Soil erosion processes and sediment sorting associated with transport mechanisms on steep slopes

Sediment size distribution greatly affects sediment transport and deposition. A better understanding of sediment sorting will improve understanding of erosion and sedimentation processes, which in turn will improve erosion modeling. To address this issue, a total of 12 rainfall simulation experiments were conducted in a 1 m by 5 m box with varying steep slopes (10°, 15°, 20° and 25°), and the simulated rainfall lasted for 1 h at a rate of 90 mm h−1. For each simulated event, runoff and sediment were sampled at 3-min intervals, which were performed to study in detail the temporal change in size distribution of the eroded materials. These data were used to interpret the real-time sequence of transport mechanisms acting in response to the simulated rainfall. Total soil loss is the sum of suspended, saltating and contact loads. The proportion of sediment <0.002 mm showed little temporal fluctuation (generally 12–14%), although it was highly correlated to instantaneous rain power (R2 = 0.452, P < 0.01, n = 120). Suspension–saltation transports the finer than 0.054 mm size sediment was the most important erosion mechanism during interrill erosion processes. However, after rill development on hillslopes, bed-load transport by rolling of medium to large-sized sediment particles (coarser than 0.152 mm) became an increasingly important transport mechanism, and it were also enhanced by increased slope. Overall, the study supports a strong relationship between the sediment transport of contact (rolling) load and stream power. The partition of soil loss into these more meaningful components appears to be essential both for initial data interpretation and for subsequent use of such data for soil loss prediction.