Ryan Moin, MESM July 2017
Take a walk along a San Francisco Bay shoreline. It wouldn’t be surprising to find concrete rubble lining the water’s edge. This is called shoreline armoring, and it serves the important purpose of providing on-shore development with protection from storm surges. But there is a big downside to this method. It is not easily adaptable to sea-level rise, nor does it provide a welcome environment for recreation or supportive habitat for wildlife.
Shorelines protected with concrete rubble are typically adapted to sea-level rise by adding more material to the structure. This is not sustainable in the long term because loss of sediment due to erosion leads to the need for larger and larger additions of concrete material. Ultimately these structures fail to contain sea-level rise.
Change may be on the way for Alameda County however, pending the results of Oro Loma Sanitary District’s horizontal levee demonstration pilot project. Sustainable Silicon Valley’s water team visited Oro Loma on July 13 and was given a first-hand look at the project.
The Horizontal Levee Project
Oro Loma’s horizontal levee project seeks to replace the coastal armoring method with the use of gradual slopes, featuring soil types and plant communities that cause the levee to act as a wetland. Benefits to this method include effective protection from future sea-level rise and storm surge events, creation of valuable natural habitat for native species, and cleaner wastewater discharges to the bay.
The project is funded by the Oro Loma and Castro Valley Sanitary Districts, and also by a $2.1 million grant from the Department of Water Resources. Additional organizations contributing research and labor to the project included: Save the Bay, UC Berkeley, San Francisco Estuary Partnership, The Bay Institute, and Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure. The project took two years to build, and the pilot is now in its first of a five-year monitoring phase.
A horizontal levee requires the addition of a layer of gravel as well as a top layer of sand. Great quantities of plants, from as many as 30 species, also have to be hand-planted. For the pilot project, volunteers transplanted 70,000 individual plants onto the horizontal levee. Save the Bay played an important role in helping Oro Loma with this task. Seed was collected, and germinated in a nursery. The seedlings were then brought to Oro Loma and installed in planter beds, where they were allowed to grow for 6 months. After this time, they were planted into the horizontal levee, and given a further 6 months to mature. Clearly this process requires a lot of patience. The staff at Oro Loma believes the wait is worth it, as the levee should become self-sustaining.
This horizontal levee was densely packed with plant growth, and was characterized by our guide as a “living levee.” A living levee is desirable in that it has the ability to absorb physical damage from storm events, and yet be able to repair itself. A living levee is also adaptive to sea-level rise provided it has space to move landward, and that it has sufficient sediment to keep up with sea-level rise.
The Oro Loma pilot horizontal levee is divided into 12 sections (or cells) separated by 3-foot mud walls. The cells feature different combinations and relative proportions of wetland plants. The goal of this experiment is to determine the ideal treatment cell that can be built to accomplish the goals of the levee. Treated wastewater is fed into each cell from a common line. The water leaving the cells is monitored for flow by meters. Sample ports also allow project staff to take samples of outgoing water and analyze their quality. Therefore, by the fifth year of the pilot, they hope to find out what combination of soils and plants make the ideal cell for the levee.
Future in the Bay Area
If the horizontal levee concept is successful in this pilot, it may be adopted by Alameda County. If it succeeds in Alameda County, there is potential for the practice to spread elsewhere in the San Francisco Bay Area. However, living levees are not possible everywhere throughout the Bay. Such levees require both space and time to work. As horizontal levees are in fact artificial wetlands, they need lots of space, which in our highly developed environment is hard to come by. They also need time and careful management to become established.
The thought-provoking presentation given to the Sustainable Silicon Valley’s water team ended with an inspiring image. It was an artist’s portrayal of what Bay Area shorelines could look like if this project is successful, and horizontal levees are able to be built elsewhere in the Bay Area. The picture painted was of a lush, green, park-like setting, where Bay Area families could come to enjoy native wildlife and recreational opportunities.
Four years from now, the results of this project should provide answers as to whether the horizontal levee can serve a role in future San Francisco Bay Area shoreline management. What is already certain is that should the pilot project be successful, horizontal levees would definitely be a “win-win” outcome for the Bay Area, its residents and native wildlife.
This article was produced based on information presented by staff of Oro Loma Sanitary District to the Water Team of Sustainable Silicon Valley.