Assessment of Bottleneck Conditions and Concluding Remarks

Streams emerge into floodplains, forming alluvial plains and fans, as they leave their mountainous course over the foreland area with huge sediment aggradation. As a result of changes in the gradient of the terrain that streams flow over, sediments typically loose material and gradually accumulate to form North Bengal plains and engineering constructions as bridges across the channels impacted in shaping the course both in upstream and downstream sections. The hydro-morphological conditions that resulted in the bottleneck state of the Gish River were caused by monsoon sediment discharge, grain size alteration, riverbed slope, the distance between the break of the slope and bridges, water flow velocity, and bridge constructions. Using both primary and secondary data, as well as positive and negative changes in river width (m), the bottleneck condition of the rivers is evaluated, and the rivers that are most affected are identified. The river’s natural flow is interrupted by the construction of bridges. It is noted that the scouring and sediment deposition at the bottom of bridge and near the bridge piers disclose that River Chel experiences a high rate of scouring during the monsoon and River Lish is getting sediment deposits of fine sediments < 75 μ that is decreasing the elevation differentiation between the riverbed and bridges. Flood modeling using HEC-RAS 6.2 software has been done to comprehend the Rivers Chel, Gish, and Lish’s potential for flooding. Flood modeling has been produced in two ways: (a) without the intervention of bridges on rivers and (b) with the intervention of bridges on rivers, in order to determine the nature and extent of flood on the three rivers under consideration.