There were a number of questions about the surge levels for Katrina, and so I have written some notes on a brief analysis that I'd already done when the FEMA reports came out. Until better information becomes available this addresses the question of what levels of surge, and how much of the coastline received each level of surge.
Also after considering the statement in the report that surge only came inland about 12 miles along rivers or bays, I understand now that they are talking specifically about the salt water; the additional flooding inland of that was caused by the surge, but does not contain salt water from the GOM.
Notes on Katrina's surge levels
Not all of the information on Katrina's surge along the MS Gulf Coast has been made available. The initial NHC Katrina report did not go into extensive detail about the surge.
High water marks do not by themselves denote surge levels. High water marks, especially outdoor ones, also include height from waves. Waves right at the shoreline can be quite high. Allegedly a DVD with amateur video which was distributed locally in limited quantities in MS, showed approximately 30 foot waves at the shore in Gulfport. Wave estimates at the shore there were already in that range; waves on top of the 20+ foot surge there were large enough to wash all the dolphins out of their 30 foot high tank at the marina. However wave action, even as small as half a foot, can travel very far inland. Tides are also a factor in computing the actual surge. Tides along the MS Gulf Coast are not a significant factor; the difference between high and low tides is only a couple of feet at most. This factor is easy to take into account as the times and heights of high and low tide are known. These factors must be subtracted to obtain the water height generated from the hurricane surge.
Just as with sustained wind values, identifying the highest level of surge is not the same thing as saying that level of surge occurred everywhere. It is important to quantify the extent of the area flooded by each category of surge.
We can make an unofficial but informed guess as to what areas of the MS coast received what categories of surge, based on information currently available, with the caveat that it may be changed later, and we are not 100% certain of its accuracy.
What tools are currently available on the internet to determine this information?
First, FEMA has published maps of the surge on the MS coastline. These maps show an extremely limited number of high water marks, mostly outside, and that information is too sparse right now to give an overall picture of the surge. Along with these selected high-water marks (which the NHC Katrina report may possibly have implied are not valid because the wave and tide levels have not been subtracted yet), the FEMA maps show all areas which were flooded, by some level of water, by the surge. This may mean only one foot of water from surge (as in the parking lot of the Jackson County sub station in Ocean Springs), to 25 feet of water from surge (as along the southern coast of the Waveland / Bay St. Louis area).
http://www.fema.gov/hazards/floods/reco ... index.shtm
Second, we can use detailed online elevation maps such as Topozone, to identify the elevation at any location along the coast.
http://www.topozone.com
Third, we can use the Hurricane Evacuation Study (HES) maps for Mississippi, from the USACE web site, or the interactive tool from NOAA Coastal Services Center (which will zoom into any specified address). These maps were generated from SLOSH runs (the parameters for the runs are documented as well), and seem to be extraordinarily accurate. These maps show what areas of each county, down to individual addresses, would be flooded for each of the five hurricane category classifications. The HES maps do not show a range of heights associated with each hurricane category, and there is a good reason for that: the height differs with each location based on surrounding topography.
http://chps.sam.usace.army.mil/USHESdat ... select.htm
http://ekman.csc.noaa.gov/website/FEMA_ ... ississippi
By comparing the elevation maps to the HES maps, we can determine at what level each category of surge begins, based on the location. Thus, Category 4 surge begins at 21 feet for BSL, Pass Christian, and Ocean Springs, but at only 15 feet for Pascagoula. This last may have been an error on the part of the model, underestimating the vulnerability of this location, because these Cat 4 levels on the map were flooded, or there may have been certain characteristics of the surge that resulted in an anomaly with higher surge here, because the flooding of the Pascagoula River Basin in general was extreme (
http://www.nasa.gov/vision/earth/lookin ... trina.html ).
Fourth, FEMA has also provided an overview map with their estimates on the height of the surge across the MS coastal region.
http://www.fema.gov/hazards/floods/reco ... erview.pdf
Finally, to back up the FEMA flood maps, we can go directly to the aerial images from NOAA taken shortly after the event (but of course this would be after most of the floodwater had receeded). It cannot always be seen from these maps which areas were flooded. In some places, such as in a large part of Hancock County, everything is covered with mud, but other places will not have any discernable sign, and you cannot always go by the debris line as an indicator either.
http://ngs.woc.noaa.gov/katrina/
Based on this information, and assuming it to be accurate, we can answer the following questions:
What areas of the MS coast experienced Cat 3 level surge from Katrina? What areas, if any, had Cat 4 level surge? Cat 5? Where did this occur, and how large were these areas?
This is not completely straightforward. It is clear that the highest level of surge did not go very far inland. All of the areas that experienced the highest levels of surge were right on the shore, and most are so low they already flood at Cat 1 level. For those we would want to rely on accurate high-water marks to determine what level surge occurred. But that information is not available right now. Luckily we can go by the FEMA overview map to determine where these areas are. Secondly we can look at the areas along the coastline that were at an elevation of Cat 4 or Cat 5, and see if any of these areas flooded. The problem with this method is that there is very little actual real estate along the entire coastline that is that high, as the MS coast is one of many areas in the US that is a flood plain. Also, these areas may not all be right on the coast.
