The World's Largest Delta: Now at the Eroding Phase

Author: Syed Ashraful Islam
(Copyright Reserved)

The present report has dealt mainly with the geomorphology and geology as well as the morphologic or coastal changes of the lower deltaic region of the Ganges-Brahmapatra river system.
The study area has been mapped initially from the remote sensing data to delineate its present geomorphologic and geologic features. Eleven major gemophormic unites have been demarcated here, namely: tidal flat, lower tidal flat/mud flat, mangrove swamp, normal depression, water logged depression, tidal depression, estuarine bar, older estuarine bar, estuarine plain, beach and beach ridges. For broad morphologic aspect the area has been discussed separately for recent and active delta regions. Along with the morphologic development, the precise geomorphic evolution during the Holocene period has also been highlighted in this report. The development of the different morphologic units is varied from place to place and their changing nature is also very prominent in the study area. From the LGM the delta morphology has been evolved here with the alternating invasion and retreat of the sea.
The surficial geology of the lower delta region has been delineated on the basis of sediment/soil characters as reflected from the satellite imageries. The area is underlain by seven major types of deposits: fluvial deposit, tidal flat deposit, tidal depression deposit, mud flat deposit, mangrove swamp deposit, estuarine deposit and beach deposit. Moreover the subsurface geology has also been constructed from the borehole information. A generalized stratigraphic succession has been established which indicates that within 1100 feet depth, the region is underlain by three major lithostratigraphic units: (i) Alluvium: clay silt sand and their admixture with organic layers and fragments (ii) Madhupur or equivalent: upper part clay and lower part sand unit and (iii) Dupi Tilla: claystone, mudstone and sandstone. The depositional history has been intended to figure out in this paper on the basis of stratigraphic pattern. In this way seven distinctive sedimentary environmental phases have been demarcated that were mainly associated with climatic fluctuations and tectonic movements.
Coastal morphologic changes from 1979 to 1995 resulted from the land erosion and accretion have been estimated in the present work selecting three representative areas: Haribhanga, Haringhata and Tetulia river mouths. The net result goes towards coastal retreat: 1.21, 1.49 and 1.57 km2/yr land lose respectively. This is mainly due to natural causes and requires especial attention and environment favorable remedial measures involving bio-geologic and engineering projects.
The geo-dynamics of the study area have been considered to determine their influences on the geomophologic and geologic scenarios. For morphologic and surficial geologic map generation, remote sensing data have been used and hence necessary knowledge and information regarding this technology have been focused in this report. Beside these, this vulnerable coastal region is also facing various natural hazards including cyclone, flood, land-loss, and drainage and salinity problem. Their extent, effect and in some cases protective measures have also been discussed here.
After all, it is expected that the maps produced in this work will play some significant roles in the planning and development activities including the land use patterns and economic mineral exploration in the lower delta region of the Ganges-Brahmaputra. Up to these, advanced research works can be hopefully carried out based on the findings regarding any of the three broad aspects of the present study.

Note: LGM= Last Glacial Maxima

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