Response of sediment flux, bridge scouring on river bed morphology and geomorphic resilience in middle-lower part of river Chel, Eastern Himalayan foothills zone, India

摘要

Dynamism of river bed morphology is continuous and an integrated process of mountain's natural outflow in Himalayan Foothill Rivers. In-stream seasonal sediment flux, bridge establishments, bridge scouring and railway-road construction have detrimental influence over the natural flow regime of the river thereby causing channel degradation in the piedmont zone of eastern Himalaya of India, specifically near the urban areas where river management strategies are very weak. Such extreme anthropogenic events have a distinctive influence in changing river bed morphology in the middle-lower part of river Chel in Darjeeling Himalayan piedmont zone of India. Huge sediment aggradation and transportation networks historically have affected River Chel and have caused channel avulsion which poses a serious threat to the physical and ecological environment of Chel. Aggradation and lifting of sediments from the river bed are fatal for the geomorphologic character of the river, though no studies have been undertaken yet, to analyse the situation quantitatively in this regard. This paper presents evidence that suggests the changing river bed morphology, the dynamic nature of the river, the poor health of the river which is principally the impact of the seasonal sediment flux and bridge construction and thus some methods of river restoration in the middle-lower part of the river have been explained. Agglomeration of sediment, flow bifurcation, the volume of seasonal water mass, bed elevation changes and scouring are a major dynamic disposition for change of river bed morphology. Point bar, sidebar, braided active channel, ripples, and rise in the bed elevation are rapid changes in upper to lower part of bridges that are examined in the present study. In the dry season (December to February), a huge amount of sediment lifting makes incommodious man-made holes and a number of emergent channels which principally control the formation of a new active channel in the next monsoon season. The avulsion of major intra-active channels and morphological changes of the river bed for the last 85 years have also been observed in this study.