Shoreline Characterization of the Geomorphological Impact of Hurricane Georges on the Chandeleur Islands: Southeast Louisiana

Abstract

ABSTRACT The Chandeleur Islands located in southeast Louisiana are the oldest and longest transgressive barrier island arc in the Mississippi River Delta plain. This north-south trending barrier island system was created by the abandonment and transgression of the St. Bernard Delta Complex of the Mississippi River (Penland et al., 1985; Suter et al., 1988). The St. Bernard Delta Complex was abandoned 1.8 ka due to the process of delta switching which resulted in the formation of the Chandeleur Island barrier arc system (Frazier, 1967). The Chandeleur Islands form an almost perfect arc facing the dominant wave approach from the southeast with sediment transport running south to north along the barrier island chain (Ritchie et al., 1992). The Chandeleur Island arc is a Stage II transgressive barrier island arc (Penland et al., 1988), and consists of the main Chandeleur Island followed by Curlew Island, Grand Gosier Island, and Breton Island to the south. The objective of this study is to develop a storm impact geomorphic classification for Hurricane Georges in order to map the geomorphic storm response patterns. This study utilized aerial videotape surveys (AVS) and will provide insight into the processes of hurricane impact and recovery in the Chandeleur Islands. Hurricanes and cold fronts are important to barrier island migration and geomorphology. Storm overwash can occur when hurricanes impact these barrier islands. The fans created by overwash form a new barrier back platform for the island to migrate onto as it continues its landward transgression. The overwash character of an island is controlled partly by its geomorphology (Sallenger, 2000; Morton, in press; Sallenger et al., in press). The geomorphic pattern seen on the Chandeleur Islands is a reflection of previous hurricane impact processes. The AVS were conducted on July 18, 1996, October 10, 1998, and August 28, 1999. Hurricane Georges passed over the Chandeleur Islands on September 28-29, 1998. The three AVS were analyzed and compared to the 1996 basemap. The basemap consists of the linework representing the mean high water line produced from mosaicked 1996 aerial photography of the Chandeleur Islands. Data obtained from AVS makes it possible to identify and map detailed geomorphic changes at a higher resolution than vertical aerial photography (Debusschere et al., 1991). Data from the field studies were used as ground truth to supplement and ground truth the AVS and linework. The information obtained from viewing the AVS was then mapped on the base map in order to create a set of three descriptor bars on the final products. The shoreline characterization includes only the Gulf of Mexico shoreline. The information obtained and mapped in the descriptor bars (one for each AVS) included the placement and relative size of washover channels, tidal inlets, fringing beaches, presence and type of dune system, washover flats and terraces, intertidal flats, outcropping marsh platforms, flood-tidal deltas, ebb-tidal deltas, and barrier destruction. The descriptor bars are made up of symbols assigned to each of the descriptors and a legend was created. Asample of the characterizations made is included in Figure 1. The scale for all plates is 1:12,000. The linear shoreline measurements made from the characterization are included in Table 1. These numbers show the change in abundance of features following the impact of Hurricane Georges (1996-1998) and upon the initial recovery (1998-1999). Storm impacts control island morphology and, in turn, morphology controls the response of an island to storms. Hurricanes may cause extensive amounts of erosion, but hurricanes create the opportunity for sediment to be ransported and deposited in the back barrier lagoons. This sedimentation allows for the creation and extension of the back barrier platform. Therefore, hurricanes help islands to maintain width as they migrate landward. End_Page 451------------------------