Lake Merced is an emergency source of water for the City of San Francisco to be used for firefighting or sanitation purposes in the absence of other sources. The San Francisco Recreation and Park Department manages the recreational areas of the Lake under a 1950 agreement with the San Francisco Public Utilities Commission. The SFPUC manages the water aspects of the lake.
Beginning in the late 1980s, Lake Merced water levels began declining, and by the early 1990s, water levels had dropped ten feet below the historic averages of the 1950s-1980s. Declining water levels generated significant concern regarding the long-term health of Lake Merced for recreational, ecological, and emergency water supply uses. In 2006, Lake Merced water levels reached 6.8 feet City Datum, a level not reached for 20 years. Lake level increases from 2002 to 2006 are a result of a combination of factors, including above-average precipitation, reductions in groundwater pumping, SFPUC system water additions in 2002 and 2003, and limited addition of treated stormwater from 2004 to 2006.
SFPUC’s Lake Merced Water Level Restoration Project
(Part of WSIP)
The Lake Merced Water Level Restoration project aimed to maintain lake levels with a combination of treated stormwater and groundwater additions. In 2009 the Lake Merced Water Level Restoration project was placed under review by the SFPUC and the City of Daly City, while additional project alternatives were being evaluated.
The Vista Grande Drainage Basin Improvement Project
During large storm events, flooding, particularly in the Vista Grande Watershed, overflows from Daly City’s Vista Grande Canal and tunnel system resulting in property damage from uncontrolled runoff into Lake Merced. In response to ongoing flooding problems, Daly City commissioned an alternatives analysis report which identified the South Lake Merced alternative for further development and study. This alternative is intended to route storm water from the Vista Grande Watershed through a treatment wetland to South Lake Merced. In addition some approved non-treated stormwater flows that meet certain water quality criteria would also be routed to South Lake Merced. As part of the Vista Grande Project, Daly City would expand the capacity of the Vista Grande tunnel in order to better manage conveyance of stormwater to the Pacific Ocean. These efforts would reduce uncontrolled overflows from the Vista Grande Canal into Lake Merced, while achieving a sustained increase in Lake Merced lake levels, improving storm water quality, and providing further restoration of the drainage basin's natural hydrology.
The project would include construction of the following:
Collection Box and Debris Screening Device
A collection box would replace the upstream portion of the existing Vista Grande Canal to collect flows from the contributing storm drains. A debris screening device would be installed downstream of the collection box to trap debris greater than 5 millimeters in diameter, which would be removed using vacuum trucks on a scheduled basis.
A reinforced concrete box culvert would replace approximately 1,500 feet of the existing canal directly downstream of the debris screening structure. The box culvert would run underneath the proposed treatment wetland described below.
Constructed Treatment Wetland
A constructed treatment wetland would be developed to improve water quality. The treatment wetland would be located along John Muir Drive, partially over the box culvert described above. The treatment wetland would be planted with emergent reeds such as cattails or bulrush, which would improve water quality by intercepting and settling out suspended particulates. After passing through the wetland, the treated water would flow through the diversion structure to the outfall at Lake Merced. During periods of very low or no flow, a recirculating pump would draw water from South Lake to maintain flow in the wetland.
Diversion and Lake Outfall Structures
A semi-automated hydraulic diversion structure would be constructed directly downstream of the box culvert and treatment wetlands to direct flows to either the Pacific Ocean or to a submerged outfall structure into Lake Merced. The specific location of the outfall structure will be determined based on further engineering and environmental review.
Lake Merced Overflow
An existing Lake Merced overflow structure, consisting of a brick and masonry riser and tunnel, connects South Lake with the Vista Grande Canal. Under the proposed project, a portion of the existing Lake Merced overflow would be replaced with an adjustable-height weir that would be used to control the lake level and allow water to be diverted back into the Vista Grande Canal.