Sediment threshold

A model is presented to compute the threshold shear stress for non-cohesive sediment (uniform and non-uniform) motion on a horizontal sedimentary bed, under a unidirectional steady-uniform stream flow. On the basis of hydrodynamical and micro-mechanical considerations, forces on a solitary sediment particle, resting on a compact bed formed by equal sized sediment particles, under slip-spinning condition were analyzed with the aid of a computational scheme. The experimental data of sediment threshold reported by various investigators were used to calibrate the model making the lift coefficient as a free parameter. An excellent agreement between the present model and the experimental data of uniform sediments was possible owing to the calibration of the model. The computational results are presented in the graphical form where the variation of normalized threshold shear stress with particle Reynolds number for various angles of repose is shown. It is revealed that the threshold shear stress increases with an increase in angle of repose of bed sediment. The results obtained using the present model are compared with the curves proposed by different investigators and also have an agreement with the experimental data of non-uniform sediments. Diagram is presented for the direct estimation of threshold shear stress from the information on sediment size and angle of repose. (C) 1999 Elsevier Science B.V. All rights reserved.