Project History and Timeline:
The Lower Santa Ana River Reach 9 Phase 2B Project (“Project”) consists of flood protection and environmental restoration along 5,700 feet of the Santa Ana River surrounding the Green Giver Golf Course in Orange County near the Riverside County line. The Lower Santa Ana River flood protection is a feature of the Santa Ana River Mainstem (SARM) flood risk management project authorized for construction by the Water Resources Development Act of 1986, Public Law 99-662. Reach 9 downstream of the Prado Dam is designated to provide environmental open space and wildlife access, along with improvements for flood risk management.
Prior to the Perennial Stream Restoration project, the south bank grouted riprap protection (close to Highway 91) was designed by the in-house staff of the Los Angeles District and construction was awarded to Harper Contracting, Inc., in 2009. This provided an opportunity for environmental restoration due to the temporary diversion of the river. An AE Services contract was awarded to WRC Consulting Services, Inc., in early 2010 for the design of environmental habitat restoration within the project boundaries bound by bank protection/Highway 91 and the golf course. This restoration project serves as environmental mitigation for the temporary and permanent loss of riparian and upland habitat due to construction within Reach 9 and other areas of SARM features. It provides stream environments and habitat appropriate to support the threatened Santa Ana sucker fish as well as riparian woodland habitat intended to support the endangered Least Bell’s vireo and other special-status birds. The Santa Ana sucker is a fish that was common in Santa Ana River streams prior to the 1970s but is now rarely found.
Project Strategies and Details:
Flood Risk Management. The flood protection measures include the south bank grouted stone bank protection to protect the highway and roadway infrastructures and plain riprap protection measures along the created meandering banks where soils are vulnerable to erosion by the river flood flow or lateral storm drain outflow (more than 15 laterals). The flood flow is a potential 30,000-cfs release from Prado Dam. The bank-full discharge is approximately 6,000 cfs, using the golf course as overflow when the flood exceeds this level.
Environmental Restoration. Flows along the Perennial Stream Project average a daily flow of 125 cfs during the non-flood season. To create a perennial stream habitat suitable for the Santa Ana Sucker fish, the following features were included:
• A stream bed consisting of various mixtures of sands, gravels, cobbles and boulders for substrate.
• The channel was designed for a non-flood low-flow inundation depth varying from 12 to 18 inches to support juvenile and young fish and from 18 to 24 inches for adult fish.
• The stream width varies with pools containing slower flows and riffles with higher velocities.
• Riparian trees and large boulders create shading and refuge areas along the stream edge and shallow inundation areas. To enhance the perennial stream habitat, restoration of riparian woodland habitat native to the Santa Ana River corridor was accomplished by the following project features:
• Detailed topographic grading of the banks with varying slopes and terraces for riparian and upland plantings. Soil was backfilled on top of the grouted stone bank protection to create opportunities for habitat establishment.
• Emergent plants were planted along the water edges along with Black Willow cuttings to help naturally stabilize the river banks.
• Arroyo Willow cuttings were planted on the banks and overbank areas where surface runoff and groundwater moisture can benefit riparian plants.
• Understory native plants were planted within the Black Willow and Arroyo Willow communities to help stabilize the ground. Other features of the project included:
• Riprap side drain channels that protect existing drainage systems for conveying stormwater runoff from the hills south of the project site under the 91 freeway and into the Santa Ana River.
• Replacement of the access driveway and vehicle bridge leading to the Green River Golf Club.
• New entry sign and landscaping for the Green River Golf Club.
• Restoration of golf course turf in an area that was needed for construction of the project diversion channel.
• A California native wildflower buffer zone between the golf course and the project restoration area.
• A bank protection maintenance road running along the top of the grouted stone bank protection.
• Relocation and improvements to the Santa Ana River Trail bikeway that runs through the project area.
Results
Plant monitoring started in 2012 one year after the first phase of construction and monitoring of the stream began in 2013 after major construction completion. The monitoring results are encouraging. Along with the installed vegetation, an impressive number of other native riparian plant species have naturally colonized into the project area. The density and diversity of the establishing vegetation suggests a high potential for this project to meet the following ultimate restoration goals:
• 9.74 acres of Perennial Stream Habitats Including stream bed, Black Willow and emergent plants
• 5.8 acres of Arroyo Willow Dominant Riparian Woodland
• 2.7 acres of Mule Fat Scrub
• 7.36 acres of transitional scrub and perennial herbaceous plants
The channel banks have experienced minimal storm flows since they were constructed and show signs of successfully withstanding erosion from flows up to a 450-cfs release from Prado Dam. Areas with dense vegetation showed signs of very minor erosion, while the recently planted areas sustained the expected minimal damage. The banks edges show signs of naturally stabilizing themselves as the vegetation continues to mature.
While the grouted stone bank protection provides flood protection for flows up to a 30,000-cfs release from Prado Dam, the Perennial Stream restoration portion is expected to change over time. Erosion and deposition are expected to occur and will likely sustain a natural stream habitat favorable to the Santa Ana Sucker fish. Vegetated sand and gravel bars have already started to establish in the wider areas of the low flow channel. The combination of armored flood control structures and a flexible restoration area will allow the Lower Santa Ana River Reach 9 Phase 2B Project area to adapt to changing conditions while meeting both flood protection and restoration goals.
Monitoring of the restoration area will continue for a 5-year period following construction completion. Surveys for wildlife habitat and water quality has been conducted on a quarterly basis since 2014 and will continue through 2019. Fish, amphibians, reptiles, birds and mammal species are already utilizing the developing stream and riparian woodland habitats in the project area. Over 60 bird species have been spotted, including special-status species like the California gnatcatcher and Cooper’s hawk. The diversity and abundance of wildlife species is only expected to increase as the project matures.
Replication
The Lower Santa Ana River Reach 9 Phase 2B Project has a high potential for replication among other natural stream restoration projects. The naturalistic grading of the channel side slopes provides greater habitat diversity by creating planting terraces at varying elevations. For example, plant species like Black Willow can be placed on terraces close to the low flow invert of the river, allowing for periodic inundation with water. Other riparian plant species like Arroyo Willow and Western Sycamore trees can be placed on terraces set at slightly higher elevations where less-frequent inundation areas are beneficial for their establishment.
Creation of the perennial stream habitat itself serves as an example of how habitat restoration can be accomplished in channels that have been highly modified from their natural condition. Modifications to traditional pool and riffle design were necessitated by constraints from protecting structures and land uses. The shallow-flow areas were created to widen the river bottom and simulate “pool” conditions without deepening the riverbed. The narrow section reaches were designed to produce “riffle” conditions without steepening the river bottom. Combining the low-flow meander pattern and varying channel geometry allows the river to restore its ecological diversity. The channel geometry ensures that diversified flow velocities are present in support of Santa Ana sucker fish habitats.
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