Levees provide strong flood protection, but they are not failsafe. Levees are designed to protect against a specific flood level and could be overtopped during severe weather events. Levees reduce, not eliminate, the risk to individuals and structure behind them.
The time of year of a failure is an important factor in determining risk. Overtopping is common during high water events in the winter. Multiple failures during large floods generally do not pose an immediate threat to water supplies outside the Delta. In contrast, a structural failure during a period of low inflow, such as summer, can draw ocean salinity into the Delta. The saline water could cause a multi-year disruption to statewide water use. Large-scale disruptions could cost hundreds of billions of dollars annually.
A levee system failure or overtopping can create severe flooding and high water velocities. It’s important to remember that no levee provides protection from events for which it was not designed, and proper operation and maintenance are necessary to reduce the probability of failure.