Linking Backbarrier Wetlands and Storm Driven Coastal Morphodynamics

Abstract:

Coastal barrier islands and mainland barriers serve as the first line of defense against oceanic waves and storm surge generated flooding. These systems are composed of different physical and ecological components, e.g. the shoreface, backshore, dunes, and are often backed by wetlands and estuaries. Within low-lying coastal zones, that are subject to frequent energetic storm conditions, overwash and coastal inundation processes may establish a morphodynamic connection between the shoreface and backbarrier environment whereby washover sediment is transported landward. The Caminada Headlands, Louisiana, USA, has historically exhibited rapid coastal morphodynamics (shoreline erosion rates ~ 20 m/yr) due to frequent tropical storm impacts and sediment deprivation. Using the Caminada Headlands as a study site, this connection is investigated using a LIDAR survey time series and an event-scale sediment transport model.

Decadal shoreline migration rates are correlated with a proxy for backbarrier wetland health. Shoreline backed by open water/deteriorated marshes exhibit greater erosion rates than those which exhibit relatively more intact backbarrier marshes surfaces. Since shoreline change is dominated by major tropical cyclone impacts, the mechanism responsible for this pattern is conjectured to be the modulation of washover sediment transport by backbarrier biophysical properties. To investigate this mechanism further, a physics-based numerical modeling system is verified for a hindcast of Hurricane Gustav’s (2008) impact to the Caminada Headlands. Analysis of computed net sediment transport patterns reveals that backbarrier topography and land cover are significantly correlated with washover volume and landward ingress. A clear gradient in washover sediment transport is discernible between area which exhibit intact, intermediately deteriorated, and absent backbarrier marshes.