Pre- and Post-Storm Photo Comparisons - Introduction
On Monday, September 15, 2008, two days after the landfall of Hurricane Ike, USGS scientists acquired oblique aerial photographs from an altitude of about 200 m from Creole in western Louisiana to Freeport, Texas, south of Galveston. These photos were compared to images at the same locations acquired on September 9, 2008, several days before landfall, and revealed remarkably diverse changes along the impacted coast.
Location index of photo pair focus areas. Each focus area includes a series of photo-pairs showing changes that occurred as a result of Hurricane Ike. The green line shows Hurricane Ike's storm track.
The primary factors that control hurricane induced coastal change appear to be the elevation of the coast and the elevation of the storm surge. These factors varied significantly across the area that was affected by Hurricane Ike, providing a unique opportunity to understand the interplay between the hurricane forces and coastal response. Here, we provide an introduction to results of the interaction between Hurricane Ike and a wide stretch of the Texas and Louisiana coastlines.
East of Landfall
Hurricane Ike made landfall at the mouth of Galveston Bay, exposing the coast lying east of landfall to the right-front quadrant of the storm, the location of its strongest winds. The highest storm surge, onshore-directed winds, and waves all focused their energy in this area. The most extensive impacts to beaches and structures occurred immediately to the right of landfall, near the right eyewall, along the sandy spit of the Bolivar Peninsula, TX. The combination of low dunes and high surge levels, made these beaches vulnerable to inundation and large-scale coastal changes during Ike landfall (see updated assessment of inundation potential for Galveston, TX).
Entire neighborhoods were destroyed by battering waves riding on top of a 5 meter storm surge (see pre- and post-storm photo comparisons for the Bolivar Peninsula, TX). Low, 1-3 meter sand dunes that were protecting those neighborhoods were flattened by erosion and sand from the beach and dunes was transported landward and ultimately deposited across the width of several blocks.
Farther to the east across the upper Texas and western Louisiana coasts, the morphology was different and so were the impacts (see Pre- and Post-Storm Photo Comparisons - High Island to Sabine Pass, TX). Here, before the storm, the elevations of the dunes (or beaches in the absence of dunes) were low, ranging from only 1 to 2 meters high in western Louisiana. Landward of the beaches were low-lying marshlands. The storm surge of 3 to 4 meters overtopped the beach systems and, in places, inundated the marshes tens of kilometers landward from the shoreline. Two days after the storm, the floodwaters were still present, held in place by what remained of the beaches that acted like levees containing the water (see Location 2: Oblique aerial photography near McFaddin Wildlife Refuge, TX). In fact, the crests of these beaches rising above the water looked like barrier islands, whereas they were actually mainland beaches. Breaches were cut through these 'barriers' and water flowed seaward slowly draining the huge lakes. During this process, the eroded sand was transported seaward into the Gulf (see Location 3: Oblique aerial photography near McFaddin Wildlife Refuge, TX). Closer to the location of peak surge though, just east of High Island, Texas, the flood waters were high enough to completely submerge the barriers as the surge flowed rapidly back into the Gulf of Mexico.
West of Landfall
West of Hurricane Ike's landfall (see Pre- and Post-Storm Photo Comparisons - Galveston, TX), differences in the storm surge, winds, and waves as well as higher coastal elevations all worked together to lessen the storm's impact on the coast. Most of the city of Galveston was west of Ike's landfall and was not exposed to the extreme winds of the right eyewall that devastated the Bolivar Peninsula. Furthermore, a 5 to 6 meter high, 16 kilometer long seawall protected much of the city from both surge and wave attack. Erosion was restricted to the beach in front of the seawall. Here, most of the severe impacts to buildings were confined to those that were built on piers that extended seaward of the seawall (see Location 5: Oblique aerial photography of Galveston, TX).
On the sandy beaches west of the seawall, peak dune elevations before the storm were 2 to 4 meters, roughly half of the elevation of the seawall. The coastal change along this unprotected stretch of Galveston Island was considerably more than on the seawalled section nearby, but less than the visible impacts on the Bolivar Peninsula. At landfall, while Bolivar was being pummeled with extreme onshore winds, much of Galveston Island experienced offshore winds, reducing both surge and, to some extent, the wave heights. Impacts were not absent, though. Waves, aided by high surge, eroded the beach, moving the shoreline landward of some houses (see Location 1: Oblique aerial photography of Galveston, TX). The elevation of the beach and dunes west of the Galveston seawall, combined with the lower surge elevations and wave heights interacted with each other such that the total devastation observed on the Bolivar Peninsula was avoided this time.