Deciphering seasonal bridge scour effects: a 1D sediment transport model study in India's tropical basin

Accumulation and sediment flow in the floodplain region belong to any river's standard, natural religion. However, the location of any structure above the river increases the flow of deposits into the river and reduces the river's capacity, helping to cause floods. The site of the bridge along with the pier in the ordinary course of the river increases the flow of sediment in the river through bridge scouring, which indirectly helps in the morphological, hydro-morphological and dynamic changes of the river and causes disasters. In the present article, along a specific stretch (Kashiyara to Gadiara) of the lower basin of the Rupnarayan River, the change in the nature of the river sediment flow and the consequent increase in the amount of sandbar has been highlighted. A 1D sediment transport model calibration of a particular stretch has been carried out using empirical equations on the HEC-RAS platform, and it has been observed that most bridge scouring occurs during the monsoon season, resulting in the highest sediment flow during that time. The decrease in velocity throughout the pre- and late post-monsoon periods has caused the material to collect in the river, resulting in multiple sandbars downstream of the bridge. Also, although changes in velocity and gauge level highlighted the effect of tide on the bridge scouring of the 1D model, it did not have much effect. By calibrating the HEC-RAS 1D sediment transport model during monsoon, sediment discharge is 285,884.64 ton/day and sediment concentration is 383 mg/l. Along the upstream and downstream of the bridge, seasonally, depth, velocity, WSE, stream power and a significant change in the increase of velocity with the river's depth was detected, which reflects the effect of the bridge scouring to a large extent. Validation of the entire 1D sediment transport model has been done with the help of some periodic statistical parameters based on sediment discharge, concentration and the effect of tide on velocity and gauge level, highlighting the change in the position of the seasonal sandbar. The data acquired from the model will assist government authorities in devising strategies to enhance navigability by mitigating the buildup of river sediments, which will be advantageous for the local stakeholders.