Calculating the Potential Effects of Large Woody Debris Accumulations on Backwater, Scour, and Hydrodynamic Loads

Potential effects of large woody debris accumulations on backwater, hydrodynamic loads on structures, and local scour at bridge piers along a section of an urban stream containing three bridge crossings were modeled using the computer program FST2DH. The two-dimensional depth-averaged flow solutions provide reliable estimates of water velocities and depths, which were used to internally calculate pressure flow and potential scour depths and hydrodynamic drag forces and impact loads on bridge piers. The effect of debris accumulations on water-surface elevations during high flows was found to be comparatively small compared to other factors such as bridge pressure flow and roughness. However, hydrodynamic drag forces acting on the blockages lead to significantly larger loads on piers. Although drift accumulations actually reduce the speeds of flows approaching piers, the most significant effect of debris accumulation on piers in this project setting is to significantly increase local scour depths. Additionally, reduced approach velocities at piers may be accompanied by increased velocities between piers, which could further increase overall bed scour.