Changes in Shoreline Change Trends in Response to a Detached Breakwater Field at Grand Isle, Louisiana

Abstract

This study examines changes in shoreline change trends at Grand Isle, Louisiana, in relation to the implementation of a detached breakwater field completed in 1999. Eight sets of aerial photographs and digital orthophoto quarter quadrangle mosaics ranging from 1973 to 2004 were analyzed to establish pre- and postconstruction shoreline trends. Preconstruction, the island experienced persistent growth at its downdrift end. Postconstruction, this accretion ceased, and the downdrift end of the island began to experience erosion. The area landward of the breakwater field was accretionary before construction and became stable, whereas accretion increased in the area immediately updrift of the breakwater field. Two key design parameters, breakwater length and distance from the shoreline, were used to assess the potential for the formation of salients. Results from an engineering model suggest that the breakwaters were constructed too far offshore for this typical response to occur. Nonetheless, results of the air photo analysis indicate a marked change in shoreline migration trends following construction of the breakwater field.