The evolution of large scale crescentic bars on the northern Gulf of Mexico coast

Abstract

Airborne bathymetric lidar is used to quantify the properties and evolution of kilometer-scale crescentic bars in the northern Gulf of Mexico between 86.0°W and 86.5°W, a distance of 50 km. Repeated lidar surveys in 2004 pre-and-post-Hurricane Ivan, in July 2005 after Hurricane Dennis, and in late 2005 after Hurricane Katrina, in addition to several satellite images, show crescentic bar growth over long time scales, and destruction over much shorter times. Morphological changes follow the Wright and Short classification to some degree, although the generation time scales appears much greater than typically reported: features with O(1000 m) longshore scales develop in months to years rather than days to weeks. The reason for this appears to be the order of magnitude difference between the extreme conditions during tropical cyclones which generate the precursor linear bars in relatively deep water, and the typical small wave conditions in the Gulf when crescentic features develop. Another unexpected feature shows that it may require more than one direct hurricane landfall to completely remove these large scale crescentic features. The crescentic bars examined here form only a small portion of the available dataset: work to be reported on shortly will examine bar properties over a 200 km section of coastline.