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Flooding in Bangladesh

(G. Falk)

 

Apart from the table flat character of the surface, high ground water tables and widespread impenetratable topsoils floods are resulting from some complex hydrological factors.

Inflow from upstream catchment areas and heavy rainfall in Bangladesh catchment areas often exceed the capacity of the natural drainage. In addition conguested affluence systems by tidal effects and storm surges increase the inundation risc.

Floods Bangladesh
Map showing areas harmed by floods in Bangladesh

Future changes are expected to increase the vulnerability on various regional and temporal scales “On a longer timescale, it is hypothesized that higher sea level tends to retard river water from the Ganges–Brahmaputra complex (which flows through the Bangladesh delta) into the bay, enhancing flooding potential. On a shorter timescale, higher sea level acts to enhance severe flooding during storm surges. (Reference - April 2007)

 

Local man made efforts to prevent flooding may turn out to show several negative impacts. Dammed river passages and a growing number of poldered areas may help to protect human habitats but likewise rise the risc of flooding elsewhere. Apart from financial reasons it seems to be inadequate to implement any massive engineering works transfering sucessfull means of land protection developed in the Netherlands without careful modelling and research. Deforestation in mountainous regions as in the Chittagong Hill Tracts lead to intensified erosion due to surface run-off and less infiltration rates. Positve future developments will highly depend on the reduction of population pressure and uncontrolled cutting of trees.


Natural floods follow an overall long term trend usually beginning with premonsoonal flash floods in the hilly eastern parts of the country. Exceptional heavy rain triggers these unpredictable local but nonetheless lethal events. Due to population pressure many unauthorised settlements have been erected at the bottom of probably affected hillslopes.


During the monsoon the effects of heavy rainfall show more regional impacts in and around the catchment areas of the mayor rivers. Before the water table of the rivers reaches a critical level vast areas in the hinterland, mainly floodplain depressions characterised by clay soils, are flooded by excessive rainfall, particularly when high groundwater levels hamper infiltration and minor rivers and artificial irrigation systems cannot drain adequately. Under normal conditions this will not lead to disastrous floodings. Flood resitant crops will survive several days up to weeks of moderate inundation. However, excessive rainfall occasionally exceeds natural infiltration capacities of floodplains, draining capacities of rivers and resistance of crops which leads to severe problems for the affected people. All in all rainwater floods do not occur unforecasted.


Regular river floodings are a consequence of snow-melt in the Himalayas and monsoon rainfall but the “synchronisation of peak flows of the mayor rivers [...] induces an unusual situation where the entire drainage system in the floodplains fail to drain all the incoming waters.” (Ahmed and Mirza 2000) Particulary the confluence regions were affected by the worst floodings in historic times. Reduced run-off in the estuarine environments south of Dhaka due to tidal evoced conguestions complicated the situation in 1998 and 2004. Along the coast embankments may lead to inland water accumulation because drainage facilities are not adequtely developed. This increases the risc of severe floodings in case of high spring tides overtopping the artificial embankments.

At the beginning and the end of the monsoon (April/ May and October/ November) storm surges associated with tropical cyclones are quite frequent events. Particularly high waves result from strong winds and the barometric effect. Due to the estuarine features of the Meghna the negative effects reach far inland.

Apart from direct devastation of property arable land is indirectly harmed by long term effects of salinization. In future the number of severe cylonic storms is supposed to increase dramatically. Thus the frequency of floodings will rise likewise.

 

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