Multi-objective projects in the Sacramento – San Joaquin Delta maximize benefits for flood control, water supply, and habitat by protecting the lands, water quality, and valuable ecosystem. Maintaining and enhancing the biodiversity of the Delta ecosystem is essential as the Delta is the largest estuary west of Mississippi. The key for success is flexibility.
The Delta, located near the San Francisco Bay, is a hub for water supply to more than 24 million Californians. Water supply reliability and flood management are directly correlated with a healthy ecosystem. Multi-objective projects are increasingly encouraged to provide habitat while protect life and valuable lands from flooding. These types of projects minimize impacts on the ecosystem and integrate restoration of habitats for fish and wildlife. Managing competing demands to protect ecosystems, save lives, and adapt to the changing climate are the goals of such project.
A project on Bethel Island, Horseshoe Bend, is prime example of how a multi-objective project can result in a successful outcome. This unique levee rehabilitation project brought together private landowners, local government, state, and federal agencies to expedite financing and construction cooperatively and innovatively. In addition to reducing flood risks, the project also enhanced and restore the river and the surrounding riparian vegetation and habitat. Our client approach to projects is to work with all stakeholders to make sure the outcome is beneficial for all that depend on the infrastructure. GEI, as the District Engineer, is the lead in planning, design, and implementation of District’s projects. We identified the funding source and applied for grants, we also assigned engineers and scientists to plan, design, and implement the project.
The waterside fish-friendly habitat enhancement areas were designed to provide benefits to fish and wildlife. Riparian and tidal wetland vegetation provides rearing habitat and covers for fish and other aquatic organisms, as well as providing hydraulic and predation refuge for migrating salmonids. It also creates the spawning habitat for Delta smelt, as well as spawning, cover, and forage habitat for other fish species. The vegetation provides a source of organic matter inputs, providing nutrients for invertebrates and other small organisms. This new habitat area was designed with an approximately 18-foot-wide bench. The bench was divided into two planting zones: an 8-foot-wide riparian zone at 1-foot above mean higher high water and a 10-foot-wide intertidal zone sloping from the riparian bench down to mean low water elevation. The project relied on the daily tidal inundation of the intertidal zone and the subsurface wetting of the soils in the riparian zone.
The project set the levee back to make wide crown to accommodate emergency services while it cut a portion of the levee on the waterside to create fish friendly habitat. The levees of bethel Island are mostly built on organic peat soils on top of layers of sand and require various methods of seepage control to maintain integrity. The project team decided to install blanket drains and collection system to capture seepage.
Among the challenges was protecting the newly created bench from erosion caused by tidal action and wind-wave and boat wake energy. The solution was two-fold: 1. natural fiber erosion blanket was installed on the bare soil on the bench, and 2. a wave break and groins approximately 1.5- to 3-feet in height and 2- to 4-feet wide were constructed with riprap. The top of the wave break was set at mean high water elevation and consisted of alternating 50-foot segments separated by 5-foot-wide openings to allow unobstructed access to the bench for fish. The perpendicular groins were spaced approximately 200 feet apart, creating breaks between planting areas to capture sediment.
FINAL AND SUCCESSFUL OUTCOME
In addition to a new and improved levee, the project delivered a new waterside habitat which is now in its third growing season and the bench is dense with vegetation and already providing valuable habitat for insects, birds, wildlife, and fish. The key to a successful design was accurately calculating the range of tidal water surface elevations, setting bench elevations to create target habitats, and maintaining elevation controls during construction. This set up the site to be planted with the appropriate species for the desired habitats without the need for supplemental irrigation during site establishment. The key to the overall success of the Horseshoe Bend Project was the integration of the engineering and ecological design teams and an openness from State agencies to creative design solutions.