Sea Level Rise and Resilient Design in Tidal Marsh Restoration Projects
Peter kobylarz & chris Zumwalt, WRA, Inc.
San Francisco Bay is the cornerstone of the environmental, economic, recreational, and social well-being of our region. The effects of sea level rise on the tidal wetlands of San Francisco Bay will have cascading consequences for the region’s inhabitants. In 2012, the National Research Council released a report predicting that the California coast, south of Cape Mendocino, could experience between 16 and 65 inches of sea level rise by 2100. Rising tides will threaten human populations, the environment, and vital infrastructure with potentially devastating effects on the state and local economy. Tidal salt marshes, which are in part characterized by their elevation relative to tidal levels, are at particular risk from sea level rise. In addition to providing habitats for endangered plant and wildlife species, salt marshes provide numerous ecological services, including erosion control along the shoreline, buffering from storm surges, and stormwater filtration. An increase in sea level of even one or two feet will have a drastic effect on existing salt marsh habitats. In some areas, salt marsh habitats may be able to migrate inland (i.e., to higher elevations) with the rising tides, but more often than not in the Bay Area, the development envelope between tidal wetlands and existing construction serves as a barrier that prevents tidal wetlands from migrating inland.
Open space groups, policy makers, and developers recognize the importance of these tidal habitats and salt marsh restoration projects are on the rise in the Bay Area. The success or failure of these restoration projects depends upon maintaining the correct relationship between tidal wetlands and sea level; a salt marsh restoration site that is successful today could be underwater in 20 years if not designed correctly. One solution to this problem is incorporating “resilient design” into restoration projects to account for sea level rise and to allow these critical habitats to migrate inland. This emerging approach is currently being applied to Breuner Marsh, a pioneering example of Bay Area resilient design strategies for tidal marsh restoration.
Case Study: Breuner Marsh
WRA was part of a team led by the East Bay Regional Park District (EBRPD) that developed a wetland restoration design for Breuner Marsh located in the Point Pinole Regional Shoreline in Richmond, California. The project stemmed from decades of community activism to preserve Breuner Marsh from development. Numerous funding partners and regulatory agencies supported the effort. As part of the project, the EBRPD required an element of public access in the form of a recreational and interpretive trail (to be included as part of the San Francisco Bay Trail), which added an additional layer of complexity to the restoration planning effort. However, the site was large enough and contained a diversity of elevations to make it suitable for developing a wetland restoration and public access plan that was resilient to rising sea levels.
WRA’s landscape architecture and GIS departments incorporated resilient design into the project to overcome the numerous challenges associated with accommodating both sea level rise and public access while ensuring the continued presence of salt marsh habitat well into the future.
To develop the design, the site was divided into three elevational areas:
- Low-lying land that could be lowered slightly to create new tidal marsh areas under current sea levels;
- Transitional areas with an elevation range that would likely convert to tidal marsh as sea levels rise; and
- Upland areas farther from the Bay where soil could be placed and the San Francisco Bay trail could be built at a high enough elevation to avoid being inundated under future predictions of sea level rise.
A significant portion of the site was used to hold fill material (soil) with the intention of filling existing wetlands in order to make it feasible for development. These fill materials were targeted for removal to return the area to tidal marsh. Rather than off-hauling the soils to the landfill or to another site, the excavated soil was used onsite to create public access in areas above the elevation of predicted sea level rise. Onsite placement was seen as a beneficial approach as it avoided significant costs, as well as increased emissions and traffic associated with off-hauling and disposal at a landfill. Onsite placement was targeted for elevations that exceed thresholds that would likely convert to tidal marsh in the next 50 to 80 years. Along this corridor, elevations were increased to be above the height of predicted future sea levels to protect the public access infrastructure.
Designing multi-goal projects that involve habitat restoration and public access is challenging, particularly in the context of sea level rise. Multi-goal projects often involve finding a balance among conflicting objectives such as habitat distribution, species protection, cost, and public access. All of these objectives become more difficult to balance when project designs must address rising sea levels. To address these issues, WRA developed a model using ArcGIS and AutoCAD that evaluates design alternatives over time in the context of sea level rise.
The model evaluates existing topography, proposed topography, habitat distribution, and the location of public access facilities. The model also emphasizes minimizing excavation, which reduces construction costs, and identifies the best locations for onsite soil placement to avoid impacting areas that might otherwise convert to tidal marsh habitat with higher sea levels. Fill from unavoidable excavation is used onsite to construct higher elevation areas, such as recreational trails and interpretive facilities.
The first step in sea level rise analysis for Breuner Marsh was to evaluate the existing topography with fine-scale survey data. To accomplish this, WRA used 6-inch contour intervals derived from LIDAR data produced by a contracted surveyor. The LIDAR dataset was converted into a digital elevation model (DEM) to represent existing topography on the site.
