Effect of rainfall intensity and gravel content on hydraulic characteristics and hydraulic parameters on soil erosion of spoil heaps: Laboratory experiments with simulated rainfall

Spoil heaps represent one of the most severe forms of soil degradation and serve as significant triggers for geological disasters. To investigate the hydraulic characteristics of runoff and dynamical mechanisms of erosion on spoil heaps slopes, we conducted a series of simulated rainfall experiments under varying conditions: rainfall intensities (30, 60, 90, and 120 mm h⁻¹) and gravel contents (0%, 10%, 20%, 30%, and 40%). The hydraulic parameters exhibited distinct patterns under different experimental conditions. These hydraulic parameters were positively influenced by rainfall intensity, gravel content, scouring time, and the interaction of rainfall intensity and gravel content, with gravel content showing the most significant impact. Under the experimental conditions, runoff on the spoil heap slopes manifested as rapid and laminar flow. The temporal evolution of the roughness coefficient paralleled that of the resistance coefficient. Exponential relationships were observed between hydraulic parameters and rainfall intensity, while quadratic relationships emerged between hydraulic parameters and gravel content in the soil-gravel mixture. The presence of gravel significantly altered the hydraulic characteristics of the spoil heaps slopes, with a critical threshold occurring at 20–30% gravel content. The Reynolds number (Re) and Froude number (Fr) demonstrated a negative logarithmic relationship (R² = 0.472, P < 0.05), while Darcy-Weisbach resistance coefficient (f) and Manning roughness coefficient (n) exhibited a positive logarithmic relationship (R² = 0.980, P < 0.01). Significant exponential relationships were found between f and Re, as well as between n and Re. Furthermore, power function relationships were established between Fr and f, and between Fr and n (R² = 0.999 and R² = 0.979, P < 0.01). The hydraulic parameters effectively predicted soil loss through power function. Fr, f, and n showed significant power function relationships with runoff rate, while Re demonstrated a highly significant linear relationship (R² = 1.0). Among all parameters, Re exhibited the most stable relationship with both soil loss rate and runoff rate, making it the most suitable indicator for characterizing soil erosion. High gravel cover on slopes reduced the erosive effect of runoff. Under all rainfall conditions, hydraulic parameters influenced soil erosion more indirectly than directly, following the pathway: rainfall ➝ hydraulic parameters ➝ runoff ➝ soil erosion.