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Some sea-level rise solutions may exacerbate coastal flooding

Some sea-level rise solutions may exacerbate coastal flooding
Researchers say that directing flood waters to less vulnerable land, rather than building taller seawalls, in places like the San Francisco Bay could help protect shorelines more effectively. File Photo by Terry Schmitt/UPI | License Photo

July 12 (UPI) -- Some strategies for combating sea-level rise can have unintended consequences, worsening flood conditions in neighboring cities, towns and neighborhoods, according to a new study.

For example, new research -- published Monday in the journal Proceedings of the National Academy of Sciences -- seawalls along the San Francisco Bay shoreline could exacerbate flood damages across the region.

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In coastal bay communities like San Francisco, researchers suggest it may be best to direct flood waters to less vulnerable land, rather than build defenses.

"It's not practical to keep building taller and taller seawalls to hold back the ocean," senior study author Anne Guerry said in a press release.

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"Our goal was to show how the threat of sea-level rise is interconnected with the whole social-ecological system of the Bay Area. Communities need to coordinate their approaches to sea-level rise adaptation so we can find solutions that are best for the whole bay," said Guerry, chief strategy officer and lead scientist at the Stanford Natural Capital Project.

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Models predict sea levels across the San Francisco Bay will rise some seven feet by the end of the century, putting millions of people and hundreds of billions of dollars worth of real estate and infrastructure at risk.

To help city leaders and policy makers develop effective sea-level rise mitigation plans, researchers at Stanford deployed complex mathematic models to simulate how seawall construction would influence the flow of floodwaters in the Bay Area.

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The models showed that should seawalls be built along the San Jose shoreline, communities throughout the region, from Napa to Redwood City, would suffer an additional $723 million in flood damage after just a single high tide.

According to the authors of the new study, seawalls can impede the movement of important bird and fish species. Seawalls can also degrade wetlands, which store carbon and remove pollutants for local waterways.

"You may be protecting your immediate community, but you may be creating serious costs and damages for your neighbors," study co-author Robert Griffin said in the press release.

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"When it comes to current sea-level rise planning, there's some incomplete cost-benefit accounting going on," said Griffin, an economist at the Natural Capital Project.

Researchers with the Natural Capital Project found communities could avoid excess flood damages by directing floodwaters toward overflow zones such as the Napa-Sonoma shoreline.

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Currently, the shoreline is home to Highway 37. Local decision makers are considering whether to build a taller embankment or construct a causeway that would raise the road while allowing excess water to flow inland.

Models suggest an embankment would worsen flooding in bayside communities, while allowing the Napa-Sonoma shoreline to serve as a strategic flood area could prevent millions of dollars in flood damage.

"It's critical to consider the regional impacts of local actions," said lead author Michelle Hummel, assistant professor at the University of Texas at Arlington. "Studies like ours can identify actions that will have large impacts, either positive or negative, on the rest of the bay and help to inform decisions about how to manage the shoreline."

Strategic flooding won't work everywhere, so decision makers considering the unorthodox approach must work with experts to ensure there are sufficient valleys and flood plains that can serve as overflow zones.

Policy makers must also think about how different mitigation strategies will affect various communities.

The authors of the latest study suggest local leaders must work with demographers to ensure flood control efforts don't negatively impact the most vulnerable communities -- neighborhoods already subject to environmental inequities.

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As such, flood mitigation planning should be coordinated with broader efforts to build just and sustainable cities.

Moving forward, researchers with the Natural Capital Project plan to model the effects of sea-level rise adaptations on infrastructure, employment and community dynamics.

"Our plans should be as interconnected as our ecosystems," said Guerry.

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