I had a phone call from a rather irate property owner demanding that Dennis appeal the recently adopted flood maps. I tried to explain that we had little scientific evidence to suggest that the maps were inaccurate. In fact, in the part of town the caller was from, there was little change in the elevation of the flood waters. Predominantly what had occurred was the water elevation was carried to the appropriate ground elevation.
On the old maps, the flood waters for a number of areas were illustrated as stopping at ground elevations well below the anticipated flood elevation. I pointed this out in an earlier post looking at Old Wharf Road in the vicinity of Camper’s Haven. In thar post I had noted that flood waters predicted to be at 12 feet above sea level mysteriously stop at Old Wharf Road, which is at elevation 5. In actuality, Hurricane Edna, in 1954, redesigned the cottage layout at Chase’s Ocean Grove north of Old Wharf Road.
The new maps carry the flood waters to approximately the correct areas. Our flyover illustrates a close correlation between the flood map and ground elevation in the neighborhood the call came from. Our flyover places the flood elevation relatively close to the home.
Here’s the rub. Flood elevations are stated in whole numbers. The actual flood elevation could be anywhere from half a foot lower to half a foot higher than the level stated on the map. The flood boundary follows a boundary based upon the model’s flood elevation transect.
Taking Base Flood Elevation 10 as a for instance, the flood elevation where the call came from. The transect places the true flood elevation as 10.4 feet. Rounded down, this results in the map stating BFE 10. Thus, for the neighborhood in question, the flood boundary extends further inland than our own mapped elevation 10.
This has an interesting ramification for flood insurance. Insurance companies base the risk upon the map, the elevation stated on the map AND the elevation certificate. The elevation certificate states the base flood elevation in the whole number. Thus, a 10.4 true flood elevation, becomes 10 feet for insurance purposes. A house could show the flood zone line pass through it and, for insurance purposes, be considered low or no risk if the elevation certificate showed the lowest land area adjacent to the house to be 10.1 feet (ie above flood elevation 10 feet), even though the true flood water elevation is 10.4 feet.
In the end, the more we learn about flood elevations, our own land forms and how all of this comes together, the less basis we have to challenge the rationale behind the flood maps.
One last thought, a one inch rise in water levels in the neighborhood in question, and the base flood elevation wound round up to 11 feet rather than down to ten feet. As with many things, it truly is a matter of inches.