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Geomorphology of Bangladesh
Since the early pleistocene the river systems have delivered a huge amount of sediment now constituting 80 % of the landmass. The average altitude of the surface is not reaching more than 10 meters above main sea level. During periods of maximum glaciation marine regression led to increased erosional processes whereas holocene transgression revitalized the sedimentation processes. Thus the surface can be subdivided into four main units: marine influenced environments, active flood plains (about 30%) influenced by rivers, old alluvial terraces (about 40 %) and more mountainous environments in the northwest and eastern parts of Bangladesh. The southernmost part is characterised by mud flats or plains more or less regularly flooded at high tide (average hight of the semi-diurnal tide ranges from 0.7 m in the west up to 5 m in the Meghna estuary).
Apart from their function as a kind of natural flood protection, intact mangrove forests keeping sediments are a decisive factor of active delta growth. Hundreds of smaller island and sandy reefs regularly changing their shape are situated further seawards. Nearly one third of the country is periodically submerged by river-made floodings.
As a result of ongoing siltation processes these active flood plains show a positive sediment balance. Documenting pleistocene sedimentation processes old alluvial terraces (eg. the Barind Tracts of Rajshahi and Modhupur Tracts of Dhaka) fertile clay soils stand some 40 m above sea level (cf. Preu 1999, Ahmet 2005). Less than 20 % of the surface are constituted by mountains and hills. The highest peaks are located in the Chittagong Hill Tracts reaching an altitude of more than 1000 m.
Where ever possible the densly settled land is intensively agriculturally used.
Rivers, Discharge and Sedimentation
The massive delta body is the intermediate result of varying sedimentation rates since the last glaciation period. The overall structural forces constituting this highly dynamic system are accumulation, erosion and abrasion. Three main rivers the Jamuna (Brahmaputra), Meghna and Ganges run through the country. Northwest of Dhaka the Jamuna and Ganges confluence and form the Padma. Together with the Meghna entering the river south of the capital one of the largest rivers of the world enters the Bay of Bengal. An enourmous annual amount of 1500 billion m² of water transports up to 1,8 billion tons of sediment downstream. Uncountable tributarian streams and braided structures spin a complex fluvial network across the alluvial plain.
Particularly the main rivers are fed by catchment areas outside the country (92,5 %). Monsoonal rainfall and snow melt determine the run-off rates. In addition regional percipitation events feed the rivers.
Apart from local floodings the confluence areas and coastal region are prone to severe floodings. Most of the rivers still remain more or less uninfluenced by human attempts of stabilization. Thus natural levees flank the streams. Frequently rising water tables within these natural obstacles do not lead to inundation. Even occassional peaks forcing the rivers to overflow the levees are no threat. On the contrary, regular inundation of the hinterland floodplains are percepted positively.
On the one hand normal floods deliver fertile clay and sill, on the other hand natural dams are likely to grow. Anyway, dramatic changes are common during floods. Ahmet emphasizes rapid bank erosion, channel shifting and bank slumping due to undercutting as some of the major threats (Ahmed 2006).
At the moment no reliable instruments exist to predict where erosion might occur or chars (small river islands) will form or vanish. Under the influence of global warming (wetter and warmer conditions) some effects on the hydrologic system like more intense riverbank erosion or higher ground water tables during monsoon can be anticipated.
Following Alam (1998) “flood prone areas will increase from about 25 per cent to 39 per cent” (Alam et al. 1998). Due to sea level rise induced backwater effects discharge flow is hampered and polders are at a higher risc of being submerged. It is also well possible that salinity intrusion during dry periods may increase. Due to the fact that there is no solid empirical database available many of the discussed impacts remain highly speculative. Refering to recent research results from the German coast more frequent flooding does not necessarily evoce sediment loss.
It is documented that the sediment complexes of coastal areas and floodplains may even benefit from inundation. (Falk 2005, Streif 2005).