Abstract: Changes in Shoreline Position Along the Northern Gulf of Mexico Between 1847 and 1994: Sabine Pass, Louisiana to Dauphin Island, Alabama

Randolph A. McBride, Mark R. Byrnes

Changes in shoreline position were quantified along the north-central Gulf of Mexico between 1847 and 1994, using National Ocean Service (NOS) topographic sheets (T-sheets), near-vertical aerial photography, differential Global Positioning System (GPS) field surveys, and existing studies. The Study area extends eastward from Sabine Pass, Louisiana, to Dauphin Island, Alabama, and consists of lake and outer Gulf shorelines that are separated into four geomorphic zones: 1) the chenier plain of southwestern Louisiana, 2) the barrier systems of the Mississippi River deltaic plain (southeastern Louisiana), 3) the Lake Pontchartrain Basin area, and 4) the barrier island system fronting Mississippi Sound (Mississippi/Alabama). The purpose of this study is to examine the regional distribution of geomorphic response with respect to relative sea level rise and sediment supply for this 700 km stretch of shoreline.

Shoreline change along the chenier plain of southwestern Louisiana depicted alternating zones of advance and retreat between 1883 and 1994. From west to east, the geographic boundaries of the six subzones are Sabine Pass, Ocean View Beach, Calcasieu Pass, Lower Mud Lake, western Mulberry Island, Chenier Au Tigre, and Southwest Pass. Average shoreline retreat between these zone boundaries are +4.9, -1.2, +3.5, -8.4, +3.4, and -2.3 m/yr, respectively. The Mississippi River deltaic plain is characterized by four barrier systems: 1) Isles Dernieres, 2) Bayou Lafourche, 3) Plaquemines, and 4) Chandeleur Islands. Shoreline change rates are much higher than adjacent geomorphic zones and average -11.1, -14.2, -5.5, and -8.5 m/yr, respectively, for the period 1855 to 1989. The four most common types of geomorphic response are landward rollover, lateral movement, breakup, and retreat The Lake Pontchartrain Basin shoreline is dominated by shoreline retreat with rates up to 4.5 m/yr and isolated zones of dynamic equilibrium. Further east, the Mississippi Sound barrier island system is composed of Cat, West Ship, East Ship, Horn, Petit Bois, and Dauphin Islands. Between 1847 and 1986, Dauphin, Petit Bois, and Horn Islands were dominated by minor cross-shore migration and rapid lateral movement to the west ranging between 30 and 90 m/yr. East Ship and Cat Islands retreated at average rates of 5.5 and 12.4 m/yr, respectively, whereas West Ship Island experienced advance averaging 0.7 m/yr for the period 1847 to 1992. General shoreline trends show west- ward sediment transport along the chenier plain and the Mississippi Sound barrier system with distinct zones of updrift erosion and downdrift accretion. In contrast, the deltaic barrier shoreline is rapidly migrating landward and deteriorating, and Lake Pontchartrain is slowly expanding in size.

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