The passage of hurricanes often results in short-period wind waves on top of the much longer-period storm surge that couple to create various types of debris lines including vegetation, seeds, dirt, man-made trash, and dislodged building material. These debris lines can be deposited on or adhere to some surfaces after the peak water level has been reached and it begins to fall. The deposited debris typically leaves a linear feature that is referred to as a High Water Mark (HWM) and this mark with experience and interpretation can be used to approximate the local peak water level. The highest quality marks for estimating storm still-water levels are those that have little or no wave effect. Some HWMs are collected where significant wave or backwater effects are present, but those effects should be noted in the field surveying notes and documentation. In this analysis, the focus was on use of HWMs as indicators of storm water level, without the effects of water level fluctuations due to wave crests wave effects, or riverine flooding.
The U.S. Geological Survey (USGS) began flagging and surveying HWMs along the Mississippi coast on September 1, 2005 to compare the peak surge to the peak surge of Hurricane Camille in 1969. An extensive post-storm effort was undertaken to identify and survey HWMs following passage of the storm in concert with the IPET. While certain
HWMs captured the peak water levels well, they did not contain information about the temporal variation of water level. It is critical in the analysis of HWMs to determine if the HWM reflects the maximum crest of the storm. An experienced field hydrologist /hydrographer can usually determine if the HWM has been affected by waves, riverine flooding or other factors.
Acquisition of HWMs following Katrina was principally performed by three federal agencies: USGS, the U.S. Army Corps of Engineers (USACE), and the Federal Emergency Management Agency (FEMA) (or a FEMA contractor). The State of Louisiana through the Louisiana State University (LSU) also helped recover HWM data. All entities participating in the IPET shared the data. Marks identified by USACE, FEMA, and LSU were also recovered by that respective agency. Most of the marks identified by USGS were recovered by FEMA (or a FEMA contractor). A subset of approximately 50 marks was also recovered by USGS field crews to confirm elevations provided by FEMA contractors. All HWMs were reviewed and assigned a reliability rating. The reliability of each HWM was assessed as "Excellent," "Good," "Fair/Poor," or "Unknown" if there was no information provided regarding the type of mark or setting in which it was acquired. Currently, there is not a standard method for determining HWM reliability. Moreover, assignment of reliability values to HWMs is not a totally objective process, but by its nature involves both objective and subjective elements. HWM recovery and determination requires experience and engineering judgment. Discussion by the IPET assigning the reliability values led to a consensus that the mark should reflect, as closely as possible, the stable ("still") or mean, storm water level. That is, the physical setting where the mark was located should approximate a tide gauge stilling-well if possible. The basis for this consensus is that storm surge models do not explicitly include wave crest or other wave effects, and one of the important uses of the HWM data is validation and verification of surge model simulations.
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