Two-Fraction Formulation of Critical Shear Stresses for Sand and Silt Mixtures

The bed sediments in most rivers and estuaries usually consist of mixtures of sand and mud formed by silt and clay particles. Past studies of mud have focused mainly on the cohesive properties of the mud due to the presence of clay particles. Only limited attention has been given to the erosion behavior of mixtures of sand and noncohesive silt particles. Although a number of experiments have shown that the presence of silt particles in predominantly sand bed results in an increase in the critical shear stress of sand fraction, the mechanism responsible for this change is not clear and no general formula is available for prediction. In this note a simple two-fraction formulation is presented for determining the relative critical shear stress of sand fraction in a noncohesive sand-silt mixture. The formulation is broadly consistent with the mechanism that the presence of silt causes the changes in bed roughness with corresponding reduction of shear stress exerted on the sand particles. The calculated relative critical shear stresses for the sand fraction at varying silt contents and the mean silt particle sizes show good agreement with the available experimental data.