Flooding on a sunny day at Carolina Beach. Photo from July 31, 2023, courtesy of Sunny Day Flood Project.
If the tides have seemed more extreme than usual over the past few days, it’s because of the king tide that began Friday. King tide is an everyday term that refers to the unusually high and low water levels that occur during a full or new moon orbiting the moon closest to Earth.
The Sunny Day Flood Project, a group of researchers studying chronic flooding in eastern North Carolina, is particularly interested in these natural, predictable tides because they show what the average tidal cycle looks like as sea levels change.
Sunny day flooding may sound counterintuitive, but it is a chronic problem that coastal communities often face, especially during spring tides.
“Coastal areas continue to experience increased local sea level rise, land subsidence, and development, resulting in increased frequency of flooding,” the project’s website explains. “Tidal cycles now occur at higher mean sea levels, resulting in ‘sunny day’ roads being flooded during high tides. Even at normal tide levels, seawater enters storm drainage systems and normal storm events cause flooding. Although these small floods receive less attention than catastrophic storms, their frequency disrupts critical infrastructure services and places chronic stress on coastal communities and economies.”
Scientists Catherine Analdo and Miyuki Hino, two of the project’s leaders, told Coastal Review in an email that sunny day flooding is flooding that occurs without rain or storms.
Analde is an assistant professor in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, and Hino is an associate professor in urban and regional planning and an adjunct assistant professor in the Environment, Ecology, and Energy Program at the University of North Carolina at Chapel Hill.
Scientists Katherine Annade (left) and Miyuki Hino lead the Sunny Day Flood Project, which studies flooding along the North Carolina coast. Photo: Sunny Day Flood Project
“Even on perfectly sunny days, ocean water levels can rise high enough to cause flooding due to tides and wind effects,” they explained. “King tides occur when perigee and spring tides coincide, when the moon is closest to Earth in its orbit and aligned with the sun, which exerts the greatest tidal force on the world’s oceans.”
The Sunny Day Flood Project was launched in 2021 and is currently funded by the National Oceanic and Atmospheric Administration, the National Science Foundation, NASA, or the National Aeronautics and Space Administration “to understand and address the occurrence and impacts of chronic coastal flooding, which includes not only sunny day flooding but also ‘flash flooding’ due to daily rainfall events coinciding with high tide.”
“We are working with communities to monitor how often floods occur, measure their impact, and assess potential adaptive responses,” Analde and Hino explained.
The team will collect measurements, conduct interviews, and conduct surveys to better understand the impacts of flooding on populations, the economy, and human health. They communicate flood risk through flood warning systems and king tide programs.
Spring tides are “not a new phenomenon,” but the combination of rising sea levels and land subsidence, known as subsidence, can result in relatively high tides that can flood low-lying infrastructure built decades ago, the researchers said.
A diagram from the National Oceanic and Atmospheric Administration shows that if the sun, Earth, and moon are not aligned, it will cause nearshore spring tides, or king tides.
“The most visible impact is flooding of roads, often caused by water flowing back through stormwater drainage networks. However, high water levels can also affect other aspects of infrastructure, such as septic tanks and underground sewers,” they continued.
Now-retired researchers at the Morehead City-based UNC Institute of Marine Science started the North Carolina King Tides Project about 10 years ago, working with the international King Tides Project to study flooding that historically occurred during king tides. This global initiative began in Australia in 2009, asking the public to record and share photos showing how storm surges affect coastlines.
The North Carolina project began publishing a calendar with high tide forecasts and encouraged communities to take photos and submit observations of shoreline water levels.
After the researchers retired several years ago, the Sunny Day Flood Project absorbed the North Carolina King Tide Project and now publishes a calendar of when king tides are expected each year. The project is also encouraging the public to upload photos and details through the free Coastal Observer app.
According to the calendar, the first king tide of the year will be from April 17 to 19, and the current king tide is expected to end on Thursday, with further tides expected from June 13 to 16, July 12 to 15, August 10 to 13, October 25 to 31, November 23 to 27, and December 24 to 25, according to the calendar.
In addition to spring tides, the calendar also lists amplified tides, which are defined as an unscientific term that refers to expected rises and falls in water levels that occur “when perigee and spring tides occur at approximately the same time but cancel each other out for more than a day.”
The team warns on the project website that the calendar “does not predict coastal flooding.” Floods are often caused by land-based factors such as wind and air pressure, seasonal changes in sea level, groundwater levels, rainfall, and existing infrastructure.
“As sea levels rise, near-shore spring tide flooding is becoming increasingly common, but in some areas flooding also occurs during calm tides,” the website states.
Researchers recommend checking the NOAA Monthly High Tide Outlook for flood chances.
The team also communicates flood risk through flood warning systems available in Beaufort, Carolina Beach, Downeast and New Bern, where water level sensors are installed. Interested parties can sign up to receive an email whenever water levels rise above the road at selected sensor locations.
On the project’s website, researchers explain that chronic flooding affects people differently than extreme events like hurricanes because it is not as deep or widespread as storm surges, but it is more frequent. High rates of persistent flooding can disrupt travel and increase infrastructure maintenance costs in coastal areas.
Analde and Hino said there are ways to deal with the effects of chronic coastal flooding, and the right response will vary by location.
“Elevating infrastructure can help keep things dry and reduce damage from salt water. Scheduling trips to avoid high tide can help avoid flooding roads and keep travelers safe. Given that these types of flooding are expected to become more frequent in the future, it’s important to consider both immediate short-term responses and long-term strategies,” they said.
