Lasbela. Uni. J. Sci., Tech., vol.3, pp. 45-52, 2014
ISSN 2306-8256
RESEARCH ARTICLE
Flood Inundation Mapping and Risk Zoning of the Swat River
Pakistan using HEC-RAS Model
Muhammad Irfan Malik*1 and Fahad Ahmad2
1. Faculty of Water Resources Management, Lasbela University of Agriculture, Water and Marine
Sciences, Uthal, Balochistan. Pakistan.
2. Department of IT, Lasbela University of Agriculture, Water and Marine Sciences, Uthal,
Balochistan. Pakistan.
Abstract:-In Swat river basin floods are very common and a constant natural threat to infrastructure and
humans. The 2010 flood event inundated and caused the highest massive losses ever recorded in the history of Swat.
Today hydrological modeling is used as an efficient tool to prevent and mitigate the damages from high flood events.
The purpose of this study was to conduct and perform hydrological modeling to determine areas inundated during
heavy rainfall events in the Swat river basin. The methodology involved: collection of hydro climatic data and Digital
Elevation Model (DEM) of the basin. The discharge values were used along with the DEM, to predict flood hazard
areas in the Swat river basin. This procedure was made using the HEC-GeoRAS in combination with HEC-RAS model
(Hydrological Engineering Center-River Analysis System). The flood zones for floods with return periods of 5, 10 and
100 years were calculated. Final results show the exact location of areas with high, moderate and low risk to be flooded
at specific high flood events. It was found that the combination of GIS with the HEC-RAS model was very powerful
and efficient approach in flood zone analysis and can also provide the location of high risk areas, so that an early
warning system can easily be located. From this study the suitable information was provided to inhabitants of the area
who are at-risk and how to prevent and mitigate the effect of flood-related damages.
Keywords: Swat, flood, HEC-RAS, modeling, disaster management.
scientific fields such as hydrodynamics. Flood
zoning maps, can be a fit and lawful tool for
determination of development and planning of
emergency strategies. By understanding the
extent of flooding and floodwater inundation,
decision makers are able to make choices about
how to best allocate resources to prepare for
emergencies. The Hydrologic Engineering
Center’s River Analysis System (HEC-RAS) is a
software package that is well-suited for
developing flood inundation maps for a variety of
applications.
The key benefit of using GIS for flood
analyses is that it not only generates a
visualization of flooding, but also creates
potential to further analyze these events to
estimate probable damage due to floods. Mostly,
studies have applied hydraulic and hydrological
models for simulating flood runoff and runoff in
low-lying flood-prone areas, in order to provide
flood risk assessment information on the
probability of flood occurrence, magnitude of the
event, location and depth of the inundation for
flood management by Booij (2005). Goel et al.,
INTRODUCTION
Flooding
is
hazardous
natural
phenomenon happening worldwide; often cause a
lot of damages on the earth’s surface including
human lives and infrastructure since time
immemorial. Floods generate rapidly, triggered
by heavy rainfall and due to accumulation and
release of runoff water from upstream to
downstream. Flood risks have also increased due
to changing climate conditions and human
interventions like social and economic
developments in the river watershed systems
(Bronstert 2003).
Hydro-meteorological
catastrophes
cannot be totally evaded, but the impacts and
after effects can be managed by developing
effective risk reduction strategies through
application of latest geospatial tools and decision
support systems (Sadiq et al., 2014). A
geographic information system (GIS) is an
appropriate tool that can be used in the different
*Corresponding author: [email protected]
41
2005 presented the technique for preparation of
flood hazard maps which include development of
DEM (digital elevation model) and simulation of
flood flows of different return periods. HEC-RAS
and GIS technologies are integrated to obtain
scientifically derived information that has been
quantified as effective in simulating, identifying
and analyzing flood events in a geo spatial
environment by Shamsi (2002). This helps in
visualizing flood simulations, and can view the
spatial impact of various scenarios along with the
critical locations to assess the vulnerability of the
area towards a flood event efficiently. Bashir et
al. (2010) used the hydraulic model HEC-RAS to
calculate the flood zones of Nullah Lai in
Rawalpindi. According to Orok 2011, that flood
risk maps should be able to identify the areas that
are most vulnerable to flooding and estimate the
number of people that will be affected by floods
in a particular area. Preparation of flood maps
provides valuable information for managers and
experts to reduce flood damages (Hassanpour et
al., 2012).
