Tiscornia Marsh Restoration:
A First Step Toward Protection Against Bay Flooding
Most of east San Rafael and much of downtown San Rafael are below today’s high tide levels and would flood today were it not for three miles of levees along the bayfront and another seven miles along the Canal. Nearly 8 of these 10 miles of levee are below today’s “100-year flood” elevation and are very vulnerable to rising sea levels flooding over the top. Were flooding to happen, there could be as much as 2 to 3 feet of water today and deeper in the future with sea level rise.
And the future doesn’t look much drier: There is a clear progression of sea level rise that scientists predict will continue. Mean sea level at the Golden Gate have risen about 10 inches since 1850. The most recent sea level rise estimates by the California Ocean Protection Council project between six to twelve inch rise by 2030, 1.1 to 2.7 feet by 2050, and 3.4 to 10.2 feet by 2100.
The Tiscornia Marsh Restoration project is dedicated to using “nature-based” components to restore habitats and to improve flood protection from sea level rise. “Nature-based” means using natural environmental systems in place of or together with more traditional engineered structures to achieve bay shoreline protection from erosion and rising sea levels.
The project also aims to provide critical education to the Canal neighborhood about the potential impacts of sea level rise within the community and involve residents in adaptation planning and completion of solutions.
The 20-acre Tiscornia Marsh property, located next to Pickleweed Park in the Canal neighborhood of East San Rafael, consists today of about 4 acres of tidal marsh, open bay, and adjacent levees. The property was donated by Mary Tiscornia in 2008 to the Marin Audubon Society. Since the late 1980s, tidal marshlands at Tiscornia have eroded as much as 200 feet, and a total of about 3 acres have been lost. Though comparatively small in acreage, the rarity of tidal marsh along the San Rafael shoreline gives this isolated small marsh outsized value as habitat for the endangered Ridgway’s rail and for migratory shorebirds.
Additionally, residents in the area have lost a significant buffer against rising sea levels that will eventually flood the Canal, Spinnaker and other inland neighborhoods without adaptation measures. If the marsh keeps eroding at its current rate, engineers estimate it will be entirely gone in 50 years.
The primary reason for marsh loss is wind-driven waves. Regional declines in sediment entering the bay and replenishing nearby mudflats may also contribute to marsh loss at Tiscornia. A small portion may be attributable to boat wake action.
Marin Audubon completed conceptual design alternatives in summer 2018 for a restoration project that will replenish the marshland with sediment, marsh plants, and a protective beach to hold back future erosion. The Marin Community Foundation funded this initial work. Marin Audubon is submitting a grant application in fall 2018 for Measure AA funds to move into the next project phase of engineering design, environmental analysis, and permitting.
Tiscornia restoration includes two separate elements. The first element is to raise the southern levee alongside Canal Boulevard by Schoen Park to strengthen this levee segment for sea level rise. Different alignments will be evaluated reflecting levee encroachment into the marsh and into the playground area.
The second element is to restore marsh that has eroded away and to bolster the marsh’s defenses against future erosion. A cobble and gravel beach would reform the 1980s marsh edge and include wood logs to help hold the beach in place. The north end would include some form of “jetty” to keep beach sediments from moving into the Canal itself. The area between the beach and the existing marsh would be filled with sediment dredged locally. The City-owned diked marsh north of the Pickleweed Park soccer fields would also be restored to the tides, which requires relocating the existing levee to the south to maintain and improve bay flood protection. The Bay Trail would be relocated atop the new and raised levees.
Funding for construction and marsh restoration will be secured following completion of environmental impact assessment and obtaining permits from several regulatory and resource agencies. Tiscornia Marsh is an important early test case for similar restoration projects around the bay.
Please visit http://www.tiscorniamarshp.org/ for more info.