| Abstract | Flooding occurs almost every year in the Ganges floodplain due to huge amounts of
flow from upstream. The MIKE-11 hydrodynamic model has been used for the flood flow
simulation along the main rivers and floodplains. Based on the frequency analysis of previous
floods, flood simulation has been made for 100 years return period. The spatial distributions of
flood depth at different locations in the floodplain have been determined using computed
maximum water level at each computational point and land level contour. Flood duration at
different locations has been estimated using Satellite images of 1988 flood.
A flood hazard map has been developed based on the hazard magnitude at different
areas of the study area. The magnitude of flood hazard depends on characteristics of flood
such as depth of flooding, duration of flooding, velocity of flood wave, rate of rise in water
level, frequency of occurrence, sediment load and timing and it is represented by the hazard
factor. The hazard factor is a resultant effect of all hcgard parameters. Two major hazard
parameters, depth of flooding and duration of flooding have been considered in determining
hazard factor magnitude. Model simulated water levels have been used to estimate the hazard
for depth of flooding while Satellite images have been used to estimate the hazard for duration
of flooding. Depth of flooding and duration of flooding were divided into four categories.
Hazard index values have been assigned for each category of flooding depth and flooding
duration. Four different combinations of hazard index values have been evaluated for flood
hazard assessment. The best set of hazard index values has been selected based on the
consistency of the results and comparison with previous research. The selected hazard index
values for four categories are 1, 2, 3, 4 for depth of flooding and 0, 1, 2, 3 for duration of
flooding. The study area has been divided into lowest, low, medium, high and very high
hazard zones based on hazard factor magnitudes. There are 128 land units in the study area.
Among them 40 units are under high and very high hazard zones, 61 units are under medium
hazard and the remaining land units under low and lowest categories. The distributions of land
units are 12.2%, 7.7%, 53.7%, 20.8% and 5.6% under lowest, low, medium, high and very
high hazard zones respectively. Most of the high hazard areas are located in between the river
Gorai and the Arial Khan and some other high hazard areas are located adjacent to the
downstream end of the Ganges.
A risk-based zoning map has been developed considering flood hazard and
vulnerability. Flood hazard is a function of flood characteristics and the vulnerability is ~
function of elements at risk. The elements at risk include population density, economical
activities, public services, utilities and infrastructures. In the present study only population
density has been considered as element at risk. All other economical activities have been_
assumed to be a proportional of population density. Risk factor, which is a product of hazard
factor and vulnerability factor, has been determined for each land unit. Based on the risk factor
magnitudes, the study area has been grouped into four risk zones namely low, moderate, high
and severe risk zones. There are 8 land units ( 4% of total area) under severe risk and 29 land
units (13.5% of total area) under high risk zones. These are the most prioritized areas for
immediate flood protection and preparedness program. |