FLOOD RISK AND VULNERABILITY MAPPING USNG GIS: A
NEPAL CASE STUDY
1
MANDIRA SINGH SHRESTHA1
International Centre for Integrated Mountain Development
Kathmandu, Nepal
RICHARD HEGGEN1, KB THAPA2, MOTI LAL GHIMIRE2 AND NARENDRA SHAKYA3
2
Tribhuvan University, Nepal
3Institute
of Engineering, Nepal
Nepal’s Ratu Khola watershed extends from elevation 800 m in the north to the IndoNepal border at elevation 61 m. Geomorphology is characterized by hill slopes and inner
river valleys, the piedmont and an upper and lower alluvial plain. Annual rainfall ranges
from 1035 to 1608 mm, 80% during monsoon. Nearly 65% of the basin is under
cultivation. The Ratu exceeds 800 m in width in the upper reaches, becoming less than 50
m near the border. The river deposits substantial sediment, resulting in aggradation,
shifting, meandering, widening and braiding. Annually many lives are lost and property
worth millions of dollars damaged. Landsat imagery at 30-m resolution and time series
aerial photographs were used to detect change in river morphology and to identify flood
plains. Photo interpretation and image analysis was field verified. Digital Elevation
Modeling was incorporated into HEC-RAS software for flood elevation estimates.
Primary data on past hazards, socio-economic condition, vulnerability, response
capability in the recent past was collected. Nearly 39% of households reside in hazard
prone area and another 22% have land and other properties in the hazard-prone area.
Roughly 20,000 houses, 70 schools and 100 other public buildings are exposed to water
induced disasters. Hazard and mitigation perception was surveyed from 30 households
from each hazard zone. Using GIS to combine the hydrologic analyses with the socioeconomic resources and constraints, subsequent planning will pursue mitigative strategies
for flood management.
BACKGROUND
Nepal with its fragile geology, steep slopes, high relief, and variable climates, is prone to
water induced disasters such as floods and landslides. Over the last twenty years from
1983-2002, floods and landslides caused 6,466 deaths and more than US $ 200 million in
damage [1]. In 1993 alone there were more than 1300 lives lost and over US$ 2million of
property and infrastructure destroyed by an individual event recording the highest 24-hr
precipitation of 540 mm.
Disaster mitigation efforts of the government are confined to rescue operation and
post-disaster recovery. In the absence of information about the nature of flood events,
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exposure of life and properties and capabilities to cope with disasters, it is difficult to
prepare and implement pre-disaster activities. Lack of information is likewise a major
constraint in implementing and coordinating the rescue and post-disaster management
activities effectively.
The large rivers in Nepal are snow-fed from the high Himalayas and are perennial.
The medium rivers originate from the middle mountains and are rain-fed. The small
rivers, originating in the southern slopes of the middle hills and the Siwaliks, have little
or no flow during the dry period. It is these small rivers that substantially contribute to
total flood damage and sediment deposit.
Though only 23% of the total area of the country lies in the Terai more than 47% of
the country’s population reside that region [2, 3]. Most of the area is covered by active
plains and alluvial fans and hence more than 40% of the total area is under cultivation.
Due to incessant rain in July 2002, floods in the Ratu Khola more than 1500 families
were displaced. Hundreds of hectares of paddy crop were adversely affected by floods
and the river also changed its course. The Ratu Khola is also called the sorrow of the
district. Such kind of damage is reported from the entire rivers originating from the
Siwaliks which impose heavy burden on the economy of the country. The necessity of
understanding the phenomenon of flooding of these “marginalized rivers” and to identify
and map vulnerable areas for proper management and mitigation of floods is becoming
essential to minimize the damages incurred annually by these rivers.
In this context a study on flood risk and vulnerability mapping using GIS with a case
study of the Ratu Khola basin was conducted by ICIMOD with the support from
UNESCO.
Study Area
The 532 km2 Ratu watershed extends from the Siwaliks at the elevation of 800 m in the
north to the Indo-Nepal border in the south at the elevation of 61 m with a total length of
82 km. The basin is located between 26o 37’ 43’’ to 27o 8’ 3’’ N and 85o 46’ 13’’ to 85o
58’ 47’’E and covers part of the Sindhuli District in the north and the Mahottari and
Dhanusa Districts in the south in the Central Development Region of Nepal. The basin
map is shown in Fig. 1. The basin’s maximum north-south and east-west aerial distances
are 58.46 and 13.14 km, respectively. Forty-eight Village Development Committees, the
smallest administrative units of the country are within this watershed.
In the Siwaliks hills, the climate is warm temperate; in the Terai, the climate is
subtropical monsoon. The basin’s average rainfall is 1400 mm. About 80% of the total
precipitation occurs between June and September. As the Ratu River is not gauged,
empirical methods have been used to compute discharge. The 2 and 100-yr floods near
the national border are 110 and 488 m3/s, respectively. The average slope of the river is
0.76%. In the upper catchment the slope is 1.65%, 0.89% in the fan zone and less than
0.2% in the alluvial fan in the lower catchment.
