A MODEL FOR PREDICTING THE TRANSITION OF A FLASH FLOOD TO A DEBRIS FLOW CAUSED BY CHANNEL EROSION AND BANK COLLAPSE

Debris flows in burned watersheds and drainages with sparse vegetation are often initiated by surface-water runoff following intense rainfall. The transition of a water-dominated flash flood to a debris flow requires entrainment of sediment exceeding the volume of sediment ordinarily transported by the flow as bed and suspended load. Mechanisms by which the additional sediment required to create a debris flow can be supplied to the flow include the “firehose effect” (Johnson and Rodine, 1984), de-stabilization of the channel bed (Takahashi, 1981), and sediment bulking from bank erosion and collapse. A process-based model for the latter type of initiation mechanism is presented. The approach combines a fully predictive (i.e. no adjustable coefficients) channel-flow model together with algorithms for sediment transport and bank-slope stability. The flow component of the model determines channel flow resistance from simple field measurements of the initial channel geometry and the physical roughness elements on the channel bed and banks. This permits the model to calculate accurately the velocity and boundary shear stress distributions appropriate for sediment transport calculations across the entire wetted perimeter of the channel. The sediment transport calculations are used to predict vertical and lateral evolution of a channel during a runoff event. As the channel incises and/or the banks are undercut, slope-stability calculations determine the timing and magnitude of sediment delivery by bank failure. The model determines whether or not the additional bank material supplied to the flow is sufficient to create a debris flow based on the volume ratio of water to sediment. A model calculation of the reach-averaged cross-sectional geometry of a gully formed during a flash flood on a burned hillslope is shown to be in reasonable agreement with field measurements. The model is then used to conduct a series of simulations of individual runoff events in hypothetical swales having different material properties. These simulations identify the flow conditions required to generate debris flows by bank failure for the various cases.