Insights into the sediment transport processes operating in rain-impacted flows.

• Differential travel rates cause particle size distributions to vary with time. • Sediment discharges peak at drop size dependent flow depths. • Sediment concentrations increase before declining as flow depth tends to zero. In laboratory experiments undertaken using sand on a sloping surface in the 1970s, it was observed that while fine material moved as suspended load, fine to medium sand was induced to saltate as the result of raindrop impacts whereas coarse sand and gravel were induced to roll. The particle size distribution of the sediment discharge was initially much finer than the original bed and coarsened with time to approach that of the bed as time passed. At the time the exact nature of the raindrop induced transport mechanisms was not well known. Later, experiments using an apparatus that enabled the depth and velocity of rain-impacted flow to be controlled over an eroding surface, showed that raindrops induced some particles to travel limited distances that depended on a number of factors including flow velocity and particle size and density. The resultant differential travel rates of particles of different size and density provided an explanation as to why particle size distributions of sediments discharged by rain-impacted flows vary temporally, and why a non-cohesive layer develops on top of the surface of the cohesive matrix soil. The formation of the non-cohesive layer is indicative of erosion being transport limited when raindrop induced saltation occurs. The experiments also showed that sediment discharges produced by rain-impacted flows peaked at a drop size dependent flow depth so that a function that accounts for the interaction between raindrop size and flow depth could be developed for drops impacting at close to their terminal velocities. One of the implications of the observed responses to variations in rainfall and flow conditions is that the assumption that rain has certain erosive power that is defined simply by the product of flow discharge and rainfall factors such as rainfall intensity or the kinetic energy flux is a false assumption. [ABSTRACT FROM AUTHOR]