Predicting sediment transport by interrill overland flow on rough surfaces

Modelling soil erosion requires an equation for predicting the sediment tra nsport capacity by interrill overland flow on rough surfaces. The conventio nal practice of partitioning total shear stress into grain and form shear s tress and predicting transport capacity using grain shear stress lacks rigo ur and is prone to underestimation. This study therefore explores the possi bility that inasmuch as surface roughness affects flow hydraulic variables which, in turn, determine transport capacity, there may be one or more hydr aulic variables which capture the effect of surface roughness on transport capacity sufficiently well for good predictions of transport capacity to be achieved from data on these variables alone. To investigate this possibili ty, regression analyses were performed on data from 1506 flume experiments in which discharge, slope, water temperature, rainfall intensity, and rough ness size, shape and concentration were varied. The analyses reveal that 89 .8 percent of the variance in transport capacity can be accounted for by ex cess flow power and flow depth. Including roughness size and concentration in the regression improves that explained variance by only 3.5 per cent. Ev idently, flow depth, when used in combination with excess flow power, large ly captures the effect of surface roughness on transport capacity. This fin ding promises to simplify greatly the task of developing a general sediment transport equation for interrill overland flow on rough surfaces. (C) 1998 John Wiley & Sons, Ltd.