Impact of 1985 Hurricanes on Isles Dernieres, Louisiana: Temporal and Spatial Analysis of Coastal Geomorphic Changes

Karolien Debusshere, Karen Westphal, Shea Penland, Randolph McBride

Catastrophic geomorphic changes occurred in the Isles Dernieres barrier island arc as a result of the direct impact of three hurricanes in 1985. The severity of the impact of hurricanes Danny, Elena, and Juan had not been equaled since the landfall of hurricanes Betsy and Camille in the late 1960s. The Isles Dernieres had not been subjected to a direct hurricane landfall since hurricane Bob in 1979. The recent hurricane impacts provided the USGS/LGS Louisiana Cooperative Barrier Island and Land Loss Study the opportunity to examine the process-response characteristics of this low-profile transgressive barrier island arc to multiple hurricane impacts in a single hurricane season.

The geomorphic changes along the Isles Dernieres were determined using four sequential airborne videotape surveys acquired in July 1984, July 1985 (pre-storm), August 1985 (post-Danny) and November 1985 (post-Juan) and mapped on 1:24000 base maps produced from concurrent vertical aerial photography. A coastal geomorphic classification was developed to describe, quantify, and map the alongshore geomorphic, sedimentologic, and vegetative character of this barrier shoreline. The classification consists of three levels of descriptors: (1) "primary morphology" to define the predominant longshore morphology, (2) "modifiers" to depict the small-scale longshore features, and (3) "variants" to locate and quantify important coastal features, not mappable at the scale used.

The comparative temporal and spatial analysis of coastal geomorphic changes in the Isle Dernieres shoreline demonstrated that prior to the direct impact of hurricanes Danny, Elena, and Juan, the Isles Dernieres had been experiencing average annual erosion conditions driven predominantly by relative sea level rise and cold front impacts. Under these normal shoreline erosion conditions, the barrier island morphology reflected the influence of constructive coastal processes; the island profiles were relatively high and storm resistant due to extensive vegetative cover, and the landscape was characterized by high-relief dunes and terraces. After hurricanes Danny, Elena, and Juan impacted the Isles Dernieres shoreline in 1985, extensive beach erosion occurred. The effects on the morphology were different for each of the four Isles Dernieres barrier islands primarily as a result of specific differences in the pre-storm morphology and vegetative cover of the individual islands. The morphology of Trinity Island for example, was predominantly dune and washover terraces under pre-storm conditions and predominantly washover flat after the hurricane impacts.

The magnitude of the hurricane process-response pattern in the Isles Dernieres is controlled by the relationship between the geomorphology of each particular area, including the adjacent back-barrier features and the storm surge overwash intensity. The general trend from the pre-storm to post-storm conditions is a lowering of the coastal profile, a reduction of vegetative cover, and the development of lower relief landforms.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.