Tides cause coastal flooding to occur at fixed times
Expert(s) |
Bas van Maren
Floods are often seen as unpredictable events. New research by Wageningen University & Research, Deltares and international knowledge organisations shows that, in many areas, coastal flooding actually occurs with striking regularity at fixed times of the day. Due to the influence of the tides, flooding in some cities consistently occurs around the morning rush hour, during office hours or, conversely, in the middle of the night.
The results of this new research help coastal areas worldwide to better prepare for sea-level rise. The research team examined the timing of coastal flooding in the United States and the United Kingdom. They analysed decades of water level measurements and mapped out the times at which flooding occurs.
Not just where, but also when
Research into flooding usually focuses on how often and how severely an area is flooded. The researchers have added a new dimension to this: the exact time at which it happens. In areas where the tide plays a major role, floods often appear to be concentrated around fixed times of day.
For example, floods in Boston often occur around midday and around midnight, whilst on the coast of Southern California they mainly occur in the morning. This pattern is determined by the way the tides move along the coast.
“The tide often determines whether a severe storm can actually lead to flooding. As a result, many floods turn out to be much more predictable in terms of timing than we previously thought,” says Joris Beemster, lead author and PhD student at WUR.
Timing determines the impact
According to the researchers, the time of day at which a flood occurs can have a major impact on its social consequences. A flood during the morning rush hour can lead to significant traffic disruption and economic damage, whilst a flood in the middle of the night may, in contrast, pose greater risks to residents and the emergency services.
The 1953 North Sea Flood is a well-known example of this: it took place at night and at the weekend, meaning the disaster was not noticed until late and emergency response was delayed. By also taking the timing of floods into account, the researchers believe a more realistic picture of flood risks emerges.
Practical application
Bas van Maren (morphology expert at Deltares): “We have known for a long time that spring tides and high tides occur at a specific time at a given location. But we have never actually analysed this timing spatially, nor has a comparison with actual flooding events been made before. This research shows that, at various locations, the timing of flooding is related to tidal dynamics, and those dynamics are highly predictable. And the more predictable a flood is, the better people can prepare.”
The extent to which the tide determines the timing of a flood varies worldwide and depends on local tidal conditions and storm surge. “In the Netherlands, storm surge is relatively significant compared to the tide,” explains Van Maren. “That is why, unfortunately, the exact timing of a flood in the Netherlands is less predictable than in, for example, the US.”
The more predictable a flood is, the better people can prepare
Bas van Maren, morphology expert at Deltares
New opportunities for climate adaptation
The researchers expect this knowledge to become increasingly important. Rising sea levels will lead to more frequent coastal flooding. It is precisely in such situations that an understanding of the expected timing can help to mitigate the consequences.
Examples include better planning of warnings, the timely closure of storm surge barriers, the adjustment of traffic measures, or preparing emergency services for the hours when the risk is greatest. This knowledge can also be incorporated into spatial planning, insurance models and climate adaptation strategies.
A new dimension to flood risk
According to the researchers, flood risk is usually described in terms of the probability of a flood and the water level. The new research shows that the time of day at which a flood occurs is also a crucial factor.
By taking into account people’s daily rhythms – when we work, travel or sleep – a clearer picture emerges of the actual impact of coastal flooding. In doing so, the researchers are adding a new dimension to our understanding and management of flood risks in a changing climate.
Flood risk is not just about location and probability, but also about time
Joris Beemster (WUR)
The results of this research have been published in the scientific journal 'Nature Communications'.
