The Medium-Term and Event-Scale Tropical Cyclone-Driven Morphodynamics of a Vulnerable Barrier System with Emphasis on the Role of Backbarrier Wetlands

Abstract

Coastal mainland barriers and barrier islands provide the first line of defense against oceanic and meteorological forces. Coastal morphological change, which may degrade these barrier's defensive capabilities, occurs over a range of time scales, from geological epochs (>1000 years) to hours and seconds. Coastal morphological change at a vulnerable, but economically strategic, barrier system---the Caminada-Moreau Headlands, Louisiana---is investigated in terms of tropical cyclone impacts and their effects over a 15 year LIDAR survey time series. Analysis of the barrier's three-dimensional morphodynamics at medium-term (decadal) time scales reveals that, while subaerial volume was approximately conserved through time, the impact of Hurricane Gustav (2008) may have exceeded a threshold for recovery due to changes in dune morphology and net elevation loss. Additionally, medium-term coastal morphodynamics were correlated with backbarrier wetland health, attesting to the tight coupling that washover processes, which transport sediment from the beach and dunes to the backbarrier, have with these environments in low-lying barriers. A numerical modeling system is developed to simulate the event scale (days) impact of Hurricane Gustav (2008) to the Caminada Headlands. Washover of beach and dune sediment is enhanced where the backbarrier environment exhibits reduced surface roughness (vegetated land cover) and elevation (increased accommodation space). The correlation between washover sediment transport and backbarrier wetland disintegration, suggests that coastlines which are backed by deteriorated wetlands will export more sediment and, over medium-term time scales, this will appear as enhanced local shoreline erosion. Therefore, the detailed sediment transport modeling results provide a more quantitative picture explaining the relationship identified between decadal scale shoreline morphodynamics and the backbarrier wetlands. The numerical modeling system is modified to incorporate a recent restoration of the Caminada Headlands to investigate its response to various storm conditions. Three restoration scenario models are forced with Hurricane Gustav's waves and water levels and a synthetic 100 year return period storm. The presence of a restored backbarrier marsh reduced dune lowering and subaerial volume losses compared to the scenario without a marsh creation component or with open water. Therefore, backbarrier marsh creation, which provides vegetated land cover and elevation to help buffer washover transport, may enhance the resiliency of coastal restorations within low-lying barrier systems.