ABSTRACT
The effects of significant instream wood accumulations at bridges on the backwater, local scour at piers, and hydrodynamic loadings during floods are calculated by the computer program FST2DH. This two-dimensional depth-averaged flow and sediment transport model provides reliable estimates of water velocities and depths at bridge crossings and internally computes the hydrodynamic drag forces on drift blockages and their effect on backwater and local scour at piers. Additional calculations deliver the impact loads on piers caused by floating logs. Use of FST2DH to evaluate the consequence of drift buildup and blockage at bridges is demonstrated by an application to an urban stream located in a steep confined canyon where a grade-separated, fixed-guideway transit system is planned. The depth-averaged flow solutions show that backwater and local scour at piers within bridge openings and at the columns supporting elevated structures in the floodplain are increased significantly by instream wood accumulations.