In addition, the FEMA overview map only lists the surge in terms of feet. How can we determine what level of surge that corresponds to? Considering it can be different depending on location, we have to first use the elevation maps in conjunction with the HES maps to find out where Cat 3, Cat 4, and Cat 5 surges begin.
Before we get into this level of detail, we find that at a high level, comparing the FEMA overview map to the HES map, solid Cat 3 surge occurred along the entire MS coastline. This Cat 3 surge finally ended, inland, just NE of Pascagoula city limits, at around 611 and Old Mobile Hwy. The extensive amount of land to the south of that location, all the way into AL and to Mobile Bay, was at Cat 1 and 2 level, and was all flooded. So the Cat 3 surge ended inland, just miles short of the AL border. Cat 1 and 2 surge extended eastward into Alabama. On the western edge of the MS coast, in Hancock County, the surge ended just south of the NASA Stennis facility on the MS / LA border, and just north of Kiln and the Diamondhead areas. The most extensive area of inland surge was on the eastern edge of the MS coastline, along the Pascagoula River Basin, and this is predicted on the MS HES maps.
This, just by itself, is really the most remarkable thing about Katrina's surge: the extent of the Cat 3 surge, not the small area that may have been affected by higher Cat 4 or 5 surge.
Did any of these areas at higher surge levels exist?
We know from the overview map that the highest levels of surge, in feet, were along the southern shore of the Waveland / Bay St Louis area, the Diamondhead bay shore area south of I-10, and an area of the coast from about Menge Ave in the Pass, east to the Long Beach / Gulfport border, so we can decide that the coast on either side of Saint Louis Bay is a good place to start identifying the highest level of surge.
Also, we can use the MS HES maps to locate any land that is at Cat 4 or Cat 5 surge level in Hancock and western Harrison Counties. We find that there are very few areas and that they are very small. Just south of the BSL bridge we find the eastern end of four small city blocks that are at Cat 4 level right along the shoreline. Another area of land that will not flood until Cat 4 or Cat 5 level is SW of there, and runs inland, parallel to the southern shoreline, cutting across the man-made lake for the water treatment plant (an easily-found location on a map). The remainder of the Hancock County shoreline, and quite a ways inland, will flood with a Cat 3. In western Harrison County, along the shoreline, we find only one narrow area, starting just west of Menge Ave in Pass Christian, that runs east along the shoreline into Long Beach, Gulfport, and Biloxi, that will only flood at Cat 4 or Cat 5 level.
Locating these areas on topozone.com, and following the brown elevation lines (which are given every five feet, for instance 15, 20, and 25 foot elevations), and comparing with the MS HES maps, we can make a general assessment at what elevation Cat 4 and 5 start for these particular locations. We find that it looks like Cat 4 starts at around 21 feet in elevation in both places, and Cat 5 at 24 feet for BSL, and 25 feet for the Harrison County area previously mentioned. This is a guess, but is probably not off by more than a foot.
Now, we can use the FEMA flood maps to see how much of each of these areas did actually flood.
In BSL, we find that everything was under water except for most (but not all) of the areas that only flood with a Cat 5 surge. We find that the 25 foot surge identified on the FEMA overview map did not reach far enough inland to affect any areas that were not already flooded. We find that all the tiny areas of land at Cat 4 did flood, and the edges of the Cat 5 area around the water treatment plant flooded. So we know that the highest surge in this area was approximately 24 to 25 feet. Perhaps the area on the Waveland / BSL coastline received higher surge, even though they were already well under water, but we will have to wait for accurate high water marks to assess that.
In eastern Pass Christian, we find that the areas that are at Cat 4 level flooded, but the strip of land at Cat 5 level (beginning at 25 feet of elevation) did not. Since this area is right on the shoreline, we can conclude the surge there reached 25 feet but did not go much over that, if any, and most likely did not reach Cat 5 level.
We can conclude that one very small area of the MS coastline, the immediate coast of Waveland and southern BSL, was subjected to a surge that just barely edged over the Cat 5 level. This area was so small compared to the total area of land inundated by surge, that it is almost negligible, statistically speaking. The elevation of this area was already so low that it was well underwater, the difference in the height of the surge was probably not a significant factor in incurring the total property devastation, but, if it can be quantified, will be useful for denoting the record height of the surge.
The areas that received Cat 4 surge were Waveland and the part of BSL that curves into the bay and GOM, and a small portion of coastline along eastern Pass Christian, to the Long Beach / Gulfport border, and Diamondhead area south of I-10.
The majority of the coastline received Cat 3 surge, and the area receiving this level of surge covered a wide range that was unprecedented, spanning the entire MS coastline, and reaching inland a considerable distance along all but a few miles close to the AL border. The area receiving Cat 1 and 2 level of surge was even more extensive, reaching completely to the northern Jackson County border, in eastern MS, and into AL, crossing Mobile Bay.
my mistake.
