Be very careful about where you build that wall

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Remember to be a child on the beach, to build walls around their sands. If you established these fortifications properly, the tide would enter and bypass your kingdom, before the walls would eventually erode. By diverting the rising water, you would save your castle – at least for a while.

Now think bigger. Imagine you are an urban planner in an area threatened by a rising sea and you have spent a fortune to build a proper sea wall. The tide is coming in and the wall is holding back, saving you billions of dollars in property damage. But: whop whop. Like the waves you once diverted around your sand castle, rising waters hit the wall and pour into communities on either side of you. You saved your residents, but endangered others.

New modeling shows how catastrophic this kind of aquatic phenomenon could be in the San Francisco Bay Area, where sea levels could rise 7 feet in the next 80 years. “These rising waters are endangering millions of people and billions of dollars in buildings,” says Anne Guerry, chief strategic officer and lead scientist on Stanford University’s Natural Capital Project, who is a co-author. paper describing the research. It was published in a magazine this week Proceedings of the National Academy of Sciences. “One of the novelties in this paper is that people have not necessarily thought about how communities, like the Gulf area, are interconnected by these common waters,” she continues.

Guerry and her colleagues modeled by breaking the coast into pieces based on characteristics such as geology. Then they showed with hydrological models where the rising water will go, if a certain part of the coast is strengthened by a sea wall. They basically imagined what would happen if the inhabitants of one area decided to protect themselves, without taking into account the hydrology that resulted from it. “That water must flow somewhere”Guerry says. “And what we’ve found is that it eventually flows into other communities, exacerbating their flooding.”

They also included economic modeling to calculate how much damage this could do. For example, they estimated that the local government, if it erected a wall around San Jose, a city in the South Bay, would flood other communities with equivalent diverted waters worth 14,400 Olympic-sized pools. San Jose would be saved, but nearby Redwood City and other communities would be screwed. “That’s $ 723 million in additional flood damage costs after just one tide during the spring, when the waters are naturally the highest,” Guerry says. “And only from the construction of one large sea wall in one small part of the bay.” And that figure of over 700 million dollars does not represent potential damage to ecosystems and fisheries, so the sum is conservative.

Additional water pushed away by the San Jose wall would accumulate even across the bays, at Napa and Sonoma, 50 miles north. The damage would go the other way: if the banks of the Napa and Sonome were walled up, South Bay would see tens of millions of dollars in damage.

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This is not great news, given that people have a habit of building big cities on the shores, which urban planners now have to to determine, and sea walls are often the best defense available. The authors of this paper note that by 2100, the United States alone is projected to spend $ 300 billion on supporting shores to sustain rising sea levels and larger storm surges that come with storms. more powerful climate change. Legislators must soon consider whether to spend them $ 26 billion to build the area around Houston. And Jakarta has to build a giant sea wall, only it can’t until the land beneath it is finished it stops sinking.

So far, policy makers have assumed that sea walls could negatively impact nearby communities, but this new research puts the numbers at potential harm, says Laura Feinstein, director of sustainability and resilience policy at SPUR, a nonprofit Gulf public policy group. (She was not included in the research.) “It’s a really quantitative and rigorous demonstration of something that people have always said about rising sea levels, and that is that regions either sink or swim together,” she says. “If an area invests resources in armoring its coast, it will only worsen sea level rise for its neighbors.”

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