North Carolinians are no strangers to flooding. As Helene and Chantal showed, that's true for more than just coastal counties. But a new study found that more than 40% of properties that have flooded in the last three decades were located outside of the designated floodplain. Even though they are still susceptible to flooding, residents or business owners there might not have flood insurance, leaving them vulnerable to economic as well as physical ruin.
"Your house can be 20 feet outside of a floodplain, and you're not required to purchase flood insurance, but you still have flood risk," said Helena Garcia, a Ph.D candidate at UNC-Chapel Hill in the department of environment, ecology, and energy.
Garcia is the lead author of a study UNC-Chapel Hill researchers published Monday in the journal Earth's Future. The findings come out one week after tropical storm Chantal swept through central North Carolina, causing mass flooding, and less than a year after Hurricane Helene put cities and towns in western North Carolina underwater. The study also noted that North Carolina experiences the fourth most hurricane landfalls in the country.
"We know people in Chapel Hill who got flooded the other day (during Tropical Storm Chantal) and they're right outside the floodplain," Garcia added.
When 100-year floods happen more frequently, the floodplain maps don't cut it
The Federal Emergency Management Agency (FEMA) designates Special Flood Hazard Areas, SFHAs, for areas that they determine to be at the highest flood risk. But they are considered by some researchers to be outdated and inadequate. This means that where people choose to live, whether or not they get insurance, and what policies protect them against flood damage are based on imperfect data.
SFHAs are based on the 100-year floodplain. That is the area that has a 1% chance of flooding every year.
"That's actually like quite a high probability," said Antonia Sebastian, a contributing author of the Earth's Future paper "So over the life of a mortgage, that's like a 28% chance that you will flood."
Sebastian is an assistant professor at UNC-Chapel Hill in the department of Earth, marine and environmental sciences and the environment, ecology and energy program.
The SFHA is based on the historical record — how high water levels have gotten in an area in the past. But, as climate change provokes higher sea levels and more intense weather events, these 100 year floods can happen more frequently. Estimates range from every 30 years to every year.
SFHAs also fail to capture the gradient of flood risk. Any area outside the 100-year floodplain is considered to be distinctly non-risky, even if it is just a few feet beyond the boundary.
The SFHA defines insurance policy
Anyone with a federal mortgage within an SFHA must purchase flood insurance. But when the SFHA doesn't cover all risky areas, those beyond its boundaries might not know that they are liable to flooding. According to the Earth's Future study, 43% of properties that flooded between 1996 and 2020 were outside of SFHAs.
"Insurance is in many ways like a homeowner's first line of financial protection against the negative impacts of flooding, and it accelerates their ability to recover," said Sebastian. "And so for folks who don't have insurance who get flooded, that can be very devastating financially."
Garcia says that there are many reasons why people don't have flood insurance. One is that they don't recognize that their homeowner's insurance doesn't cover flooding.
"Your house can be 20 feet outside of a floodplain, and you're not required to purchase flood insurance, but you still have flood risk." — Helena Garcia, a Ph.D candidate at UNC-Chapel Hill
Another is that they don't recognize that their property could flood, perhaps because it's not in a SFHA. Or, because their "flood memory" is faint — the last time that they experienced a flood was a long time ago, so their perception of risk is lower.
The third reason is affordability. Flood insurance is another bill to pay.
Since the Earth's Future study revealed areas where repeat flooding occurs, Sebastian says that one potential policy outcome could include subsidized flood insurance policies in those hazardous areas.
A new database for flood risk
Stuart Brown manages the Flood Resiliency Blueprint, a statewide flood resiliency plan within North Carolina's Department of Environmental Quality. The plan leverages data about who is most at risk for flooding to suggest programs and policy. He said that at the outset of this project, "the biggest gap we identified was the flood data that we have."
The North Carolina State Legislature funded the Earth's Future study to create a new database of flooding incidents in North Carolina. The study looked at the impact of 78 weather events across approximately three-quarters of the state's land area between the years 1996-2020. These included 18 FEMA-declared disasters, but also some smaller storms.
"A big piece of this study is that we've mapped all of those smaller events. And so we now have this comprehensive record of flooding over time which wasn't previously available. And I think that type of information can provide a lot of value to local governments," said Sebastian.
The researchers departed from existing sources of flood records, which are not precise to the address-level, and can be incomplete if not every homeowner filed an insurance claim.
Researchers could predict whether a building got flooded even without a record its damage by using machine learning. To do this, they combined what they knew about the geography of a neighborhood with what they knew about how neighbors experienced flooding.
Sebastian explained, "You can imagine: I live 100 meters from the stream. My neighbor lives 100 meters from the stream. We got the same rain, like, we probably both flooded, but they didn't have insurance. So now we're going to predict (that a flood occurred) there."
Helena Garcia has studied North Carolina flooding extensively for her Ph.D. Yet, she was still surprised by the amount of repetitive flooding that occurs here, according to the study's results.
"It's not just concentrated at the coast. We're seeing it further inland as well," she said.
Stuart Brown said that he has worked with members of the Earth's Future research team on the Flood Resiliency Blueprint before, including Sebastian. He says the machine learning techniques they employed in this study could be useful refining the data that his team uses.
Speaking for the Blueprint team, he said, "Any ability to better understand and reduce our uncertainty about how we're going to see flooding is going to be a huge benefit to us and to the citizens of North Carolina."
