Case Study: Oro Loma
Horizontal Levee Project

Restored Wetlands Case Study Image

This unique experimental project is located on the property of the Oro Loma Sanitary District treatment plant that is jointly owned by the Oro Loma and Castro Valley Sanitary Districts. The project includes a wet weather treatment basin and a horizontal levee, in addition to an on-site nursery to supply plants for the horizontal levee. The wet weather treatment basin is designed to capture and hold excess stormwater runoff. The horizontal levee is designed as an experiment to test the concept that a gently sloped horizontal levee can provide flood protection and needed habitat at the edge of the Bay, as well as improve water quality by polishing treated wastewater all at a modest cost compared to traditional flood protection alternatives.

Unlike the earthen sea walls of a traditional levee, the horizontal levee has a wide, gentle slope that mimics historic wetland habitat. This slope will build elevation with the accumulation of living and dead biomass.  This gradual increase in elevation can help provide flood protection during storms and extreme high tides and hedge against sea level rise.

Construction of the Horizontal Levee project began in Spring 2015 and was completed in April 2017. The levee slope was constructed as a full-scale pilot project simulating natural transition zone habitat historically found at the Bay’s margins. For the purposes of experimentation, the levee was divided into twelve sections or cells and each cell contained a specific composition of plant species and soil treatments. The division into experimental cells allows scientists and students at UC Berkeley to conduct water quality tests to determine which combination of soil types and plants best remove nutrients.

Save The Bay’s Work on the Horizontal Levee

Save The Bay collected seed to create a mix for plant cover that could compete with weedy invasive species, and also collected  rhizomatous (underground stem plant parts) to grow and multiply into the 70,000 plants needed for the project. Collection occurred in Fall of 2014 for planting in the Winter of 2015 – 2016.  We collected seeds and rhizomes from sites along the eastern side of the Bay, close to the project site, and in areas where we maintain collecting permits. These sites included East Bay Regional Parks: Point Pinole, Coyote Hills, Lake Chabot, and Garin Park. To collect the seeds for the annual seed mix, we collected from additional sites outside the East Bay included Sears Point in Sonoma and Foothills Park in Los Altos Hills. Over twenty plant species comprised the ultimate goal of 70,000 plants and over three million seeds were collected for the seed mix. Save The Bay grew the plant material in 17 raised beds that we constructed at the Oro Loma Sanitary District. These beds accommodated the propagation of both the rhizomatous plant species and some of the species grown for the annual seed mix.

To install the ~70,000 plants required for this project, Save The Bay utilized a combination of staff and volunteers. Save The Bay’s restoration staff led a total of nine volunteer programs and committed 28 staff workdays to outplanting the Oro Loma site from mid-November 2015 until early February 2016. Over 220 volunteers helped contribute to the planting work.

Results and Lessons Learned

The propagation methods used for this project were the first of its type for Save The Bay as most of our restoration work has been focused on growing plants in containers within our two native plant nurseries at the Palo Alto Baylands and the Martin Luther King, Jr. Shoreline. This was an opportunity to try new methods of plant propagation and to test methods to understand how we scale up methods to meet larger project demands.

Growing the plants for a project of this size in raised beds was far more successful than we anticipated. Initially, we were concerned whether 17 beds were adequate to grow the large number of plants needed for the project. However, the number of plants remaining in the beds after the slope planting was completed suggest that we could have vegetated several projects of this size. The raised beds continue to provide plant material for several of our other projects.

The high quality soils, irrigation in the initial stage of plant installation, and subsequent water flowing through the slope resulted in rapid establishment of high quality habitat. The vegetation establishment was more rapid than expected by Save The Bay staff, and the quality of the plant community mosaic has exceeded expectations. The annual seed mix of native weedy species in addition to the dense cover of rhizomatous perennial species was very successful in limiting establishment of any significant populations of typical weedy species such as mustard, fennel, or other problematic species. In fact, the site comprised approximately 90% cover of native species only 18 months after implementation.

Preliminary water quality results indicate that the levee slope is very efficient at reducing nutrients and some recent monitoring showed that the slope is providing habitat for bird species. The project team is continuing to monitor the project and will share results with the public as they become available.

Implications for Management

The predictions for increase in sea levels associated with climate change make it critical that we plan for habitats along the edge of the Bay that can provide space for marshes to migrate and to protect the shorelines. This project demonstrated that successfully creating this type of habitat is possible and Save The Bay’s work shows that nursery operations can be scaled to meet that need.

In addition, most of the Bay Area’s wastewater treatment plants are located near the shoreline and as treatment plants seek out ways to efficiently treat wastewater, this horizontal levee concept may provide an attractive alternative in areas where than can be appropriately located.