After months of questioning certain parts of FEMA’s preliminary floodplain maps, the Wrightsville Beach Board of Aldermen received answers from the representatives of the state floodplain mapping program.
Since the maps were released in August, residents and town leaders have expressed confusion over much of Harbor Island moving to the high-risk VE zone while certain oceanfront property remains in the AE zone. Residents living in high-risk zones have higher flood insurance rates and more building regulations.
Floodplain mapping program (FMP) lead coastal engineer Tom Langan and FMP outreach coordinator Randy Mundt came to Wrightsville Beach town hall council chamber Thursday, Dec. 11 to explain the methods of creating the maps, outline the appeals process, and answer questions from the aldermen.
How FEMA determines flood risk
Coastal flood risk is determined based on predicted storm surge and wave behavior, Mundt explained. FMP made these predictions using a model that took into account the topography of land, topography of the ocean floor, hurricane tracks, tides, wind modeling and wave modeling.
FEMA’s wave modeling predictions have been an area of contention for Harbor Island residents, as that area’s VE designation implies it would see a 3-foot wave during a 100-year storm.
“I live on Harbor Island,” Alderwoman Lisa Weeks said, “and most of the claims in the neighborhood were just from rising water, not from structural damage from a 3-foot wave.”
Mundt explained wave action was predicted using FEMA’s Wave Height Analysis for Flood Insurance Studies (WHAFIS) model. FMP runs the WHAFIS model on transects, or strips of land perpendicular to the coast that they believe are representative of the area as a whole.
Langan said the WHAFIS model for transects running through Harbor Island indicated wave regeneration. Waves traveling inland would first be hindered by the primary frontal dune and then begin to build again as they passed over the waterway.
“I think that regeneration is what everyone is having trouble with,” town manager Tim Owens said, pointing out the body of water separating the beach strand from Harbor Island did not seem large enough to regenerate a 3-foot wave.
Langan said another factor was storm surge. A new, more accurate storm surge predictor called ADCIRC indicated water levels were actually higher further inland. He likened it to sloshing water in a bathtub.
“On the other side of the bathtub, the water’s higher. The water that’s being pushed inland has to come from somewhere,” Langan said.
Another important factor in determining flood risk is ground elevation, Langan said. FMP determines its ground elevation points with LIDAR technology, which uses a laser to measure elevation. Langan admitted the LIDAR data has a 1.5-foot margin of error, leading the aldermen to realize a potential first step for appealing the maps.
Potential for appeal
Because of the difficulty of challenging abstract variables such as wave regeneration and storm surge, the aldermen discussed testing the margin of error in FEMA’s ground elevation data.
“We could survey that transect and give you the [actual elevation] data,” Owens said, “and have you guys re-run the model.”
Mundt said in addition to producing more accurate results, new data could possibly lower the flood risk for certain areas if the actual points were higher than the LIDAR points. However, he warned providing new data could also have the opposite effect.
“If that survey data shows that the ground elevation is lower than what the LIDAR data shows it’s possible that the wave heights that WHAFIS will generate will actually be higher than what we show on the preliminary,” he said. “If we get better topography that shows a higher flood risk, I don’t think FEMA would want us to just forget about that.”
Regardless, Mundt and Langan stressed ground elevation was just one of many components affecting the flood zones, and there would be no guarantee finding errors in the LIDAR elevations would make any difference in the zones.
The first step, they said, would be to test the sensitivity of the WHAFIS model to a 1.5-foot change in ground elevation. That would determine whether it was worth the town’s effort and money to survey the land.
Should the town proceed after that, it could run the WHAFIS model using the more accurate ground elevation data or on a slightly different transect to attempt to change the flood zones.
Mundt said there would be a 90-day appeal process, which would begin in February or March. The town should also consider how going through a lengthy appeal process would affect the rest of New Hanover County, he added, because maps were adopted county-wide and the new maps would grant many areas of the county lower flood insurance rates.
Hiring help
During the board’s regular meeting Dec. 11, the aldermen discussed hiring a firm to get an honest assessment as to whether the town stood a chance of winning an appeal. Owens recommended the town hire Applied Technology and Management (ATM), which proposed a phase 1 review for $24,700.
“[The firm will] generally look at the data that’s in these models and try to look at it in the respects as far as how it would improve your situation,” Owens explained. “It may be coefficients, it may be elevation; they look at all the different data.”
The aldermen expressed a desire to meet face to face with ATM before hiring the firm, so they asked Owens to contact ATM to seek a meeting in January.
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