Reading the Aftermath of Hurricane Matthew

Reading the Aftermath of Hurricane Matthew 
October 20, 2016 

Hurricane Matthew may not have been the strongest storm to ever hit the Atlantic, but it ranks among the most destructive. Recent estimates from major banks total almost $10 billion dollars in damage and lost revenue from Miami to Virginia.

Matthew clocked category five winds of more than 157 miles per hour. But it was water, not wind, that was the most devastating aspect. Between storm surges at the coast and extreme rainfall inland, parts of eastern North Carolina received record flooding.

Satellite Image of Hurricane MatthewAfter more than a week of flooded rivers and roads and 25 confirmed deaths, the water is beginning to recede. And as it does, scientists are beginning to examine the data from Matthew, to determine how these floods happened and what their long term effects might be.

By the time Matthew reached Wilmington on Saturday, October 8, its sustained winds were 70 miles per hour—less than half of what they had been at the storm’s height. Even at that speed, however, Matthew’s wide, sweeping arms still had the power to generate a sizeable storm surge.

The National Hurricane Center defines storm surge as any water level on a shoreline higher than normal high tide. Hurricanes like Matthew wield an enormous amount of energy in their winds and rotation and all that energy can drive massive amounts of ocean water onto shore and up through estuaries, causing wave and flood damage.

Matthew caused storm surges from Florida to Virginia Beach. The worst storm surges hit Florida and Georgia at more than nine and eight feet respectively. The Carolinas received less than six feet of storm surge at their worst, but that level was still enough to damage the Oak Island and Jacksonville piers, while Cape Fear River levels reached a higher level than any seen in the last 60 years.

Storm surges can be truly devastating. In 1999, Hurricane Floyd caused a 15-foot storm surge along the Outer Banks that tore houses off stilts and destroyed roads. Scientists can predict storm surges using computer models, depending on the strength of the storm and the path it takes along the coast.

Flooding in Charleston SCLuckily, North Carolina was spared as destructive a surge as it got from Floyd. The same cannot be said, however, for rainfall and inland flooding. According to stream gauges operated by the United States Geologic Survey (USGS), Matthew set flooding records at 28 separate locations in eastern North Carolina.

Jeanne Robbins, the assistant director for data at the USGS South Atlantic Water Science Center, says several conditions led to the flooding being as devastating as it was.

“In addition to the tremendous rainfall associated with Hurricane Matthew,” Robbins said. “Rain that fell prior to Matthew in September led to higher than normal streamflows and wet soils that had limited capacity to absorb more rainfall.”

When rain falls, it only has two places to go: into the soil where it can be absorbed by plants or into a stream, river or other body of water where it can flow into the sea. Land is basically a sponge in that can only take on so much water before it is completely saturated.

The problem with saturated land, like a saturated sponge, is that once it becomes saturated, water just runs off it. The effect is worse in developed areas with lots of asphalt and buildings where the land is blocked off from absorbing any water at all—you can read more about how water travels from rain to rivers here and how to give water more time to absorb.

Matthew dropped anywhere from 2.5 to 15 inches of rain throughout central and eastern North Carolina, and thanks to a rainy September, the soil was not ready to handle much of that water. It swiftly ran over roads and lawns and through drains and culverts and subsequently into rivers.

Radar Image of Hurricane MatthewRobbins says the fact that Matthew’s rains covered such a large swathe of land contributed to the severity of floods. When rain runs off asphalt or soaked ground, it will run far and wide to reach a river or other body of water. Matthew dropped nearly 13.6 trillion gallons of rain on the United States, about two-thirds of the volume of Chesapeake Bay. While only a fraction of that fell on North Carolina, it was more than enough to overwhelm the Neuse River and others that drain North Carolina into the ocean.

In towns like Lumberton, Princeville, Kinston, Goldsboro and Smithfield, that manifested in the Neuse, Tar and Lumber Rivers spilling several feet of water over farms and through towns, but all that water did not come at once. Robbins says that it takes time for runoff to flow over the land and down through the river systems, so floods can remain or even increase in strength for days after the rain stops. While the storm passed North Carolina on October 8, the crest of the flood did not hit Goldsboro until the 12th, or Kinston until the 14th. You can see an animated map of rainfall and flooding here, which shows many places along the east coast getting more and stronger floods in the week following Matthew's passing.

“In some cases we saw the rivers rise with the influence of rainfall from the areas in close proximity to the river and then continue to rise as floodwaters from tributaries traveled downstream and contributed even greater volumes of water to the already swollen rivers,” she said.

The USGS is continuing to monitor water levels and stream flow rates at more than 260 locations. The data will not only serve as a record of one of the most powerful natural disasters in North Carolina history, but it can guide planning and preparation efforts for storms, which unlike the heavy rains and storm surges themselves, we can actually do something about.

“Prudent planning and readiness that is based on reliable data can help minimize the losses associated with phenomena like floods,” she said. “The USGS's role is to help gather the critical, real-world measurements and scientific interpretation that can be used to support those efforts.”

Matthew may follow Floyd, May and Irene as one of the worst natural disasters in North Carolina’s history. It will not be the last, but as flood waters recede, rebuilding commences and data come in, North Carolinians can learn from Matthew and prepare for what comes next.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.

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