Rainfall-surge events are a growing threat on the East Coast

Many models of climate change predict that hurricanes will increase in frequency and intensity in future, bringing devastating consequences for both human life and property. In this connection, the effects of both sea level rise and storm intensification have been well documented over the past decade, but always separately from one another. A new study from Princeton University now shows that the two sources of water, when acting jointly, can produce flooding events that will have far worse impacts than those from storm surge or rainfall alone. 

The study, published today in the journal Nature Climate Change, is among the first to assess how climate change could impact the frequency of extreme rainfall-surge events. 

“Most studies have looked at the hazards of rainfall and storm surge separately, but in this study, we looked at the joint hazards from the Gulf Coast up through New England,” said study senior author Professor  Ning Lin.

In order to assess the effects of joint extreme events, the research team used eight different global climate models that simulate the way in which the world’s climate is being altered due to increasing greenhouse gas levels in the atmosphere. 

Since each model weighs climate variables in different ways, the scientists calculated a reliable average. They did this by comparing each model’s predictions of past hurricane development and strength to historical simulations. The researchers then downscaled these models to generate tens of thousands of synthetic storms for the region of interest along the western Atlantic and the Gulf of Mexico. 

“This way, by comparing the models’ predictions of the past, we know we have a reasonable basis for the models’ predictions of the future,” said Professor Lin.

The researchers next modeled the powerful winds produced by the hurricanes and the distances that the winds extend outward from the storm. It is these powerful winds that drive storm surges by pushing great volumes of water toward land. The storm winds also affect the amount of rainfall associated with a hurricane. If a storm has a slower translation speed (the speed with which it moves over land and water), then there will be more time for rain to fall in any particular area. 

In these ways, the two hazards of storm surge and rainfall are intertwined and can add to the catastrophic effects. For example, increased rainfall can add to the local storm surge flooding, while slower storms may give gale-force winds more time to pile up higher, more damaging surges.

Using these climate models, along with physics-based hurricane hazard models and statistical analyses, the researchers predicted a dramatic rise in how often joint 100-year events – that usually only have a 1 percent chance of happening in any given year – will occur by the year 2100.

For example, along the Gulf of Mexico, extreme rainfall-surge events have historically occurred on average every 200–500 years. But by the end of the 21st century, the study predicts, these extreme events may occur on average every 10–30 years. And in New England, where extreme joint events historically occurred less than once in a thousand years on average, they are predicted to occur approximately every five years.  

The analysis also enabled the researchers to gauge the relative contributions of sea level rise and storm changes to the joint rainfall-surge hazards. Sea level has risen dramatically in the past 30 years (by around 0.13 inches per year), and this has been considered a primary cause of more frequent floods. For the eastern seaboard of the US, though, the new study implicates increasing storm intensity and slowing translation speeds as the main reasons why joint extreme rainfall-surge events look set to spike.

“It’s a surprising finding that says we should not neglect changes in storm climatology, with enhanced rainfall projected to play a bigger and bigger role in driving compound flooding,” said Lin. She and her colleagues hope these findings will help better inform policymakers as well as bolster overall preparedness for the severe storms to come.  

Thomas Wahl, an assistant professor at the University of Central Florida who was not involved in the study, offered comments on its significance. “Based on the results presented in this study and others, it is paramount for decision-makers to properly account for compound flooding events in their coastal management and adaptation plans.”

Wahl noted that the U.S. Army Corps of Engineers and the Federal Emergency Management Agency (FEMA), two agencies deeply involved in flood mitigation and disaster response, have both recently created task forces to better understand the impacts of compound flooding.

This growing awareness of the threat of compound flooding dovetails with Lin’s own work on hurricane risk analysis, where the hazards of storm surge and rainfall are no longer being treated as separate phenomena. 

“We’ve been learning that as joint hazards, surge and rainfall really should be studied together, and right now that’s where my work is going,” said Professor Lin. “The current and future joint hazard posed by hurricanes has not been well-quantified, but with this study, we’re now getting a clearer, and unfortunately sobering picture.”

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By Alison Bosman, Earth.com Staff Writer