Effects of flow velocity variation on rain driven transportation and the role of rain impact in the movement of solids

Rain-flow transportation in, and airsplash from, shallow sheet flows of varying velocity were investigated experimentally by using four drop sizes at constant intensity (64 mm h-1). Whereas airsplash yields were negligible, rain-flow transportation reached rates of up to 3 g m-1s-1 and exhibited two distinct modes of operation, or regimes, depending on whether the unimpacted flows were laminar or transitional to turbulent. In the first of these regimes, clouds of particles, suspended by individual drop impacts, were moved downstream before settling back to the bed. In the second regime, general turbulence, imposed by drop bombardment, made the flow everywhere able to entrain and transport particles without direct dependence on individual drop impacts. These findings are combined with those of previous studies to make an assessment of the roles of rain and flow in water erosion.