Mud from bottom of bay could be relocated to help fight sea level rise
Protecting the Bay Area from sea level rise may all come down to mud.
That’s the finding of a new report from San Francisco Estuary Institute that tries to address a twopart problem related to the looming threat of sea level rise: the lack of natural sediment coming into the bay and the need to reinforce its shorelines to protect the region from rising seas.
There’s a fairly straightforward solution, the nonprofit research organization proposes: Take the sediment that’s dredged from the bay’s shipping channels and barged out to sea or to deep parts of the bay — 2½ to 3 million cubic yards of mud a year — and use it to restore wetlands on the perim
eter. The estimated cost is $25 million to $35 million a year, which the authors say is far less expensive than building seawalls or making emergency fixes to flooded highways and airports.
“Sea level rise is just a big challenge, but really because of the power of mud we can put our ecosystems back together and make them still resilient,” said Letitia Grenier, a coauthor of the report. “It’s just that we have to undo some of our former practices that don’t end up being sustainable.”
One of those practices is widespread damming of rivers and streams that empty into the bay, which prevents natural sediment from trickling down as it used to. The amount of sediment needed to protect the bay from sea level rise by 2100 is more than 450 million cubic yards — more than twice the volume excavated to build the Panama Canal — according to the report.
Sea level is expected to rise by at least 1 foot by 2050, according to several models, though storms and surges could cause it to go higher. The California Ocean Protection Council recommends coastal communities make their shorelines sufficiently resilient to handle 3½ feet of sea level rise by 2050 and predicts the California coast could face sea level rise of 7.6 feet by 2100.
By 2100, 600,000 people and $150 billion in property could be at risk in the Bay Area if nothing is done to stop carbon dioxide emissions, the U.S. Geological Survey says. Threats include flooding to residential areas and to infrastructure like airports and water treatment plants.
Plenty of sediment used to wash into the bay, especially during the Gold Rush, when whole mountainsides were blasted for mining. That natural sediment has long since worked its way through the system, but dams were built that blocked sediment from coming into the bay.
Amy Hutzel, deputy executive officer at California State Coastal Conservancy, an agency that works on wetlands restoration and wasn’t involved in the report, said government agencies have long recognized that there isn’t enough sediment in the bay.
“That, along with sea level rise, is going to create challenges for wetlands in the future,” she said. “But also, there are things we can do. This is not happening tomorrow or next year. But there are things we need to be doing this decade to prepare for this.”
The state’s goal is to have a total of 100,000 acres of marshlands in the San Francisco Bay, which means restoring an additional 60,000 acres to add to the existing 40,000 acres. Hutzel said that is a lofty goal, but there are already plans and funding in place to restore at least 36,000 acres by 2030.
“The longer it takes to get these projects into place, the less chance the wetlands have of keeping up with sea level rise,” she said. “We need to restore them now so they can accumulate sediment and start to vegetate and then keep pace with sea level rise.”
Some wetlands restoration projects have used the mud that gets scooped up from the bottom of the bay, including sites at Sonoma Baylands near Sears Point and Hamilton Field in Novato. Sediment was pumped behind existing levees put up for agriculture or military uses and then the levees were breached. Eventually plants took root and marshland returned.
The U.S. Army Corps of Engineers is responsible for dredging most of the main shipping channels around the ports of Oakland, Redwood City and Richmond, the latter used mostly for oil refineries. It also dredges San Pablo Bay and Suisun Bay, which allows ships to head up to Stockton and Sacramento.
The federal government, which funds the dredging at around $50 million to $80 million annually, requires it be done in the most lowcost and ecological way possible. The least expensive disposal options are shipping the sediment 50 miles out to sea or dumping it at a deep site near Alcatraz Island, where some of it may eventually drift back into shipping channels and need to be dredged again.
The sediment sent for “beneficial reuse” or wetlands restoration, requires specialized equipment that adds to the cost. It also has to be tested for heavy metals or other contaminants. State bonds and parcel taxes have funded some of these projects.
“In my opinion the SFEI is correct, that sediment is going to be an increasingly valuable resource for the bay,” said Edwin S. Townsley, deputy district engineer for project management at the Army Corps. “The big question, as it often is for government programs, is who pays.”
In addition to buffering the coast against sea level rise, the ecological benefits of wetlands restoration include providing habitat for migratory birds and fish. Natural bay lands are also places where urban residents can easily get in touch with nature, unlike massive sea walls that separate them from the bay, report author Grenier said.
At Bedwell Bayfront Park in Menlo Park, visitors can see a mature marshland on one side and an old salt pond on the other that will be restored to its original state, thanks to wetlands restoration projects being done to prepare for sea level rise.
“It is expensive to move mud around,” Hutzel said, “but in the long run I think it’s worth the investment.”