Implementing similar approaches for
flood management and monitoring system can
possibly help in mitigating flood-induced
hazards. Today state-of-the-art flood forecasting
and warning systems have made a significant
impact to reduce the losses. By using these
advance technologies we can better design for
flood mitigation measures, forecast earlier and
issue possible warnings to the peoples living in
low lying areas which will be affected. Robust
techniques that integrate both in-situ and
remotely sensed data can help in improving the
flood monitoring system in the country.
After 2010 devastated flood in Pakistan,
government proposed several structural measures
which include construction of dams, retaining
ponds, dikes, embankments, flood walls and
buildup of some pond areas. But non-structural
measures like preparation of Flood hazard and
Risk maps were not included in their programs.
Therefor the aim of this research is to determine
the extent and depth of flood and producing flood
zoning maps by using HEC-RAS model to check
flooding risks along the Swat River at different
return periods. This research has been done in
downstream of Swat River at Munda
Headwork’s. Discharges with different return
periods and floodplain characteristics (situation
of bed, river banks, etc.) are introduced into the
HEC-RAS
model
and
extracted
flow
characteristics were analyzed using ArcView to
determine flooding zones. The accuracy and
precision of these flooding maps are evaluated by
comparison with information and local
investigations of 2010 flood. This paper presents
a case study while addressing the steps taken to
construct an HEC-RAS model and to resolve the
output into flood inundation maps.
Purpose and Need
The frequency and intensity of floods in
the last few decades have increased in the Swat
River basin. During the monsoon season,
precipitation is an important meteorological
factor because the excessive rainfall is likely to
coincide with the peak discharge in the river from
upper basin, which increases the probability of
floods. In Swat floods are mostly generated due
to intense rainfall, melting of snow or glaciers
and some other intensifying factors (Sultan-iRome, 2005). The Swat River has an average
annual flow of 7.047 million acre-feet at the
Munda Headworks in Malakand (WAPDA,
1970). During peak flood season river Swat
carries huge load of sediments.
The highest recorded floods produced in
the Swat River are those of 9th August, 1992 and
25th July, 1995 and maximum discharge has been
passed through river Swat stream gauges. But
2010 flood was the most catastrophic event that
the Swat valley had ever experienced in its
history. The situation got worse as the heavy
shower continued for two days in Swat. The
overall precipitation recorded during 28th July to
3rd August 2010 was 390 mm whereas the annual
rainfall in Khyber Pakhtunkhwa is 962 mm
(PMD, 2010), from this size of the catastrophe
can be visualized. This rainfall resulted in the
peak water discharge in the river.
According to Federal Flood Commission
(FFC) report the 2010 flood surpassed the records
of past flood events severely damaging the
Amandara Headwork and destroying the Munda
headwork, both major irrigation structures on
River Swat. About 5876 residential houses were
smashed due to flooding out of total 18,359 ; this
indicates that 21% of households were affected in
Swat valley only (Save the Children, 2010), as
shown in Fig. 1. The business establishments in
42
the area were also destroyed and about 26
connecting bridges between villages were
completely ruined; the extent of damage was
severe as serious disruptions occurred to
transportation and communication network that
blocked the valley for couple of days. Mostly the
loss of life was due to an inundation of flood
water on low-lying areas and a lack of
communication between the places where people
took shelter immediately after the flood hit. Now
there is an urgent need to address flood
preparedness and mitigation along with natural
resources management of Swat River Basin.
Basin is located in the northern part of Pakistan
between 34˚ 00´ to 35˚ 55´ North and 71˚ 10´ to
72˚ 50´ East and has a catchment area of about
13,650 km2. The valley is surrounded by
mountains and forests from all sides excluding
the south-west, which is an outlet of the river
Swat. The area is mountainous with elevation
ranges from 500m to 6000m above sea level.
Climatically, in Swat summer is short
and mild whereas winter is long and cool.
Maximum temperature recorded is 33˚C in the
month of June. The annual rainfall fluctuates
from 700 mm to 1,630 mm. The valley is
susceptible to flooding from the Swat River and
its important tributary Panjkora River which
drains the Dir Area and a number of small
streams that flow through the valley (Fig. 2). It
has high potential for tourism and other natural
resources. Poor management of land and water
resources is significant, and the area is highly
vulnerable to the impacts of natural disasters.
