SF Flooding King Tides


“100-Year Storm Flood Risk Map” pinpoints neighborhood trouble spots that could become even more troublesome with climate change.

The last several months could be an alarming preview of what climate change has in store for the Bay Area. In November, residents couldn’t leave their houses without an N-95 air respirator due to heavy smoke from Northern California fires. Last week, torrential rains drowned Guerneville and overwhelmed Santa Rosa’s sewage system with five times the amount of average water flow. While we don’t yet know how much rainfall the future has in store, it’s likely to be too much, too fast, and demand local responses from government agencies.

SFPUC has responded by insuring prospective homeowners have some clue to what they’re getting into during this rapid evolution of climate change. Using modeling software to anticipate where storm runoff would flow during a “100 Year Storm” (a torrential storm that has a 1% chance of occurring every year), the agency created the “100-Year Storm Flood Risk Map” to anticipate “deep and contiguous flooding,” or flooding that is at least “6-inches deep spanning an area at least the size of half an average city block.”

“These maps not only help prospective homeowners make informed decisions about their future, they also provide existing homeowners with the steps they need to take to protect themselves from flooding events,” SFPUC General Manager Harlan L. Kelly, Jr. said. “In this era of climate change, when weather patterns are increasingly more uncertain, we want to provide as much information and assistance as possible to our residents regarding flooding in San Francisco.”

The Board of Supervisors passed an ordinance in late February to require homeowners in the flood map zones to disclose that information to potential buyers or tenants. The ordinance is waiting for approval from the Mayor. 

“We are combining our capital work to reduce storm flood risk with our programmatic work, which is exciting,” SFPUC Utility Specialist Sarah Minick said. Minick believes this partnership of long-term capital funded work, $539 million over ten years to improve storm drainage and other aging infrastructure, coupled with the shorter, immediate needs of public awareness within programmatic work is the best way to tackle the issue of flooding.

“For example the Warriors stadium will have a 250,000 gallon cistern to meet non-potable demands,” Minick said. “This uses storm water for flushing and irrigation, so this is water not going into the sewer.”

For many residents – especially those homeowners who formed the group Solutions Not Sandbags – the news that they live in a high-flood area is not exactly news. (17th and Folsom is notorious for having flooding issues, for example.)

“We’d like to see people wet-proof or dry-proof their lower stories,” Minick said. “Residents need to recognize that water may come in at that level.”  Minick recommends residents in these flood-prone areas put up a flood barrier, wet-proof these areas and keep valuable items such as electronics out of basements or garages.

It might not feel fair to these homeowners, but it’s scientifically accurate: the homeowners most at risk are in areas where naturally occurring streams and marshes used to be. When heavy rains come, the water naturally heads down to these areas – and even more of it, since so much of the city has been covered by concrete and made less permeable.

“It’s similar to our seismic regulations; we can’t stop earthquakes, we have to be informed and be ready,” Minick said. “The 100-year storm is an act of nature that we can’t accommodate. It’s just too much water.”

Creating more permeability in the cityscape to account for heavier storms is the goal of the grant program recently created by SFPUC. Although it’s for large parcels that manage at least 0.5 acres of impervious surface, the grant will cover the costs of design and construction of approved stormwater management features, such as rain gardens, permeable pavement, cisterns, and vegetated roofs. And the more permeable ground in the city, the less chance the excess water will end up in flood-prone neighborhoods.

As far as the historic inundation in Santa Rosa, Will Reisman of SFPUC explained that large storm events affect each sewage collection and treatment system differently, because each system is configured differently. Santa Rosa separates its sewage from its storm lines, but these sewage lines can be affected by “inflow and infiltration,” where storm water enters the separate sewer line through cracks in the pipes, deteriorating manholes, or other ways, and can overwhelm the pump station built for sewage only.

“In contrast, San Francisco is a combined sewer system. Our pipes and the treatment plants are designed to intentionally accept storm water,” Reisman said.  “When we experience intense rainfall, some pipes in the collection system may surcharge and cause street flooding. These discharges do contain sewage, although they are mostly (95-100%) stormwater.”

SFPUC posts warning signs during severe flooding: “Floodwater can contain contaminants washed off the street, including bacteria and parasites. Try to avoid coming into direct contact with floodwater and keep children and pets away.”

Predicting the coming increase – or possible decrease – of rainfall for the Bay Area due to climate change is an incredibly difficult science. SFPUC is currently working on an advanced precipitation model with Lawrence Berkeley Labs to predict how much rain is in the long term forecast.

“Creating a permeable landscape is helpful but will not protect residents against a 100-year storm,” Minick said. “In the event of 100-year storm, a rain barrel will not save you.”

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