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I. ENVIRONMENT AND ASSESSMENT OF FLOOD IMPACTS
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A. Main features of the Bangladesh delta
Bangladesh is basically a land of rivers. It is criss-crossed by around
200 rivers most of which are either contributory or distributary to the
three major rivers: the Ganges, the Brahmaputra and the Meghna. Fifty seven
rivers originate outside the boundary of Bangladesh. The total length of
the river courses is approximately 24,000 km and their area covers 9,770
km2 or 7 per cent of the country area. The ecological characteristics
of the Bangladesh Delta are strongly influenced by the hydraulic and morphological
processes associated with this delta system of the three major rivers.
The catchment inside the country can be inundated to about 6 meters by
river water carried from outside the country each year, which is further
aggravated by rainfall inside the country which can add another 2 meters
of water. The flow in the river is shown in Figure 2. The rivers undergo
changes in courses which have significant influences on the morphology
of the alluvial floodplain. Bank erosion and deposition is a continuous
process associated with the rivers of Bangladesh. River bank erosion, which
is associated with floods, is a common and severe hazard in the country.
Every year almost one million people are affected by eroding banks along
75 rivers at about 130 different locations (Mott MacDonald, 1993a). On
average about 87 km2 of mainland was lost each year due to erosion
by the major rivers during 1984 to 1993 while about 50 km2 of
land accreted per year (ISPAN, 1993). Between 1970 and 1990, at least 7
million people were displaced by riverbank erosion (Mott MacDonald, 1993a).
Figure 2. Variation in mean monthly discharge in the Padma downstream
of the confluence of the Ganges and Jamuna
There are several estuaries in the coastal region, the principal estuaries
having 4 to 16 km width at their entrances. The inter-connected estuaries
and rivers form an intricate network. The coast of Bangladesh is subjected
to astronomical tides which originate in the Indian Ocean and travel through
the Bengal. The tides are pre-dominantly semi-diurnal. Tidal action is
felt up to 325 km during dry season and up to 225 km during wet season.
Most of the area in the Ganges Delta is below the high water level of spring
tides. High salinities are present in estuaries in the western part of
the Ganges Delta during dry season. Salinity in the Meghna Delta is low
due to large fresh water supplies from the Padma and the Meghna. The Bay
of Bengal is the breeding place for tropical cyclones associated with storm
surges. Approximately 12,000 km2 of coastal land is prone to
cyclonic storm surge floods. In SW region, storm surge can intrude as far
as 60 km through the network of estuaries (Karim and Chowdhury, 1995).
The funnel-shaped coast line configuration makes the country more vulnerable
to the surge hazards. The most important economic feature of the Bay of
Bengal lies in the sea port communication facilities, in addition to which
it is a source of huge marine fisheries resources.
Huge amounts of sediments are carried by alluvial rivers which build
new land, fill in subsidence and improve soil fertility. The annual sediment
transports are 590 and 548 million tons in the Jamuna and the Ganges respectively
and 897 million tons per year downstream of their confluence in the Padma
river (Delft, 1996a). Floodplain sedimentation takes place as floods spread
out onto the floodplain, with coarser sediments (sand) being deposited
near the bank with the highest thickness, eventually forming natural levee
standing up to 3 meters above floodplain over 1 km width as cited from
Bristow (1987) in Delft (1996b). The seasonal flooding characteristics
have an important influence on the physical and biological properties of
soil and, as a result, a significant bearing on land-use and agricultural
potential.
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