Geomorphological Controls on Road Damage during Hurricanes Ivan and Dennis

County Road 399 along Santa Rosa Island in northwest Florida was largely impassable following Hurricanes Ivan M, and Dennis, but the damage was highly variable alongshore, ranging from areas with little or no damage to areas where the road was completely breached. Damage was categorized and correspondence analysis was used to examine levels of damage were affected by the proximity of the road to the shoreline, the elevation and width of the island, and the offshore bathymetry. Depending on the specific combination of these variables along the island, they can have either a compounding or competing influence on the overwash penetration and the damage to the road. Sections of road with little to no damage were located close to the shoreline and associated with an above-average island elevation (large foredunes or berm). These areas tended to be located in wider sections of the island and equally distributed between areas with a steep and a gentle offshore profile. During Hurricane Ivan, sections of the road that sustained major damage or were completely destroyed tended to be in narrow sections of the island where the offshore profile was steep, while half the level of damage was associated with a more gently sloping bathymetry. In contrast, major damage during Hurricane Dennis occurred in areas where the road was located in the backbarrier, where the prestorm elevation of the island was relatively low, and half the level of damage occurred in narrow sections of the island where the offshore bathymetry was steep. Based on the observed relationships it is concluded that damage to the road is forced by the island geomorphology and that areas where the road received extensive damage will continue to be areas of concern despite changes in road design and placement.