# Rising Seas and Storms Could Seriously Damage California’s Coast within 30 Years

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In less than 30 years the combination of sea-level rise plus an extreme storm along California’s coast could flood more than 150,000 residents and impact over $30 billion in property value. By 2100, up to 600,000 people and$150 billion in property could be at risk from coastal flooding.

Waves flood across a coastal road near Santa Cruz, California. (Credit: Amy Foxgrover, USGS. Public domain.)

In less than 30 years the combination of 10 inches of sea-level rise plus an extreme storm along California’s coast could flood more than 150,000 residents and impact over $30 billion in property value. By 2100, when the oceans could be 6.6 feet higher, up to 600,000 people and$150 billion in property could be at risk from coastal flooding—potentially worse than Hurricane Katrina in destruction.

USGS scientists and collaborators combined sea-level rise scenarios, climate-change forecasts, and economic and social data to show the dollars and lives that could soon be at risk in California. Their analyses focused on highly developed seashore counties in Southern California and the San Francisco Bay area, which include 95 percent of the state’s coastal population. The journal Scientific Reports recently published these peer-reviewed findings.

“This study is unique in that we’ve looked at the complete impacts of climate change on the California coast,” said USGS research geologist Patrick Barnard, lead author of the paper. “It’s not just sea-level rise associated with climate change that we need to consider, but also the combination of that with the storms we experience along the coast.”

Most climate-change scenarios forecast 10 inches of sea-level rise in California within 30 years. At every high tide, that could flood areas where 37,000 people live and 13,000 people work, exposing $8 billion worth of property to damage. Add a “100-year storm,” with a 1 percent chance of happening every year, and the numbers jump to 155,000 residents, 86,000 employees, and$32 billion of real estate. Emergency managers often use 100-year storms for planning purposes.

Forecast of potential California coastal flood risks in the 2040s after 10 inches (25 centimeters) of sea-level rise and an extreme “100-year” storm with a 1 percent chance of happening each year. (Credit: Rex Sanders, USGS. Public domain.)

Potential impacts to California coastal property values compared to the economic damage from other disasters. (Credit: Rex Sanders, USGS. Public domain.)

Coastal flooding from a combination of sea-level rise and storms is an economic threat similar to some of the nation’s costliest disasters to date. The potential economic impacts in these forecasts are many times higher than the $10 billion cost of the 1989 Loma Prieta earthquake in California, and comparable to Hurricane Katrina’s$127 billion in destruction.

And the actual damages could be significantly larger. The forecasts don’t include possible failures of levees, sea walls, and similar shoreline protection. This study also doesn’t account for the ripple effects from damaged ports, roads, sewage-treatment plants, and other critical coastal infrastructure. “You quickly see how this can have impacts beyond the coastal communities experiencing direct flooding,” said Juliette Finzi Hart, a USGS oceanographer and co-author on the journal article.

Barnard and his colleagues started building the detailed Coastal Storm Modeling System (CoSMoS) several years ago to forecast the effects of coastal storms on top of sea-level rise. Soon they partnered with scientists and developers at Point Blue Conservation Science to release the data in an easier-to-use form through the Our Coast, Our Future (OCOF) web site.

Pale blue and green areas could be flooded in this screen shot from the Our Coast, Our Future web site, showing a flood-forecast map for the southern San Francisco Bay Area with 10 inches (25 centimeters) of sea-level rise and a “100-year” storm with a 1 percent chance of happening each year. (Public domain.)

Philip Gibbons, program manager for Energy and Sustainability at the Port of San Diego uses CoSMoS data to understand threats to port assets. “We are also utilizing the flood depth and duration data to properly ‘tell the story’ of what future impacts sea-level rise will have within our jurisdiction,” he said.

The OCOF maps succeeded in bringing this information to many government officials and coastal residents. But the team soon realized that the web site did not show the full impacts of coastal flooding or meet everyone’s needs. “We weren't reaching a wide enough audience,” said Barnard. “Policy people need to know the dollars and lives affected.” So he reached out to Nathan Wood, a USGS research geographer.

“We added the number of residents and employees in a town that could be impacted by a certain scenario,” said Wood. “We get into demographics, types of businesses, and infrastructure.” His group created the Hazard Exposure Reporting and Analytics (HERA), an interactive online dashboard that lets someone pick the community and hazard scenario, then explore potential population, economic, land-cover, and infrastructure vulnerabilities. Translating scientific data into accessible, usable, and relevant information helped many more California coastal planners and residents.

Screen shot of the Hazard Exposure Reporting and Analytics (HERA) web site showing southern San Francisco Bay Area flooding and building replacement value impact forecasts with 10 inches (25 centimeters) of sea-level rise plus a storm expected every year. (Public domain.)

“CoSMoS, OCOF, and HERA are integral tools in The Bay Foundation’s adaptive management planning for our coastal resources,” said Karina Johnston, science director at The Bay Foundation in Los Angeles. “The maps and visualizations have been particularly helpful for the public to envision impacts to our coastal areas and start conversations about adaptation and mitigation.”

The new paper titled “Dynamic flood modeling essential to assess the coastal impacts of climate change,” is available at https://doi.org/10.1038/s41598-019-40742-z. Researchers from the USGS, Coastal Carolina UniversityDeltares, the University of Illinois at Chicago, and Point Blue Conservation Science collaborated on this study.

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