Assessing Causes of Bank Erosion Using CFD Modeling

Abstract

Significant bank erosion has occurred along the right bank of Pulgas Creek Channel near 1091 Industrial Road in the City of San Carlos, California. The erosion site is located downstream from a City storm water pumping station. During significant storm events, the pump station discharges water into the channel through three discharge pipes. Accelerated bank erosion that occurred after recent storm events raised concerns that the storm water discharge pipes may be contributing to erosion at the site. A computational fluid dynamics (CFD) modeling study was conducted to assess the likely causes of bank erosion at the site. A CFD model was developed using the volume-of-fluid (VOF) model available in the FLUENT software package for predicting the three-dimensional flow patterns in the channel and the distribution of shear stress along the channel bed and banks for a range of flow conditions. The model includes details for the three storm water discharge pipes and the Pulgas Creek channel section between Industrial Road and the concrete bridge at the upstream of HWY 101. CFD simulations were performed for a representative storm event in January 2008, with and without the pumps in operation. Three-dimensional flow patterns and shear stress distributions along the channel bed and banks for several combined channel flow and pumping scenarios were evaluated. Results from the CFD modeling study suggest that pump station discharges contribute to the accelerated bank erosion during significant storm events; however, during high flows, higher velocities and shear stresses occur along the right bank whether pumping is occurring or not. Therefore, bank erosion results from the combined effects of high flows with or without pumping.