Changes in particle size distribution of suspended sediment affected by gravity erosion: a field study on steep loess slopes

Abstract

Gravity erosion (mass movement) generates an enormous volume of sediments on steep slopes throughout the world, yet its effect on the particle size distribution of suspended sediments (PSDSS) remains poorly understood. The objective of this study is to quantify the effects of gravity erosion on soil loss, especially the changes of particle size distribution of the suspended sediment. In this study, experiments were conducted in a field mobile laboratory in which mass movements were triggered on steep slopes under simulated rainfall. During the experiments, water-sediment samples were collected in polyethylene bottles directly from the gully and channel flows before and during the mass movements. The volumes of each mass failure during and 20 min after the rainfalls were measured by a topography meter. The particle fractions of the samples were analyzed by combining the sieving and photoelectric sedimentometer techniques. A suite of indexes such as median sediment size (d50), sediment heterogeneity (H), fractal dimension (D), and enrichment/dilution ratio (Red) were then used to evaluate the effect of mass movement on PSDSS. (1) Gravity erosion had a significant influence on PSDSS. After the mass movements occurred, the proportion of sand-sized particles was decreased from 71 to 51%, whereas the proportions of clay and silt were increased from 1 to 7% and 28 to 42%, respectively. (2) The d50, H, and D were significantly correlated with slope failures. The d50 was decreased from 0.084 to 0.051 mm, whereas the H and D were increased from 5.6 to 26.8 and from 2.60 to 2.78, respectively. This implies that mass movements make the PSDSS more nonuniform and irregular. (3) The suspended sediment tended to be enriched in the silt and clay fractions and diluted in the sand fractions after mass movements. Reds for clay, slit, and sand fractions were 13.9, 1.4, and 0.7 respectively. The changes of PSDSS after mass movements reflected a combined complex effect of soil sources, erosion types, selective detachment, and deposition processes. Mass movements led to a drastic increase of sediment concentration and the enrichment of fine particles, which developed into hyperconcentrated flows.