Impacts of bridge constructions on channel dynamics and sediment aggradation: An integrated discussion, West Bengal, India

Channel dynamics, including channel migration, erosion, and deposition, is a significant geomorphological phenomenon. It has been noted that the Gish River exists with its widest extension between 1913 and 2021, causing a bottleneck situation. The section G4 (Gish-4) experienced the most significant squeezing character over time. The catchment landslide-induced sediments, mostly transported in the monsoon by 1st and 2nd order streams, deposited at the beds near the rail and road bridges of the rivers Lish, Gish, and Chel. Landslide susceptibility mapping (LSM) and sediment transport index (STI) maps specify the zones of sediment generation and aggradation. Results of low ‘b” value 0.77 + /−0.04 specify active tectonics and instability of the area. A petrographic analysis of landslide samples and river bed sediments justifies that the landslide connected streams are continuously transported and deposited in the river bed. In the Himalayan foothill rivers like Lish, Gish, and Chel, channel planform dynamics may be the result of landslide occurrences, that induce huge sediments in the rivers and sediment transport index indicates the deposition of the sediments near rail and road bridges in spatio-temporal scale. • The seven sections from each river have been taken into consideration as piedmont zone (L1,G1,C1), far upstream from the rail bridge (L2,G2,C2), upstream from the rail bridge (L3,G3,C3), between the rail and road bridge (L4,G4,C4), downstream from the road bridge (L5,G5,C5), far downstream from the road bridge (L6,G6,C6) and confluence (L7,G7,C7) [C- Chel, G-Gish and L-Lish]. This study reveals the channel shifting from 1913-1936, 1936-1943, 1943-1965, 1965-1984, 1984- 2008 and 2008-2021. • Landslide susceptibility mapping and 1st and 2nd order stream connecting landslides are marked. AHP model has been performed to assign weightage to the parameters to prepare the landslide susceptible map of the considered study area. • Sediment Transport Index (STI) shows maximum sediment has been concentrated in the upstream and downstream section of river Lish and Gish. • Gutenberg-Richter Relation with b-value across the study area and its adjacent zones specify the area to be active. • Petrography and digenetic features disclose the same mineral composition of landslide sediments connection streams and river bed sediments near bridge.