A Preliminary Assessment of Hydrodynamic Responses to Detached, Segmented Breakwaters, South-Central Louisiana

As one of the efforts to mitigate the effects of coastal erosion and preserve Louisiana\u27s barrier islands and coastal wetlands, detached, segmented breakwaters were constructed along the east flank of Raccoon Island, south-central Louisiana. A monitoring program was established to quantify the influence of the breakwaters on nearshore waves and morphology. Beach profiles, extending from the edge of the vegetation line to the crown of the breakwaters, were surveyed across the centers and the tips of the breakwaters, and across the gaps between the structures. Nearshore wave conditions were measured using pressure transducers deployed seaward, in the lee, between the gaps, and at a control site away from the influence of the breakwaters. Preliminary results based on the first six months of monitoring indicate that initial shoreline adjustment, in the form of beach cusps, reached quasi-equilibrium approximately three months after construction. Significant sand accumulation directly landward of the breakwaters was measured during the six-month period. The influence of the breakwaters on the wave field was significantly different under normally and obliquely incident waves. A wave-height reduction of approximately 90% was measured during normal waves, when compared to an unprotected site, while a 70% reduction was measured during oblique waves. Wave propagation through the gaps between the breakwaters was not influenced by the breakwaters during normally incident waves, while a reduction in wave height of approximately 50% was measured during oblique waves. The data presented are unique in that they describe among other phenomena, the rapid development of tombolos which originated in the immediately lee of the breakwaters and propagated onshore due to an apparent influx of sediment from offshore