Sediment incipient motion on sandy seabed considering relative density effect: Laboratory experiments and mathematical analysis

Abstract

The threshold of the sediment initial movement is closely related to marine structure scour. In complex marine environment, the relative density of seabed is usually changed by liquefaction and scour that is caused by currents and waves. It affects the scour depth and scour time scale which have been noted recently. However, the effect of relative density on sediment movement was mostly ignored in previous studies. In this paper, a series of tests were conducted to investigate the incipient motion under different relative density conditions on different seabeds. Subsequently, the internal friction force was introduced to describe the relative density effect, and an incipient motion model was built. Accordingly, a unified sediment incipient motion velocity function that considers relative density is proposed. Based on this function, the critical bed shear stress was assessed. A new theory was then evaluated in terms of the critical bed shear stress. Finally, the percentage of internal friction force to the whole resistance was discussed. Results indicated the significance of considering the relative density effect in predicting small-sized sediment movement. The new function provides better prediction accuracy with an acceptable error of 20%. Additionally, the discussion of the internal friction force gives a potential avenue to quantify interparticle force.