Sea Walls Might Just Make Floods Someone Else's Problem, Study Suggests

An anonymous reader writes: Protecting the coasts in the United States from the impacts of climate change comes with a hefty price tag. But new research shows that using sea walls to safeguard land can just make the rising tides a problem somewhere else. The paper, published in PNAS, looks into the effect of erecting sea walls in one location and what that means for other places along the coast. Using the San Francisco Bay as a case study, it also assesses the economic impacts of flood scenarios in the nonprotected regions. According to the paper, defending individual parcels of the shore can increase flooding elsewhere by as much as 36 million cubic meters. This can result in $723 million in damages for a single flooding event in the most dire situations -- costs can even exceed the damages that would have resulted otherwise in the protected region.

Robert Griffin, an assistant professor at the University of Massachusetts, Dartmouth's School of Marine Science and Technology, decided to look into what happens to unprotected areas. Griffin and his team combined hydrodynamic and economic modeling to investigate flood damages in the San Francisco Bay under a variety of different scenarios -- with different parts of the shore protected by walls with different lengths, for instance. (For the sake of the experiment, the sea walls were modeled as being infinitely high.) The team focused on problems brought on by tidal events, rather than storms, and broke the results down by amount of sea-level rise: 50 cm, 100 cm, 150 cm, and 200 cm above 2010 levels. The study 'can be useful in a variety of outcomes through time,' Griffin told Ars, adding that the 200 cm scenario is close to the high end of current projections for the year 2100. 'Displacement effects relate to the morphology of the land. Places that are low-lying, and valleys, can potentially accommodate more water in a tidal flooding scenario. If you block those places in the case of a flood, those waters go elsewhere. If those other places aren't also similarly defended, then it can increase the damages on those places,' Griffin said.

For example, if you protect the Napa-Sonoma shoreline, the Santa Clara Valley and San Leandro in the South Bay can expect to experience $82 million and $70 million in flooding damages, respectively, with a sea-level rise of 200 cm. San Rafael would also be hit with an additional $53 million in damages in the case of a flood. On the positive side of things, protecting parts of the South Bay could lead to small but widespread damage reductions. Protecting Alameda, for instance, could reduce flood damages in areas south of there, including San Lorenzo and Newark. It would also cut down damages on the opposite side of the shoreline, near Palo Alto and Silicon Valley, the paper notes. Though the modeling done in this research focuses on the San Francisco Bay, Griffin noted that other parts of the world's coasts could see similar effects. Further, around 468 million people live close to bays and estuaries, according to the paper. Considering sea walls are already in place along many coasts, these displaced damages could already be happening -- though potentially to a lesser extent than if the sea level reached the paper's more dire levels.

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