RANKING THE GEOMORPHIC FACTORS THAT CONTROL COASTAL HAZARDS

In order to understand and delineate the risk to any geologic hazard at a particular location, one must have a thorough understanding of all of those factors that may contribute to or mitigate against the impact of the hazard. Coastal hazard specialists have done an adequate job of inventorying these ?risk factors? in the coastal zone (e.g. dune height, beach width, vegetation cover). The difficulty with hurricane risk mapping to date is that we haven?t had an adequate picture of which risk factors are more important than others. In an effort to remedy this shortcoming, we have implemented a GIS-based, quantitative technique for ranking the various geomorphic parameters (risk factors) that control the intensity of storm processes along the coast. A first test of this methodology was performed using pre- and post-storm data collected from the impact of hurricane Opal along the Florida panhandle in 1995. Pre-storm parameters are digitized and entered into a GIS database. These parameters include factors such as: dune height, beach width, vegetation cover, inlet proximity, elevation, substrate, offshore slope. A post-storm map of overwash penetration was hand digitized for quantifying storm impact. A grid was then placed over all layers and landscape data was converted to a yes or no value and exported to a spreadsheet. Logistic regression analysis was performed to compare pre-storm patterns to post-storm overwash penetration. Preliminary results have produced the following ranking: site elevation provided the best protection followed by dune height (in front of the site) and beach width. We are in the process of evaluating data sets from other storms in other regions to explore how consistent these results will be. Having a quantitative understanding of coastal hazard risk is critical for producing accurate risk maps, as well as, for prioritizing spending on mitigation.