Horizontal Levee: Preferable Solution

Foster City levee under construction in 2021

We've been posting about the Foster City levee project since 2016. Now that it's complete, the Chronicle says that our sea wall is the template for the Bay Area for the rest of the century:

the $90 million, 6.2-mile project is worth a visit for anyone wondering what the future might hold — in this case, a glum wall with great public access....

There’s a bleak simplicity to the levee, and its intent is clear — protection from whatever the bay and natural forces might dish out in decades to come.

Illustration by Oro Loma Sanitary District

Not so fast: horizontal levees can also protect Bay communities from floods, have more pleasing esthetics than our sea wall, and reduce algae blooms that are toxic to fish. They're also cheaper to construct.

Instead of a vertical wall to protect against storm surges, Horizontal levees couple traditional levees with a gentle, vegetated slope leading down towards the Bay in order to protect communities from sea-level rise while also removing nutrients from the water. The slope can be built with local materials like dredged sediment overlain by native plants. A buried layer of gravel and sand within the horizontal levee conveys treated wastewater underneath the soil while wetland plants can extend their roots into the permeable layer to get water.

Basically, horizontal levees are constructed marshlands that are built with a gradient and have native plants that filter wastewater. They have their own environmental concerns because of dredging and construction right on the shoreline, but given that the models say that rising seas are inevitable, horizontal levees are a viable, some would argue preferable solution to the Foster City levee.