San Francisco’s Seawall

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If you listen carefully near the metal grate beneath your feet, you might hear water sloshing about. You're actually standing over the waters of San Francisco Bay. And if you look straight down through the grate, you might be able to see water down there crashing into a concrete barrier. That's San Francisco's seawall.

Listening to those waves, you might imagine that the seawall's main purpose is to hold the water out. But it's not that kind of wall. Bay water seeps in through the sandy muddy soils beneath the wall. The seawall doesn't keep water out. It holds the land in. It prevents erosion of the landfill behind it, the soils and debris that were dumped there in the late 1800s. Without the seawall, waves from the Bay would gradually wash away this unconsolidated fill.

In the 1840s when the city was first developing, merchants and other businesses desperately needed more flat lands to build on this very hilly city of sand dunes. So they began dumping the sand into the shallow waters of Yerba Buena Cove, an inlet that spanned much of what is now downtown San Francisco. As the newly added sand formed new land, developers could build warehouses, hotels, saloons, entire new city blocks. The filled land had the added benefit of being closer to the deep waters of San Francisco Bay, so large cargo ships could more easily unload their goods. Throughout the 1850s, even in the wake of devastating fires, workers filled more and more of the cove, says archaeologist James Delgado.

JAMES DELGADO
That sand—dumped on top of burnt out pilings, collapsed buildings, and even ships—extended San Francisco's waterfront by blocks. That filling would continue from 1851 all the way through 1855. Until finally the state legislature passed a bill that said San Francisco can only fill to this line. And that's the line that San Francisco ultimately met. We know that line because today it's the Embarcadero.

NARRATOR 
But the newly created land was unstable. Waves washed away the sand causing some early buildings to crack and sink. So in 1867, the State of California's Board of Harbor Commissioners began to design and build the first seawall. One of the first segments ran along Front Street just beyond the parking lot across from the Exploratorium. This new seawall made of rock ran along city streets forming a jagged sawtooth shape. The flaw in this design soon became obvious. Silt started to collect in the inlet of the sawtooth, making it too shallow for ships to get close to shore. In 1870, the Port of San Francisco mapped out a new seawall in a straight line, some four miles along today's Embarcadero. This new seawall, the one we have today, ran along the edge between two underwater landscapes, says environmental scientist Lauren Stoneburner from the San Francisco Estuary Institute.

LAUREN STONEBURNER 
San Francisco's modern shoreline today actually ends at a point where shallow bay turned into deep bay. And that was about 18 feet deep. Eighteen feet is the cut-off for photosynthetic activity in the water column. And so you're gonna have a lot more productivity in that shallow bay and beyond 18 feet, it's a different landscape. So this seawall is being built right at the edge of the border between shallow bay and deep bay.

NARRATOR 
Today, that shallow bay landscape is gone, but water from the Bay still seeps in underneath the Financial District, says Jim Delgado.

JAMES DELGADO
The landscape of today defined by concrete and asphalt and steel and glass that rest atop an earlier landscape of mud and natural organisms and of tides, which despite the fact the modern city overlays them continue to rise and fall beneath the streets and sidewalks of the Financial District. At every stage, as we have excavated we constantly have to pump because the tide still comes in and rises to the original level that it did long before that area was filled to become the Financial District.

NARRATOR 
It took more than 30 years to finish the seawall. For each section, crews had to dredge a trench 40 feet deep into the Bay mud. They then filled that trench with boulders from Telegraph Hill, piling more and more rocks on top to re-raise it to the level of the city streets. This was state-of-the-art engineering at the start of the 20th century. One hundred years later, we've learned that this seawall design is highly vulnerable to earthquake damage. Continue on to the next track to learn more.

NARRATOR
While the current seawall design was state of the art at the time it was constructed in the early 20th century, we've since discovered that the wall holding in downtown San Francisco is in fact highly vulnerable to earthquakes. Dr. Rufus Catchings, a geophysicist at the US Geological Survey explains that one of the main earthquake hazards is a process called liquefaction. That's when sandy soils like those in the downtown area behave more like a liquid.

RUFUS CATCHINGS
It's a lot of sand; you get liquefaction, which causes the sand to lose coherence, and whatever is built on top of it basically collapses. Because the foundation collapses, the seismic waves are amplified and the shaking becomes much more violent than it would, say, on solid rock.

NARRATOR
The seawall itself is not built on landfill, but it sits on a substrate that's only a bit better, something called young Bay mud. This too can amplify earthquake shaking. So a strong earthquake could cause the seawall to slide toward the Bay by as much as a foot. The Embarcadero roadway could crack open and some buildings along the waterfront might even collapse. Any other buildings in downtown San Francisco are also at risk. 

Any large skyscraper is vulnerable unless it's anchored into the bedrock some 200 feet below ground. The city's Seismic Safety Code for new buildings says they should have a less than 10% chance of collapsing during a major earthquake. This might seem reasonable. But with more than 100 tall buildings downtown it's not entirely reassuring. The infamous Millennium Tower has been sinking and tilting by many inches even without an earthquake. And some geologists have reported that all the landfill in downtown San Francisco is gradually sinking or subsiding because of the weight of so many skyscrapers. 

As for the seawall, the Port of San Francisco and the city are working hard on a plan to upgrade the seawall, making it stronger and better able to withstand a powerful earthquake. It's a huge and hugely expensive project. Life-saving upgrades alone could cost more than half a billion dollars, and full scale renovation might cost $5 billion. But such costs are easy to justify given the lives and economic resources at stake. 

When the Exploratorium moved to the waterfront in 2012, engineers were already planning to prevent earthquake damage. They designed a seismic joint—a flexible connector to physically separate the museum's pier from the land. During an earthquake the joint lets the pier shake separately from the seawall and the rest of the land. This makes it a lot safer. You can find the seismic joint about 50 feet back from the building's front entrance. It looks like a vertical accordion fold in the middle of the wall near the entrance to the Exploratorium store. 

The seawall, like most structures in downtown San Francisco, has never really been tested by a major earthquake. The 1906 quake happened before the current seawall and most current buildings were finished. In the 1989 Loma Prieta quake, the center was much further away. Seismologist Rufus Catchings. 

RUFUS CATCHINGS
Loma Prieta was 80 to 90 kilometers from where we sit now. So if it were much closer, I think you would have a much different result. Certainly the 1906 epicenter was very close, and that was quite catastrophic. You would see some of the same things, maybe not the fire, but you would see some of the same things happening to buildings just because of the nature of the materials that we are sitting upon.

NARRATOR
For now, we can only do our best to prepare and support city and Port leadership in doing the same. To learn more about the geology that's beneath your feet, proceed to stop number two, near the base of Telegraph Hill.