Sediment selective behaviors of colluvial deposit materials on steep slopes and under heavy rainfall conditions in southeastern China

Characterizing sediment selectivity during transport will improve understanding of soil erosion processes, but few studies have focused on steep slopes and soil with loose, coarse material. This study examined colluvial deposits, which contain loose materials with large amounts of coarse particles. These soils were subjected to simulated rainfall in a 5 m² (5 m by 1 m) box at heavy rainfall intensities (1, 1.33, 1.67, 2, and 2.33 mm min⁻¹) and on five steep slopes (20°, 25°, 30°, 35°, and 40°). During the experiments, the particle size of the eroded material gradually coarsened, then stabilized and often showed short-term fluctuations as the rainfall progressed. During interrill erosion processes, the sediment became enriched with finer particles because of the low runoff energy of interrill flow; the dominant transport mechanism was the suspension-saltation transport of sediment with a grain size finer than 0.537 mm. However, the sediment was coarser after rill development than before, and increased stream power meant that bed-load transport by rolling of the larger sediment grains (coarser than 0.957 mm) became an increasingly important mechanism; bed-load transport also increased with rainfall intensity, so the particle-size distribution for high-intensity rainfall resembled that of the original soil. Finally, during rill erosion processes, rill bank collapse occured and the original soil contained coarse, heavy particles that could more easily be rolled on steep slopes. These factors could increase the relative importance of bed-load transport. The results of this study have important implications for assessment and modeling of erosion processes in disturbed soils with large amounts of coarse particles.