Purpose Spoil heaps have emerged as a significant source of soil and water loss, posing a serious threat to regional ecological security and sustainable development. Rill development is recognized as a critical process of soil erosion on spoil heap slopes. However, the temporal and spatial variations of rill morphology on spoil heaps and their underlying mechanisms remain poorly understood. This study aims to unravel the spatiotemporal evolution characteristics of rill morphology and the intrinsic mechanisms governing erosion processes. Materials and methods A field runoff plot (15 m × 1 m × 0.5 m, slope gradient of 28°) was used for inflow simulation experiments with four flow discharge rates (7, 13, 19, and 25 L min⁻¹). Results and discussion Spatially, rill width exhibited predominant downward extension, while rill depth displayed bidirectional development (both downward and upward) with increasing flow rates. Furthermore, temporal analysis revealed that the initiation timing for rapid width expansion and depth acceleration occurred progressively earlier with higher flow rates. Cumulative increased rate could quantify the development speed of rill morphology indicators over time. In sections with rapidly increased rill erosion, the cumulative increased rates reached 80% for width and 90% for depth, respectively. Rill width predominantly developed during the initial 6-minute phase, attaining 80% of final width by 15 min. In contrast, depth evolution peaked during the 6–24 min phase, reaching 80% of maximum depth. The sorting process of sediment particles was insignificant in rill erosion because the composition of sediment particles was essentially consistent during rill erosion. Spatially, the distribution of average flow velocities on the slope exhibited a pattern of differentiation (initial increase followed by decrease), consistency, and re-differentiation with the increasing flow rates. Temporally, the average flow velocities exhibited a decreasing trend across all measured sections on the slope. Spatially, the flow regime and flow type in different measured sections became consistent as flow rates increased. Additionally, the flow regimes transitioned from laminar-transitional flow to turbulent flow, while the flow types shifted from supercritical-subcritical flow to subcritical flow as flow rates increased. Rill surface area (Rs) could characterize the severity of the morphological development of rills. Rill surface area (Rs) could be used as a morphological parameter to characterize the degree of rill erosion within a range of 1.5 m. Conclusions The rill morphological development exhibited a characteristic phased pattern that showed rapid and stable widening at first, and slow furrow deepening later. The Reynolds number was the best hydraulic indicator to characterize the soil erosion mechanism within 1.5 m. Graphical abstract

Spatiotemporal variations of the rill morphology and hydraulic characteristic on spoil heaps

期刊论文
发表日期：

2025年9月11日

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Yaobin Niu,
Xu Wu,
Zhaoliang Gao,
Fenwu Liu,
Zhiping Yang

摘要

Purpose

Spoil heaps have emerged as a significant source of soil and water loss, posing a serious threat to regional ecological security and sustainable development. Rill development is recognized as a critical process of soil erosion on spoil heap slopes. However, the temporal and spatial variations of rill morphology on spoil heaps and their underlying mechanisms remain poorly understood. This study aims to unravel the spatiotemporal evolution characteristics of rill morphology and the intrinsic mechanisms governing erosion processes.

Materials and methods

A field runoff plot (15 m × 1 m × 0.5 m, slope gradient of 28°) was used for inflow simulation experiments with four flow discharge rates (7, 13, 19, and 25 L min⁻¹).

Results and discussion

Spatially, rill width exhibited predominant downward extension, while rill depth displayed bidirectional development (both downward and upward) with increasing flow rates. Furthermore, temporal analysis revealed that the initiation timing for rapid width expansion and depth acceleration occurred progressively earlier with higher flow rates. Cumulative increased rate could quantify the development speed of rill morphology indicators over time. In sections with rapidly increased rill erosion, the cumulative increased rates reached 80% for width and 90% for depth, respectively. Rill width predominantly developed during the initial 6-minute phase, attaining 80% of final width by 15 min. In contrast, depth evolution peaked during the 6–24 min phase, reaching 80% of maximum depth. The sorting process of sediment particles was insignificant in rill erosion because the composition of sediment particles was essentially consistent during rill erosion. Spatially, the distribution of average flow velocities on the slope exhibited a pattern of differentiation (initial increase followed by decrease), consistency, and re-differentiation with the increasing flow rates. Temporally, the average flow velocities exhibited a decreasing trend across all measured sections on the slope. Spatially, the flow regime and flow type in different measured sections became consistent as flow rates increased. Additionally, the flow regimes transitioned from laminar-transitional flow to turbulent flow, while the flow types shifted from supercritical-subcritical flow to subcritical flow as flow rates increased. Rill surface area (R_s) could characterize the severity of the morphological development of rills. Rill surface area (R_s) could be used as a morphological parameter to characterize the degree of rill erosion within a range of 1.5 m.

Conclusions

The rill morphological development exhibited a characteristic phased pattern that showed rapid and stable widening at first, and slow furrow deepening later. The Reynolds number was the best hydraulic indicator to characterize the soil erosion mechanism within 1.5 m.

Graphical abstract