Coastal Change Hazards: Hurricanes and Extreme Storms

Hurricane Rita

Assessment of Potential Coastal Impacts

Posted Friday, September 23, 2005

Hurricane Rita predicted path.

As of the 2:00 p.m. EDT National Hurricane Center (NHC) public advisory #24A, the central Gulf coast from Port O'Connor, TX to Morgan City, LA is under a hurricane warning, while the region from Galveston Island, TX to the eastern boundary of Cameron Parish, LA falls within the forecast cone for hurricane landfall. The potential impacts of Hurricane Rita on this segment of Gulf coastline are predicted with a USGS-developed storm-impact scale. In the storm-impact scale model, four impact regimes are defined by comparing predicted elevations of storm surge and wave runup to known elevations of coastal topography. For Hurricane Rita, vulnerability assessments focus on the most extreme impact regime (level 4), i.e. where storm surge exceeds the elevation of the seaward dune, and the coastal regions approach complete inundation.

In a cooperative research program between the U.S. Geological Survey and NASA, this stretch of coast was surveyed in 1999 and 2001 using airborne laser mapping techniques (lidar). Lidar surveys provide detailed topography of the coastal areas, and the elevation of the seaward dune or berm are extracted. These surveys do not reflect changes that have occurred between the survey dates and the time of impact of Hurricane Rita.

Areas susceptible to inundation during landfall of Hurricane Rita are assessed by comparing lidar-derived dune elevations to simulated storm surge values. Storm surge values follow the 2 p.m. NHC Public Advisory #24A that open coast surge may reach 4.6 meters (15 feet) above normal tide levels near, and to the east of, where the center makes landfall. For comparison, we also show results using a surge level of 3.0 meters (10 feet) that may be more applicable to areas further removed from landfall. Inundation commonly leads to the landward transport of massive volumes of sand and the possibility of opening new breaches that sever barrier islands.

When interpreting these maps (shown below), it is important to understand that the assessments assume a fixed surge value at each location. That is, a red area on the 4.6 meter (15 foot) surge map may inundate only if it is located near or immediately to the right (east) of landfall. A red location on the 3.0 meter (10 foot) surge map is less sensitive to its position relative to landfall.

Note: This experimental product is based on research results of the USGS National Assessment of Coastal Change Hazards Project and applies to potential inundation caused by storm surge only. This vulnerability assessment does not directly consider potential property damage or the impacts of high wind speeds and heavy rain. This analysis only considers the vulnerability along the open coast and does not extend to shorelines 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. This discussion simplifies the problem to some of the most important aspects, but a full analysis of all processes would be required to fully understand the magnitude and spatial variability of storm-induced coastal change of Hurricane Rita.

[larger version] [larger version] Maps showing storm surge minus dune elevation, which indicates the potential for level-4 coastal change (inundation in meters), for surge levels of 4.6 m (top map) and 3.0 m (bottom map). Red indicates that NHC advisory storm surge elevations are higher than the seaward dunes, which suggests greater vulnerability to inundation and associated coastal change. Green indicates that the surge value is lower than the elevation of the dune, suggesting less vulnerability to inundation. See discussion above for further explanation of the surge values.