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Sea-level rise is a global reality. In the San Francisco Bay Area, sea levels are estimated to rise by almost seven feet within the next 80 years. In Boston, MA sea levels have risen by more than half a foot over the last 70 years, and the rate of sea-level rise will continue to increase dramatically, according to the . “Those rising waters put millions of people and billions of dollars in buildings at risk. This is typical of many coastal regions facing threats from sea-level rise and coastal storms,” says Robert Griffin, �������� Assistant Professor in the Department of Fisheries Oceanography at UMass Dartmouth’s School for Marine Science & Technology (SMAST) and Economist for the Natural Capital Project at Stanford University.

“The typical defense for sea-level rise is to build seawalls or levees to hold back the water. But we can’t keep building taller and taller walls,” Griffin and his colleagues mapped what happens to other communities when one community builds a seawall along the San Francisco Bay. The team divided the shoreline into segments and modeled changes in flood depth and economic damages along the whole shoreline of the Bay. Depending on where the seawall was placed communities around the Bay could face additional flooding equivalent to about 14,400 Olympic-sized swimming pools and $723 million in additional flood damage costs after just one high tide during spring, when waters are naturally highest,” Griffin says. By identifying, which areas will flood and associated damages, the study was able to identify which places will be flooded more by installing new seawalls and areas that would create the most damage around the bay if they were protected by seawalls.

“What we found is that the water ends up flowing into other communities, in some cases making their flooding much worse. Seawalls can also cause all kinds of other damages beyond economic—they can cut off critical habitat for birds and fish, reduce nature’s ability to store carbon, and create water quality issues,” he explains. This research makes the case that regional planning for sea-level rise adaptation will allow communities to address the threat of sea-level rise while mitigating these unintended effects on their neighbors. 

While the  was conducted in San Francisco Bay, Griffin says globally, approximately half a billion people are at risk of experiencing the effects of flooding. “This particular phenomenon is mostly an issue for embayments and estuaries that are affected by tidal flooding. Bays and estuaries represent 21% of overall global shoreline length and 54% of global population at risk from sea-level rise and flooding—nearly half a billion people.” In the Northeast, tidal flooding ("sunny day flooding") already impacts places like Boston and Providence eight to 18 days a year, according to a  from NOAA. “Places like Narragansett Bay and Boston Harbor where dense development is at risk from tidal flooding may also benefit from a regionally coordinated adaptation approach.” While the extent of the interrelated flood risk in these coastal areas has not been evaluated, this study suggests the costs of ignoring the issue can be large and will be increasing as sea levels rise.

“The good news is that traditional approaches like seawalls aren’t the only answer to combat sea-level rise. Instead of trying to keep the waters out, in many cases it’s more practical and more economical to strategically choose areas that can absorb the water,” Griffin says. “We can guide the floodwaters to natural areas that can act as overflow zones.” These can be things like marshes, and ponds, but they can also be parks and golf courses or other semi-natural areas where intermittent flooding will cause less damage. “Of course, not every community has natural areas that make sense for strategic flooding. Collective action can help us think strategically about avoiding the building of seawalls in places that will make matters much worse for remote neighbors and thinking creatively about where making room for the water will benefit an entire region.”