Study of clay–sand network structures and its effect on river bank erosion: an experimental approach

Evolution of the riverbank morphology due to the removal and collapse of bank sediment results in substantial loss of fertile land and human property. At the lower stretch of a perennial river, clay–sand mixed bank sediments are predominant. Existing literature reveals that large shear stresses are generated due to the interaction between the hydrodynamic forces and bank roughness. This may lead to the dislodgement of the sediment particles from the bank surface. The generation of shear stress of a turbulent flow depends on the scales of turbulent fluctuations and the clay–sand network structures (size, shape, and arrangement of clay and sand particles). However, information on the modulation of turbulent flow structures due to the variation of clay–sand network structures of the bank sediment remains unrevealed. In the present experimental investigation, an attempt is undertaken to study the effect of sediment surface roughness and the associated turbulent scales of the interacting flow. This is achieved by fabricating artificial banks of five different proportions of clay in the clay–sand mixture in a laboratory flume. Scanning electron microscope (SEM) and 3D profilometer measured the surface roughness parameters of clay–sand network structures for different clay proportions. Information on near bank turbulence characteristics was quantified using a 16 MHz acoustic Doppler velocimeter (ADV). Results from the present study show that the surface roughness of bank sediment decreases with increasing clay fraction in the clay–sand network structure of the bank sediment, which also lowers the associated turbulent velocity fluctuations or eddy scales of the flow. Furthermore, small-scale turbulent fluctuations decrease the sediment entrainment rate from the bank face.