Increased Compound Flooding Risks for Major US Cities

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Flooding

This image shows extensive flooding in Bangkok, 2011. Image by Paul 012.

Are we likely to have more floods in the future? According to new research, floods resulting from multiple events are now more likely than ever before.

Researchers from University of South Florida studied the incidence of compound floods (floods caused by the contemporary occurrence of storm surge and heavy precipitation) in the US.

They reported that compound floods are more likely to happen now than they had been in the past, with increased impact for the society and the environment.

US Coastal Areas

In the United States, coastal counties are the most densely populated; indeed, nearly 40 % of the population lives in these areas. Moreover, many of these people live in big urban centers, as in coastal areas there are 17 cities with more than 1 million inhabitants.
For these areas/cities, flooding can cause serious problems, affecting both the economy and the environment.

Floods: Compound Events

Floods generally occur through two different mechanisms, heavy rainfall and storm surge. In the latter, the water level rises due to anomalous atmospheric conditions – i.e. strong winds or very low pressure.

If these two mechanisms take place at the same time (a compound event), the probability of having floods is much higher; moreover, it is more likely that the floods will cause much more damage.

Storm surge

This is what a storm surge in Florida looks like. Image by tpsdave.

Compound Flooding Occurrences

In recent years, compound flooding was observed in several countries all over the world, including Thailand (2011), Australia (2011), UK (winter 2013/2014) and the US (hurricane Isaac, 2012).

Scientists performed some studies to understand the probability of compound flooding; in the majority of cases, however, the research was carried out at a local level, on a small scale.

New Storm Study

Researchers from University of South Florida (US) recently performed a more systematic study to assess the probability of compound flooding in the US. They worked in cooperation with the Universities of Maine (US) and Siegen (Germany). They published their results in Nature Climate Change on the 28th of July 2015.

In their study, they considered both surge data recorded by tide gauges in 30 selected US coastal areas all over the country, and the precipitation which occurred within 25 km of these areas. They had two approaches:

  • Case I: they considered the highest storm surge(s) and the corresponding precipitation within ± 1 day of the surge.
  • Case II: they considered the heaviest precipitation and the corresponding storm surge(s) within ± 1 day of the precipitation.

They studied a period spanning several years, from 1990 onwards.

Different Storm Mechanisms

The combination of storm surge and precipitation can exacerbate the floods through different mechanisms; those included in the study were:

  • In estuary areas, heavy precipitation taking place at the same time as a storm surge can lead to higher water levels and, hence, cause (more severe) flooding.
  • A storm surge which already caused flooding occurring at the same time as moderate precipitation; although the precipitation normally may not be dangerous, combined with the storm surge it can make things worse.
  • Moderate storm surge which can block water drainage; any precipitation occurring at the same time or after this is likely to cause more serious flooding.

Flood Study Had Interesting Results

Decoded Science spoke to Dr. Thomas Wahl, one of the scientists of the study, who explains and comments on the results.

“What we saw was that in the majority of cases, a high storm surge was occurring at the same time as precipitation. In some areas, however, the compound event was more probable than in others.
It is also interesting to note that the probability of compound events got much higher in recent decades; the probability for case I events, for instance, becomes almost double considering two different 30-year periods.”

Rain

Compound floods are becoming more common. Image by werner22brigitte.

Understanding Compound Events Better

The reasons for the occurrence of the compound events, and for them to become more common, are not clear.

According to Dr. Wahl, it is important to understand better why this happens.
“This study we performed is important as nobody ever looked at the compound event in a systematic way; now we know that often – almost always – storm surge takes place at the same time as precipitation and that this tendency increased substantially in recent years.
This was the first step; now we have to do more to understand why this happens. To do this, we need to perform studies with more complex and complete modeling, which include parameters such as rainfall, river discharges, ocean-atmospheric pressure coupling, etc. We also need to assess at a local level the impact these compound floods have on society.”

Compound Flooding: Their Consequences and Management

The study by Dr. Wahl and his coworkers showed that compound floods are more frequent than originally thought, and that they are becoming more common. This research is essential to assess the dangers associated with the flooding and help the whole community to manage / minimize the risks when flooding happens.

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