Erosion Countermeasure Design In Complex Flow Field: A Case Study On The American River

The American River Common Features project in Sacramento California identified a portion of a levee on the North side of the floodplain in the vicinity of multiple bridges that needs erosion countermeasures to protect a large at-risk population. These erosion countermeasures are needed to protect the levee from high velocities and turbulent flow around the bridge piers and abutments for the design flood event (1/325 Annual Chance Exceedance). The site on the American River includes two highway bridges and one railroad bridge. Some of the columns, posts, and abutments of these bridges are near the toe of the levee, increasing turbulence near the levee. Understanding the erosion risk to the levee, including effects of flow through and around the bridge structures, is needed to minimize environmental impacts and reduce cost while still protecting the large at-risk population living behind the levee. The risk-informed design at this site utilized the Bank Stability and Toe Erosion Model (BSTEM) developed by USDA-ARS to evaluate bank retreat. A Monte-Carlo simulation was evaluated within BSTEM to provide quantitative bank retreat estimates for risk-informed design of the erosion countermeasures. BSTEM model scenarios were developed to simulate the increased shear stress around the bridge piers as a result of the increased velocities/turbulence, specifically the impacts of having a larger scour hole develop because of the more erosive hydrodynamics. BSTEM scenarios were also developed without the additional scour caused by the bridge effects so a comparison could be made on the relative impact of pier scour to the erosion countermeasure design. The scenarios represent extreme cases that bracket the expected condition. These collective BSTEM results were used by the design team to inform the performance and risk reduction of the proposed erosion countermeasures. This case study demonstrates the use of BSTEM for risk-informed design of erosion countermeasures for a levee near bridges with complex flow. This helped to optimize the design to meet flood risk reduction objectives.