After years of study, Daly City has a plan to protect neighborhoods against seasonal flooding while restoring Lake Merced’s water level. The Vista Grande Basin Improvement Project is designed to eliminate bottlenecks as rainwater drains from Daly City to the ocean, diverting water to Lake Merced as needed.
The Vista Grande Basin is a roughly 2.5-square-mile area encompassing about one-third of Daly City. Floods occur there when storm drains back up.
The city proposes to improve the capacity of Vista Grande Canal, which carries water from three drain systems to Fort Funston. The project also would construct a wetland to treat water before it’s piped into Lake Merced.
Daly City Department of Water and Wastewater Resources Director Patrick Sweetland says the project is a win for both Daly City and San Francisco, which owns Lake Merced.
“We get the benefit of finding a way to recharge the lake while providing our residents with flood-control protection,” Sweetland said. Alison Kastama of the San Francisco Public Utilities Commission agreed.
Lake Merced’s water level has decreased as its surrounding watershed has become urbanized, Sweetland noted.
The Vista Grande Canal runs west from Lake Merced Boulevard along John Muir Drive, emptying into the Vista Grande Tunnel. The tunnel, which carries stormwater to the beach, is the system’s weakest link. Built in 1897, it has a water flow capacity of just 170 cubic feet per second. Sweetland said this is inadequate, and the canal’s 500-cubic-feet-per-second capacity also falls short. When it is exceeded, untreated water floods across John Muir Drive into Lake Merced, and residential areas flood as storm drains back up.
That is what happened in 2004. On the morning of Feb. 25, Westpark Drive homeowner Richard Swan watched the water level outside his home office’s sliding glass door climb to 3 feet in just 20 minutes. Then he heard a “boom” as his garage door blew in and his home was inundated, sustaining $50,000 worth of damage.
“About every 20 years, we have a significant flood. They hire one of the big-dog engineering firms to do a study, and then everybody goes back to sleep,” said Swan, expressing frustration with the city.
Swan said he worries that the city’s engineering benchmark, which prepares for a hypothetical 10-year storm event dumping water into the canal at a rate of 680 cubic feet per second, doesn’t set the bar high enough to protect against a recurrence of the 2004 flood.
Sweetland countered that no storm drain system could reasonably have been expected to prevent that flood.
“Mr. Swan experienced a once-every-100-years superstorm that was beyond the capacity of any system to absorb,” Sweetland said.
When asked whether climate change might cause such storms to occur more often, Sweetland declined to speculate, but said that is something the project’s ongoing study will be looking into.
Public comment ends June 7, and letters should be addressed to Sweetland at 153 Lake Merced Blvd., Daly City, CA 94015.