Coastal Change Hazards: Hurricanes and Extreme Storms

Hurricane Isaac

Pre-Storm and Post-Storm 3D Lidar Topography: Dauphin Island, AL, and Chandeleur Island, LA

The USGS acquired an airborne lidar survey of post-storm topography between the 8th and 18th of September, 2012 along the barrier islands of Louisiana, Mississippi, and Alabama to measure coastal change resulting from Hurricane Isaac. Comparisons of the post-storm elevation data to lidar data collected prior to Isaac's landfall are used to characterize the nature, magnitude, and spatial variability of hurricane-induced coastal changes, such as beach erosion, overwash deposition, and island breaching. We have focused our preliminary lidar analysis on Dauphin Island, AL, and on Chandeleur Island, LA.

Location index for pre-storm and post-storm lidar elevations on Dauphin Island, AL and Chandeleur Island, LA. Each highlighted location includes pre- and post-storm topography as well as topographic change. The green line shows Hurricane Isaac's track. Red patches indicate locations included in the post-storm airborne lidar survey.

Below are pre-storm elevation maps, post-storm elevation maps, and elevation difference maps at each highlighted location. In the pre- and post-storm elevation maps, orange and red colors indicate higher elevations while yellow and green colors indicate lower elevations. In the elevation difference maps, red colors indicate erosion such as shoreline retreat, and blue-green colors indicate accretion, such as overwash deposits where waves and surge have moved sand landward. Location numbers are consistent with those from the photo pair comparison and maps are oriented with the Gulf of Mexico on the bottom.

Location 1

Pre-Storm Elevation		Post-Storm Elevation
Elevation Difference		Location 1: Western Dauphin Island, AL. The western extent of the inhabited portion of Dauphin Island experienced shoreline erosion and overwash. In the post-storm elevation map, a line of berms running west-east along the road form higher elevation features that were not present in the pre-storm elevation map and appear as regions of deposition in the elevation difference map. These berms were constructed after the pre-storm lidar data were collected and prior to Isaac's landfall. Overwash deposition appears north of the main access road. And the deposition at the western end of the map documents the construction of the structure that closed Katrina Cut (see photo pair Location 1). Houses were removed from the lidar data and are not represented in these maps.

Location 3

Pre-Storm Elevation		Post-Storm Elevation
Elevation Difference		Location 3: West of Katrina Cut, Dauphin Island, AL. The inlet that formed during Hurricane Katrina was closed with a rock structure (see photo pair locations 2 and 3) after collection of the pre-storm lidar and prior to Isaac's landfall and appears as a depositional feature. During Isaac, the island to the west end of the cut was breached and experienced shoreline erosion and overwash. The spit feature behind the rock structure formed prior to landfall (see photo pair Location 3).

Location 5

Pre-Storm Elevation		Post-Storm Elevation
Elevation Difference		Location 5: Chandeleur Island, LA. The remnants of the sand-berm constructed in front of the Chandeleur Islands is visible in pre-storm elevation map and were completely eroded by Hurricane Isaac. (The footprint of the pre-storm berm remnants are depicted with light shading in the post-storm elevation map.) The remnant island behind the berm experienced erosion along the outer edge and some deposition in the low region at the center (see photo pair location 5).

Note: The results presented here are considered preliminary and have not been thoroughly reviewed or edited. They are based on preliminary data that may be updated as the survey accuracy improves in future processing steps such as application of the precise ephemeris for GPS satellites, offset checks using ground control surveys, checks for laser calibration and mounting errors, and full application of data quality control, assurance and editing procedures.