Damages to House
Hold Infrastructures
Completely Damage
Partially Damage
No damages
0% 20% 40% 60% 80%100%
Number of Houses damaged
Fig. 1. 2010 Flood damages to housing and infrastructure in
Swat valley (Source: Save the Children. 2010)
The destructive consequences of 2010
flood reveal the necessity of conducting flood
risk assessment in the Swat valley so that the
vulnerability of the region to a higher magnitude
flood event can be well represented in a spatial
format. This procedure was made using the HECRAS model (Hydrological Engineering CenterRiver Analysis System). The information
produced by analyzing the potential losses that
might occur due to such inundation can play an
vital role in the decision making process of the
planning authorities by assisting them in making
informed
decisions
while
promoting
developmental activities in the Swat valley.
Fig. 2. Location of Swat river basin
MATERIALS AND METHODS
The methodology started with the data
collection process. Primarily the data is collected
from the secondary sources. The study requires
spatial and hydrological data, channel geometry,
boundary conditions and channel resistance are
required for conducting flow simulation through
HEC-RAS. The Survey of Pakistan has provided
the topographic maps and the land use map of the
reach from National Agricultural Research Center
The Study Area
The valley of Swat with a population of
1,257,602 (1998 Census), located in Khyber
Pakhtunkhawa Province (KPK) is chosen as the
study region. Geographically, the Swat River
43
(NARC) for present study. The statistics of
discharge has been taken from Water and Power
Development Authority (WAPDA) and climatic
data from Pakistan Meteorological Department
(PMD). Total 19 lateral cross-sections at 1000 ft.
of intervals at various important locations are
mapped along 21 km river length were used in
this study.
In order to show the surfaces, both digital
formats of Digital Elevation Model (DEM) and
Triangular Irregular Network (TIN) was used.
The Digital Elevation Model (DEM) for the River
Swat area derived from ASTER data having 30 m
spatial resolution (Fig. 3) is used as an input for
terrain processing.
using computerized procedures. The processing
of DEM to delineate watersheds is referred as
terrain processing. River flow direction was also
determined to further use it as a model input
variable. (Fig. 4).
HEC-GeoRAS uses the functions
associated with Spatial Analyst and 3D Analyst
extensions of ArcGIS.
Fig. 4. Watershed delineation of Swat River basin
from the Digital Elevation Model (DEM)
It is specially designed to process the
geospatial data. It is used to create a HEC-RAS
import file containing geometric attribute data
from an existing digital elevation model (DEM)
and complimentary data sets. The geometric data
developed in HEC-GeoRAS includes; stream
center line, reaches (tributaries), cross sectional
lines, cross sectional surface lines, cross sectional
bank stations, downstream reach lengths, main
channel, right over bank, left over bank (Fig. 5).
Upon successful creation of these layers, the
geometric data was exported to HEC-RAS as an
import file for simulating the flood event. The
single manning’s value “n” used in this study was
n = 0.6.
ArcGIS
was
used
to
generate
Triangulated Irregular Network (TIN) required in
GeoRAS environment in order to prepare data
sets required as input data to the HEC-RAS
Fig. 3. DEM for the entire Swat river basin
For hydrologic model development, the
River Swat watershed was delineated through
Digital Elevation Model (DEM). ArcView 3.2
with its extension HEC-GeoHMS was used for
terrain analysis uses surface topography to
generate the stream network. Now with the
availability of DEMs and GIS tools, the
properties of watershed can easily be extracted by
44
simulation. The created themes and TIN were
joined together and made a geometric data file
and then exported and use as an input for HECRAS for further process.
For steady flow analysis simulating
different water surface profiles can be conducted
by using HEC-RAS model. This model could be
used to perform one -dimensional steady flow
utmost significance. HEC-RAS model has been
simulated for different possible discharge values
for various return periods, to mark inundated
areas. Model has been simulated for the probable
water discharge values for 5, 10 and 100 years
return period. DEM was used as the initial terrain
input in hydraulic modeling with HEC-RAS.
Table I. Peak Discharge at various Return Periods at
Chakdara Gauging Station
Return Period
Probable Peak
(Year)
Discharge (m3/sec)
5
300
10
100
370
1510
For this study, HEC-RAS model was
simulated for different peak discharge values as
shown in (Table I) and inundation prone areas
were identified. Map of the inundated areas for
the different discharge cases are given below in
(Fig. 6).
