FIELD EXPERIMENTS ON EROSION BY OVERLAND-FLOW AND THEIR IMPLICATION FOR A DIGITAL TERRAIN MODEL OF CHANNEL INITIATION

Dietrich et al. (1992, 1993) proposed a digital terrain model for pred icting the location of channel heads on the basis of the assumption th at they occur where saturation overland flow exerts a boundary shear s tress in excess of a critical value. Flume experiments were conducted in the modeled field site to evaluate the threshold hypothesis and to constrain critical shear stress and flow resistance parameters. Under complete grass cover, microtopography and grass stems were found to pr event significant sediment transport at all but the highest flows. Whe n the grass stems were cut close to the ground, flow resistance and cr itical shear stress for significant sediment transport were reduced by up to an order of magnitude, but the remaining dense root mat prevent ed deep scour. These field experiments support the threshold assumptio n and the model estimations of the critical shear stress if local topo graphic convergence of flow is taken into account. The experiments als o support the interpretation that significant degradation of vegetatio n cover is required for channel incision.