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Horizontal Levees Are Spreading Across the Bay Area

The Bay Area is increasingly adopting horizontal levees, a type of green infrastructure that blends with natural landscapes to provide flood mitigation and habitat restoration. Unlike traditional steep, rock-clad seawalls, these levees feature gradual slopes covered in native plants.

The Palo Alto Horizontal Levee Pilot Project, seeded with over 35 native plant species, is designed to interact with Bay waters like natural estuaries. Its gradual slope and dense vegetation help break waves, similar to a beach, as explained by lead engineer Mark Lindley. This approach contrasts with traditional levees that use steep faces and large rocks for protection.

The Oro Loma Horizontal Levee near Hayward, completed in 2017, served as a model for the Palo Alto project, with about a dozen similar projects in various stages of development around the Bay. The Palo Alto pilot is unique in its hydrological connection to the Baylands Nature Preserve via plumbing from the Palo Alto Wastewater Treatment Center. Treated wastewater irrigates the levee's plants, which then remove nitrogen before the water filters into the Bay, improving water quality.

By mimicking natural environments, horizontal levees aim to restore habitats crucial for endangered species like the salt-marsh harvest mouse, which relies on vegetation to escape rising tides. Wildlife biologist Katie Smith believes the levee design offers optimal potential for these mice. Although the 500-foot pilot project restores a small habitat area and doesn't provide full flood control, it will generate data over five years from researchers monitoring its benefits.

The collected data on recreational use, biodiversity, and water quality could help justify the higher costs often associated with green infrastructure. The Palo Alto project, nearly a decade in the making, faced delays due to complexities in connecting the wastewater treatment plant's plumbing. Despite not being fully operational, its gentle slope is already functioning as a flood break, a critical feature as sea levels are projected to rise significantly by 2100, with temporary rises expected from weather systems like El Niño.

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