Coastal flooding is happening far more often than we thought

Flooding has become an unwelcome visitor to many coastal neighborhoods. For a long time, official records suggested these floods were rare. But new research shows they are happening much more often than previously believed – and we may have been measuring them incorrectly.

The risks extend beyond storm surges and times of heavy rain. Even calm, sunny days can bring rising water into streets and homes.

Scientists from North Carolina State University and UNC Chapel Hill have taken a closer look. The study reveals that relying on marine water-level data – the standard method for tracking floods – misses a big part of the story.

Flood data under scrutiny

Miyuki Hino is a corresponding author of the study and an assistant professor of city and regional planning at UNC.

“Government agencies and researchers use data from tide gauges to measure water levels in coastal areas, then use that data to estimate flood frequency in the region,” said Hino. “Those estimates are used both to assess how often flooding has taken place and to predict how often it may take place in the future.”

The study reveals that current flood assessments underestimate both the frequency and duration of flooding events. This matters now more than ever.

“Due to sea-level rise, we’re now seeing flooding in coastal areas outside of extreme storms like hurricanes,” said Katherine Anarde, assistant professor of coastal engineering at NC State.

“There can be flooding during everyday rain showers or at high tide on sunny days. It’s important that the methodology we use to monitor and predict flooding reflects this reality, since sea-level rise means these flooding events are going to become even more common.”

Better tools for flooding

Tide gauges have been the backbone of flood measurement in coastal areas. Agencies often rely on thresholds from the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS) to infer inundation based on water levels.

However, these measurements are made offshore – not where people live, work, and drive. The researchers, part of the Sunny Day Flooding Project, wanted to change that. They built a network of land-based sensors that can detect floods on roads.

These new sensors were placed across three North Carolina communities: Beaufort, Carolina Beach, and Sea Level. Over the course of a year, the sensors picked up a surprising number of high-water events: 26 days in Beaufort, 65 days in Carolina Beach, and a striking 128 days in Sea Level.

The numbers don’t add up

“These numbers were very different from what the HTF and NWS thresholds tell us based on water levels at tide gauges,” said Anarde. “In general, the thresholds drastically underestimated the number of floods.”

“For example, when you subtract floods associated with extreme storms, we recorded flooding on 122 days in Sea Level.”

Professor Anarde noted that tidal gauge data significantly underestimated flood days, showing just 31 (NWS threshold) and 9 (HTF threshold).

The results were inconsistent. The NWS threshold sometimes overestimated the number of flood days as at Carolina Beach, for example. The researchers recorded 65 days of flooding using their system, while the NWS data from the closest tidal gauge identified 120 days of potential flooding.

But it’s not just about how often this happens. The duration of these events was also longer than is captured by the HTF and NWS thresholds. “Essentially, the thresholds don’t adequately account for how long it takes water to drain off of land,” explained Hino.

Better data helps protect coasts

When communities are left with poor information, they can’t plan effectively.

“More accurate information on coastal flooding can inform where and how we invest resources in building more resilient communities,” said Anarde. “It’s hard to design an efficient solution when you don’t know the scope of the problem.”

“Every community is unique, so there’s no one-size-fits-all solution,” added Hino. “But with more accurate data, we can help communities assess what response strategy is best for them, now and in the future.”

With a clearer understanding of the risks, towns can make smarter choices about infrastructure upgrades and emergency planning. It also helps residents better understand the changes happening around them.

In the long run, having the right information can mean stronger, safer communities for generations to come.

The full study was published in the journal Communications Earth & Environment.

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