An appraisal of hazards prone areas in parts of Brahmaputra valley, Assam
Vimal Kumar
Geological Survey of India,
Laitumkhrah, Shillong - 793 003
Brahmaputra basin is one of the most thickly populated river basins of India. The valley of the basin is formed by the alluvial sediments of floods of Brahmaputra river and its different tributaries. It is most haphazardly developed, populated and is prone to varied natural hazards such as recurring floods, bank erosion, water logging and heavy siltation etc. For geoenvironmental appraisal of the valley an area between silghat (in the west) and Dhemaji (in the east) has been studied, using various multidated data that mainly includes topographical maps, LANDSAT-FCC, TM-FCC and IRS geocoded sub scenes.
Geomorphologically, the study area is characterised by various fluvial landforms like flood plains, alluvial fills, back swamps, cut off meanders, abandoned channels etc. By conjunctive utilisation of various remotely sensed data and available geological / geophysical information the mapping of different linear structures, including a few active faults, has been attempted. The study has facilitated the understanding of the neotectonics of the area. These structures guide the major drainage pattern of the area and also have a distinct bearing on the geomorphic expression of the landforms. Oscillatory behaviour of the rivers in the recent historic times has been studied, as Brahmaputra and also most of its tributaries have a tendency of meandering and shifting their courses during floods. This tendency of oscillation causes bank erosion which, in turn, results in shifting of centres of human settlement. Besides being located in seismic zone, the area suffers with the floods of very high magnitude, which ultimately lead to heavy loss of both land and lives, destruction of crops etc. Factors contributing to this severity of the floods include the presence of ubiquitous clayey strata under valley floor and seasonal rush of water from the catchment of several tributaries. The areas, where the valley floor is made up of clayey material, suffer hazards like water logging and drainage congestion. All these affect the economic development of the area. In order to mitigate these hazards, a few projects are being envisaged in different parts of the area by various agencies. During the planning of these developmental projects an environmental management policy has to be initiated. It must consider the aspects like the maintenance of suitable vegetal cover and confining the settlement in the less flood-prone / flood-free areas. The hazards like water logging and drainage congestion could most effectively be improved upon by having better drainage for which the resuscitation of abandoned channels might provide a partial solution.
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