| “One of the findings was that in order to stop this trend, fairly substantial actions were required.” | |
| Bryan Largay, Elkhorn Slough National Estuarine Research Reserve |
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An experimental underwater retaining wall called a “sill” was recently constructed at a California estuary to slow fast tidal currents that were causing tidal scour and marsh dieback. Early results indicate that the project is a success—and could be a model for coastal resource managers looking for ways to address similar impacts caused by rising sea level.
“To our knowledge, a sill has never been used for this purpose before,” says Bryan Largay, tidal wetland project director at Elkhorn Slough National Estuarine Research Reserve, a 1,700-acre tidal salt marsh inland of Monterey, California.
The project is the result of more than six years of study by more than 100 scientists, resource managers, and conservationists participating in the Elkhorn Slough Tidal Wetland Project, a collaborative, ecosystem-based management effort. The group looked at the habitat changes that tidal scour and marsh dieback were causing in the estuary and then designed the sill as a cost-effective solution.
“What’s happening here looks a lot like the sea level rise that is predicted to happen over the next 50 or 60 years,” Largay says. “The hope is we can be considered a long-term monitoring site for detecting the effects of climate and sea level rise on estuaries.”
Too Much, Too Fast
The problems in the estuary began in 1947 after the construction of a stable inlet with jetties at the mouth of the estuary. Increased currents began washing fine mud from the tidal channels.
In the 1980s tides were returned to Parsons Slough, a large branch of the estuary once reclaimed for agriculture, which had subsided by several feet. When the tides returned, the amount of water in the system increased, accelerating the currents further. Over the past 60 years, the estuary gradually eroded, deepening from about 5 to 10 feet, to 20 to 30 feet.
One of the effects was that the elevation of mean high water rose by about a foot over the past 60 years. The additional water in the system drowned marshes, causing them to die back. As a result, the estuary has lost 15 to 20 percent of its marshes—about 150 acres total, Largay says.
The impacts of excess water experienced at Elkhorn Slough are not unlike predicted impacts of sea level rise on marsh environments. The Intergovernmental Panel on Climate Change 2007 report suggests that by 2080, sea level rise could convert as much as 33 percent of the world’s coastal wetlands to open water.
Critical Habitat
Wetlands provide critical habitat for many species, play a key role in nutrient uptake, serve as the basis for many communities’ economic livelihoods, provide recreational opportunities, and protect local areas from flooding.
Elkhorn Slough is a diverse ecosystem hosting 340 bird, 550 marine invertebrate, and 102 fish species—two dozen of which are listed as rare, threatened, or endangered—and is a designated Globally Important Bird Area.
Research shows that some offshore species, such as sole, flounder, and halibut, also rely on the estuary’s waters during their early development. Moss Landing Harbor, located at the entrance to the slough, is one of the state’s most active fishing ports.
Grappling with the Issues
Realizing that the erosion and marsh dieback would only get worse, the reserve initiated the Tidal Wetland Project in 2004 by bringing together prominent scientists, managers, regulatory agency staff members, and local community leaders.
“We brought a couple of dozen people to the table for about 30 all-day meetings where we grappled with what was going on, what we thought was causing it, the impacts not only to the environment but to the surrounding communities, and what could be done about it,” Largay says. “One of the findings was that in order to stop this trend, fairly substantial actions were required.”
Complicating the situation was the concern that reducing the tidal flushing in the estuary would cause water quality problems because of excess nitrogen in area runoff.
Finding the Solution
The project team came up with three potential solutions for reducing tidal energy, Largay says. “We needed to either create a long, narrow sinuous channel, create a bottleneck at the mouth of the estuary, or elevate the land to restore Parsons Slough.”
Adding enough sediment to Parsons Slough to restore tidal marsh would have cost $50 million, Largay says. Constructing a sill that acts as a “speed bump” at the mouth of Parsons Slough to slow the flow of the tide and reduce erosion was the most efficient and lowest risk approach.
With a $4.5 million grant from NOAA through the American Recovery and Reinvestment Act, construction of the 200-foot-wide and 10- to 15-foot-tall sill started on November 10, 2010, and was completed in February. The structure is two to five feet below water at low tide.
The engineering, permitting, and construction of the structure—which is in an already-disturbed area near an existing railroad bridge—were all conducted under rigorous scrutiny from regulators and the local community.
Environmental monitoring ensured that wildlife species were unharmed. Long-term monitoring will continue for a number of years.
Early Results
Early monitoring indicates the project is a success. The sill is working to slow the flow of the slough’s waters, and there is also no evidence of water quality problems.
Long term, the sill is expected to prevent thousands of cubic yards of sediment from washing into the bay each year. The additional sediment is anticipated to rebuild seven acres of tidal marsh around Parsons Slough.
Largay notes that the project also created or saved approximately 107 jobs through contracts and direct hires.
While Largay is “very pleased with how everything came together” and believes that others will be able to use the lessons Elkhorn Slough is learning from the project, he emphasizes that a sill is “not a solution to every problem. It needs to be thoroughly evaluated.”
He adds, “Better planning in advance rather than coming in to build a structure to counter the effects decades later is really the way to go. The big lesson was that advance planning of potential environmental impacts is key to good management.”
For more information on the Elkhorn Slough Tidal Wetland Project, go to www.elkhornslough.org/tidalwetlandproject/. You may also contact Bryan Largay at (831) 234-1177 or bryan@elkhornslough.org.
