DANICA SCHAFFER-SMITH & JULIE DEMEESTER: Thinking beyond the 100-year flood plain for hurricane protection

After both storms we all saw images of the flooding’s devastation. And there were lots of stories of wastewater treatment plants and hog lagoons overwhelmed by rising waters. But those anecdotes must be backed up by the data we need to make good decisions to avoid future damage.

We wanted a more comprehensive measure of what areas flooded during both storms. So, we turned to satellite imagery, coupled with a computer algorithm, to map the extent of inland flooding from the two hurricanes.

Our findings show that the state’s flood hazard maps underpredicted the extent of flooding from recent storms, their effect on vulnerable human communities, and resulting environmental damage.

The “100-year” floodplain is often the benchmark used to keep people out of harm’s way. This delineation is based on historic flood data. It makes less sense today when climate change is leading to more frequent and intense storms and rapid development is funneling more water downstream.

Our study found Matthew and Florence impacted a much greater area -- at least 23 percent bigger than the 100-year floodplain.

The study overlaid the flood maps with the Centers for Disease Control’s Social Vulnerability Index, which ranks census tracts to predict how well they will respond to disturbances like flooding, and found that the hazard maps systematically underpredicted impacts on communities with senior citizens, people with disabilities, unemployed people, and people living in mobile homes.

The study provides the first assessment of the two storms’ potential impact on water quality in the region where 118 municipal water intakes and 206 public water supply wells lie in the repeatedly flooded areas. Another 40 hazardous waste sites, 339 industrial wastewater facilities, and 218 municipal wastewater treatment plants were likely compromised by the storms. North Carolina environmental regulations do not permit the construction of Confined Animal Feeding Operations in the 100-year floodplain, but we identified 91 swine feeding operations and their associated waste lagoons in the repeatedly flooded area and another 36 poultry feeding operations where animal waste is dry composted.

Our results can help to target buyouts in places that have been, and are likely in the future to be, seriously affected by hurricanes. Our maps can also be used to assess whether infrastructure such as water intakes should be sited differently. They can serve as a frame of reference to determine the need for additional regulations to make communities more resilient to future storms.

In 2020, we have the information we need to make smarter decisions to protect people and nature from future storms. We know that with climate change, storms will come more often and the rainfall will be more intense. We hope that this study, which can be replicated in other flood-prone areas of the Southeast, enables better decision making that will make people and nature more resilient to the inevitable coming storms.