Geomorphology is characterized by hillslopes and inner river valleys with 2-3 tiers
of terraces, piedmonts and fans; and alluvial plain comprising marsh and spring areas and
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old and active flood plains. The upper
reach of the basin is Siwalik rocks.
The lower reach is composed of
brown sand stones and clays. The
depositional zone, of very low relief
and gentle slope, is the northern
extension of the Indo-Gangetic plain.
The alluvial plain consists of fine silt
and clay. As the water table is high,
rain inundation is common during
monsoon.
About 66% of the basin is under
cultivation and about 23% under
forest cover. Only 3.3 % of the area is
occupied by built up areas, orchards
and ponds. The 2001 population of
the basin was 310,994 in 53,323
households.
The average male literacy rate is
39% and female, 16%. These figures
are very low compared to the national
average of 60 and 48% respectively.
Agriculture, wage, trade, business
and service are major sources of
income.
Figure 1. Ratu Basin
Many families are poor. Nearly
16% of the family annual income is less
than US $280 and 38% between $280-560. Nearly 21% of the families are landless.
About 12% of families do not produce food grains sufficient to meet their own
requirement.
METHODOLOGY
Flood Risk assessment requires an understanding of the causes of a potential disaster
which includes both the natural hazard of a flood, and the vulnerability of the element at
risk. The terms hazard, vulnerability, and risk are defined as follows [4].
 Hazard (H) means the probability of occurrence, within the specified period of
time and within a given area, of a potentially-damaging phenomenon. This
definition indicates a given time period in which flooding is likely to occur.
Hazard in the present paper is simply defined as the probability of the
occurrence of a potentially-damaging phenomenon within a given time since
there is an absence of time series data of discharge due to lack of gauging
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station. Therefore, a high-hazard zone characterizes the high probability of the
occurrence of a damaging phenomenon.
 Vulnerability (V) means the degree or loss to a given element or a set of
elements at risk resulting from the occurrence of a natural phenomenon of a
given magnitude.
 Risk (R) can be defined as the expected degree of loss due to a particular natural
phenomenon.
Risk = (H) hazard * (V) vulnerability
Sources of Information
Both primary and secondary sources of information were used in this study. Secondary
sources include data published by the Department of Hydrology and Meteorology, His
Majesty’s Government (HMG), maps, aerial photographs and satellite images. Following
are the sources of spatial data used:
 Topographic maps (scale 1:25,000) compiled from 1:50,000 scale aerial
photography taken in 1992 and field verification done in 1995; published in
1996, Survey Department, HMG [5],
 Aerial photographs of, 1953-54, 1978/79 and 1992 scale 1:50,000, Survey
Department, HMG,
 Landsat imagery, Thematic Mapper, 30 m resolution (path 141 and row 41)
obtained in March 10, 1999 from the Department of Forestry, HMG.
Primary data on past hazards, socio-economic condition, vulnerability, response
capability and efforts made by local people for mitigation of flood and other water
induced disasters in the recent past were collected by field survey.
Methods and tools
Flood risk mapping has been conducted in three different stages; first the mapping of
flood prone areas, secondly the mapping of hazard and vulnerability and thirdly the
preparation of risk maps.
Flood prone areas were mapped by using three different techniques:
 HEC-RAS
The US Army Corps of Engineers Hydrologic Engineering Center River
Analysis System (HEC-RAS 3.1) was used to calculate water surface profiles.
ArcInfo and ArcView GIS 3.1 were used for the GIS data processing. HECGeoRAS 3.1 for ArcView is a GIS extension to process geospatial data [6][7].
 Topographical maps, aerial photographs and satellite images
Software used included ArcInfo, MapInfo 5.0, ArcView 3.1, and ILWIS 3.0.
ArcInfo and MapInfo were used to enter, edit, and geo-process the data. Data in
vector format required for flood hazard and risk assessment were converted into
raster format. ILWIS 3.0 was used to produce a Digital Elevation Model (DEM)
 Field inspection, group discussions and household survey
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ANALYSIS AND MAPPING
Flood Prone area Mapping
Inundation mapping using HEC-RAS model
Based on spot heights from 1:250,000 maps and digitized data, an integrated Digital
Terrain Model (DTM) was prepared. The spatial data required for a HEC-RAS import
file with a 3-D stream network and 3-D cross-section was GeoRAS pre-processed. River
cross-section geometries obtained from the HEC-RAS GIS were imported into HECRAS. With the Manning’s “n” coefficients, specified discharge and boundary conditions,
the model was run to estimate the flood levels of 2 to 100-yr return periods. Water
surface profiles were post processed using Geo-RAS to prepare the flood inundation
maps. The model was run for 50% and 100% of flow concentrated in a single main
channel. However it must be recalled that the river is a braided channel with tributaries
and hence this assumption is a major limitation of this analysis. The 100-yr inundation
map with 100% flow concentration is given in Fig. 2.
