Effects of sediment size on transport capacity of overland flow on steep slopes

Sediment transport capacity is a key concept in determining rates of detachment and deposition in process-based erosion models, yet limited studies have been conducted on steep slopes. We investigated the effects of sediment size on transport capacity of overland flow in a flume. Unit flow discharge ranged from 0.66 to 5.26 x 10(-3) m(2) s(-1), and slope gradient varied from 8.7 to 42.3%. Five sediment size classes (median diameter, d(50), of 0.10, 0.22, 0.41, 0.69 and 1.16 mm) were used. Sediment size was inversely related to transport capacity. The ratios of average transport capacity of the finest class to those of the 0.22, 0.41, 0.69 and 1.16 mm classes were 1.09, 1.30, 1.55 and 1.92, respectively. Sediment transport capacity increased as a power function of flow discharge and slope gradient (R-2 = 0.98), shear stress (R-2 = 0.95), stream power (R-2 = 0.94), or unit stream power (R-2 = 0.76). Transport capacity generally decreased as a power function of sediment size (exponent=-0.35). Shear stress and stream power predicted transport capacity better than unit stream power on steep slopes when transport capacity was <7 kg m(-1) s(-1). Sediment transport capacity increased linearly with mean flow velocity. Critical or threshold velocity increased as a power function of sediment size (R-2 = 0.93). Further studies with fine soil particles are needed to quantify the effects of sediment size on transport capacity of overland flow on steep slopes.