San Francisco’s seawall has held the line for more than a century, but the next version may do more than block water.
The Port of San Francisco and the West Coast team of the Smithsonian Environmental Research Center, or SERC, are working together on the Living Seawall Pilot Project, inspired by living seawalls in places including Seattle’s Elliott Bay.
Much of San Francisco’s current shoreline is manmade, and the Port says that leaves it with little natural protection from wave energy, coastal erosion and flooding. Brad Benson, the Port’s waterfront resilience program director, said a more natural shoreline would have a “sloped, gradual area where you can have robust, intertidal growth and activity.”
He said steep, manmade shorelines are harder on marine life. Andy Chang, PhD., one of the project’s lead scientists, said that matters across the ecosystem, including birds, fish and marine mammals.
Even in places where the shoreline cannot be sloped, the project is testing ways to make seawalls more habitable. The idea is that adding protective areas could support more biodiversity and help native seaweeds, mollusks and phytoplanktons resist invaders including brown kelp and European green crabs.
San Francisco’s seawall stretches about three miles. The concrete structure was built between 1878 and 1915 and has so far avoided major disrepair. But its age, along with erosion of the weak soils beneath it, has added urgency to rebuilding plans. Sea-level rise is also a growing concern.
“We’ve had approximately nine inches of sea-level rise over the last century, so during king tide events, we start to see flooding in and around the Ferry building, which is the low point along the Embarcadero,” Benson explains.
Chang said seawalls leave limited engineering choices in places where they are required, but there is still room to improve habitat.
“In places where a seawall is a requirement, the options are limited engineering-wise,” notes Chang. “We can add features that make it better habitat, such as adding texture, or shelving or small tidepools … which promotes a greater diversity of species.”
To test that idea, SERC trialled three kinds of tiles in three separate parts of the bay. One set used standard concrete. Another used ECOncrete, a concrete developed by a company founded by marine biologists, but kept the surface smooth. The third used ECOncrete with a textured design of ridges and small shelf-like protrusions.
ECOncrete supplied the tiles with what it describes as a proprietary admixture designed to reduce the more toxic effects of industrial grade concrete. The company’s co-founder, Dr. Ido Sella, said at least 70 percent of the world’s marine infrastructure is built from concrete, and that has a major effect on coastal marine life over time.
The problem is not only the material itself, but the way it is usually used. Standard concrete seawalls like San Francisco’s are often smooth and ecologically “grey,” with no surfaces that organisms can easily inhabit. The manufacture of standard industrial concrete is also a known source of carbon emissions.
SERC monitored the submerged tiles over several years. Team members visited each site at low tide to photograph and record the marine life they found, test salinity and water temperature, and collect samples.
The fieldwork was demanding. The visits sometimes happened before dawn or after dark, and the sites were often difficult to reach. Chang described the work as “exhilarating” and also cold and wet.
SERC is still preparing its final report, but the early signs are encouraging. The seaweed-covered tiles recently displayed along the Embarcadero showed that treated, textured tiles attracted and supported small marine communities. Those included seaweeds and shellfish, sea snails and small fish. Many of the organisms that did well were native to the bay, which Chang said was a bonus for “one of the world’s most invaded bays.”
The project also points to another role for marine life along hardened shorelines. According to the source material, marine biota can help protect industrialised habitats by acting as buffers against weathering, as part of a carbon sink and as natural water purifiers.
The Port is still waiting for the official scientific findings from the pilot, but Benson said the work is already shaping long-term plans. He estimates that nearly three miles of the Port’s coming 7.5-mile flood plan will include nature-based engineering solutions, including living seawall features.
“We just want to be able to demonstrate to the San Francisco public that as we make these resilience investments, we’re not only just reducing flood risk and reducing earthquake risks, we’re leaving the waterfront a better place,” he says.
