Peninsula sea-level study to focus on flood threats surrounding SFO 

click to enlarge flooding
  • Courtesy Robert M. Riechel
  • Heavy rain flooded the 500 block of Seventh Avenue in San Bruno in January 2008. A resident of the street says he sees flooding once or twice a year.
Neighborhoods, infrastructure and endangered-species habitats adjacent to the San Francisco International Airport are threatened by flooding connected to sea-level rise, and not enough planning has been done to protect them.

That’s the word from San Mateo County Supervisor Dave Pine, who recently helped secure funding from the California Coastal Conservancy to study risks to parts of South San Francisco, San Bruno and the airport.

The “SFO/San Bruno Creek/Colma Creek Resilience Study” is a joint effort by the airport, county and affected cities. It coincides with a study already being conducted by SFO to assess its vulnerability and identify protection strategies.

San Bruno Creek and Colma Creek both figure prominently in the study zone, which includes an area west of SFO and terrain and shoreline north of the airport. Although the creeks are natural features, both are enclosed by manmade culverts and play crucial roles in draining storm water from surrounding communities into San Francisco Bay. The 15,257-acre study zone also includes habitats for the California clapper rail bird, the San Francisco garter snake and the California red-legged frog, all of which are endangered. Public infrastructure in the zone includes U.S. Highway 101, BART and Caltrain lines, two sewage treatment plants, and multiple flood-control channels and pump stations.

Before the study can assess vulnerabilities and suggest mitigation strategies, Pine said, the first step is to assemble a working group with representatives from all concerned organizations.

“One of the most important things about this effort is to get all the stakeholders in the same room talking to each other,” Pine noted.

The list of stakeholders is long, because the area in question is under the governing jurisdiction of multiple entities — including San Mateo County and its Flood Control District, along with SFO, San Bruno, South San Francisco, the Bay Conservation and Development Commission and Caltrans.

While the separate but related resiliency study being conducted by SFO is focused on protecting the airport from flooding that could occur during a 100-year storm, nearby neighborhoods have experienced more frequent flooding.

Robert Riechel, who lives on Seventh Avenue in San Bruno, said he’s seen flooding about once or twice per year since 2002.

Certain parts of the new study zone, such as Riechel’s neighborhood and the Tanforan shopping center, rely on electric pumps to convey storm water to San Bruno Creek and Colma Creek. The creeks themselves, however, rely on gravity to get that water to the Bay.

Coastal Engineer Dilip Trivedi of Moffat & Nichol, the consulting firm that is handling both studies, said the main issue is not that sea-level rise might cause San Francisco Bay to directly encroach upon the vulnerable areas, but that the gravity-based system can’t work unless the Bay’s water level is lower than the storm water traveling through the system. He explained that if the Bay’s water level is too high, the stormwater will have nowhere to go and flooding will occur.

Trivedi said installing more pumps would not be a sustainable solution because pump stations have significant personnel, monitoring, maintenance and energy requirements. One possible mitigation strategy would be to develop the system’s overflow capacity. For example, Trivedi said that if water could be diverted to local wetlands during a major storm event and temporarily held there during high tide, that might be enough to prevent flooding.

One such wetland, he said, is west of the airport, adjacent to U.S. Highway 101 and the BART tracks.

The new study is expected to last one year, with Moffat & Nichol releasing its final report in June 2015.

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