Sink or swim

For two decades, local climate change experts have been warning what rising sea levels would mean for the Bay Area. So why doesn't San Francisco even have a flood map?

Click to view the San Francisco Public Utilities Commission subsidence map (PDF)

Will rising seas destroy San Francisco's sewers? Should condos South of Market be on stilts? Could the huge Coca-Cola bottle at the Giants' ballpark one day bubble with seawater? Can anyone explain why San Francisco still doesn't have flood insurance?

As temperatures rise, snow packs vanish, and sea levels surge, San Francisco is waking up to its own inconvenient truth: surrounded on three sides by water, paved with concrete throughout, and erecting condo towers faster than you can say "bamboo," the city by the bay is particularly vulnerable to climate change.

With a recent California Climate Change Center report predicting sea levels will rise between four inches and three feet by 2100, San Francisco can expect increased flooding and damage to vital infrastructure and the destruction of fragile ecosystems and low-lying neighborhoods.

The evidence of impending doom is already there.

Addressing a climate change summit last month, Tom Franza, assistant general manager of the San Francisco Public Utilities Commission, revealed that seawater already tops the city's weirs for about an hour during very high tides. Franza expects this salt water intrusion, which threatens to kill helpful microbes that digest our solid waste, to get worse as sea levels continue to rise.

So what steps is the city taking to combat climate change?

The SFPUC is already building safety valves on floodgates and pushing for environmentally friendly development toward a future where green roofs, grassy swales, and permeable sidewalks will help stop rainwater from inundating already stressed sewers. It's also working with the Departments of Planning and Public Works to map blocks and lots that are already sinking — known officially as subsidence — and therefore especially vulnerable to flooding from rising seas.

It comes as a shock to learn that the Planning Department doesn't already have maps of areas that are prone to floods, but zoning administrator Larry Badiner told the Guardian, "In the past, floods were related to free-running streams, and since there aren't any in San Francisco anymore, it wasn't an issue."

Senior planner Craig Nikitas did confirm for us that city planners are working with the SFPUC and the DPW to flag blocks and lots prone to sinking, a phenomenon associated with rising seas that city officials don't quite understand.

"If I had to guess, I'd say [they're sinking] because most are on sandy soils or fill and over time there's been a settling of sand or because of subterranean flooding," Nikitas said.

As the city's subsidence map shows, the problem is biggest in SoMa and along the bay — where concrete-intense development is on the rise.

In the future, Nikitas told us, "If a developer comes in to do something in those areas, the system will flag it, and builders should pay extra attention to drainage and elevation, using raised entrances three steps up from the street and trench drains and installing sump pumps if there's a subterranean garage."

As small a step as subsidence mapping sounds, it's a sea change for city planners. SFPUC principal engineer Jon Loiacono recalls how in the past he was trained to say, "If flooding happens on your property, it's your problem."

Loiacono remembers only one instance when the SFPUC built a pump station in response to a developer's concerns. That was almost a decade ago.

Advising developers about the perils of building in flood-prone areas sounds obvious, but with that step comes responsibility that threatens to drown the city fiscally.