New Study Examines Nature-Based Horizontal Levees to Reduce Flood Risk in San Francisco Bay

A recent paper by USGS and University of California, Santa Cruz researchers explores how hybrid nature-based climate adaptation measures, such as horizontal levees, can reduce the risk of overtopping in San Francisco Bay.

San Francisco Bay relies heavily on traditional engineered levees and berms for flood protection, but these solutions will not be sufficient for future sea level rise (SLR) without significant investment. Sea levels are expected to rise and challenge existing flood defenses in the area, since current low-probability events, like a 1:100-year storm, will occur much more frequently. The potential cost for the San Francisco Bay to protect assets and people is estimated at around $335 billion.

Nature-based horizontal levees, as described by the researchers, are a hybrid solution involving a wide sloping area of vegetated wetland built between a traditional levee and the water. The plants within the nature-based horizontal levee are a wave attenuation mechanism for reducing the probability of wave-driven levee overtopping.

Through hydrodynamic analysis, the study supports that moving from steeper to more gradually sloping and wider horizontal levees reduces the magnitude and probability of overtopping. For example, 1:100 sloped horizontal levees, the most gradual and widest tested, can reduce the risk of exceeding structural design thresholds by up to 30% with 1m of SLR. Steeper 1:20 sloped designs can offer up to a 20% reduction. While wider horizontal levees offer greater risk reduction, the study also notes that they require a larger spatial area and incur higher land purchase costs.

The study also notes that risk reduction is maximized in areas where the current levee is fronted by relatively deeper water with little to no existing marsh platform.

Read the entire study, Nature-based solutions extend the lifespan of a regional levee system under climate change, in nature.com Scientific Reports.