IT IS HARD TO THINK of a tidal marsh as being in danger of sea level rise. After all, it is already surrounded by water. But a tidal marsh — like the 100-acre wetland at China Camp State Park in southern Marin County — is essentially a water garden that depends on sunlight and shallow shores. Its grasses shelter unique fish that feed birds and special status species like the salt marsh harvest mouse and golden eagles.
As seas rise, marshes can drown and become open water, wiping out entire ecosystems that people and animals depend on unless they are replenished with new sediment.
A pilot project underway in Marin County could open the door to a new, lower-cost and lower-carbon method of wetland restoration by sending waves of new sediment from areas of human settlement into nearby marshes.
The experiment, planned for Gallinas Creek, would test a small-scale version of an innovative dredging technology used in large shipping ports around the world.
“We are the first people to do this in a tidal flood-control channel, a whole new application,” said Roger Leventhal, senior civil engineer for Marin County. “We are working in water that is smaller and shallower.”
According to Leventhal, people living along Gallinas Creek, which sits just north of China Camp, have been wanting the county to dredge the creek since it was last dredged in 1992. The buildup of fine grain clay and mud is making the creek shallow and causing the water to become stagnant, and many residents are upset because they can’t access their homes by boat.
“There was a whole East Marin culture built around the creek,” he said. “Not everyone in Marin is a bazillionaire, in fact, Santa Venetia has a lot of people who lived there forever and they’re not rich. They had a boating culture, and all those docks are silted in. I think that part of the culture is lost, which is too bad.”
Under the concept, a small, shallow-draft dredging vessel would liquify the sediment in Gallinas Creek by injecting water into its upper layers. Through the magic of physics, the injection creates something called a turbidity current, a train-like cloud of suspended clay and silt particles with a head, body and tail. It generates its own momentum, crawling across the floor of a lake, bay or sea, leaving a deepened trench. The train of particles that leave Gallinas Creek into San Pablo Bay would then join the daily ebb and flow of tides, passing through the China Camp marsh, where much of the sediment would settle, according to preliminary studies.
Sentiment over sediment
Leventhal said that the computational modeling done by the U.S. Army Corps of Engineers shows that the sediment mostly stays within the marsh. Eventually, the pilot water injection dredging would be done during periods when there are more wind and waves, when even more sediment is suspended in the water.
The idea is that the sediment will nourish bayfront marshes using the forces of nature, rather than requiring heavy machinery to mechanically scoop, barge and dump the material.
“With the caveat that we’re not controlling it at that point,” Leventhal said. “Some of it might go out to the Bay or through the Golden Gate. Not every particle’s going to go onto the marsh. It’s going to go where it goes, and it might be decades. That’s part of how nature works. You have to think in geologic time a little bit.”
Water-injection dredging has been used in places such as the Mississippi River system and ports on the East Coast, as well as in Europe, but those systems typically operate in deeper harbors and reservoirs.
Because Gallinas Creek is shallow, the pilot project will require the design of a new vessel, perhaps a small skiff rigged with a water injection device.
Leventhal has already been working on another project, funded by a grant from the San Francisco Bay Restoration Authority, a regional tax and grant agency created to fund projects that protect and restore the Bay shoreline.
That grant provided the county $1.3 million to evaluate a smaller version of a more traditional method called hydraulic dredging, which vacuums the sediment into a pipe to place it directly.
“One problem with the hydraulic dredging is that it has a little cutter head, so they keep finding fish,” Leventhal said, noting that there are a lot of rare species in the Bay marshes. “You can’t grind up an endangered fish.”
Last month, he secured an additional $640,000 from the same agency for the innovative Gallinas Creek water injection pilot project.
“This grant doesn’t build anything,” he said. “It’s just design and permitting, but we’ll have everything we need in order to be able to build it with the next grant.”
Modeling and monitoring
Regulators may require extensive modeling and monitoring to ensure the sediment plume does not create unacceptable results, spread contaminants or damage habitat. Multiple agencies would likely review the project, including the U.S. Army Corps of Engineers, the San Francisco Bay Regional Water Quality Control Board and the San Francisco Bay Conservation and Development Commission.
The big hook for the granting agencies is to see if sediment can benefit the natural marshes’ sustainability against sea level rise.
“There was a whole East Marin culture built around the creek. … They had a boating culture, and all those docks are silted in. I think that part of the culture is lost, which is too bad.”
Roger Leventhal, Marin County senior civil engineer
There are other potential applications around the Bay Area. Cities and counties might afford to manage their own creeks and channels in ways that benefit nearby marshes.
The Army Corps of Engineers will only dredge areas where there is some commercial or navigational interest, but maintenance for smaller channels falls into the hands of local governments. If successful, the Gallinas Creek pilot could provide a new nature-based tool for managing sediment in tidal channels across San Francisco Bay while helping restore marshes that play a critical role in flood protection and wildlife habitat. “There’s tons of channels in the South Bay, San Mateo, Santa Clara that are silted in that nobody can afford to dredge,” said Leventhal.
The estimate for the build-out and execution of the future pilot project — including fabrication of a small dredging vessel, monitoring equipment and reporting — could be around $2 million.
Supporters say the project reflects a growing movement toward “engineering with nature,” an approach that attempts to harness natural physical processes rather than override them with heavy infrastructure.
