FEMA Flood Map Follies: Part II

This fall, FEMA released their preliminary coastal flood maps for the State of New Hampshire.  The maps were highly anticipated amongst municipalities and developers in light of the release of the costal flood maps in Massachusetts and Maine, wherein it was revealed that FEMA had utilized wave patterns of the Pacific Ocean in developing its flood maps.

Following the release of the preliminary New Hampshire flood maps, I had the opportunity to speak with Steven Michaud of Doucet Surveys, Inc. and Nathan Dill of Ransom Consulting, Inc. regarding the development of the FEMA flood maps, both of whom provided their unique insight and experience to this matter.

Steven Michaud is an experienced land surveyor well versed in the techniques and methods for mapping topography of the State of New Hampshire.  Mr. Michaud explained to me that the issue with the revised FEMA flood maps is not with the technology utilized to determine the topography.  The first FEMA flood maps were generated in 1978 utilizing the technology of the time, primarily long range photography that could be obstructed by treetops and man-made structures.  The photos were limited resolution — accurate to about ten feet—thereby, limiting the accuracy of the resulting maps.

Mr. Michaud informed me that to generate their revised flood maps, FEMA employed Laser Imaging, Detection and Ranging or “LiDAR.”  LiDAR instruments are fitted to aircraft and satellites to map an area, which can generate a resolution within centimeters and is not obstructed by vegetation or structures.  The topographical mapping of the New Hampshire coast in the revised flood maps, therefore, is superior to prior maps.  It is not the geological mapping that is the issue with the FEMA flood maps; rather, the issue lies with the determination of the base flood elevation aka the 100 year flood level.

Nathan Dill is a coastal engineer at Ransom Consulting, who studied storm surge modeling in Louisiana in the aftermath of Hurricane Katrina.  He understands floods and flood plains.  Mr. Dill explained to me that the generation of the base flood elevation involves a consideration of several factors.  First is a consideration of the “stillwater elevation,” which is determined by considering the high tide and “storm surge” (what the wind does to water without the effect of waves).

The next factor, and the one that Mr. Dill informed me is FEMA’s ultimate error, is “wave setup,” i.e. the effect of waves upon the stillwater elevation.  To determine “wave setup,” FEMA relied upon the Direct Integration Model (“DIM”).  The DIM method relies upon empirical data from the West Coast, where waves are bigger and the shores are straighter.  The DIM method is not appropriate for the East Coast, where smaller waves and crooked, more sheltered coastline exists.  The result of using the DIM model is a “wave setup” that can be approximately five to six feet in some communities—overly conservative for this region.  Curiously, in non-New England states along the Atlantic and Gulf Coast, “wave setup” is determined in conjunction with storm surge using more advanced and technically accurate modeling techniques, which apply super computers to simulate storm surge and wave setup simultaneously.  Mr. Dill informed me that these more accurate modeling techniques can result in a “wave setup” roughly half of that under the DIM method.  Similarly, a model called the Simulating Waves Nearshore Model (“SWAN Model”), which does not require massively parallel super computers, can also be applied to calculate wave setup more accurately than the DIM method.

In short, there are specific and technical grounds to challenge the FEMA flood maps.  The ninety-day appeal period for challenging the preliminary flood map was triggered on September 2, 2014, thus the appeal period will expire, and the maps will become final, on December 2, 2014.  Those wishing to challenge the FEMA flood maps must provide scientific and technical data to have FEMA consider the preliminary maps.  Those who do not file an appeal within the ninety-day appeal period may still submit scientific and technical data to FEMA, which may still result in a revision to the maps if warranted.  Moreover, individuals can challenge the FEMA maps as applicable to their specific piece of property through a “letter of map amendment”—a process wherein a property owner can challenge the location of the base flood elevation as it relates to the topographical characteristics of the individuals property.  DTC is available to assist those wishing to take advantage of the appeal, comment, or letter of map amendment process.

The author would like to thank Steven Michaud and Nathan Dill for their insight, comments, and assistance in writing this article.  For more information about filing a comment or challenge to the FEMA flood map, please contact Eric A. Maher, Esq.