The influence of raindrop induced saltation on particle size distributions in sediment discharged by rain-impacted flow on planar surfaces

A mechanistic model was used to simulate the downstream movement of particles travelling in suspension, by raindrop induced saltation and flow driven saltation on planar surfaces ranging from 2 m to 30 m in length. Results were produced for two rainfall events, one of high intensity, the other of moderate intensity, and a mixture of particle sizes ranging from fine material to 0.9 mm sand. Initially, the composition of the sediment discharge was controlled by the composition of the soil matrix and the rate the raindrop driven transport systems were capable of transporting the particles from zones near the downstream boundary. As time progressed, material transported from upslope provided a degree of protection in these zones and controlled the composition of the sediment discharge. The protection provided reduced the discharge of fine material but had little effect of the composition of the discharge of the coarser material during a given rainfall event so that it remainder finer than that of the soil matrix. However, in the long term, the slow movement of the coarsest material into the zones controlling sediment discharge led to coarsening of the sediment discharge so that it tended to the composition of the soil matrix at the steady state. The time taken to reach that steady state may be considerable, and increases with plot length, so that many many storms may occur before the steady state condition is reached.