In a recent study published in Nature Communications, Haigh and a team of international researchers demonstrate that coastal flooding is prone to occur at certain times of day, reflecting the timing of the tides. While previous research has largely focused on the seasonal timing of river and coastal flooding, this is the first study to show that impact-causing floods have predictable daily timing across regions.
“Floods have traditionally been viewed as primarily weather-driven and therefore relatively unpredictable in their exact timing,” Haigh said. “But surprisingly little attention has been paid to what hour of the day flooding occurs. Another reason is the way we measure flooding. We commonly talk about the number of ‘flood days’ per year. Once an event is reduced to a flood day, the information about whether the flooding happened at 3 a.m., 8 a.m. or 3 p.m. disappears.”
Using tide gauge data and flood event records from the US and the UK, the researchers found clear patterns in the appearance of floods. For example, in Boston, floods occur predominantly around noon and midnight, while in Southern California, they are more common in the morning. In Florida, the timing can vary along the coastline because the tides differ from location to location. Along the Atlantic Coast and down toward the Florida Keys, coastal flooding is moderately predictable, but along the Gulf Coast, flooding is less predictable.
Haigh says this predictability tends to be determined by the tides.
“In many locations, tides determine whether storm surge is sufficient to push water levels above flood thresholds,” Haigh said. “As a result, even storm-driven flooding often inherits the timing of the tides.”
The researchers also found that this phenomenon isn’t unique to locations that experience semidiurnal tides, which consist of two high tides and two low tides each day. It also happens in places with diurnal tidal systems, with one high tide and one low tide per day, because of the way different tidal signals interact.
As sea levels continue to rise, Haigh says that coastal flooding is increasingly occurring without storms to spur it on, making the findings of his research more significant.
“This matters because timing influences impacts,” Haigh said. “The same flood can have very different consequences if it occurs during the morning commute, business hours or in the middle of the night. Understanding these temporal patterns could improve flood forecasting, emergency planning, infrastructure operations and long-term coastal adaptation.”
Haigh says that this work could make existing flood forecasts more predictable, providing more warning before disasters occur. This can help people, businesses and emergency managers anticipate disruption and plan accordingly.
This information is also likely to become more useful as sea levels rise.
“Higher mean sea levels signify that it will take only a relatively small weather contribution or sometimes simply a very high astronomical tide to cross a flood threshold,” Haigh said. “Understanding exactly when those critical tidal windows occur could therefore become an increasingly important component of coastal flood forecasting and preparedness.”
