Effect of clay fraction on turbulence characteristics of flow near an eroded bank

Abstract

Stream-bank erosion in the alluvial channel is a matter of great concern particularly for the irretrievable loss of fertile land. This in effect has direct impact on the agriculture, environment, economy and social issues at large. A series of laboratory experiments were carry out to study the turbulence structure over eroded bank topography. Modulation of mean, turbulent flow characteristics and the benefaction of turbulent bursting structures to the total Reynolds shear stress due to the changed geometry of the eroded bank wall are described for three different clay fraction variations. A side looking micro-acoustic Doppler velocimeter (ADV) was used to record the detailed high frequency velocity data near the bank wall to understand the coupled dynamics of flow and erosion on sediment-water interface. The information on clay and fine sand content of natural river bank was obtained from field observation. Results from the present study indicate that higher turbulence intensities and Reynolds shear stress have prominent role in the removal of bank materials from the bank wall and continues the dislodgement of material from the bank face until mass failure. Quadrant analysis to the random velocity fluctuation in the vicinity of eroded stream-bank shows that downstream bank material movement is strongly correlated to the presence of sweep events. Probability density function (PDF), kinetic energy fluxes and coefficients of skewness and kurtosis of velocity fluctuations show significant changes with the change in clay fraction of the bank material composition. Though the velocity spectral densities are in general observed to be similar for all the three different bank undercut depths for different clay fraction of bank; however, spectral energy shows signature of higher energy peak for lower clay fraction of the bank material.