Coastal Change Hazards: Hurricanes and Extreme Storms

Hurricane Katrina

Pre- and Post-Storm 3D Topography: Dauphin Island

In a cooperative research program, the USGS, NASA and the U.S. Army Corps of Engineers (USACE) are using airborne laser mapping systems to survey coastal areas before and after hurricanes. As the aircraft flies along the coast, a laser altimeter (lidar) scans a several hundred meter wide swath of the earth's surface acquiring an estimate of ground elevation approximately every square meter. The elevation data from different flights can be compared to determine the patterns and magnitudes of coastal change (erosion, overwash, etc.) and the loss (or gain) of buildings and infrastructure. Results shown below come from two lidar systems, the USACE's Compact Hydrographic Airborne Rapid Total Survey (CHARTS) and NASA's Experimental Advanced Airborne Research Lidar (EAARL).

Presented here are three lidar data sets from Dauphin Island, AL (Figure 1). The first data set was acquired in May 2004 as a pre-hurricane season baseline survey using USACE CHARTS (Figure 2A). The second survey, acquired in September 2004 by NASA EAARL, documents coastal change in response to Hurricane Ivan (Figure 2B). Another NASA EAARL survey was completed in September 2005 after the landfall of Hurricane Katrina (Figure 2C).

Figure 1. Location map.

Figure 2: Dauphin Island, AL. Three-dimensional views of island topography (A-C), post-Katrina oblique aerial photograph (D), and differences in topography from Hurricanes Ivan (E) and Katrina (F). The view is along the island, looking from east to west, with the Gulf of Mexico to the left and Mississippi Sound to the right. Lidar elevation images (A-C) show elevations above mean high water. The rounded appearance of houses is an artifact of the spatial resolution of the data and the gridding process. [larger version]

The May 2004 survey (Figure 2A) shows how Dauphin Island looked prior to the 2004 hurricane season. The main road on the island, Bienville Blvd., is easily seen running along the axis of the island. The impacts of Hurricane Ivan, which made landfall 50 km to the east of this location, are evident in the September 2004 survey (Figure 2B). The shoreline has receded and, in some places, is located landward of the first row of homes. Overwash deposits have buried the roads and some homes are missing. The September 2005 survey data (Figure 2C) shows that this section of Dauphin Island, located approximately 150 km east of where Hurricane Katrina made landfall, was devastated by the storm. Notice that sand has been stripped from the island and deposited in Mississippi Sound, and that most of the houses have been completely destroyed. The red arrow in each frame points to one of the few houses that remains standing after both storms. The oblique photo (Figure 2D) taken after Hurricane Katrina on August 30, 2005 provides visual reference.

Consecutive data sets have been differenced to show elevation change between the surveys (Figure 2E-2F). In these images, red indicates a decrease in elevation (erosion), green indicates an increase in elevation (accretion), and gray indicates no change. For each differenced set, the vertical change data has been draped over the post-storm topography (post-Ivan in Figure 2E, post-Katrina in Figure 2F). The May 2004 - September 2004 difference image (Figure 2E) clearly shows the impact of Hurricane Ivan; sand was eroded from the Gulf side of the island and deposited on Bienville Blvd. The gray mounds are houses that remain after Ivan. The September 2004 - September 2005 difference image (Figure 2F) shows the devastating impact of Hurricane Katrina. Sand has been eroded, transported across the width of the island, and deposited in Mississippi Sound, essentially shifting the entire island landward. Each red square is a destoyed house.

Map views (Figure 3) of the difference images provide a different perspective of the coastal change resulting from Hurricanes Ivan (Figure 3A) and Katrina (Figure 3B). The aerial photo (Figure 3C), acquired by NOAA on August 31, 2005, dramatically illustrates the magnitude of overwash.

Figure 3: Dauphin Island, AL. Map views of post-storm difference grids for Hurricanes Ivan (A) and Katrina (B), and vertical photography of the same location (C). The lidar images show elevation gains (green) and losses (red). [larger version]