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Abstract: Historical Shoreline Dynamics Along the Chenier Plain of Southwestern Louisiana

Mark R. Byrnes, Randolph A. Mcbride, Qiang Tao, Lisa Duvic

Processes affecting coastal change in Louisiana have caused the highest rates of erosion and wetland loss in the United States. Eustatic sea level rise, subsidence, hurricane impacts, sediment supply, and regional geology dictate the magnitude of shoreline movement for the Gulf coastline. Although rates of change are high and relatively consistent for much of the Mississippi River Deltaic Plain, shoreline response along the marginal deltaic environment of the chenier plain in southwestern Louisiana illustrates great variability in direction and magnitude. The purpose of this paper is to quantify changes in high-water shoreline position for the period 1883 to 1994 within a framework of process-response variables affecting short- and long-term changes, and to identify the r lative importance of natural and human-induced influences on coastal evolution.

A computer-based shoreline mapping methodology, within a framework of a geographic information system, was used to compile and analyze changes in historical shoreline position between Sabine and Southwest Passes in southwestern Louisiana. The data base consists of three cartographic-based shorelines (1883/86, 1923/35, and 1948/54 National Ocean Service topographic maps), a series of air photos from the 1960s to the 1990s, and a global positioning system field survey completed in May 1994. In addition, beach profile data for a 15-km-stretch of coast between Holly Beach and Ocean View Beach are used to address short-term response to engineering activities in this area relative to long-term trends. Regional patterns of change illustrate systematic shifts between shore retreat and advance in relation to sediment supply from the westward-directed Atchafalaya River mudstream, as well as local sources, and shoreline orientation relative to storm and normal wave conditions. The eastern 15 km of the study area shows net retreat of about 2 m/yr until a position where the modern chenier plain is prograding at a rate of about 3.5 m/yr. West of this area, shoreline orientation changes to a northwest-southeast direction and high retreat rates are encountered (-6 to -12 m/yr). Upon reorienting to a more east-west direction, the western 85 km of the chenier plain becomes primarily progradational to stable, with minor net retreat indicated between Holly Beach and Ocean View Beach (-1 m/yr). The western 25 km of the study area shows net progradation of about 4 m/yr. Although shoreline etreat is common over much of the chenier plain, long-term trends indicate a near balance between areas of erosion and accretion since 1883.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana