The Port Commissions says that a suite of fixes to safeguard San Francisco’s northern seawall against earthquakes and sea level rise over the next century will cost a whopping $5 billion.
The seawall stretches from Fisherman’s Wharf to Mission Creek. It is essentially the foundation that keeps the waterfront in place while protecting the city from incoming water.
But this essential structure is made up mostly of landfill that now dates 100 years old. Latter-age engineers dug a trench 100 feet wide and 30 feet deep near the waterline, then filled it with rock to create a "pyramid-shaped dike up to 40 feet tall, capped with a ‘bulkhead wall,’" which remains in place to this day.
"Constructed between 1879 and 1916 , the Seawall made possible the transformation of three miles of shallow tide lands into a world-class maritime waterfront that was key to the development and prosperity of San Francisco," reads the report the commission submitted to the city today.
Landfill, of course, is notoriously unstable in an earthquake, hence the crumbling of the Marina District during the 1989 Loma Prieta quake. But despite the damage, Loma Prieta was technically not big enough to be considered a major seismic event, the report reminds us. Right now, there is a 72 percent chance of a 1906-level shaker happening in the next 30 years, something the seawall in its present state might not be able to withstand.
Also, the soil underneath the seawall is just plain rotten, consisting mostly of "young bay mud, a weak, saturated, and highly compressible marine clay that tends to amplify shaking."
The commission warns that if the Big One were to hit tomorrow, the waterfront would most likely spread, shifting anywhere from one foot to several feet from where it sits now.
While we are assured that "complete failure" of the seawall isn’t likely, the aforementioned spreading would inflict billions of dollars of damages, on top of the rest of the earthquake’s wallop. (The folks at the port sure know how to kick off a Monday.)
The wall can be fixed up through a variety of means, including a process called jet grouting: Workers insert hoses into the ground and blast the dirt until it's syrupy, then pour concrete in so that the whole thing dries into a column of concrete-reinforced earth.
But thanks to how crummy the soil is — not to mention the neighborhood, home of the Ferry Building and AT&T Park, being so busy — the process would be long, expensive, and a serious pain. The seismic retrofitting alone could cost up to $3 billion, but if you add in the cost of also raising the wall to account for predicted rising sea levels, the bill approaches $5 billion.
Nevertheless, the port recommends that the bulk of the work be completed within the next ten years, one-third of the way through our projected earthquake clock. Putting it off longer than that just doesn’t seem prudent for a city of non-amphibious residents.
- Seawall memo [Port Commission]
- SF Preps For Sea Level Rise [Curbed SF]