On the evolution of turbulent characteristics of an eroding cohesive riverbank

Adequate understanding of the flow structures in the proximity of an eroding riverbank is of great importance in protecting and controlling the bankline progression. An earlier initial field reconnaissance along the upper regime of the Hooghly River in India showed that the sediments comprise of clay-silt-sand mixture of different proportions. The present study experimentally investigates the temporal evolution of the coherent structures of turbulent flow as observed over the eroded bank topography in the Hooghly River for different compositions of the bank material in a laboratory flume. Artificial riverbanks have been fabricated using three different bank material composition in a laboratory flume to systematically understand the flow modulation inside the undercut region. The flow characterization is carried out to acquire sound knowledge on turbulence characteristics, such as mean velocity, intensity, shear stress, and power spectral density. It is evident from the turbulent coherent structure that the ejection events result in sediment pick up and the sweep events result in transportation of the sediment particles from the bank face. To quantify the eddy size, length scales are determined within the undercut region. Further, the anisotropy invariant maps are also computed to characterize the anisotropic structure of flow for different undercut roughness conditions. Results from the present study indicate that the near-wall streamwise integral length scales, turbulence intensity, and energy dissipation rate are significantly influenced by the different undercut morphologies. The Reynolds shear stress value appears to be strongly negative at the near-bank wall region and changes to positive at the mid-depth of the undercuts. The value of the integral length scale near the wall decreases with an enhancement of the roughness dimension. Furthermore, at the near-wall side bank region, the turbulence dissipation is found to be strongly dependent on the undercut morphology.