Geomorphological hazard mapping using GIS and Remote Sensing
Since the morphology of the Ratu Khola in the depositional zone is very dynamic, it is
questionable to reliably map the hazard zone for longer time scale simply relying on
Figure 2. Inundation Map for a 100-year
using HEC_RASevent
Figure 3. Flood Hazard Map using GIS
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some theoretical inferences. So the geomorphologic approach to hazard mapping based
on remote sensing and aided by GIS has been attempted here. For this, flood affected
areas, river morphology, old channels interpreted from time sequence aerial photos of
1953-54, 1978-79 and 1992 and the imageries of 1999 were considered as essential
parameters. In addition, the floodplains interactively interpreted from 1978-79 and 1992
aerial photos, and the areas of sheet flood and the damp and marsh areas interpreted from
the 1999 imageries were also incorporated in the hazard map. The part of the foot slope
characterizing the mouth of the gullies or debris fan has been considered as the sites of
the debris flow. Such sites were interpreted from 1978-79 and 1992 aerial photo aided by
3D DEM display.
Besides, potential sites of river bankcuts calculated from DEM analysis has also been
integrated in the hazard map as shown in Figure 3. For identifying potential sites of bank
cutting, slope map derived from DEM has been classified into two categories showing
major topographic break of depositional zone: one is the general slope of the depositional
zone, and the other indicating the cutbank sites or the local cliffs. A value of 2 percent
slope is considered as the threshold that defines the general slope. Cutbanks or local slope
breaks exceed 2 percent. The cutbanks are considered as those that are adjacent to the
riverbed. All the potential bankcutting sites areas fall within flood affected areas.
Similarly, an inundation hazard map has been calculated for the lower Terai where
inundation is a severe problem during prolonged monsoon rain. The calculation of
inundation map is based on the assumption of a 1m embankment at the Nepal – India
border. The inundation has been computed using the DEM by iterative processes of
neighborhood operation available in ILWIS 3.0 software.
The methodological scheme of flood hazard zonation is shown in Fig. 4. The
cumulative area of flood affected areas of 1978-79, 1992, and 1999 including the river
channels are classified as high hazard. Similarly areas under inundation, and potential
bankcut sites and the areas of active colluvium are considered under high hazard. An area
Data source
Satellite imagery
(30 m Res) 1999
Aerial photo ( 1:50,000 )
1953/54 1978/79
Topographic map 1:25000
(DEM)
1992
Parameters
River
Old
Flood affected
area (Cumulative) morphology channels
(Cumulative)
Flood
plains
Sites of
Bank Others
Moist area Sheet flood debris flow Inundation cut
Lower Middle
Hazard type
H
H: High
H
M
H
MH: Moderately high
MH
Upper
L
M
M: Moderate
MH
H
H
H
L: Low
Figure 4. Hazard mapping scheme based on remote sensing and GIS
L
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under moderately high hazard includes middle floodplain, moist areas depressions, damp
and marsh sites. Moderate hazard zone is characterized by old channel course and the
areas under sheet flooding. The rest are
included in the low hazard zone.
Field Mapping
Areas prone to floods and other water
induced diasaters were delineated in
consultation
with
residents
and
demarcated on 1:25,000 topographical
map as shown in Figure 5.
Mapping of Vulnerability and Risk
Vulnerability
Four sets of parameters were chosen:
Houses, Built-up areas, Land use and land
cover and Road Infrastructure. Distance
to house or built-up areas refers to buffer
distance. The closer the distance, the
higher the vulnerability. A surface map of
distance was prepared in ArcView. The
Figure 5. Hazard Map based on field
rating scheme of vulnerability is given
Information
in Table 1.
Table 1. Vulnerability rating scheme
Distance to house and built-up areas (m)
Above 80
40-80
Rating
5
3
< 40
1
Land use/land cover (excluding built-up)
Cropland and orchard
3
Vulnerability
Low
Moderate
High
Moderate
Forest, shrubs, grass, wasteland, channel bed and sand bar
1
Low
and so on
Distance to cart tracks or roads (m), canals, transmission lines
< 15
5
High
15 – 30
> 30
3
1
Moderate
Low
Land use and land cover refers to all other use and cover types except the built-up
areas. As orchards and crops have economic value; vulnerability is moderate. The
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economic role of forest, grazing and wasteland is relatively low. Vulnerability for these
features is considered as low in relative sense.
