Be Very Careful Where You Build That Seawall

Think back to being a kid at the beach, building walls around your sandcastles. If you engineered those fortifications properly, the tide would come in and flow around your kingdom, before the walls eventually eroded away. By redirecting the rising water, you would have saved your castle—at least for a little while. 

Now think bigger. Imagine you’re a city planner in an area threatened by rising seas and you’ve spent a fortune to build a proper seawall. The tide comes in and the wall holds, saving you billions of dollars in property damage. But: whomp whomp. Like the waves you once redirected around your sandcastle, the rising waters hit the wall and flow into the communities on either side of you. You’ve saved your residents, but imperiled others.

New modeling shows just how catastrophic this wayward-water phenomenon might be in the San Francisco Bay Area, where sea levels could rise 7 feet in the next 80 years. “Those rising waters put millions of people and billions of dollars in buildings at risk,” says Anne Guerry, chief strategy officer and lead scientist at Stanford University’s Natural Capital Project, who coauthored a paper describing the research. It was published this week in the journal Proceedings of the National Academy of Sciences. “One of the things that’s new about this work is that people haven’t necessarily thought about how communities, like in the Bay Area, are connected to one another through these shared waters,” she continues.

Guerry and her colleagues did the modeling by breaking up the shoreline into sections, based on characteristics like geology. Then they used hydrological models to show where the rising water would go if a given section of coastline was fortified with a seawall. Basically, they imagined what would happen if the residents of one area decided to protect themselves without fully considering the resulting hydrology. “That water has to flow somewhere,” says Guerry. “And what we found is it ends up flowing into other communities, making their flooding much worse.” 

They also incorporated economic modeling to calculate how much damage this would do. For example, they estimated that if the local government was to throw up a wall around San Jose, a city in the South Bay, it would inundate other communities with the equivalent of 14,400 Olympic-size pools’ worth of redirected waters. San Jose would be saved, but nearby Redwood City and other communities would be screwed. “That equates to $723 million of additional flood damage costs after just one high tide during spring, when the waters are naturally highest,” says Guerry. “And that’s just from building one large seawall in one small part of the bay.” And that $700 million-plus figure doesn’t account for potential damage to ecosystems and fisheries, so the tally is a conservative one.

The extra water pushed back by San Jose’s wall would even accumulate clear across the bay, in Napa and Sonoma, 50 miles north. The damage would go the other way too: If the Napa and Sonoma coasts were walled off, the South Bay would see tens of millions of dollars in damages.

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That’s not great news, considering that humans have a habit of building big cities on coasts, which urban planners now have to fortify, and seawalls are often the best defense available. The authors of this paper note that by the year 2100, the US alone is predicted to spend $300 billion on buttressing shorelines to hold back both sea level rise and the bigger surges that come with storms made more powerful by climate change. Lawmakers must soon consider whether to spend $26 billion to wall off the area around Houston. Jakarta, too, needs to build a giant seawall, only it can’t until the land underneath it stops sinking.

Up to this point, policymakers have assumed that seawalls might negatively affect nearby communities, but this new research puts numbers on the potential harm, says Laura Feinstein, sustainability and resilience policy director at SPUR, a nonprofit public policy group in the Bay Area. (She wasn’t involved in the research.) “It’s a really quantitative and rigorous demonstration of something that people have always said of sea level rise, which is that regions either sink or swim together,” she says. “If one area pours resources into armoring its shoreline, that’s just going to exacerbate sea level rise for its neighbors.”