The next step was to survey elevations of the existing tidal wetland vegetation and transitional habitat features using a laser level. The survey data helped to determine the appropriate elevation for planting tidal marsh species which are sensitive to slight elevation changes. WRA identified the elevation at the upper and lower distribution of several key tidal marsh species corresponding to different ecological zones within the tidal marsh. Tidal marsh zones and associated dominant plant species typical of San Francisco Bay salt marshes include:
- Mud Flat, characterized by a lack vegetation
- Low Marsh, dominant species include California cordgrass (Spartina foliosa)
- High Marsh, dominant species include pickleweed (Salicornia pacifica)
- Transition Zone, dominant species include coastal gumweed (Grindelia stricta), marsh lavender (Limonium californicum), salt grass (Distichlis spicata)
Figures 1 and 2 show the habitat profiles in 2010 and 2080. Over time the low marsh habitat will expand and the high marsh and transition zone shift as a result of sea level rise.
After establishing existing conditions at the site, elevation ranges for each tidal marsh community were used to create vegetation maps using raster datasets in ArcGIS. These vegetation rasters were then converted to AutoCAD vector format to use as the basis for the development of several potential restoration designs. This analysis was also used to identify adjacent, low-lying areas which are expected to convert to tidal marsh under elevated sea levels. These low-lying areas were prioritized for protection as the basis for this resilient design. In addition, the model helped to predict the amount of fill needed to elevate the new public access facilities above predicted future sea levels. The elevation contour data from the restoration designs were then converted back into a DEM with ArcGIS to create vegetation maps representing the post construction project site under each design alternative. Figures 3 and 4 show the marsh habitat ranges under different sea level rise scenarios.
Data from the National Oceanic and Atmospheric Administration on projected sea level rise, as well as site-specific data on sedimentation rates, helped to determine how the site might respond to rising sea levels. WRA used the following estimates of high and low tide elevations to predict the change in the types and quantity of tidal marsh habitat by 2080, when almost one meter of sea level rise is projected for the site. These estimates were based on data available during the planning phase of this project in 2012 and accepted by the applicable regulatory agencies. More recent data on sea level rise is available and may suggest a more accelerated rise of tidal waters.
These tidal elevation zones were used by the GIS team to isolate sections of the elevation raster and create polygon features which corresponded to the appropriate marsh species habitat ranges. These features were used for area calculations and cartographic purposes. Finally, the team created new vegetation maps representing conditions under elevated sea levels so the project design alternatives could be analyzed by all project stakeholders, including the permitting agencies.
Following successful acceptance by the stakeholder team, the design phase was completed in 2013. Construction of the tidal marsh component of project was completed in late 2014 using the designs derived from WRA’s landscape and restoration model. Several weeks later, the levees used to protect the site from tidal inundation during construction were breached, and the marsh was inundated with seawater. Since the breach, the restoration site has demonstrated several indicators of success, including a rapidly growing community of pickleweed and other tidal marsh species throughout the marsh plain.
The public access components of the project, including parking, ADA-compliant trail, picnic facilities, and several bridges are being constructed in the fall of 2015 and 2016. The park is anticipated to be open to the public by 2017.
Working with a range of stakeholders, WRA used a multi-faceted approach to developing plans for the restoration of tidal marsh habitat at Breuner Marsh. The restoration included large areas that were preserved at an elevation that will convert to tidal marsh habitat as sea levels rise. Excess soils excavated to create new tidal marsh habitat were used to create areas for public access infrastructure that will be protected from inundation as sea levels rise. The excavated material was shaped into natural landforms and at grades to facilitate ADA access along a new segment of the San Francisco Bay Trail.
While the science of tidal marsh restoration has progressed greatly over the years, incorporating sea level rise and tidal marsh migration into restoration projects is still an evolving practice. As one of the first projects in the San Francisco Bay to implement a resilient tidal marsh restoration approach, the Breuner Marsh project will be important to watch over the next several decades as the wetlands develop and change. There will likely be many lessons to be learned as we see the site adapt to sea level rise, lessons which should be incorporated into future planning efforts for both habitat restoration and infrastructure protection projects along the Bay. The full extent of sea level rise remains a troubling unknown for our region. By using creative solutions to the problems we face today, we can try to preempt the consequences of sea level rise and ensure that restored habitats such as the one found at Breuner Marsh will endure for decades to come.
National Academy of Sciences. 2012. California sea level projected to rise at higher rate than global average. Online at: www.dels.nas.edu; Accessed September 23, 2015.
WRA, an environmental consulting firm with nearly 35 years of experience in the Bay Area and throughout California, has used this progressive GIS model to influence several projects in California including Breuner Marsh Restoration and Public Access in Richmond; Yosemite Slough Restoration in San Francisco; Corte Madera Marsh Restoration in Marin County; and San Dieguito Lagoon Wetland Restoration in San Diego.
For questions regarding WRA’s sea level rise and project design model, please contact our GIS team: Peter Kobylarz at (415) 524-7375, firstname.lastname@example.org or Chris Zumwalt at (415) 524-7550, email@example.com.
Check out the Map Gallery of this issue of the BayGeo Journal for more information about Breuner Marsh.
Fall 2015 Volume 8 Issue 2