I went back and read the FEMA methodology again, and the on/off flag is an indicator of whether the HWM was used in determining surge inundation or not. Wave HWM are never used in determining inundation. The reason the surge HWM was dropped out is unknown, but it is in the same range as other nearby HWM, so that didn't change anything one way or the other.
Here is part of an email I received from FEMA's contractor last year based on questions I had about the specifics of determining the surge inundation:
"The Katrina surge inundation limits were mapped solely based on high water marks (HWMs) surveyed after the storm and Light Detection and Ranging (LIDAR) topographic data collected by the National Oceanic and Atmospheric Administration (NOAA) in 2004. More than 400 HWMs were collected in the 3 Gulf counties of Mississippi, an unprecendented number of post-storm points for an unprecedented storm. The 400+ HWMs consist of points reflecting surge-only flooding and points that include local wave effects (wave heights and wave runup). Tide was not factored out of any HWM elevation. To map Katrina's surge inundation, we used the surge-only points and mapped/labeled all of these points on the Katrina Recovery Maps."
"...The detailed appearance of the inundation limit that you perceive is more a function of the high resolution of the LIDAR data than the HWMs. In other words, we're very certain of our ground elevations to a high degree of precision, but there is inherent variability and uncertainty in the HWMs, hence our reference to the inundation limits as "approximate." That's not to say the HWMs are not accurate -- each point surveyed was required to be +/- 0.25 foot."
Actually the ABFEs that temporarily replace the existing FIRMs, before the new FIRMs are in place, are a lot lower than would be required to prevent flooding from Katrina. But they are a lot better than the ones FEMA had been working on for the last couple years, which had to be thrown out completely, because they were as much of a sham as the existing FIRMs from the 80s (which were all to the advantage of the insurance companies, which subsequently would not have to pay out very much when the 'big one' came).
Still, these new ABFEs are having trouble being accepted by many coastal communities. A lot of places simply don't want to have to build up at all, because it will change the look of the community.
HF -- yes, 15 foot waves on top of 25 feet of surge is possible for the Waveland area. But pine trees bend, and possibly they were bent under by the force of the current, rather than hit by waves. Hopefully over the next couple months I'll be able to obtain more specifics on that.
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