Role of Salt-Marsh Erosion in Barrier Island Evolution and Deterioration in Coastal Louisiana

Denise J. Reed

Barrier shoreline erosion in Louisiana reaches over 10 m/year, and island area decreased by 40% between 1880 and 1979. Salt-marsh erosion is an important factor in evolutionary barrier shoreline development and is presently contributing, both directly and indirectly, to the deterioration of Louisiana's barrier islands. The marshes originally developed as fresh marshes associated with regression of Mississippi River delta lobes. After abandonment, salinity gradually increased and natural habitat change occurred as subsidence of deltaic sediments and transgression of the coastline by marine processes proceeded. The marsh surface is subjected to relative sea level rise and unless there is sufficient sedimentation to maintain marsh elevation, erosional processes become domina t. Increased inundation of marsh vegetation stresses even halophytic vegetation and leads to plant death. Examination of variations in marsh topography over an area of approximately 1 ha. revealed marked variations in the frequency and duration of tidal inundation. Increased flooding of lower areas can be sufficient to cause plant death and the opening of marsh ponds. As small ponds expand and coalesce to form larger areas of open water, wave action becomes important in eroding pond banks and mobilizing sediment from the bed causing pond deepening. Fragmentation of the marsh by these subsidence-induced processes is part of the evolution of morphostratigraphic forms in the Mississippi deltaic plain from erosional headland with flanking barriers to barrier island arc.

Coastal bays in the Mississippi deltaic plain, many of which originated as interdistributary bays before transgression, are also expanding at the expense of the surrounding salt marshes. Measurements on the margins of Terrebonne and Timbalier Bay, approximately 200 km² in area, show that the rate of marginal erosion varies according to the exposure of the bay shoreline to prevailing southeasterly winds and the fetch length across the bay. In the most exposed areas, erosion rates of over 4 m/year have been monitored and even the north-facing bay shorelines of barrier islands experience erosion rates of up to 2 m/year. The expansion of coastal bays and the fragmentation of interior marshes by pond formation leads to escalation of marsh erosion and increased threats to barrier islands. Bay margins disintegrate to a series of small islands that are rapidly eroded by wave attack. As islands disappear, wave fetch across the bay lengthens and wave attack on these island shorelines and on back-barrier marshes increases. Erosion of back-barrier marshes both decreases island area and reduces the foundation for landward migration of the barrier sand body. Plans to restore Louisiana's barrier islands must face the problem of bay expansion and back-barrier marsh erosion in order to maintain a viable barrier-beach system.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.