Assessment of Potential Coastal-Change Impacts - Isles Dernieres, LA
Posted: 08/30/2008 9pm
The Isles Dernieres consist of narrow barrier islands that have long been a site of concern for Louisiana due to severe and chronic erosion that was diminishing the islands. If the islands were to disappear, the mainland would be exposed to larger waves and storm surge. In the late 1990's, most of the islands were restored to be both higher and wider. Prior to restoration, the islands rose only 1-1.5 m high; after restoration, some were rebuilt to 2.4 m, an elevation sufficient to contain a Category 1 surge (area classified as blue on the map). If Gustav makes landfall nearby as a Category 3 hurricane or higher, nearly all of the Isles Dernieres are expected to experience inundation and extensive coastal changes.
Inundation potential (I, storm surge minus dune elevation) for the Isles Dernieres, LA for the direct landfall of Category 1-5 hurricanes. Reds indicates areas where NOAA SLOSH-modeled surge exceeds the elevation of the primary dune, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicates that the expected surge elevation is lower than the dune crest and that the potential for severe coastal change is lower than in areas likely to inundate. [larger version]
The potential inundation, I, of the beach system was defined by calculating the difference between the elevations of SLOSH-modeled storm surge (Maximum of the Maximum Envelope of Water values at the shoreline) and dune/berm crests measured from a 2006 lidar survey. Positive values (reds) indicate that water levels are predicted to be higher than the dune crests and that the beach is more likely to be inundated by storm surge during the direct landfall of a hurricane. Negative values (blues) signify areas where water levels are predicted to be lower than the dune crest. These estimates assume landfall at mean astronomical tide and do not include the effects of wave setup. Landfall at high tide and significant wave setup would increase the potential for inundation. Additionally, the maps represent the vulnerability of the beach system as it was during the time of the pre-storm lidar survey. Changes to beach morphology between the survey date and the date of hurricane landfall can affect the predicted potential inundation.
When interpreting the potential inundation maps for each region, it is important to remember that they describe only one scenario (inundation) that is relevant to coastal change due to the approaching hurricane. Coastal change due to other scenarios (collision and overtopping) can be extreme even if there is not inundation. Additionally, the map represents a worst-case scenario with respect to storm surge levels. This analysis assumed hurricane landfall immediately to the west of a specific location; consequently, the presented inundation potential is localized and will not occur along the entire stretch of coast for a single storm landfall.
Disclaimer: This experimental product is based on research results of the USGS National Assessment of Coastal Change Hazards Project and is intended to indicate the potential for coastal change due to inundation caused by storm surge only. This product is based on an analysis that simplifies the problem to include some of the most important aspects (estimated dune elevations and predicted surge levels). This product does not directly consider potential property damage or the impacts of high wind speeds and heavy rainfall. This product applies to open coast environments and does not consider potential coastal change along bays, passes, or inland lakes. The actual changes that occur during extreme storms are complex functions of a number of processes and variables including ocean waves, currents, and tides. The public should not base evacuation decisions on this product. Citizens should always heed the evacuation advice of local emergency management authorities.