Relationship between Sediment Particle Size and Transport Coefficient on Steep Slopes

Abstract

Transport capacity of overland flow is related to sediment particle size. The WEPP (Water Erosion Prediction Project) model considers soil particle size in the sediment transport capacity equation; however, the equation does not work well for steep slopes. This study introduces sediment particle size into the determination of transport capacity on steep slopes by quantifying the relationship between sediment particle size and transport coefficient using the modified sediment transport capacity equation for steep slopes from the WEPP model. The relationship is based on the measured sediment transport coefficient associated with different sediment particle sizes in a hydraulic flume. The experiment used five sediment particle sizes (0.10, 0.22, 0.41, 0.69, and 1.16 mm), on five slopes (8.7%, 17.4%, 35.9%, 34.2%, and 42.3%), and with five flow rates (0.66, 1.32, 2.63, 3.95, and 5.26 10-3 m2 s-1). The results showed that the sediment transport coefficient decreased significantly with increasing sediment size. The relationship was well described by a power function with transport coefficient as dependent variable and sediment particle size as independent variable. The sediment transport capacity equation could be modified using two independent terms: the sediment particle size power function, representing sediment properties, and flow shear stress, representing flow hydraulics. However, both the WEPP transport capacity equation and the modified equation did not predict the measured transport capacity well for steep slopes. Further studies are needed to improve the performance of the modified equation using fine sediment particles beyond the range of this study.