- By Patrick Hoge | Examiner staff writer |
- Apr 21, 2024 Updated Apr 21, 2024 (SFExaminer.com)
Long before rising seas wash over San Francisco’s shores and flood its streets, rising groundwater mixed with salt water from the bay could touch and degrade underground structures like sewage lines and building foundations.
SAN FRANCISCO vs. CLIMATE CHANGE
In a year featuring a contentious presidential re-match, a heated mayoral race, and horrific warfare playing out in Ukraine and the Middle Eas…
That’s the implication of a study released this week by scientists at the University of Hawaii at Manoa. They compiled research from around the globe showing that as sea levels rise, coastal groundwater is lifted closer to the surface while also becoming saltier, more corrosive and potentially more destructive to subterranean systems.
“Depending on how elevated a city is, it could be a problem today or decades out. It’s a problem before it reaches the surface,” said Shellie Habel, a coastal geologist with the university and the paper’s lead author.
Habel’s publication aligns with a growing body of data from Bay Area researchers and others about the risks posed by rising groundwater as sea levels are projected to rise on average by 0.8 feet statewide over the next 30 years and between 1.6 and 3.1 feet by the year 2100, if not higher, according to new draft estimates from the California Ocean Protection Council.
The response of shallow groundwater to sea level rise is a relatively young field of study. One challenge with gauging the effects of rising groundwater is that they typically happen out of sight. Still, potential effects — including leaking basements, spreading toxins and increased liquefaction hazards in earthquake country — are real, particularly in areas where people filled the Bay with earth, experts said.
“Water is the most powerful substance on Earth,” said Kris May, founder and CEO of Pathways Climate Institute, a San Francisco consulting firm focused on climate adaptation and resilience. “It is more powerful than concrete, so it can do a significant amount of damage.”
Pathways released a report last year — together with the San Francisco Estuary Institute and UC Berkeley associate professor Kristina Hill — mapping shallow groundwater and identifying potential threats for Alameda, Marin, San Francisco and San Mateo counties.
“We designed infrastructure for static historic conditions, and now our changing climate is changing everything,” said May, who is currently working on mapping shallow groundwater conditions in Contra Costa County.
Ibbi Almufti, a San Francisco-based principal and risk and resilience expert with Arup, a global development consultancy, said that builders already consider groundwater, especially in some parts of San Francisco, but the time horizon for thinking about designs is getting longer.
“We’ve always designed for it,” Almufti said.
“It’s just changing.”
Almufti said that for one building he worked on in the Financial District, the foundation was secured to the ground with many “micropiles” to prevent groundwater from pushing up and causing “floating, almost.”
Kevin Befus, an assistant professor in the Department of Geosciences at the University of Arkansas who has mapped groundwater elevations across California, said he wants planners to have rising groundwater on their radar as they grapple with how to manage threats such as surface flooding.
“If we are not careful, there could be water rising behind us,” Befus said.
One area of concern with rising seas pushing up groundwater is the potential to increase liquefaction severity in the San Francisco Bay Area, as documented in a U.S. Geological Survey study Befus helped produce.
A spokesperson for the San Francisco Planning Department referred questions to the Port of San Francisco, which in January released a draft of an estimated $13 billion waterfront protection plan in concert with the U.S. Army Corps of Engineers.
With the plan, the Corps and the Port issued an appendix about shallow groundwater that said there was “a high level of uncertainty” about groundwater issues and that further testing and study were needed to assess the need for inland drainage or groundwater management systems.
The City’s waterfront has little topography to accommodate a rise in the water table, so groundwater emergence and saltwater intrusion are likely to occur with higher sea levels, it said.
Steven Reel, the port’s engineering lead on the waterfront resilience program with the Corps of Engineers, said rising groundwater “is a relatively new issue,” and there had not been much study done about its effects on buildings but called it “a slow-moving risk.”
Rising groundwater could increase pressure on building foundations — particularly basements, which in the downtown area are often below water level and thus designed to account for saltwater and to resist the tendency of water to make bodies float, Reel said. Increased pumping into The City’s drainage system could potentially lead to capacity problems — but it might be a nonissue, he said.
“There is going to be additional groundwater,” Reel said. “The volume of that water needs to be considered.”
In addition, the report from the Army Corps of Engineers and the Port of San Francisco said pumping can draw salt water inland from the shore and up from below.
At her house in Alameda, meanwhile, May said she believes she can already see the impact of sea-level rise in her basement, where she has two sump pumps.
“The groundwater table is often higher than the floor of my basement,” May said. “I’m getting more cracks in my foundation. I’m getting groundwater seepage into my basement, and if it gets high enough, it can destabilize the foundation of my house.”
