Flow-driven soil erosion processes and the size selectivity of sediment

The processes and mechanisms of entrainment and transportation of soil particles by surface runoff was investigated in the laboratory by a 0.05 × 0.2 × 3 m flume for two contrasting soil samples. The results show that there are at least two different mechanisms affecting particle transport by flow, with their relative importance relating to hydraulic condition, especially flow streampower. Different particle size classes are transported mainly by one mechanism. Suspension/saltation was observed to be the only mechanism at low streampowers (<0.1 W m−2). Bed load transport, probably involving rolling was shown to be activated at the streampowers greater than a specific threshold of about 0.1–0.15 W m−2, becoming the dominant transport mechanism at these modest streampowers. The relative importance of the two transport mechanisms was also related to soil types. Particle density seemed to have a main role in this context. Comparison with theoretical prediction of sediment concentration provided some support for the increasing dominance of bed load transport mechanisms with increasing streampower in these experiments.