An evaluation of bed shear stress under turbid flows

Abstract

This paper investigates the effect of various turbidity levels on the drag coefficient and bed shear stress in decelerating flows over a smooth bed. Turbid flows were simulated in a laboratory annular flume (Lab Carousel) using clay suspensions of different known mass concentrations. Flow velocity measurements were taken in the turbulent boundary layer and the values of bed drag coefficient and shear stress were calculated using the method of flow deceleration. Results showed that the bed drag coefficient responds significantly to an increase in the suspended sediment concentration (up to 20 g\L) showing a decrease of up to 50% in drag due to turbulence suppression. However, subsequent increases in mass concentration resulted in an increase in the drag coefficient reaching the clear water value at approximately 200 g\L because of the influence of viscosity. Thus the application of the law of the wall in flows of high concentrations of suspended cohesive sediments up to 200 g\L results in overestimation of the bed shear stress. Copyright 2006 by the American Geophysical Union.