Source: US Geological Survey
Researchers from the University of California, Santa Cruz and USGS have found that these hybrid structures—wetland buffers that slope gently out in front of traditional levees—can reduce the risk of floodwater overtopping by up to 30%, while also supporting valuable marsh habitat.
Reimagining Levees for a Changing Climate
Unlike conventional levees, which often rely solely on height and bulk to hold back water, horizontal levees incorporate a gradual, vegetated slope on the water-facing side. This natural buffer helps absorb wave energy, reducing the pressure on the levee itself. The result: a more resilient system that blends engineering with ecosystem restoration.
To test the effectiveness of these designs, the team used a hydrodynamic model (XBeach non-hydrostatic) to simulate storm-driven waves and flooding in a range of scenarios across San Francisco Bay, both under current sea levels and future projections.
Their findings showed that as sea levels rise, the risk of levee overtopping during storms will grow significantly. However, horizontal levees with wider and more gradual slopes can significantly reduce this risk. These designs not only buffer storm surge, but also provide new intertidal wetland habitat, which is crucial for ecosystem function and carbon storage.
Maximizing Benefits with Nature-based Design
The study also offers insight into how to design horizontal levees for maximum impact: broader, gently sloped wetland areas are far more effective at wave attenuation than narrower or steeper versions.
The benefits of these nature-based systems go beyond flood control. Wetlands can filter pollutants, support fisheries and wildlife populations, and serve as carbon sinks—while also providing scenic green spaces for nearby communities.
San Francisco Bay, with its mix of dense urban development and tidal marsh ecosystems, is an ideal testing ground for this kind of climate adaptation. The study’s approach could also apply to other wave-exposed urban estuaries around the world.
With billions of dollars projected to be spent on sea-level rise adaptation over the coming decades, horizontal levees offer a cost-effective and ecologically sound alternative or complement to traditional infrastructure.