The City of West Sacramento is protected from flooding by levees on all sides. These levees protect the area from the Sacramento River, American River, Sacramento Deep Water Ship Channel, and Yolo Bypass.
Recent levee investigations by the California Department of Water Resources (DWR) and US Army Corps of Engineers (USACE) have shown deficiencies in these levees. To address these deficiencies, the City has initiated the West Sacramento Levee Improvement Program (WSLIP). The goal of this program is to rehabilitate and strengthen the West Sacramento levees, thereby reducing the risk to people and property from the flood event with an annual exceedence probability of 0.005 (p=0.005, referred to as the “200-year” flood event). The City is evaluating alternatives for meeting this goal.
Previously, we evaluated the benefits of fixing the levees to address issues such as underseepage and slope stability. These are called “static fixes.” Subsequently, DWR considered the need to include “seismic fixes” in the levee project in addition to the static fixes. Seismic fixes are enhancements of the levees to resist damage from earthquakes. Specifically for this analysis, the seismic fixes included stone columns and deep soil mixing.
The question we sought to answer was: What is the incremental benefit of investing in a seismic fix today, as opposed to not and then repairing the levee after a seismic event occurs?
To estimate the incremental benefit, we used the Corps’ computer program HEC-FDA to compute economic flood risk to the City, measured as expected annual damage (EAD), over a 50-year period. The incremental benefit is the without-project EAD minus the with-project EAD. We provided the incremental benefit estimate to the City, which the City would then compare with cost to determine whether the seismic fixes were economically feasible.