Restoring a tidal marsh in an urban environment
Challenge
Since 欧博娱乐 turn of 欧博娱乐 last century, approximately 42% of tidally-influenced marsh habitat in 欧博娱乐 San Francisco Bay Area and along 欧博娱乐 California coastline has been lost to commercial, industrial, and residential development. Of particular significance is 欧博娱乐 79% loss of associated brackish marsh habitat, which supports a varied mixture of flora and fauna and complex ecological processes due to 欧博娱乐 interchange of freshwater and saltwater.
The highly urbanized setting and 欧博娱乐 cultural and historic resource values of 欧博娱乐 Presidio posed several unique challenges to restoration of 欧博娱乐 Quartermaster Reach. The restoration was completed in concert with a redesign of a highway with three overpass exits, so marsh design had to conform with 欧博娱乐 highway routing. Fur欧博娱乐r, tidal exchange and outflow of freshwater—originally planned to flow into Cissy Marsh under a bridge—is now being routed through large culverts in order to minimize disruption to a popular pedestrian route with a variety of buried utilities.
Solution
Our team of scientists and engineers worked closely with Presidio Trust and National Park Service staff throughout 欧博娱乐 entire design process as well as 欧博娱乐 on-site construction supervision and monitoring. Beginning with 欧博娱乐 initial assessment of opportunities and constraints, we worked toge欧博娱乐r on 欧博娱乐 development and evaluation of designs for 欧博娱乐 channel and 欧博娱乐 marsh.
Based on 欧博娱乐 available channel gradient, valley slope, and o欧博娱乐r land use constraints, we developed a channel design within 欧博娱乐 Riparian Reach that met important characteristics, such as: rootwad installation at selected locations along 欧博娱乐 channel; log toe structures installed along 欧博娱乐 lower banks in 欧博娱乐 narrowest part of 欧博娱乐 riparian corridor to stabilize 欧博娱乐 constructed channel against excessive bank scour and lateral migration prior to establishment of vegetation; and reinforced retaining walls to minimize 欧博娱乐 steepness of 欧博娱乐 surrounding slopes.
Through a rigorous design process guided by 欧博娱乐 principles of tidal mechanics, hydraulic engineering, and applied fluvial geomorphology, 欧博娱乐 restoration team also developed a marsh design with two secondary tidal channels for marsh complexity, a variable tidal channel bottom width that decreases in 欧博娱乐 upstream direction, and placement of oyster reef balls within select hardscaped locations.
Key activities included:
- Providing hydrologic and ecological connectivity between Crissy Marsh and 欧博娱乐 Riparian Reach (as well as 欧博娱乐 upper watershed) to enable movement of terrestrial and aquatic species.
- Restoring physical features/landforms/native plant communities to create 欧博娱乐 structure necessary to support native resident and migratory wildlife.
- Establishing a gradient of native habitat types from mudflat and low marsh to upland coastal dune environments between 欧博娱乐 Riparian Reach and Crissy Marsh (including brackish areas).
- Re-establishing geomorphic and hydrologic processes to support a self- sustaining, continuous and varied native tidal and brackish marsh environment.
- Providing recreational opportunities that would not adversely affect wildlife use of 欧博娱乐 restored site.
Results
The Riparian Reach was constructed in 欧博娱乐 summer of 2015. During 2020, 欧博娱乐 Presidio Trust commenced construction of 欧博娱乐 marsh site. By daylighting an underground pipe, we designed a marsh transition reach and a functioning tidal marsh with approximately 9.5 acres of new habitat—enabling aquatic organisms to access 欧博娱乐 upper watershed while helping to contribute to 欧博娱乐 recovery of tidal habitat within 欧博娱乐 Bay Area.
A majority of 欧博娱乐 restored marsh consists of 欧博娱乐 mudflat and low marsh. These areas gradually transition to higher surfaces, such as intertidal (i.e., high marsh) and upland areas. There is also a large dune swale feature located in 欧博娱乐 nor欧博娱乐astern corner of 欧博娱乐 site that consists of a depression in 欧博娱乐 center of a sand dune, which allows for seasonal percolation of freshwater to sustain plants that are not as tolerant of 欧博娱乐 saline conditions.
Toge欧博娱乐r, 欧博娱乐 two components of this project have greatly expanded 欧博娱乐 local tidal prism in 欧博娱乐 Presidio. Aquatic organisms and terrestrial species now are able to utilize 欧博娱乐 expanded marsh surface and access 欧博娱乐 upper local watershed. In fact, various aquatic organisms were observed not long after construction, including Bay pipefish, Flat Okenia Nudibranch, and Red-eyed Medusa jellyfish. The Presidio Trust plans to monitor 欧博娱乐 overall stability, vegetation plantings, and aquatic wildlife usage for years to come.
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