Fig. 5. Mapping cross sections at the river channel,
Swat River basin.
computations. Information attained from channel
geometry and discharges values were used to
create HEC-RAS channel flows. Through HECRAS modeling water surface profiles were
develop from corresponding discharge values.
The HEC-RAS model calculates water surface
profile at all locations of interest.
After a successful simulation of river
hydraulic results in HEC-RAS, these were then
exported to HEC-GeoRAS to generate inundation
extent. This information allowed us to visualize
(spatially) the areas which are at high risk of
inundation.
RESULTS AND DISCUSSION
After the computations performed by
HEC-RAS software, river flood modeling results
are shown in graphical and tabular formats. These
results include longitudinal profile, cross
sections, flood zoning maps and inundated areas
using GIS.
As there is no measured discharge data
and inundation depth available for any other point
in the study region. Therefore, the results
produced by integrating HEC-RAS and GIS for
such a higher magnitude flood event attains
Fig. 6. Flood inundation areas of different return
periods
The result in (Fig. 6) shows the similarity
in the inundation area of 5 and 10 year return
period because of smaller difference between the
calculated peak discharge values namely 300 m³/s
45
and 370 m³/s, respectively. The result generated
by HEC-RAS model has similar flood inundation
patterns and are close to the real situation in 2010
(Fig.7). As the results produced by the HEC-RAS
model are close to the observed data of 2010
flood, the model can be assumed as valid to
perform the hazard assessment analysis of the
study area.
The inundation map shows the flood extent at
peak flow of 100 year return period. The spatial
distribution of the flooded area was located at
areas with low relief and covered an area of about
Fig. 7. Relationship between 2010 and 100 years
flood event.
A flood of 100 year return period largely
affects the agricultural lands and communities
located in the study area. There are great numbers
of built-up areas and urban settlements those are
at-risk of inundation. The HEC-RAS model
forecast that an average 30.39 km2 area will be
inundated by these discharges, as given in (Table
II). It also shows how the high risk area is
flooded by the event.
Table II. Inundation extent for different probabilities of
occurrence
Return
Probable
Probable
Inundation
Period
Peak
Peak
Area
(Year)
Discharge
Discharge
(Km2 )
(m3/sec)
(ft3/sec)
5
300
10594
22.13
10
370
13066
23.56
100
1510
53325
45.48
Fig.8. Flood water depth simulated by HEC-RAS model for
5 years flood event
46
45.48 km2. The flooded area was also graphically
overlaid on the Google Earth. The outcome of the
overlay which has been shown in (Fig. 7) clearly
identified the affected settlements including both
the agricultural land and houses.
Flood depth map is an important output
of the model showing the vulnerability of the area
by indicating the water depths (levels). The
result shows inconsistency in the flood water
depths of the inundated area, which depends on
geometry, topography and the hydraulic
conditions of the river. Generally, water depth is
higher along the main channel and lower at the
floodplains. The depth of water can be calculated
by subtracting grid maps of water surface and
terrain. The flood depth or risk maps are given in
(Fig. 8-10).
CONCLUSION
The research study has demonstrated
integration of GIS technology with computer
based flood modeling technique to identify high
risk of flood inundation areas for disaster risk
management. The GIS helped in data updating,
visualization of different scenarios and risk
mapping. HEC-RAS model was calibrated to
assess flood inundation at different return periods
for the Swat river basin. The model predicted
flood inundation extents of about 45 sq km at
100-year return period. At 100 year return period,
the maximum increase in floodwater elevation
predicted in the Swat River is over 25m. Due to
increase in flooding frequency, population
residing near the river banks, agricultural land
and other valuable infrastructure like roads and
bridges are found at - high risk of flood
inundation. Hydraulic modeling using GIS
technique proved useful in simulating flood water
depth and inundation areas for various return
periods in high mountainous area of Swat River
basin.
Fig.9. Flood water depth simulated by HEC-RAS model for
10 years flood event
47
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198.
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ERNAL/COUNTRIES/SOUTHASIAEXT/PAKIS
TANEXTN/0,,contentMDK:22797764~menuPK:5
0003484~pagePK:2865066~piPK:2865079~theSit
ePK:293052,00.html.
Fig.10. Flood water depth simulated by HEC-RAS
model for 100 years flood event
(Received 14thAugust 2014, Accepted 3rd November 2014)
Article can be viewed online at www.luawms.edu.pk
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