Site-6Site-7

Distribution
GEV
GLO
GNO
GPA
GLO
PE-IIIPE-III

Parameters of the Distribution
k = -0.025
ξ = 0.856
α = 0.238
k = -0.002
ξ = 0.999
α = 0.174
k= 0.208
ξ = 1.022
α = 0.210k= 0.269
ξ = 0.230
α = 0.978k = -0.316
ξ = 0.905
α = 0.162µ = 1.000
σ = 0.339
γ = -0.166µ = 1.000
σ = 0.433
γ = 1.010

Fig. 2 Digital Elevation Model (DEM) of the Study Area

Fig. 3 Flood plain area inundated and depth of flooding for 1000 year return period floodfor the study area

Inundation areas for floods of various return periods

8000

Area (Sq. Km)

7000
6000
5000
4000
3000
2000
1000
0
2

10

25

50

100

200

500 1000

Return Period (year)

Fig. 4 Flood plain area inundated for various return periods for the study area

Fig. 5 Flood risk zone map for a risk (R) of 25% over a period (n) of 25 years

CONCLUSIONSIn this study flood inundation maps, flood hazard maps and flood risk zone maps have beendeveloped for a river reach of a basin. The flooded areas for some of the years for the study
area simulated by the HEC-RAS package have been compared with the flooding mappedusing the satellite data employing the GIS package ERDAS. These maps may be used forland use planning, flood insurance purposes and flood damage reduction. The calibrated andvalidated hydrologic model, as described in the study may be coupled with a distributed
rainfall-runoff model using the antecedent rainfall forecasts based on radar, satellite based
instrumentation and high resolution Numerical Weather Prediction (NWP) models and may
be used for simulation of flood inundation, depth of flooding and risk associated with theflooding in real time for flood mitigation and management. Presently, there are many
uncertainties in forecasting heavy rainfall and the uncertainty should be minimised,
quantified and presented as an integral part of the forecast. It would help in providing
improved flood hazard warning and lead to better flood management and flood damage
reduction.
REFERENCE
Berga, L. (2000) Dams and Floods. Paris: ICOLD, http:// www.icold-cigb.net
Hosking, J. R. M. & Wallis, J. R. (1997) Regional frequency analysis-an approach based on Lmoments. Cambridge University Press, New York.
IWRS (2001) Theme paper on Management of floods and droughts, Indian Water Resources
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Robson, A. & Read, D. (1999) Flood Estimation Handbook: 3 Statistical procedures for flood
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