Local Scientists Say Expect More Rainfall, Flooding With Hurricanes

Aug 14, 2019

 


 

NASA/USGS Landsat images of coastal North Carolina centered on the Neuse River Estuary and Pamlico Sound before and after Hurricane Florence.
Credit UNC Institute of Marine Sciences

Climate change and warming ocean temperatures may explain why catastrophic flooding during hurricanes and tropical storms seems to be happening more frequently in Eastern North Carolina.

A team of researchers analyzed data from the National Oceanic and Atmospheric Administration that listed rainfall totals from major storms that struck the North Carolina coast dating back to 1898. They found that six out of the seven wettest storm events on record have occurred in the last two decades. 

“That certainly was an eye-opener for us,” said Dr. Hans Paerl, the lead author of the study and Kenan Professor of Marine and Environmental Sciences at the University of North Carolina Institute of Marine Sciences. “We did have a major hurricane in I think 1933. It was unnamed, but it was the storm of ‘33. It was a big one. And then everyone remembers Hazel which occurred in, I believe, 1954. But even those storms didn’t have as much rainfall as the more recent storms that we’ve seen in our area.”

Hurricane Floyd in 1999, Hurricane Matthew in 2016, and Hurricane Florence in 2018 inundated Eastern North Carolina with torrential rainfall and disastrous flooding. According to the study, published July 23 in Nature Research’s Scientific Reports, the probability that three significant cyclone-driven flood events occurred within a 20-year period is less than 2%.

Students in Dr. Paerl's lab collecting water samples in the Neuse River.
Credit UNC Institute of Marine Sciences

“We now have a long-term data set that can verify what a lot of the models have been saying over the last five to ten years,” said Paerl. “Expect to see more wet events that are associated with tropical systems.”

The study suggests that the increased intensity and quantity of rainfall during hurricanes and tropical storms in the past 20 years is a consequence of the greater moisture carrying capacity of tropical cyclones due to the warming climate. 

“Because of warming of ocean waters, there’s more evaporation going on. And that evaporation gets caught up in these major storm events and as a result, they basically dump more rain down when they make landfall or at least impacting the coastal zone.”

It’s not just hurricanes, Paerl warns. Tropical storms or subtropical depressions also have the potential to cause very high precipitation events and catastrophic flooding. 

Dr. Paerl with a student at the UNC Institute of Marine Sciences in Morehead City, N.C.
Credit UNC Institute of Marine Sciences

“In 2006, we had tropical storm Ernesto and folks in New Bern probably remember that. They had about 10-15 inches of rain from just that tropical storm event and it is one of the ones that comes out in the data set as being one of the six wettest storm events that we have seen over the last 20 years.”

The study also found that the higher rainfall totals associated with tropical systems cause major water quality impacts throughout estuaries and coastal waters in North Carolina as large amounts of organic matter and stormwater runoff wash into nearby water bodies. 

“The loading of nutrients (nitrogen and phosphorus) into our estuarine and coastal systems is increasing dramatically with these storm events, and we know very well that is linked to accelerated growth of algae as algal blooms.”

Since May, state environmental officials have urged the public to avoid toxic algal blooms in the Chowan River. Algal blooms were also reported in water bodies in Bertie, Chowan, Pasquotank, and Perquimans counties less than nine months after Hurricane Florence.

High water levels in the Neuse River after Hurricane Florence
Credit Dr. Hans Paerl

“The big question is of course what can we do about this,” Paerl said. “The reduction of omission of greenhouse gases is the number one thing we can all contribute less to in order to minimize this warming trend we’re seeing. In terms of the impacts that we’re seeing in our coastal watershed, it really emphasizes the importance of retaining nutrients and organic matter on land, in order to keep it from flushing into estuarine and coastal systems and degrading our water quality.”

Some methods to prevent nutrients from reaching nearby water bodies have been implemented in Eastern North Carolina, Paerl said, including stormwater retention in urban areas, no-till agriculture, and riparian buffers. The study suggests that state and federal governments should better prepare for acute and cumulative water quality, fisheries resources, and overall socio-economic effects associated with the increase in flooding from more tropical cyclone development.