BREAKWATERS IN COASTAL LOUISIANA: PANACEA OR PLACEBO?

Interpretation of a multi-year time-series of hydrodynamic measurements and beach-profile survey data has revealed a unique morphological response of the beach-nearshore to detached, segmented breakwaters along Raccoon Island, Louisiana (northern Gulf of Mexico, U.S.A.). Rapid and persistent sedimentation occurred in the immediate lee of the structures shortly after construction, resulting in salient formation and shoreward growth over an initial 12-month monitoring period. These findings are highly unique in that the beach-nearshore response documented here has seldom been reported in the published literature. We refer to these features as “Reverse Salients” to distinguish them from the more conventional deposits that accrete offshore towards the structure, and which have been well documented in the literature. Ongoing monitoring of these features indicates the importance of an offshore source, supplied through cross-shore transport of material as opposed to the more conventional sediment trapping of longshore transport from an updrift, alongshore site. In addition, the data appear useful in refining attempts to predict the shoreline response to construction of segmented breakwaters given certain design criteria. The data presented here will be compared with shoreline response at other Louisiana sites where an entirely different response has been documented. The importance of better understanding coastal process-breakwater response is critical in this rapidly eroding environment given the impetus for coastal protection using hard structure approaches.