Over the past 60 years, a network of levees has been constructed along the lower Santa Clara River downstream of the Los Angeles County line. The levee network includes both public levees and private levees designed to protect agricultural lands, urban development, and floodplain mining pits from floodwater during all commonly occurring flood events. By constraining floodwaters locally and preventing floodplain inundation, the levees can act to increase flooding potential downstream and can also have a negative impact on channel and floodplain geomorphic and ecological processes. In an effort to understand the flood risk management and geomorphic/ecological benefits associated with levee setback, a hydrodynamic model was developed to examine the impact of targeted levee setback on flow hydraulics (i.e., flow depth and velocity) and floodplain inundation during high flow conditions. The modeling analysis included examining the impact of levee setback on flow hydraulics at four discrete floodplain parcels along the lower 8 miles of the Santa Clara River for current and proposed future flood flows that account for climate change. The modeling showed that the effects of levee setback on flow depth and velocity were localized around the setback areas primarily due to limited floodplain storage capacity and downstream hydraulic controls (e.g., bridges and the ocean), and confirmed that setback at the largest floodplain parcels with the lowest average ground elevation had the greatest impact. Moving forward, the developed model can be used to assess the impact of other proposed channel/floodplain modifications (e.g., levee removal or installation) on flood risk management and geomorphic/ecological processes.

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