Storm Erosion and Partial Recovery of Barrier Islands in Deltaic Settings

Trinity Island, Louisiana, is a rapidly eroding barrier island composed of a sandy shoreline backed by marsh overlying a substrate of silt and clay. Ten beach profile surveys (1986 through 1993) and sediment core data (1987) from Dingler and Reiss (1991, 1995) quantified the cross-shore response to two hurricanes (Gilbert in 1988 and Andrew in 1992) and approximately 30 cold fronts per year at Trinity Island. These data documented rapid landward migration of the shoreline and loss of subaerial volume, with the shoreline retreat (-12.9 m/yr) slightly greater than the long-term rate calculated by Penland et al. (2005) (-11.7 m/yr, 1887-2002), but less than the short-term rate (-19.1 m/yr, 1988-2002).

Reanalysis of the Dingler and Reiss data set indicates average erosion of the subaerial beach above Mean Sea Level (MSL) equal to -67 cu m/m/yr, whereas the sub-aqueous portion of the profile (above-1.25 m MSL) increased by 22 cu m/m/yr. The deficit of 45 cu m/m/yr most likely represents loss of fine sediment from the littoral system. The percentage of this loss (~67%) is approximately equal to the percentage of fines (69%) estimated by List et al. (1997) based on sediment core data for non-reworked deltaic sediment.

These data also provide quantification of non-storm processes. Prior to Hurricane Andrew, from 1986 through 1991, the dune crest was increasing in elevation at 9.2 cm/m/yr presumably through minor overwash events and eolian transport. Andrew flattened the dune and exposed the cohesive substrate. Following Andrew, the exposed cohesive substrate eroded from Nov 1992 to Nov 1993 at a rate of -16.8 cu m/m/yr while the remainder of the profile was stable, indicating only minor storms occurred during this period. These data are being applied to test a conceptual model of non-storm erosion and post-storm recovery for barrier islands.