Flood Hazard Mapping and Vulnerability Analysis …
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of sediments from river, and change in land use are increasing the vulnerability of
people during floods.
To reduce the vulnerability due to flood hazards, mitigation is a must. Mitigation
can be done by structural or non-structural measures. Flood hazard mapping is one of
the non-structural measures of mitigation to estimate the areas which are at a risk of
flooding under extreme flooding conditions under different scenarios. This study aims
to prepare flood hazard map of Pokhara Metropolitan City using one-dimensional
hydraulic model HECRAS, Arc GIS, and HEC-Geo RAS. Various scenarios are
considered to quantify the flooding and flood/inundation, and hazard prone areas
along Seti River would be identified and processed to prepare hazard maps.
2 Methodology
One-dimensional steady hydrodynamic modeling was used using HECRAS 5.0 to
calculate the water surface profiles for various return periods. The geometry was
created using HEC-GeoRAS extension in Arc GIS, and Arc GIS was used to prepare
the hazard and vulnerability maps. The details of the methodology are shown in
Fig. 2. To obtain the flood hazard maps, DEM ALOS Palsar 12.5 m resolution [5],
hydrological data (Discharge Data from DHM), land use data of 2010 (Obtained
from ICIMOD [6]), population data (obtained from Central Bureau of Statistics),
household data (Obtained from Open Street Map) were used.
Flood frequency analysis for the return periods of 2, 5, 10, 50, and 100 years was
carried out using Gumbel’s method, Fuller’s Method, Log-Pearson-III method and
Log Normal method. For the ungauged tributaries, catchment area ratio method was
used to calculate the discharge which was used as input to HEC-RAS. The manning’s
roughness coefficient ‘n’ was used by using trial and error method and comparing
the rating curve obtained from Department of Hydrology and Meteorology, Nepal
data and output of HEC-RAS. The value of n was taken as 0.045 for channel and
0.06 for left and right over banks.
The goodness of fit test was performed for the given values using chi square
test, and Gumbel distribution was found to be suitable for the basin, and hence,
the discharge for the various return period was used from Gumbel’s method. The
discharge used in the study for 2, 5, 10, 50, and 100 years return periods was
401.38 m
3 /s, 706.34 m
3 /s, 908.25 m
3 /s, 1353.62 m
3 /s, and 1540.48 m
3 /s, respectively.
3 Preparation of Flood Hazard and Vulnerability Maps
Flood depth is considered as the most important indicator of intensity of flood hazard
[8]. So, to quantify the flood hazard; three levels of hazards, low, moderate, and high
are categorized in this study according to the flood depth. Flood depth less than 1 m
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of sediments from river, and change in land use are increasing the vulnerability of
people during floods.
To reduce the vulnerability due to flood hazards, mitigation is a must. Mitigation
can be done by structural or non-structural measures. Flood hazard mapping is one of
the non-structural measures of mitigation to estimate the areas which are at a risk of
flooding under extreme flooding conditions under different scenarios. This study aims
to prepare flood hazard map of Pokhara Metropolitan City using one-dimensional
hydraulic model HECRAS, Arc GIS, and HEC-Geo RAS. Various scenarios are
considered to quantify the flooding and flood/inundation, and hazard prone areas
along Seti River would be identified and processed to prepare hazard maps.
2 Methodology
One-dimensional steady hydrodynamic modeling was used using HECRAS 5.0 to
calculate the water surface profiles for various return periods. The geometry was
created using HEC-GeoRAS extension in Arc GIS, and Arc GIS was used to prepare
the hazard and vulnerability maps. The details of the methodology are shown in
Fig. 2. To obtain the flood hazard maps, DEM ALOS Palsar 12.5 m resolution [5],
hydrological data (Discharge Data from DHM), land use data of 2010 (Obtained
from ICIMOD [6]), population data (obtained from Central Bureau of Statistics),
household data (Obtained from Open Street Map) were used.
Flood frequency analysis for the return periods of 2, 5, 10, 50, and 100 years was
carried out using Gumbel’s method, Fuller’s Method, Log-Pearson-III method and
Log Normal method. For the ungauged tributaries, catchment area ratio method was
used to calculate the discharge which was used as input to HEC-RAS. The manning’s
roughness coefficient ‘n’ was used by using trial and error method and comparing
the rating curve obtained from Department of Hydrology and Meteorology, Nepal
data and output of HEC-RAS. The value of n was taken as 0.045 for channel and
0.06 for left and right over banks.
The goodness of fit test was performed for the given values using chi square
test, and Gumbel distribution was found to be suitable for the basin, and hence,
the discharge for the various return period was used from Gumbel’s method. The
discharge used in the study for 2, 5, 10, 50, and 100 years return periods was
401.38 m
3 /s, 706.34 m
3 /s, 908.25 m
3 /s, 1353.62 m
3 /s, and 1540.48 m
3 /s, respectively.
3 Preparation of Flood Hazard and Vulnerability Maps
Flood depth is considered as the most important indicator of intensity of flood hazard
[8]. So, to quantify the flood hazard; three levels of hazards, low, moderate, and high
are categorized in this study according to the flood depth. Flood depth less than 1 m