Study uncovers underground flood risk in Bay Area due to storms, climate change

ByDan Ashley and Tim Didion KGO logo
Wednesday, February 1, 2023
Study uncovers underground flood risk in Bay Area
A new study uncovers underground flood risk in the Bay Area due to storms and climate change.

SAN FRANCISCO (KGO) -- Powerful storms offer clear evidence of how vulnerable parts of the Bay Area are to flooding. But now, a new study is raising concerns about a kind of "double threat" right underneath our feet.

It's being driven by sea level rise that's literally pushing saltwater inland and raising the water table -- and potentially threatening to undermine the system of levees and seawalls that protect our shoreline.

"They're definitely vulnerable because they are, you know, the groundwater is running under them. So they're not preventing the groundwater from -- or sea level rise from -- still causing groundwater rise in the inland areas. So even though you have a levee, you're still gonna get flooded from below as the groundwater rises," said Kris May with the Pathways Climate Institute.

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May, along with Ellen Plane of the San Francisco Estuary Institute, analyzed groundwater data for low-lying areas in four counties: San Francisco, Marin, Alameda and San Mateo.

Their mapping suggests that key infrastructure will be increasingly vulnerable to being flooded from below, especially during storm events.

"Yeah, so most of our infrastructure is within six feet of the ground surface. And so we're talking about, you know, buried sewer lines, both wastewater and stormwater sewers. In some places, buried electrical lines, all sorts of different things that are under the ground," Plane said.

In the case of stormwater and water treatment systems, they say water entering broken or degraded sewer pipes could overwhelm capacity, increasing flood risk during storms. The report also raises concerns about the former military or industrial sites in areas like Hunters Point, Richmond and Alameda, where rising groundwater could push toxic materials to the surface.

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"Because as that groundwater rises, it can cause that contamination to move and spread in new ways, potentially in new directions," Plane said.

With the pace of sea level rise expected to accelerate as glaciers melt, the report suggests an urgent need for resilient solutions, including expanded tidal marsh and horizontal levees in place of shoreline development.

"That vegetation, it drinks the groundwater, so it can actually help depress the groundwater, at least somewhat and give you more of a little bit more time before the groundwater becomes more of a problem. And so really creating more green space between communities and the Bay is just so needed," May said.

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