Mechanisms for the particle sorting and transport dynamics of eroded sediment in spoil tips under scouring conditions

Particle sorting and transport dynamics of eroded sediment can be expected to clarify the erosion mechanism of spoil tips in soil and water conservation. This study aims to investigate the response of sediment particle sorting and transport to hydrodynamic parameters during erosion. Two spoil materials, namely the Lou soil and aeolian sandy soil, were selected as research objects. Runoff scouring experiments were conducted under two slopes (28°and 32°) and four inflow rates (8, 12, 16, and 20 L/min). A systematic analysis was also implemented on the particle-size distribution of eroded sediment, the sorting features of sediment particles, and the transport mechanisms under different soil types. In addition, the relationships between sediment particle characteristics and hydrodynamic parameters were examined to identify the key controlling factors of sediment sorting and transport during erosion. The results showed that the eroded sediments from both the Lou soil and aeolian sandy soil spoil tips were dominated by the 0.002~<0.050 mm size fraction, which accounted for more than 46% of the total. There was great variation in the response patterns of sediment particle composition to slope and inflow rate. In the aeolian sandy soil spoil tips, the particle-size composition of eroded sediment was more sensitive to slope, indicating that the slope played a more significant role in the detachment and transport of sediment particles. In the Lou soil, the transport of coarse particles in the 0.250~2.000 mm size fraction was controlled by the inflow rate. There was a significant correlation between slope and inflow rate. The influence of hydraulic conditions on sediment transport was strongly dependent on the soil type and particle composition. Compared with the Lou soil spoil tips, the eroded sediment from the aeolian sandy soil spoil tips shared a larger mean weight diameter, ranging from 0.061 to 0.085 mm, and a smaller fractal dimension, ranging from 2.494 to 2.561. The coarse particles in aeolian sandy soil were more easily detached and transported during runoff scouring, whereas the sediment sorting was weak. In contrast, the relatively stronger sorting was observed in the smaller mean weight diameter and larger fractal dimension of the eroded sediment from the Lou soil. Fine particles were also enriched in the transported sediment. Therefore, there were differences in soil texture and particle composition between the Lou soil and aeolian sandy soil, leading to sorting responses during erosion. Among the hydrodynamic parameters, stream power was identified as the optimal predictor for the sorting characteristics of eroded sediment from the aeolian sandy soil spoil tips. Specifically, stream power showed the strongest relationship with mean weight diameter and fractal dimension (R²= 0.92 and 0.55, respectively). In the Lou soil spoil tips, runoff shear stress was the optimal hydrodynamic parameter for sediment sorting (R²= 0.80 and 0.31, respectively). The contribution rates of suspension-saltation exceeded 61% for the aeolian sandy soil spoil tips and 83% for the Lou soil ones. Most eroded sediment was also transported in the form of suspended and saltating particles rather than as bed load. Moreover, the power and logarithmic functions were used to quantitatively describe the contribution rates of suspension-saltation and bed load in the eroded sediment from the Lou soil and aeolian sandy soil spoil tips under stream power. The stream power served as an effective hydraulic indicator to partition the different transport modes during erosion. Dynamic erosion mechanisms of spoil tips can provide a solid theoretical basis to optimize the differentiated soil and water conservation using specific regional soil types.