Flowing sidewalks in San Francisco: is climate change to blame?

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By Jessica Wolfrom

Examiner editor

The city’s sidewalks began to warp, crack and tilt in one of San Francisco’s newer neighborhoods, causing tripping hazards for pedestrians and frustrating residents forced to navigate the pavement. uneven city.

Sidewalks in some blocks of the Mission Bay neighborhood have sunk four to six inches, and in some places even up to a foot, KPIX first reported.

But the neighborhood’s hollow sidewalks aren’t the only thing to change in this densely developed area. Nearly two miles from SOMA, the still tilting Millennium Tower also struggled to balance on the sagging ground that supports the weight of several floors of the building.

One of the possible culprits could be climate change. As a historic drought continues to dry out the state, it can also exacerbate subsidence or subsidence of the land surface, as the city’s groundwater that normally flows from higher elevations like Twin Peaks or Bernal Heights towards the sediment below the lower quarters slowed to a trickle.

“When there is a lot of precipitation, the water goes into the clay. It’s slow, but it increases the water table, ”said Lawrence Karp, a geotechnical engineer based in the Bay Area. “In times of drought like the one we’ve been experiencing for a long time, the water table drops… and the clay becomes denser.

When the soft clay consolidates due to the loss of groundwater, he said, its weight increases, causing it to entrain the structures that are above it, including the sidewalks and streets of Mission Bay.

The redeveloped waterfront district was built over a former rail yard and salvaged landfill that was historically used as a dumping ground for industrial debris, including burnt remains from the 1906 earthquake.

Today, it is home to a large, state-of-the-art medical campus, high-rise apartment buildings, a hotel, school, and the recently completed Chase Center.

But the sidewalks here have become a dangerous headache for residents and local businesses. The gap is so steep past the Café Réveille on Long Bridge Street that two ramps, bright yellow paint, and quirky signage guide the way to the café, warning patrons to, “Please be careful!” “

A man walks past a “watch your steps” sign alerting customers to a step forward from a sinking sidewalk outside Café Reveille in Mission Bay on Friday, September 24, 2021 (Kevin N. Hume / The Examiner)

It wasn’t always that bad. “Over the past two months, this has been a noticeable change,” said Aaron Nelson, barista at Café Réveille.

Others say that leaky sidewalks are old news. “The streets have been crumbling since we’ve been here,” said longtime resident Peggy Fahnestock, who moved to Mission Bay in 2009.

A few years ago, she says, a member of her walking group tripped and broke his arm in front of Café Réveille. “It’s been so bad for a long time,” said Fahnestock, a board member for the neighborhood association. “I tripped over there and I fell… you have to watch where you are going.”

While more research is needed to draw concrete conclusions about the links between drought and sloping sidewalks, scientists say this is an unsurprising result given the conditions.

“From a scientific point of view, we don’t have any evidence yet, but we do know that (this) process has happened in other places,” said Manoochehr Shirzaei, professor of geophysics and remote sensing at the Virginia Tech University, which has studied subsidence in the Bay Area in depth. “When we change anything in one part of the hydrological system, you start the chain of events,” he said, calling the city’s groundwater a “connected system”.

And right now, this system is under increasing stress due to the ongoing drought.

“Drought impacts groundwater by temporarily reducing its natural replenishment of precipitation and runoff,” said Will Reisman, spokesperson for the San Francisco Utilities Commission. However, he said, “the total amount of groundwater stored in the aquifer remains significant and replenishes in subsequent wet years.”

Still, water is clearly on the minds of many at Town Hall. Last week, in a bid to further conserve the city’s water supply, the supervisory board unanimously passed a measure that more than doubles the amount of water new large buildings will need to collect and reuse. on the spot.

“This summer of intense drought and terrible forest fires reminds us that the climate crisis is now and will not go away on its own,” Supervisor Rafael Mandelman said. “Even as we pursue zero carbon goals for our city and our world, water reuse and recycling will be increasingly necessary for our survival. “

Mission Bay isn’t the only neighborhood to suffer from subsidence, but it is one of the most recent, which makes the cracking infrastructure remarkable. Other areas built on landfills like San Francisco International Airport (SFO) and Treasure Island have also sunk as the materials in the landfill slowly compact on themselves, according to research, making these areas more vulnerable to flooding and sea level rise.

A cyclist rides on raised sidewalk slabs outside a residential building along Mission Bay North Boulevard in Mission Bay on Friday, September 24, 2021. (Kevin N. Hume / The Examiner)

The sag on its own will not sink a building or a sidewalk, Shirzaei said. What matters, he said, is what’s called differential motion, or unequal rates of subsidence in the basement.

“Differential movement, which can cause significant damage to any infrastructure,” he said. “In fact, there is no human-made structure that can withstand this kind of stress. So the pipelines, the concrete pillars, all would break and eventually lose their integrity. “

So far, however, the buildings in Mission Bay have steadily remained in place as they are built on stilts, Karp said. But the question for locals like Fahnestock remains, who will pay to repair the damaged walkways?

The City said such corrections are the responsibility of residents. “Sidewalk maintenance is the responsibility of the owners of adjacent properties under state and city codes,” said Rachel Gordon, spokesperson for Public Works. “Public Works inspects sidewalks and notifies homeowners if there is a problem that needs to be addressed. “

The goal, said Gordon, is not to be punitive, but to ensure a safe path for pedestrians.

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A pedestrian walks on raised sidewalk slabs outside a residential building along Mission Bay North Boulevard in Mission Bay on Friday, September 24, 2021 (Kevin N. Hume / The Examiner)

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