Floods on a Schedule: Coastal Flooding at Consistent Hours

Flooding is typically perceived as a sudden and unpredictable hazard. Here, we show that recurrent flooding can occur at highly predictable times, in tidally dominated coastal systems. This predictability arises from the fortnightly alignment of the sun and moon, which causes spring tides that consistently peak at the same time of day. The exact timing varies geographically because of the propagation of the tidal wave through the world's oceans and seas. Here, we quantify the intraday timing of coastal flood events by examining tide gauge records from the United States and the United Kingdom. In many locations, floods cluster around specific hours of the day, especially where semidiurnal or mixed tides dominate the water level spectrum. For example, floods in Boston cluster around noon and midnight, whereas in Southern California they occur in the morning, with possible differences in societal risk. Sites with stronger meteorological influence show weaker temporal clustering. Crucially, this predictability extends beyond nuisance flooding to larger, consequential events involving extensive inundation, road closures, and damage to coastal infrastructure. These findings show that in tide-dominated settings, flood timing is highly predictable and location-specific, enabling anticipatory risk communication and time-sensitive adaptation.