Flood Risk Management and Ecosystem Restoration Project
11 September 2015
ABSTRACT: The South San Francisco Bay Shoreline Project is a multi-purpose project that will reduce tidal flood risk along South San Francisco Bay, restore approximately 2,900 acres of former salt production ponds to tidal wetland habitat, and enhance existing opportunities for recreation associated with the restored habitat. The non-Federal sponsors for implementation of the project are the Santa Clara Valley Water District for the flood risk management features and the State Coastal Conservancy for the ecosystem features. Recreation features will be implemented by both non-Federal sponsors.
The project is located on the southern bay shoreline along portions of the City of San Jose and Santa Clara County, between the Alviso Slough/Guadalupe River and Coyote Creek. The area includes existing remnant commercial salt ponds--Ponds A9-A15, owned by the U.S. Fish and Wildlife Service (USFWS) and Pond A18, owned by the City of San Jose. The pond dikes are non-engineered structures originally constructed by heaping soft bay mud, and are maintained and repaired by the USFWS and City of San Jose in a similar manner. These dikes were never intended as flood risk management structures, and present a high probability of failure from tidal flooding and sea level rise and increased flood risk for the inland areas behind them.
Flood Risk Management -- The area behind the ponds is heavily urbanized and includes extensive infrastructure associated with commercial, industrial, and residential developments. The project would reduce flood risk for a population of over 5,500 residents and commuters, as well as structures within the community of Alviso, the San José-Santa Clara Wastewater Facility, and the Silicon Valley Advanced Water Purification Center. The project area has a history of fluvial flooding, which has been addressed by other local and Federal projects. There are no historic tidal floods; however, the project area is deeply subsided (up to thirteen feet in some areas) due to historic groundwater withdrawals, and given the high probability of failure of the pond dikes, is considered highly susceptible to tidal flooding. Flood risk will increase with future sea level change.
Ecosystem Restoration -- San Francisco Bay Area has lost 90% of its historic tidal marshes, which provide essential habitat for the Federally endangered salt marsh harvest mouse and Ridgway’s rail, as well as aquatic habitat for other bay species. The flood risk management features would allow ecosystem restoration to proceed by constructing an engineered levee to protect the areas behind the existing ponds, thereby allowing the pond dikes to be breached to restore tidal action and establish tidal wetland habitat.
Recreation -- Existing recreation in the study area is primarily provided by the USFWS’s existing pond dikes. The City’s Wastewater Facility lands are closed to public access, leaving gaps in the San Francisco Bay Trail. The recreation features would create key connections in this regional trail system by creating a publicly accessible trail on top of the new levees (on both USFWS and City lands) and new pedestrian bridges to connect the levees where they cross over an existing railroad and the Artesian Slough. These features are compatible with the proposed flood risk management features and would enhance the public’s experience with the restored environment.
The Recommended Plan is a Locally Preferred Plan (LPP), with a higher levee and a broader, more sloping ecotone transition to the bay providing more flood risk benefits and habitat benefits than the NED/NER plan. The flood risk management features of the Recommended Plan include four miles of new levee along the southern boundary of several former salt production ponds, a flood gate across Union Pacific Railroad tracks, and a tide gate across Artesian Slough. Non-structural measures, such as flood warning systems, evacuation planning, and flood proofing critical infrastructure are also recommended for local implementation. The ecosystem restoration features include measures to prepare the existing ponds for eventual reconnection to the bay (such as dike lowering, ditch blocks, borrow ditches), phased breaching of pond dikes, and monitoring and adaptive management to ensure the project is meeting ecosystem restoration objectives. Recreation features include public access on top of the new levees, two pedestrian bridges, benches, viewing platforms, and signs.
The estimated total first cost of the Recommended Plan, based on October 2015 price levels, is $174,000,000, including $91,688,000 for flood risk management, $75,997,000 for ecosystem restoration, and $6,315,000 for recreation. The flood risk management features have a benefit-to-cost ratio of 4.2, 5.3, and 9.4 at a discount rate of 3.375 percent under the USACE Low, Intermediate, and High sea level change scenarios, respectively. The recreation features have a benefit-to-cost ratio of 1.11. The ecosystem restoration features provide 2,900 acres of tidal wetland habitat and 48,308 average annual habitat units.
The Federal share of the total project cost is about $70,261,500, consisting of 65 percent of the NED/NER Plan cost for flood risk management; 61 percent of the NED/NER Plan cost for ecosystem restoration (due to lands, easements, relocations, right-of-way, and disposal sites (LERRDs) costs in excess of 35 percent of the total first cost for the ER component of the NED/NER Plan); and 50 percent of the cost for recreation. The non-Federal cost share is about $103,738,500, consisting of the remaining percentages of the NED/NER Plan, plus 100% of the costs of the LPP above the cost of the NED/NER Plan. The non-Federal sponsor will not seek reimbursement for the excess LERRDs costs.
REPORT DOCUMENTATION: Pertinent documentation on the project, the results of the Civil Works Review Board (CWRB), and subsequent Washington-level review actions, are listed below (items not linked will be provided when available):
- CWRB Agenda
- Project Map/Placemat
- Project Summary
- CWRB Briefing Slides
- CWRB Lessons Learned
- CWRB Meeting Record
- State & Agency Review Comment Letters
- Documentation of Review Findings
- Signed Chief of Engineers Report
- Advance Copy to Congressional Committees
- ASA(CW) Memo to OMB
- OMB Response
- ASA(CW) Transmittal to Congress
- Signed Finding of No Significant Impact (FONSI)