Investigations on undercutting processes of cohesive river bank

Abstract

River bank failure is the result of prolonged and repetitive erosion processes caused by the interplay between the flow turbulence with that of bank sediment. These interactions of affecting factors become more complex when the river bank sediment is cohesive in nature. The present study attempts to understand the erosion of cohesive river banks under controlled laboratory conditions, starting from an uneroded bank to the state just before failure. Experimental runs are carried out over the cohesive river bank, fabricated in a laboratory flume. Instantaneous real-time transverse erosion depth within the bank is recorded using ultrasonic ranging system (URS) and the corresponding velocity data by acoustic Doppler velocimeter (ADV). The results from the experimental runs with varying flow characteristics and cohesive sediment properties reveal that the turbulent shear stress combined with velocity fluctuations close to the bank increases with the flow Reynolds number. The size of the dislodged cohesive aggregates from the cohesive bank depends on the shear stress fluctuations. The transverse erosion depth as a function of time and flow Reynolds number are the significant parameters on which fluctuations of shear stress depend. These fluctuations in shear stress and flow velocity are high near the upstream and downstream sides of the undercuts, resulting in a longitudinally elongated undercut zone.