Prediction of Effects of Woody Debris Removal on Flow Resistance

A simple technique for predicting the Darcy-Weisbach friction factor for river channels with varying amounts of large woody debris was developed. First, debris density is determined based on measurement or visual estimation of cross-sectional areas of debris formations in the plane perpendicular to flow. The Darcy-Weisbach friction factor is then computed using debris density, channel geometry, and the debris drag coefficient. The debris drag coefficient may be computed from a power function with experimentally determined coefficients. For verification of the proposed procedure, debris density and friction factors were measured in river reaches in western Tennessee, and southeastern New South Wales, Australia. Friction factors computed using the procedure were within 30% of measured values for straight, sand-bed reaches and within 38% of measured values for sinuous, gravel-bed reaches. The computational procedure explained 84% of the variance in observed values.