By Jessica Wolfrom
Examiner staff writer
City sidewalks have started to buckle, crack and slope in one of San Francisco’s newest neighborhoods, causing tripping hazards for pedestrians and frustrating residents forced to navigate the city’s uneven pavement.
Sidewalks on some blocks of the Mission Bay neighborhood have sunk four to six inches, and even as much as a foot in some places, KPIX first reported.
But the neighborhood’s sunken sidewalks are not the only thing shifting in this densely developed area. Nearly two miles away in SOMA, the ever-tilting Millennium Tower has also struggled to keep its balance on the sinking ground that bears the building’s multi-story weight.
One possible culprit could be climate change. As a historic drought continues to parch the state it may also be exacerbating subsidence, or downward settling of the ground’s surface, as the City’s groundwater which normally flows from higher elevations like Twin Peaks or Bernal Heights to the sediment beneath the lower-lying neighborhoods has slowed to a trickle.
“When there’s a lot of precipitation, the water goes into the clay. It’s slow, but it builds up the water table,” said Lawrence Karp, a Bay Area-based geotechnical engineer. “In periods of drought like we’ve had for quite a while now, the water table drops… and the clay densifies.”
When soft clay consolidates from loss of groundwater, he said, its weight increases, causing it to drag down structures that sit on top of it, including the sidewalks and streets of Mission Bay.
The redeveloped neighborhood at the water’s edge was built over an old rail yard and reclaimed landfill that has historically been used as a dumping ground for industrial debris, including the scorched remnants of the 1906 Earthquake.
Today, it’s home to a sprawling state-of-the-art medical campus, high-rise apartment buildings, a hotel, a school, and the recently completed Chase Center.
But the sidewalks here have become a hazardous headache for residents and local businesses alike. The gap is so steep in front of Cafe Réveille on Long Bridge Street that two ramps, bright yellow paint and quirky signage guide the way into the cafe, cautioning patrons to, “Please watch your step!”
It wasn’t always this bad. “In the last couple months, it’s been a noticeable change,” said Aaron Nelson, a barista at Cafe Réveille.
Others say the sinking sidewalks are old news. “The streets have been sinking ever since we’ve been here,” said longtime resident Peggy Fahnestock, who moved to Mission Bay in 2009.
A few years ago, she said, a member of her walking group tripped and broke her arm in front of Cafe Réveille. “It’s been that bad for a long time,” said Fahnestock, a board member of the neighborhood association. “I’ve tripped out there and fallen… you have to watch where you’re going.”
Although more research is needed to draw any concrete conclusions about the drought’s connections to the sloping sidewalks, scientists say that this is an unsurprising outcome given the conditions.
“From a scientific perspective, we don’t have any evidence for it yet, but we know that (this) process has happened in other places,” said Manoochehr Shirzaei, professor of Geophysics and Remote Sensing at Virginia Tech University, who studied subsidence in the Bay Area extensively. “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, that system is under increasing stress from the ongoing drought.
“The drought does impact groundwater by temporarily reducing its natural replenishment from rainfall and stormwater runoff,” said Will Reisman, spokesperson for the San Francisco Public Utilities Commission. However, he said, “the total amount of groundwater stored in the aquifer remains large and is replenished during subsequent wet years.”
Still, water is clearly on the minds of many at City Hall. Last week, in an effort to further conserve The City’s water supply, the Board of Supervisors unanimously passed a measure that more than doubles the amount of water that new large buildings will be required to collect and reuse on-site.
“This summer of intense drought and terrible wildfires reminds us that the climate crisis is now, and it is not going away on its own,” said Supervisor Rafael Mandelman. “Even as we pursue zero carbon emissions goals for our city and our world, water reuse and recycling will be increasingly necessary for our survival.”
Mission Bay is not the only neighborhood to suffer from settling ground, but it’s one of the newest, which makes the cracking infrastructure noteworthy. Other areas built on landfills like the San Francisco International Airport (SFO) and Treasure Island have also been sinking as the landfilled material slowly compacts onto itself, research shows, making these areas more vulnerable to flooding and sea-level rise.
Subsidence on its own won’t cause a building to sink or a sidewalk to crack, said Shirzaei. What matters, he said, is something called differential movement, or the uneven rates of sinking of the subsurface ground.
“Differential movement, that can cause significant damage to any infrastructure,” he said. “In fact, there’s no human-made structure that can sustain that kind of a strain. So, pipelines, concrete pillars, all would fracture and eventually lose their integrity.”
So far, however, the buildings in Mission Bay have remained steadily in place because they are constructed on piles, said Karp. But the question for residents like Fahnestock remains, who is going to pay to repair the damaged walkways?
The City said that such fixes fall on residents. “Sidewalk maintenance is the responsibility of the adjacent property owners, under state and city code,” said Rachel Gordon, spokesperson for Public Works. “Public Works inspects the sidewalks and notifies property owners if there is a problem that needs to be fixed.”
The goal, said Gordon, is not to be punitive, but to ensure a safe path for pedestrians.