Assessment of Potential Coastal-Change Impacts - Chandeleur Islands, LA
Posted: 08/30/2008 9pm
The Chandeleur Islands are extremely low barriers consisting of a mixture beach, low dunes, and marsh. Dunes in this chain were destroyed by severe erosion during 2005's Hurricane Katrina. The islands were completely inundated and nearly all the sand was removed from the beaches and dunes, leaving just fragments of the back-barrier marshes. Some recovery of low beaches has occurred as some island marshes continued to erode; but only modest growth of dunes has occurred, leaving this island chain with very low elevations. If Gustav comes ashore near this location as a Category 1 hurricane or higher, inundation and extreme coastal change are expected over the entire area.
Inundation potential (I, storm surge minus dune elevation) for the Chandeleur Islands, 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 lidar-derived dune/berm crests measured from a June 2008 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.