Bed level variation and channel sensitivity analysis of a river channel to the extreme flood event

Sediment transport is a fundamental process to understand river morphodynamics. Bedload sediment transport during high flow energy governs channel geometry, though with variable response to flood events. Therefore, the sensitivity of bedload sediment transport to flood events is an important geomorphic query. We analysed the bedload transport process during the extreme flood event in the Purna River, a partial-bedrock river in peninsular India. The Purna River originates from an elevation of ~900 m and drains ~18,450 km² area. This major tributary of the Tapi River flows ~360 km. The flood events were characterised through flood frequency analysis using the Gumble distribution on peak discharge data from 1980-2016. The bedload sediment transport was assessed for the highest flood event using the Mayer-Peter Muller equation. Daily data of discharge, wetted area, wetted perimeter, grain size (D₅₀) data of pre- and post-monsoon, and the cross-section was obtained from the Central Water Commission (CWC), India. The bed slope was analysed using Manning’s equation. Our analysis shows that the return period for the highest flood at upstream station (Gopalkheda) is 35 years, while it is 136 years for downstream station (Yerly). The average bed shear stress was ~1.04 and ~1.55 times more than the critical shear stress using D₅₀ during the flood event for upstream and downstream reaches, respectively. The average bed shear stress exceeded the equal mobility condition (τ_eq≈1.45τ_c) in the downstream reach leading to full mobilisation. Therefore, it causes high bedload transport and scouring of bed level by more than 1 m in the downstream reach. However, at upstream reach, there was low bedload sediment transport and insignificant change in the bed level due to partial mobilisation. Also, the Maximum Flow Efficiency (MFE) at the downstream station is 6-7 times more than the upstream reach, representing high erosion in the downstream reach. Therefore, Purna River is characterised by reach-scale variability in the channel process to the same flood event. The downstream reach is more sensitive than the upstream reach and hence more prone to morphological change. These alterations have implications for designing hydraulic structures, water management, and river ecology.