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San Francisco's Aging Infrastructure Isn't Ready for Its Wetter Future

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Two people in raincoats and boots use tools to try to open a drain on a flooded street.
Mission District residents work to open a clogged drain on Mission and 21st streets in San Francisco on Jan. 10, 2023. (Beth LaBerge/KQED)

San Francisco’s future looks a whole lot wetter, thanks in part to human-caused climate change.

That’s according to a new city-funded study that predicts that San Francisco will be hit by increasingly intense storms in the coming decades, and needs to dramatically update its stormwater infrastructure to try to handle the deluge.

“We’re gonna see more areas that flood that have never flooded before,” said Kris May, founder of the Pathways Climate Institute, a San Francisco-based consulting firm, who helped lead the study. “I don’t think we have nomenclature anymore for what is coming with climate change.”

The report, which was released weeks after KQED filed a public records request about it, predicts that storms in San Francisco, and throughout the Bay Area, could become 37% wetter by the end of this century.

“Our infrastructure is not designed for these big storms, and we’re never going to really be able to design it to handle them,” said May, noting that the study stops short of recommending how the city should adapt its sewer system and water-related infrastructure.

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“We don’t know how to solve this yet and that’s what’s scary for most of the folks I’ve been working with,” she said.

While San Francisco has its own unique challenges, May added, it’s among scores of coastal cities that are now being forced to address storm-related threats.

“I don’t think any city is really in the shape to prepare for the storms that are coming,” May said. “It’s just going to be a big change that the country as a whole has to deal with.”

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Unlike typical climate studies that cover larger geographic areas, this report focuses on only 3 kilometers (just under 2 miles), in an effort to identify which parts of the city are most vulnerable to flooding.

“It enables us to look at extreme weather in ways we hadn’t before,” said study co-author Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory. “If you use the old techniques, you’re underestimating how bad the future is.”

For San Francisco, that future periodically brings the heightened risk of intense flooding in a city with aging infrastructure that’s bordered by water on three sides.

“The public needs to know that dangerous climate change is already here,” he said, pointing to the intense atmospheric river storms that battered the city earlier this year. “This is not our grandchildren’s problem or our children’s problem. It’s ours.”

In addition to overall wetter conditions, the study predicts increasingly intense bursts of heavy rain during storms — up to two-thirds wetter by the end of the century — the type of brief torrents that can easily overwhelm sewer systems, swamp cars and cause significant property damage and even loss of life, said Michael Mak, a Pathways water resources engineer.

San Francisco got a preview of that this winter, when massive amounts of rainfall in short periods left thousands without power, turned roads into rivers and downed scores of trees across the city.

If the city does not adapt, “we’re going to see more events like we saw over the past few months, except it might be much more frequent than once every few decades and might be every other year, or it might be multiple times a season,” said Mak.

According to Mak, San Francisco’s sewer system and flood infrastructure, designed to clean and push water out to the bay during storms, simply don’t have the capacity to handle the extreme influxes of water that are expected to become more frequent.

“At first, it was like, ‘Wow, OK, these extreme storms are going to be much more extreme than what we’ve seen,’” said Brian Strong, San Francisco’s chief resilience officer. “Then this past year, we’ve seen some of that come true.”

Strong, whose office helped commission the study, recognizes there are limits to how San Francisco can physically adapt its infrastructure to deal with substantially more rainfall. But he hopes the study will help guide future development decisions.

“We can’t build streets without thinking about where the water is going to go,” he said. “We can’t completely engineer our way out of all of these things. So, we will have to work together and figure out how to do a better job capturing water and reducing runoff.”

New infrastructure, Strong said, can only help so much.

“It doesn’t make sense to keep building a bigger pipe if, ultimately, it’s still not going to be big enough,” he said.

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