SEDIMENT TRANSPORT MODELING FOR BRIDGE SCOUR DOWNSTREAM OF A DAM

Abstract

A sediment transport modeling study was made to determine stream channel scour at the proposed Rancho SantaFe Road Bridge on San Marcos Creek in Carlsbad, California. The study was made to determine the required depth of bridgepiers to prevent undermining due to channel bed scour. San Marcos Creek at the bridge crossing is located downstream ofLake San Marcos. Since the lake detains nearly all the sediment transported in the stream, the channel bed at Rancho SantaFe Road is susceptible to scour in the future. To determine the general scour, it is necessary to consider the flood hydrograph,channel geometry, and sediment delivery in the stream channel. The FLUVIAL12 model was employed to simulate spatialand temporal variations in watersurface elevation, sediment transport, and channel geometry. Scour and fill of the streambed are coupled with width variation in the prediction of stream channel changes. Computations are based on finite differenceapproximations to energy and mass conservation that are representative of open channel flow. General scour at the bridgepier was estimated to be 2.6 m (8.5 ft) following an assumed flood series, with the maximum total scour (general scour pluslocal scour) at the bridge pier of 3. 8 m (12.5 ft) occurring at the peak flood.