The Vista Grande Drainage Basin Improvements Project, located in the Cities of Daly City and San Francisco, is designed to upgrade the regional Vista Grande storm drain system, which serves northwest Daly City. The project will reduce flooding, improve stormwater and water quality, support Lake Merced’s ecological and recreational value, enhance habitat, and contribute to regional water supplies.

Delve Underground provided design services to the City of Daly City to develop a flood control solution that both reduces the risk of flooding and property damage and provides beneficial uses for stormwater.

The existing brick-lined canal and tunnel were built in 1896 to divert stormwater away from Lake Merced in San Francisco, which was at the time being used as a municipal drinking water supply. The canal and the 3,000-foot-long (914 m), 7-foot-high by 4-foot-wide (2.1 m by 1.2 m) brick-lined tunnel are undersized for peak stormwater flows, and they regularly back up and flood low-lying portions of Daly City and occasionally overflow and spill into Lake Merced. Furthermore, the original construction of the Vista Grande canal and tunnel cut off Lake Merced from much of its original watershed, and the lake has seen diminished water levels without imported water additions.

The project leverages the existing infrastructure to reduce flooding and restore Lake Merced to its original watershed. The existing tunnel will be replaced with a 12-foot-diameter (3.7 m), cast-in-place concrete-lined tunnel to increase system capacity. The tunnel will be mined primarily through the Merced and Colma Formations, with dune sand encountered at the east portal. A new outlet structure will also be constructed on the beach to replace the existing outlet.

Additional improvements include replacing portions of the existing brick-lined canal with subsurface box culverts and adding a diversion structure that will allow stormwater to be routed either to Lake Merced or diverted through the new tunnel to the beach outlet.

The project also includes trash screening devices, a treatment wetland, and a new lake overflow structure to control the lake level and promote water circulation to improve water quality.

Delve Underground’s work included the planning, conceptual, and final design of the tunnel, intake and outlet structures, and connecting pipelines, as well as preparing a construction schedule and cost estimates. We are supported on this project by Brown and Caldwell, ESA, Moffatt and Nichol, and Langan.

The City of Daly City has secured project funding, and construction bids are set to be received in February 2026. The total estimated construction cost is $137 million.

Pictured Above: Electrical control building and wetlands rendering