Applicability of WEPP Sediment Transport Equation to Steep Slopes

摘要

This study was carried out to evaluate the transport capacity equations of the Yalin equation and the WEPP model for steep slopes, and to recommend the best-fitting exponent value of shear stress in the WEPP transport capacity equation. The transport capacity was measured in a 5 m long, 0.4 m wide hydraulic flume, and the diameter of test sediment varied from 20 to 2000 � m with a median diameter of 280 � m. Flow discharge ranged from 0.625 � 10 -3 to 5 � 10 -3 m 2 s -1 , and slope gradient ranged from 8.8% to 46.6%. An averaged dimensionless critical shear stress of 0.052 was used for T c calculation in the Yalin equation. The relationship between transport coefficient K t and shear stress was graphically determined for the transport capacity equation of the WEPP model. The transport capacities predicted by the Yalin equation and the WEPP model were compared with the measured T c to qualify their suitability on steep slopes. The results showed that the Yalin equation overestimated the measured T c by 109%. The error increased as a power function of shear stress (r 2 = 0.97). The transport capacity of the WEPP model underestimated the measured T c by 65%. The absolute error increased as a linear function of shear stress (r 2 = 0.91). Paired t-tests showed that the transport capacities calculated using the Yalin and WEPP equations were different significantly from measured T c at the 0.05 level. Sediment transport coefficient K t , calculated with the WEPP equation using the measured T c of this study, varied with shear stress. However, K t converged to a steady value of 0.053 when the best-fitting exponent of 2 for shear stress was used (r 2 = 0.98). Thus, K t t 2 is considered when estimating T c for steep slopes. More studies are needed to further evaluate the WEPP T c equation as well as this new equation using various soils on steep slopes.