Erosional Patterns of the Isles Dernieres, Louisiana, In Relation to Meteorological Influences
Keywords:Barrier island, beach-face erosion, land loss, overwash, sediment volume, wave attack
Over the past 100 years, the Isles Dernieres, a low-lying barrier-island chain along the central Louisiana coast, has eroded extensively. This erosion has resulted in more than 1 km of northward beach-face retreat and the loss of 71 percent of the total island area. The primary causes for the erosion are wind and wave attack, diminished sand supply, and relative sea-level rise. Over the short term, wind and wave attack, which produce backshore transport, beach-face erosion, and overwash deposition, are the major morphodynamic processes modifying the Isles Dernieres. These processes are driven by recurrent cold fronts that move through the area every few days between autumn and spring and by hurricanes that impact the area every few years during the summer and autumn.
Five years of detailed topographic surveys show that a beach on the central Isles Dernieres changed significantly in both shape and sediment volume; however, the pattern of change was not the same each year. Between August 1986 and July 1988, cold-front-generated waves eroded the entire beach face and caused an average of 35 m of retreat at mean sea level and a sediment loss of about 60m3 per meter of beach width. In contrast to the relatively slow erosion caused by cold fronts, hurricane Gilbert, a category 5 hurricane that passed about 800 km south of the Isles Dernieres in September 1988, produced differential beach-face retreat of about 9 m at mean sea level and 40 m at an elevation of 0.5 m. Most of the sediment eroded from the beach face was deposited on the backshore, which resulted in only a small loss of sediment from the beach and a noteworthy decrease in beach-face slope. During the two years following hurricane Gilbert, the mean-sea-level contour remained stationary while the beach face slowly returned to its pre-Gilbert shape. Having regained that shape, the beach face began again to retreat at a rate that approximates the earlier cold-front-driven rate. That magnitude of retreat is expected to continue until another large hurricane alters the erosional pattern. The average rate of berm-crest retreat for the five-year period was 0.5 cm/day; the actual retreat, however, primarily occurred during the winter.