INITIATION OF SEDIMENT MOTION INLAMINAR OVERLAND FLOW

Due to its extremely shallow in depth, the overland flow hydraulics is affected by surface roughness. raindrop impact, and in the case of laminar flow, fluid viscosity. The initiation of sediment in overland flow is subsequently influenced by these factors and differs greatly from that in river flows. This study is to investigate, by means of flume experiments, the effects of stone roughness, raindrops and fluid viscosity on the initiation of sediment motion in laminar overland flow. It was found that the critical dimensionless shear stress in laminar overland flow is much smaller when surface roughness and rainfall are present than when they are not. Surface roughness and rainfall generate disturbances in the flow which entrain otherwise immobile sediment. As a result, the threshold of motion in laminar flow on a rough surface with rainfall is similar to that in turbulent flow. It was found that critical flow power in laminar flow on rough surfaces in positively correlated with fluid viscosity and negatively correlated with rainfall intensity and surface roughness.