Quakes, floods forecast to ruin S.F. Embarcadero
The Embarcadero faces severe threats, with regionwide repercussions from both earthquakes that could undermine the city’s seawall and a rise in bay waters that could flood downtown streets and inundate BART and Muni tunnels, according to an exhaustive new study from the Port of San Francisco.
A temblor on the scale of 1989’s Loma Prieta earthquake could dislodge the seawall beneath the Embarcadero and liquefy portions of the soil that it holds in place, says the 82page report. That would not only damage the waterfront’s crowded sidewalks and historic buildings but utility and transit lines affecting the entire city. The danger is
particularly severe in the area around the Ferry Building, but portions of Fisherman’s Wharf are also at risk.
The impact of sea level rise would also be extensive, though not in the immediate future. An increase in the average daily tides of as little as 2 feet, which some scientific forecasts anticipate as soon as 2050, would send water spilling onto downtown blocks and make the subway tunnel beneath Market Street susceptible to flooding in a major storm, the study released Friday warns.
The analysis follows a $425 million bond for seismic upgrades to the Embarcadero that voters approved in 2018. Its findings will be used to prepare a list of recommended projects to strengthen the most vulnerable areas along the seawall, which was constructed between 1878 and 1921 and runs 3 miles from Mission Creek to Pier 45.
A full makeover of the Embarcadero, one that prepares for sea level rise as well as earthquakes, could cost upward of $5 billion and take decades to complete.
“We need to make decisions now that we can feel good about in the future,” said Lindy Lowe, the port’s resiliency director. “The priorities in this first round are life safety and reduction of risk.”
The findings are in line with years of warnings that large earthquakes could cause permanent damage to the seawall, which consists of a crude dike of rocks topped by concrete. Similarly, there are numerous projections that daily tides could spill across the Embarcadero as sea levels rise in coming decades because of climate change.
What’s new is the emphasis on precisely what might happen — and determining the exact mix of the soils behind the seawall, which was built to extend San Francisco’s northeast waterfront into the bay.
“Understanding what’s under the (Embarcadero) surface really is essential,” said Steven Reel, the port’s project manager for the study. “It’s a complicated puzzle.”
To gather information, engineers on the consultant team led by the firms CH2M and Arcadis drilled as deep as 215 feet to get soil samples, which were inspected visually or sent to a laboratory. Other borings were made by pushing small cones past the Embarcadero’s surface. There were 90 borings in all.
The study found that the soils south of the Bay Bridge to where the seawall ends at Mission Creek are more stable than expected. Landfill rests on firm sand and bedrock that is relatively close to the surface.
Not so in the Ferry Building area: The bedrock is as much as 240 feet below the water’s surface, topped by a thick ooze of young bay mud. This has caused the seawall from Folsom Street north almost to Broadway to settle and become less stable.
The biggest surprise was the fragility of the wharves beneath the aged restaurant buildings on Taylor Street between Jefferson Street and Pier 45 at Fisherman’s Wharf. It turns out that loose sand lies below the fill — creating the potential for what Reel called “rollerskating,” when wharves and the structures atop them would shimmy back and forth.
All this means that from the Bay Bridge north, the seawall and the soils behind it might settle and lurch in certain conditions with catastrophic results. A temblor similar to Loma Prieta, but with an epicenter only 10 miles away instead of 60 miles as was the case then, could be enough to destabilize the waterfront.
“It wouldn’t take much more than a 1989 earthquake to liquefy the Embarcadero,” Reel said.
The challenge with sea level rise is preparing for the distant future, rather than shoring up against a natural disaster that could strike at any time.
If proactive steps are not taken, the study anticipates that pervasive impacts could begin to affect the Embarcadero between 2050 and 2080, depending on the pace of change. Even the combination of an extreme storm and 1.1 feet of sea level rise — a scenario that some projections say could be just 15 years off — could flood the foot of Market Street and extend far beyond the waves that now sweep ashore during extrahigh “king tides” in winter, what the report calls “nuisance flooding.”
The twin threat of earthquakes and climate change are the reason for starting work now on planning for changes that could far exceed the $425 million allocated by voters, port officials say.
“Our policy is, we’re the custodian of the Embarcadero,” said Elaine Forbes, the port’s executive director. “It’s our intention to preserve and protect the waterfront for as long as possible.”
The new study will be presented to the city’s Port Commission on Tuesday. A set of recommended projects for the first set of upgrades would be prepared this winter, with a final selection in the spring.
The port’s schedule calls for design and environmental work to continue into 2024, when construction would begin.