Building on the USGS Louisiana Barrier Island Study (Williams and others, 1992) and the USGS Hurricane Andrew impact assessment (Penland and others, 1998), this data layer depicts the shore-normal transects used to measure shoreline change.
The Isles Dernieres are located about 120 km southwest of New Orleans. This barrier island shoreline is 32 km long and extends from Caillou Bayou east to Wine Island Pass. The Isles Dernieres represent a barrier island arc developed from the reworking and erosion of an abandoned distributary of the Lafourche delta complex within the Mississippi River delta plain (Penland and others, 1988). Previous work by McBride and others (1992) documented a long-term (1887-1988) gulfside erosion rate of -11.1 m/y and a short-term (1978-1988) gulfside erosion rate of -19.2 m/y. The long-term bayside (1906- 1988) erosion rate was measured at -0.6 m/y and the short-term (1978-1988) bayside erosion rate was measured at -5.2 m/y. The Isles Dernieres decreased in area at a rate of -28.2 ha/y between 1887 and 1988, suggesting the long-term date of disappearance would be 2015. The rate of area loss between 1978 and 1988 was measured at -42.7 ha/y suggesting a short-term disappearance date of 2004. In the 1992 Hurricane Andrew impact assessment by Penland and others (1998), the Isles Dernieres experienced a brief and intense period of increased erosion. The average gulfside erosion rate accelerated to -59.37 m/y and the average bayside erosion rate accelerated to -10.8 m/y during the year of the 1992 Hurricane Andrew impact. The Isles Dernieres rate of area change accelerated to -155.7 ha/y during the 1992 hurricane season. Hurricane Andrew accelerated the long-term area loss rate to -28.5 ha/y and the shortterm area loss rate to -49.54 ha/y. The Hurricane Andrew impact on the Isles Dernieres produced 3-5 years of erosion in a matter of days. After the Hurricane Andrew impact, the projected long-term disappearance date of the Isles Dernieres was reduced from 2015 to 2012 and the short term disappearance date was reduced from 2004 to 2003.
In this shoreline change update, the authors used the methods and transects used by McBride and others (1992) to insure data compatibility of the new measurements and analysis.
To evaluate change in the shoreline position, shore-normal transects were constructed at approximately 15-second intervals of longitude or latitude depending on shoreline orientation. Isles Dernieres, Bayou Lafourche and Plaquemines barrier systems (east-west shroelines) were analyzed using 15-second (about 404 m) intervals of longitude, while the Chandeleur Islands (north-south shorelines) were examined by using 15-second (about 462 m) intervals of latitude. Measurements of shoreline movement and change in island width were taken along transects perpendicular to the composite shoreline trend. Average rates of movement and area change were calculated by dividing absolute measurements by elapsed time (year, month, and day - where available.) Shoreline change maps were procuded to determine the spatial and temporal distribution of shoreline movement (magnitude, direction, and rate of change) and document geomorphologic evolution. (McBride and others, 1992).
To up-date the shoreline change analysis by McBride and others (1992) and Penland and others (1998) for the Isles Dernieres to 1996.