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A Megascale Systems Approach for Shoreline Change Analysis and Coastal Management Along the Northern Gulf of Mexico
Randolph A. McBride, Mark R. Byrnes
Long-term changes in shoreline position were quantified along the north-central Gulf of Mexico for the period 1847 to 1994, using National Ocean Service (NOS) topographic sheets (T- sheets), near-vertical aerial photography, and differential Global Positioning System (GPS) field surveys. The study area extends eastward from Sabine Pass, Louisiana, to Dauphin Island, Alabama, and consists of outer Gulf shorelines that are separated into three geomorphic zones: 1) the chenier plain of southwestern Louisiana, 2) the barrier systems of the Mississippi River deltaic plain (southeastern Louisiana), and 3) 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 illustrates alternating subzones 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, Cheniere au Tigre, and Southwest Pass. Average shoreline change rates between these subzone boundaries are +4.3, -1.2, +2.8, -8.7, +2.8, and -2.9 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 for deltaic barriers are landward rollover, breakup, retreat, and lateral movement. 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 rapid lateral movement to the west ranging between 30 and 90 m/yr and minor cross-shore migration. East Ship and Cat Islands underwent retreat averaging 5.5 and 12.4 m/yr, respectively, whereas West Ship Island experienced advance averaging 0.7 m/yr for the period 1847 to 1986. In general, shoreline trends show westward 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 migrating landward rapidly and deteriorating. The megascale shoreline response classification implemented for this study is critical for developing realistic research and management strategies regarding process/response relationships in coastal depositional systems around the world.
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