As hurricane season approaches, NC now has ways to predict when roads, bridges will flood

Flooding caused by Hurricanes Matthew and Florence washed out more than 1,000 state roads, culverts and bridges in North Carolina in 2016 and 2018. The N.C. Department of Transportation didn’t know which ones until someone saw the flooding first-hand and reported it.

Now, NCDOT and the state Division of Emergency Management are developing systems that use about 400 stream gauges, rainfall data, software and computer models to show where road flooding is likely to happen in advance and monitor flood waters at key locations in real time.

Combined with eyewitness accounts, the systems should help NCDOT and local officials keep people from driving on flooded or damaged roads, which is one of the most common ways people die during and after storms.

“That’s the main goal here is to save lives,” said Matt Lauffer, NCDOT’s hydraulics design engineer. “But then also just respond better and recover quicker. I think ultimately it will just help us run the transportation system more efficiently.”

A system specifically for roads

The state developed an early-warning system for flooding several years ago. But the Flood Inundation Mapping and Alert Network or FIMAN was designed for buildings not roads.

As NCDOT officials watched FIMAN being used during Hurricane Florence in 2018 they asked why there couldn’t be a similar system for roads.

The storm dumped 20 to 30 inches of rain over large parts of Eastern North Carolina, inundating interstates 40 and 95 and cutting off all roads in and out of Wilmington. As the waters rose, NCDOT had no way of monitoring its highway network remotely, Lauffer said.

“And that was blatantly obvious when I-95 and I-40 were going under,” he said. “We didn’t have the gauges all tied in to data so we could easily report out. And so that was a rude awakening. That was a hard place to be.”

Now there’s FIMAN-T, which takes data from rain and stream gauges, folds that into models that predict flooding and then shows on maps what’s expected to happen on the state’s streets and highways. The system covers about 3,000 miles of road so far, mostly east of the Triangle, Lauffler said, but will grow as more gauges are installed.

NCDOT and Emergency Management also have developed a system that monitors rainfall data generated by National Weather Service radar. The system alerts the department when the volume and intensity of rain reaches levels that historically cause flooding at particular bridges and culverts, helping inspectors prioritize which ones to keep an eye on and potentially close to traffic.

Because the rainfall data covers the entire state, that part of the system allows NCDOT to monitor potential flooding at 15,000 bridges and culverts statewide.

For thunderstorms and hurricanes

And NCDOT also is testing the use of computer models that predict storm surge and wave heights to determine which coastal roads might be flooded by an approaching storm. The models, created by UNC-Chapel Hill’s Renaissance Computing Institute and the U.S. Department of Homeland Security’s Coastal Resilience Center of Excellence, predict what a storm might do 48 hours in advance, giving NCDOT and local officials time to prepare, Lauffer said.

The early warning systems will help NCDOT after thunderstorms, when flash flooding washes out roads quickly, as an intense storm over Wake and Franklin counties did in June 2019. But the most widespread use may come during hurricanes and other coastal storms, when floodwaters can rise slowly over several days in Eastern North Carolina and take just as long to recede.

Lauffer said all the gauges, software and computer models aren’t a substitute for eyewitness accounts of highway flooding.

“You combine that with the eyes-on-the-ground stuff,” he said. “And now you’ve got two really good sources of information to compare and contrast and make decisions with.”