To develop the vulnerability map, parameter maps were transformed into weight
maps by assigning a weightage to each class of parameter. Values for the combined
weight map ranged from 3 to 18. The vulnerability map then required classifying the
combined weight map into three i.e. high (above 8), medium (5-8), and low (below 5)
classes. The threshold values were obtained by trial-and-error manipulation.
Risk map
Risk is obtained by the multiplication of cost, vulnerability, and recurrence interval of the
natural phenomenon. Owing to the absence of information on costs and recurrence
interval, the risk map for the Ratu basin was prepared by combining the flood hazard map
with the vulnerability map. Vulnerability and risk mapping is presented in Figs. 6 and 7.
Figure 6. Vulnerability Map
Figure 7. Risk Map
RESULTS AND RECOMMENDATIONS
Results
The geology of the Ratu Khola is driven by the Siwaliks. As a result, the sediment
production is very high. Wide valley and relatively larger colluvial and alluvial fans are
developed. The dynamic river morphology is evidenced in time-sequenced aerial photos
and field survey. Bankcutting and river shifting are common in the inner valleys and fan
zone. Inundation and sheet flow is an additional hazard in the middle and lower Terai.
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The inner valleys and the Terai are densely populated and infrastructure continues to
be built. As a result, flood risk to life and property continues to increase.
Application of HEC RAS to assess inundation using 1:25,000 topographic
information is possible. The inundation hazard is confirmed in the field during normal
and severe flood events. HEC-RAS used cross-section profiles from DEM derived from
5-m contours. More-accurate cross-sections based on field measurements would have
enhanced the result. Another limitation is that flooding was assessed only in the main
Ratu channel. Several tributaries also contribute to flood disaster.
Flood hazard assessment based on a geomorphic approach using GIS and remotesensing was verified by field inspection. Information of flood-affected areas derived from
aerial photos since 1954 shows that large areas have been affected by overbank flow,
bank cutting and river shifts. While people have generally avoided settling on the floodprone area, damage and risk to life and property remains high. Twenty-six percent of the
watershed is under high hazard, where 14 and 18% of the houses and built up areas now
lie. Likewise, 25% of the agricultural land is under high hazard. Similarly 16 and 15% of
the infrastructures falls in the high and moderately high hazard zones. In general, 11% of
the total watershed area is under high risk.
Since the Ratu is highly dynamic and inter-basin flow is occasionally exacerbated by
roads, embankments and other infrastructure, risk will rise with increased development.
The low resilience and response capability to flood disaster is exacerbated by a socioeconomic backdrop of ill-distributed landholding, low income, food deficiency and low
literacy. While flood relief and rescue measures have been carried out, these efforts are
insufficient to reduce the vulnerability.
Recommendations
It is recommended that gauging stations be established on major control points,
preferably on bridges in the upper, middle and lower catchment. There is need for highresolution digital elevation data and imageries. Microwave remote sensing during a flood
would help to assess damage and support post-disaster mitigation.
Any further development expansion or extension should be based on planning and
building codes. River training programmes should be based on the understanding of
geomorphic process.
Infrastructure pertaining to flood control, irrigation, or roads across the International
border may increase the risk. Hence they should be planned according to bilateral
agreement based on risk analysis.
In each settlement, at least one public granary and rescue house/camp should be
established to reduce loss of crops and property. Aforestation should be promoted.
Community participation should be encouraged to prepare for and mitigate the hazards.
Local institutions should be strengthened by the support of governments, NGOS and
INGOS to increase coping capacity and enable existing mechanisms.
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REFERENCES
[1] Disaster Review 2002, His Majesty’s Government of Nepal, Ministry of Water
Resources, Department of Water Induced Disaster Prevention, (2003), pp 4.
[2] Chhetri M.P.B. and Bhattarai D., “Mitigation and Management of Flood in Nepal”,
His Majesty’s Government of Nepal, Ministry of Home Affairs, (2001).
[3] Sharma K.P., Adhikari N.R., Ghimire P.K and Chapagain P.S., “GIS-based flood
risk zoning of the Khando river basin in the Terai region of east Nepal”, Himalayan
Journal of Sciences, Vol. 1, Issue 2, (2003), pp103-106.
[4] Tianchi L., “Hazard and Risk Mapping”, Internal Report, Mountain Natural
Resource Division, International Centre for Integrated Mountain Development
(ICIMOD), (2001).
[5] Survey Department, His Majesty’s Government of Nepal (1993) Topographic maps
of Nepal. Produced in cooperation with the FINNIDA. 1996
[6] USACE [2001], “HEC-RAS River Analysis System”, Hydraulic Reference Manual,
US Army Corps of Engineers (USACE), Hydrological Engineering Center, Davis,
California.
[7] USACE [2001], “HEC-RAS River Analysis System”, User’s Manual, US Army Corps
of Engineers (USACE), Hydrological Engineering Center, Davis, California.