Evaluation of the applicability of three sediment transport capacity equations on steep colluvial slopes and their modifications

Accurate estimations of the sediment transport capacity (T_c) are essential for soil erosion modelling. However, the applicability of existing T_c equations to colluvial soils with high steep slopes and high gravel content is limited. In this study, the variation of T_c with shear stress (τ), unit stream power (P) and stream power (ω) is investigated for different slopes and flow discharge. We also evaluate the applicability of the Yalin, Govers and GUEST equations (based on τ, P and ω, respectively) for estimating T_c on steep–slope colluvial deposits. Experiments were conducted using colluvial soil in a non-erodible rill flume. The results reveal that T_c follows a power function with τ and ω and a linear function with P. The regression results of the three hydrodynamic parameters and T_c agree with the T_c equation forms of the corresponding equations. The Yalin equation, developed based on gently sloping erodible bed conditions, simulates overall low T_c values for steep sloping non-erodible bed conditions (P.O._0.5–2 = 42.8%). The accuracy is significantly improved by correcting the sediment transport coefficient K_t (P.O._0.5–2 = 100%). The accuracy of the Govers-simulated T_c values under steep slope conditions, based on gentle slope conditions, decreases with increasing slope gradient (P.O._0.5–2 = 37.14%), which is attributed to the large amount of coarse-grained sediment present in this study. Thus, we retained the original form of the equation and further improved its accuracy by adjusting the coefficients (P.O._0.5–2 = 94.29%). As T_c increases, the GUEST equation can accurately simulate T_c. The accuracy is improved by calibrating the F-value (P.O._0.5–2 = 100%). Using dimensional analysis, the equations built based on hydraulic conditions (excess current power, shear stress, flow velocity, etc.) and median particle size can accurately simulate T_c values (P.O._0.5–2 = 100%). These findings provide a basis for the development of erosion models for avalanche deposits on steep slopes.