The City Dock neighborhood of Annapolis, Maryland, is a quaint cluster of colonial brick buildings nestled around a small inlet of the Severn River. Just uphill is Maryland's historic State House, where George Washington resigned his post as general of the Continental Army after defeating the British. Much of the architecture remains untouched since then, but the waters of the inlet have risen well over a foot, and today the waterfront floods frequently.

Flooding in the Annapolis area is so common, in fact—63 times in 2017 by one measure—that it's beginning to have a noticeable impact on local businesses by driving away customers.

"They can't drive near it, they can't park near it, they can't walk to it," said Nancy McPherson, who manages a City Dock gallery where the sidewalk outside floods regularly, preventing customers from reaching the front door. "Unless they're local and they know they can get in from behind us, they just see it's flooded and they go away."

In a study published Friday in the journal Science Advances, a group of researchers from Stanford University put numbers to the economic impact, estimating that flooding robbed the area's businesses of nearly 3,000 customers in 2017, a nearly 2 percent hit. In terms of direct costs, the flooding already costs the businesses roughly 1 percent of annual revenue, the study says.

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While that may seem small, the problem will grow precipitously worse unless something's done to protect the area. Just 3 more inches of water rise would double the number of customers lost, the researchers estimated, while 1 foot of sea level rise—expected in a matter of decades as the planet continues to warm—would mean a loss of about a quarter of the businesses' annual customers.

Miyuki Hino, a Ph.D. student at Stanford and lead author of the study, said coastal communities are beginning to wake up to the costs of increasingly frequent flooding driven by rising seas, "but we really didn't have any hard numbers, any good measurement of what it really meant to those communities."

"Those kinds of numbers are important not only because they help us get our arms around the size of the problem, but also because we need to know where and how much we need to invest in these issues," she said. "Adaptation is a big challenge, and there's a lot of ways we could spend the limited money we have."

Storm surges on top of rising sea level add to the City Dock neighborhood's flood risk. Credit: Jonathan Newton/Washington Post via Getty Images

The average number of days with high-tide flooding has more than doubled since 2000 across the Southeastern United States, while in the Northeast it has jumped 75 percent. All around the nation's coasts, those numbers will continue to rise throughout the century. Many cities that hardly ever flood today are projected to do so on a daily basis by 2100, according to NOAA modeling.

All that flooding is beginning to take a toll on local economies, and many cities are beginning to plan for how to deal with it. Annapolis is spending millions of dollars on a mitigation plan for City Dock that would pump the floodwater back into the river. That would solve only a piece of the city's problem. An assessment published in December said Annapolis needs to develop a plan for how it will cope with rising seas more broadly, including developing financing tools to fund expensive infrastructure projects. Next door to City Dock, the Naval Academy recently announced it will build up an existing seawall by more than 2.5 feet to protect against rising seas.

While adaptation projects like these and similar ones in Boston, Norfolk, Miami and other coastal cities are coming up with cost estimates of hundreds of millions or billions of dollars each, planners are still largely guessing about the costs of doing nothing.

Seeing the Big Picture of Nuisance Flooding

Hino chose City Dock for several of reasons, including the frequency of flooding. Perhaps most important, she had access to minute-by-minute parking data from a lot that serves as the primary parking for 16 businesses, allowing her to measure people's behavior in detail.

Hino and her colleagues found that business owners and others had a wide range of beliefs about how often the area flooded. Some said flooding occurred two or three times a week. Others said once or twice a month. Some said flooding was a drag on business. Others thought it attracted gawkers to the area. One restaurant said it would announce closures on Facebook for days with flooding. Another said it alerted its main supplier when nuisance flooding was expected.

The researchers combined tide gauge data with surveillance photos and social media posts to draw a detailed picture of when and how the parking lot flooded over the course of 2016 and 2017.

They learned that the flooding was driven almost entirely by the tide—the lot sits on the waterfront but tends to flood when the inlet bubbles up through storm drains. The researchers found that minor flooding in the lot cut visits by nearly 40 percent, moderate flooding by more than 60 percent, and major flooding by nearly 90 percent. In 2017, they found the lot had some degree of flooding during 44 days.

Coastal flooding at City Dock blocks access to many of the businesses and can get inside the buildings, as well. Credits: Alex Davies and Luis Rodriguez

Hino also examined data the city had collected from eight businesses, which showed a drop in revenue on days with flooding. The authors use the data to estimate that in 2017, the 16 businesses in the area lost between $86,000 and $172,000 out of a total of $12.2 million in annual revenue.

McPherson had no doubt that flooding costs them money, and will cost even more in the future. And she also hinted at some of the larger costs of living with regular flooding.

"Since I live south of here, I need to come across from the drawbridge," she said. "And if it's flooded here, it's flooded there. And I can't get to work without doing a big loop around here."

A Wider Problem for Coastal Cities

Joshua Behr, a research professor who studies the social and economic impacts of flooding at Old Dominion University in Norfolk, Virginia, said it is these harder-to-study effects that lead to the much larger, fuller picture of the costs of rising seas.

"Things like damage to vehicles, missed work," he said, "those are the real interesting things."

Behr suspects that if you were to add up all these ongoing costs of so-called nuisance flooding, they'd likely be greater in some places than the costs of major storms. The data is difficult to collect and quantify, however.

In Portsmouth, Virginia—which like much of the Hampton Roads region suffers from some of the worst tidal flooding in the country—Behr has conducted surveys that found that about a quarter of residents reported missing work and losing wages as a result of flooding. Nearly one in five said they had suffered property damage.

Flooding means days of school missed, he said, which can hurt children's achievement over time. And Behr has correlated higher rates of asthma with flooding.

"There are a lot of broader impacts on achievement and access," he said.

Hino said solutions like sea walls, pumping stations and planned wetlands all come with their own costs, but it's impossible to make informed decisions about those without knowing the costs of doing nothing.

"There's a lot we can do to prevent the impacts of high-tide flooding, prevent them from escalating quickly," she said. "I don't think of this as a doom and gloom story. I think of it as, look at what we can gain if we adapt and find solutions to these problems."