ABSTRACT: Shoreline Evolution from 1945 to 1988 at Grand Isle, Louisiana

Celeste A. Bencaz, Richard U. Birdseye

Louisiana is losing over 55 mi² of its coast annually owing to natural processes and many forms of human intervention. Sand bodies are especially impacted in the Mississippi Delta Plain where Grand Isle, the state's only inhabited barrier island, is located.

Trends in erosion and deposition on Grand Isle were evaluated using aerial photographs acquired in 1945, 1949, 1956, 1958, 1963, 1964, 1965, 1966, 1971, 1976, 1978, 1982, 1986, and 1988. Shoreline changes in 14 different zones on the island were calculated for each set of aerial photographs and summarized on maps and in tables. Geological interpretations, meteorological data, and historical records helped to explain the patterns of change.

The major factors contributing to shoreline changes at Grand Isle are:

1. The nature of the unprotected sediments forming the margins of the island. Unstabilized shorelines include slopes composed largely of sand, tidal marsh, sand and shell beaches, and recently dredged fill.
2. Shoreline orientation with respect to wave fetch, prevailing wind directions, and approaching storms.
3. Duration and magnitude of major storms.
4. Local weather, along with regional and worldwide climate.
5. Relative sea-level rise, owing to subsidence of the Mississippi Delta Plain sediments from compaction, consolidation, and isostatic adjustment, as well as to a currently rising global sea level. The rate of relative sea level rise at Grand Isle averages 0.41 in./year.
6. Sediment deprivation, largely owing to human influences such as dams, reservoirs, artificial levees, and control structures located in the Mississippi drainage basin, alluvial valley, and delta plain.
7. Disruption of longshore drift by construction and orientation of jetties, groins, and breakwaters.
8. Beach nourishment and marsh infilling by humans.

In the 43 year interval studied, naturally induced shoreline retreat exceeded accretion on both the north (bay) and south (beach) sides of the island. However, largely because of beach nourishment and jetties, the area of Grand Isle actually increased by 33.9 acres between 1945 and 1988.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990