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High-Tide Floods Deter Visitors, Cut Historic Maryland Town Revenue

A paddler passes through a flooded downtown Annapolis, Md., in Oct. 2017. | Will Parson/Chesapeake Bay Program

In the historic Maryland town of Annapolis, tidal flooding driven by the rising sea led to a loss of about 3,000 visitors and up to $172,000 lost revenue for eight businesses in 2017, after the floods deterred potential customers from reaching the businesses’ popular parking lot.

Without adaptive measures to counter sea level rise, high-tide flooding’s effects on downtown Annapolis businesses could become much worse, the authors predict.

The analysis, published on February 15 in Science Advances, highlights how sea level rise can affect a seemingly natural event – the tides – which can in turn negatively impact the economy of coastal cities like Richmond, Virginia; San Diego, California; and Miami, Florida, among others. The study’s innovative economic evaluation will be increasingly important to coastal communities along East Coast, the authors said, as officials seek to develop adaptation plans at the local level.

“It’s important that communities address high-tide flooding along with a larger plan for how they see their coasts evolving in the future,” said Miyuki Hino, a Stanford University researcher and co-author of the study. “We are really right on the cusp of seeing a sharp increase in the impacts of high-tide flooding. It’s been a gradual increase until now, but we’re at a tipping point where the losses are going to accelerate soon. It’s clear that there’s an urgent need for adaptation.”

Sea level rise is a significant threat to coastal cities around the world. Most studies of the phenomenon have focused on the occurrence and damage of sea level extremes, such as from hurricanes and storm surge. Experts have predicted that around 170 coastal communities in the U.S. will flood more than 26 times per year by 2030, making it important to understand flooding’s effects at the local level.

“The East Coast is on the leading edge of feeling these high-tide flooding impacts at the moment, but in some ways these impacts are a preview of the future for a lot of other locations,” Hino noted.

High-tide flooding – or tidally-driven flooding, also known as nuisance flooding or sunny-day floods – due to sea level rise is less studied due in part to the nature of high-tide floods. “It is really hard to actually detect high-tide flooding and identify what is flooding when, because the floods sometimes only last a few hours at a time,” said Hino. These floods also rarely leave lasting damage.

The city of Annapolis uniquely addresses high-tide flooding research challenges, according to Hino, making it the perfect city to study the effects of high-tide flooding at the local level. In one of the first analyses to go beyond measuring physical exposure to higher water levels, Hino, Christopher B. Field, a fellow Stanford University researcher, and colleagues sought to quantify the effects of high-tide flooding on social and economic activity in Annapolis, which is experiencing sea level rise at a rate two to four times greater than the global average.

Miyuki Hino speaks at a 2019 AAAS Annual Meeting press briefing on Thursday. | Robb Cohen Photography & Video

The authors first used social media, government resources and photos across 21 days in 2016 and 2017, and videos from 2018, to create a visual record of high-tide flooding in the City Dock parking lot of downtown Annapolis, the main parking area for 16 businesses in the downtown area. “City Dock is well documented because it’s such an important part of the downtown,” Hino said.

The researchers then matched that evidence to water level observations from federal tide gauge to identify a threshold water level at which flooding begins. This allowed the authors to confirm that the flooding is due to seawater level and not precipitation. “Flooding due to precipitation would also affect visits, interfering with our estimates of how high-tide flooding affects visits. We know that high-tide flooding is a major concern for the city right now, and it is likely to get worse in the future, so we wanted to pinpoint that effect,” explained Hino.

They found that Annapolis exceeded the threshold water level during 63 days in 2017.

Hino and her colleagues then estimated the impacts of high-tide flooding on local economic activity by using visits to City Dock as an indicator. To do this, they used records of parking transactions in the City Dock parking lot from May 2016 to November 2017 and matched each hour of records to hourly tidal and precipitation data. Hino said the parking meters allowed the authors to measure visits at a very fine time scale.

By analyzing this data, Hino and her colleagues found that minor floods, 1.73 to 2.03 feet above the threshold, reduced visits by 37% to 38%, while moderate floods, 2.03 to 2.33 feet above the threshold, reduced visits by 63% to 65% and major floods, more than 2.33 feet above the threshold, reduced visits by 88% to 89%. This decrease in visits continued for many hours after the water receded. This finding indicates that, in 2017, high-tide flooding led to the loss of 2,916 visitors to City Dock, or a 1.7% decrease.

“The relationship is clear: higher sea levels mean more high-tide flooding, which means more disruption in these coastal communities,” said Hino.

To supplement their data, the authors also reported on data from the City of Annapolis, which collected daily revenue data from eight of the businesses around the City Dock affected by high-tide flooding and compared revenue on a flood date to the same day in previous years. Flooding decreased revenue by approximately 22.5%, costing the eight businesses studied between $86,000 and $172,000 in revenue that year. The researchers estimated that a 1% loss in visits translated to a 0.61% loss in revenue and that the City Dock businesses lost 0.7 to 1.4% of their annual revenue in 2017 due to high-tide flooding.

To anticipate how the impacts of high-tide flooding are likely to progress in the future, the authors calculated what might happen when sea levels rise higher than 2017 observed water levels. They discovered that just three inches of increased sea level results in around 3,221 lost visits. One-foot sea level rise led to a loss between 28,098 to 45,559 visits at City Dock, their analysis predicted, which is equivalent to a 24% (19 to 28%) decrease relative to a year with no floods.

“Looking forward, these impacts grow very, very quickly,” Hino said at a press briefing Thursday at the 2019 AAAS Annual Meeting.

Future studies should focus on a more complete understanding of high-tide flooding impacts that can help guide investments in drainage infrastructure, pumps and structure elevation and more. In an interview conducted by Samanthe Belanger, a co-author of the paper also from Stanford University, and Hino, a City Dock business representative said of flooding, “We just deal with it. Cross our fingers and hope that sea levels aren’t rising too much.”

Each community’s adaptive measures will vary, Hino added. “It is really going to depend on the local context to know what the right answer is,” she said, requiring coastal communities to decide on a path forward because the impacts of high-tide flooding can already be seen.


[Associated image: Will Parson/Chesapeake Bay Program]