100-year floods will be annual events by the end of the century

By the end of this century, many coastal communities will face “100-year floods” on a yearly basis, according to a study published by the American Geophysical Union. 

The experts suggest that this forecast may be inevitable, reporting that even if carbon dioxide emissions peak by 2040, extreme floods will likely become an annual event.

100-year flood 

The term “100-year flood” refers to a severe flooding event that historically had a one percent likelihood of occurring each year. The terminology may be misleading because these floods can happen in consecutive years or might not occur at all within a century. 

Regardless, the new study indicates that these historical patterns may no longer be useful for predicting future flood events. The research was led by Hamed Moftakhari, a civil engineer and professor at the University of Alabama.

“The threshold that we expect to be exceeded once every hundred years on average is going to be exceeded much more frequently in a warmer climate until they are no longer considered 100-year events,” said Moftakhari.

Focus of the study

Storms and waves cause severe flooding in coastal communities, but the researchers focused their study on the longer term impacts of sea-level rise.

“The gradual increase of global mean sea level (MSL), known as sea-level rise (SLR), is one of the biggest climate change-related concerns of the 21st century. SLR poses a threat to seaside communities and can lead to an increase in the frequency and severity of coastal flooding. Thus, evaluating the future expected number of extreme sea-level events and their magnitudes is critical for sustainable coastal flood risk management,” wrote the study authors.

The experts set out to estimate future extreme sea levels under two carbon emission scenarios: if carbon dioxide emissions continue to rise through the end of the century, or if carbon dioxide emissions reach their peak by 2040 and then decline. 

Proactive planning is needed

The researchers analyzed trends based on data from 375 tide gauges located all over the globe. The results showed that in both carbon emission scenarios, sea level rise will lead to an increase in 100-year flood events in most of the locations that were examined.

Moftakhari said that a proactive approach to land planning, urban development and coastal protective measures could help communities reduce flooding and avoid disaster, and that starts with realistic forecasts of future coastal conditions.

“What makes it so challenging is that the majority of tools, design guidelines, manuals of practice and more are all based on the assumption of stationarity,” said Moftakhari. “They need to be updated to enable us to keep pace with the rate of change.”

Unique solutions 

According to a study published in the journal PLOS ONE, more than 600 million people live in low-lying coastal regions, and that number is expected to rise. In order for coastal communities to withstand extreme flooding, well-designed coastal defense structures are critically important. 

According to Moftakhari, coastal communities will require unique solutions based on local information to match their needs. Some areas are experiencing much faster rates of sea level rise. This is the case in parts of Texas and Louisiana on the Gulf coast where the land is gradually sinking.

Enhancing the resilience of coastal communities 

“We know that mean sea level is rising, the question is: how are we going to deal with it?” said Moftakhari. “We’ve already seen that many portions of the coast are permanently inundated and losing land, and many coastal cities and islands are experiencing flooding much more frequently than in the past – it’s time to learn how to deal with non-stationarity.”

Moftakhari is determined to remain optimistic, noting that disasters are the outcome of human decision-making, not hazards alone. 

“Don’t forget that this is all about the level of water that we expect to experience without mitigation measures. There will be technological advancements that could enhance the resilience of communities.”

The research is published in the AGU journal Earth’s Future.

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