Led by PhD researcher Joris Beemster, scientists from Wageningen University & Research, together with international partners, investigated the timing of coastal flooding in the United States and the United Kingdom. They analysed decades of water level observations to identify the times of day when flooding is most likely to occur.

Because of the influence of the tides, flooding in some cities can repeatedly occur, for example, around the morning rush hour, during office hours or in the middle of the night. Understanding these patterns could help coastal communities better prepare for a future with rising sea levels.

Not only where, but also when

Flood research usually focuses on how often an area floods and how severe those floods are. The researchers add a new dimension: when flooding happens.
In areas where tides strongly influence water levels, flooding tends to occur at fixed times of day. In Boston, for example, flooding often occurs around midday and around midnight, while along the coast of Southern California it is much more likely to happen in the morning. These patterns are determined by the way tidal waves travel along the coast.
“The tide often determines whether a severe storm actually results in flooding,” Joris Beemster explains. “As a result, many coastal floods are far more predictable in time than we previously realised.”

Timing determines the impact

According to the researchers, the timing of a flood can have a major influence on its societal impact.
Flooding during the morning rush hour may cause severe traffic disruption and economic losses, while flooding in the middle of the night can pose greater risks for residents and emergency services. The North Sea Flood of 1953 is a well-known example. It occurred during the night and over the weekend, meaning the disaster was not immediately recognised and emergency response was delayed.
By considering not only where flooding occurs, but also when, the researchers argue that flood risk can be assessed more realistically.

New opportunities for climate adaptation

The researchers expect this knowledge to become increasingly important. As sea levels continue to rise, coastal flooding will become more frequent. Knowing when flooding is most likely to occur could help reduce its impacts.
These insights could, for example, help authorities improve the timing of flood warnings, the operation of storm surge barriers, traffic management measures and emergency response planning during the hours when the risk is greatest. It could also inform spatial planning, insurance models and climate adaptation strategies.
Beemster says: “As sea levels rise, these tide-driven moments will be the first to exceed critical flooding thresholds.”

A new dimension of flood risk

Flood risk is usually described in terms of the likelihood of flooding and the expected water level. This study shows that the timing of flooding is another important factor.
By taking into account people's daily routines - when we work, travel and sleep - the researchers provide a more realistic picture of the actual impact of coastal flooding. In doing so, they add a new dimension to understanding and managing flood risk in a changing climate.
“Flood risk is not only defined by location and probability,” Beemster concludes. “It also has a clock.”