I. ENVIRONMENT AND ASSESSMENT OF FLOOD IMPACTS
A. Main features of the Bangladesh deltaBangladesh 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).
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