Hurricanes bring not only intense rainfall, but also high winds and flooding. This flooding is powered by the hurricane storm surge: a rise in coastal sea level caused by lowered barometric pressure and by wind blowing the ocean onto the land. The result is that waves and currents affect areas that seawater does not normally access. The turbulent surge devastates the landscape twice: first, as it rushes inland, and then again as it flows back to the ocean. Low areas often remain flooded for weeks or months, and saltwater kills vegetation and impacts water wells. Coastlines frequented by hurricanes can retreat significantly during just a single storm event, taking development literally “off the map.” When Hurricane Ike made landfall along the Texas Gulf Coast on September 13, 2008, Galveston Island and the Bolivar Peninsula were directly in its path. Storm surge reached 3 meters over much of Galveston Island, and 5 meters along the Bolivar Peninsula. While the city of Galveston was protected by a 5- to 6-meter high seawall, the northeast end of Galveston Island had unprotected development immediately landward of the beach (Figure 1).
Figure 1: Before and after photographs of the Bolivar Peninsula show how the developed area was stripped of vegetation and homes were severely damaged or destroyed. Image Credit: USGS
Selecting the Imagery
Photographs taken from aircraft immediately after the storm supported real-time rescue response and also enabled aide to those left stranded where storm-surge debris had blocked roads. Remote sensing imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer, or ASTER, supplied data for mapping environmental changes at local scales; cloud-free imagery of the hurricane impact area became available two weeks after the event.
Applying the Imagery
Before and after images from the ASTER system (Figure 2) clearly show the extent of the storm surge impact, including vegetation damage and flooding. The red colors in these false-color images indicate vegetation; the pre-Ike image (Figure 2, left) shows healthy vegetation across the whole area. In the post-Ike image taken two weeks later, shown on the right of Figure 2, areas where vegetation died or was removed because of seawater appear brown. Dark blue and black areas on the mainland north of the Bolivar Peninsula indicate standing water in the image on the left of Figure 2. Many more areas appear dark blue and black in the post-Ike image showing that, even two weeks after the hurricane, flooding persisted on significant parts of the mainland. Remote sensing images such as these can also be used to re-map affected areas and monitor vegetation regrowth and habitat recovery.
Figure 2: Pre-Ike (left) and post-Ike (right) ASTER imagery of Galveston Island, the Bolívar Peninsula, and the mainland in August 2006. Healthy (red) and dead (brown) vegetation shows storm surge inundation effects. Image Credit: Jesse Allen, NASA Earth Observatory.
Case Study Author: Rebecca L. Dodge (Midwestern State University)
Case study from: Dodge, R.L and Congalton, R.G. 2013. Meeting Environmental Challenges with Remote Sensing Imagery, p. 30-31. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.