Interactions between barrier islands and backbarrier marshes affect island system response to sea level rise: Insights from a coupled model

Abstract

Interactions between backbarrier marshes and barrier islands will likely play an important role in determining how low-lying coastal systems respond to sea level rise and changes in storminess in the future. To assess the role of couplings between marshes and barrier islands under changing conditions, we develop and apply a coupled barrier island-marsh model (GEOMBEST+) to assess the impact of overwash deposition on backbarrier marsh morphology and of marsh morphology on rates of island migration. Our model results suggest that backbarrier marsh width is in a constant state of change until either the backbarrier basin becomes completely filled or backbarrier marsh deposits have completely eroded away. Results also suggest that overwash deposition is an important source of sediment, which allows existing narrow marshes to be maintained in a long-lasting alternate state (~500 m wide in the Virginia Barrier Islands) within a range of conditions under which they would otherwise disappear. The existence of a narrow marsh state is supported by observations of backbarrier marshes along the eastern shore of Virginia. Additional results suggest that marshes reduce accommodation in the backbarrier bay, which, in turn, decreases island migration rate. As climate change results in sea level rise, and the increased potential for intense hurricanes resulting in overwash, it is likely that these couplings will become increasingly important in determining future system behavior.

Second-Pass Assessment of Potential Exposure to Shoreline Change in New South Wales, Australia, Using a Sediment Compartments Framework

“…Our model does not support continuous (rollover) or discontinuous (drowning, overstepping) dynamic barrier behaviours [64,65], and is therefore only applicable to simulating shoreline recession on relatively steep, moderate to high energy wave-dominated coasts, where the existing beach and dune morphology is well-developed. The model is not suitable for simulating shoreline change on low-lying barrier island coasts, where dynamic barrier behaviour controls shoreline change [64][65][66].…”

Impacts of Seagrass Dynamics on the Coupled Long‐Term Evolution of Barrier‐Marsh‐Bay Systems

“…The wave power from equation (4) is used to calculate the volume of marsh edge erosion ( E m ), also following Marani et al () and Mariotti and Fagherazzi (): where k e is an erodibility coefficient set equal to 0.14 m 3 year −1 W −1 (Lauzon et al, ) and h is the height of the marsh platform. Based on volumetric organic content estimates from VCR marshes by Walters et al (), the marsh unit above sea level in the model is composed of 50% organic matter and 50% mineral sediment. To represent decomposition and dispersal, all organic matter eroded from the marsh unit is lost from the system.…”

“…Recent studies have highlighted the importance of interactions between adjacent coastal subsystems in determining overall system behavior and evolution (McGlathery et al, ; Walters et al, ). For example, in modeling experiments the presence of a back‐barrier marsh reduces the rate of island migration by reducing accommodation space in the back‐barrier bay (Brenner et al, ; Lorenzo‐Trueba & Mariotti, ; Walters et al, ).…”

“…Recent studies have highlighted the importance of interactions between adjacent coastal subsystems in determining overall system behavior and evolution (McGlathery et al, ; Walters et al, ). For example, in modeling experiments the presence of a back‐barrier marsh reduces the rate of island migration by reducing accommodation space in the back‐barrier bay (Brenner et al, ; Lorenzo‐Trueba & Mariotti, ; Walters et al, ). Using GEOMBEST+, an extension of the GEOMBEST model coupled with components from the marsh‐tidal flat model of Mariotti and Fagherazzi (), Walters et al () find that overwash from barrier islands can also be an important source of sediment for marshes, allowing for the maintenance of narrow fringing marshes in a long‐lasting, metastable state under conditions in which they otherwise would not occur.…”

“…The potential bistability of seagrass systems coupled with their significant hydrodynamic impacts on sediment dynamics and waves suggest that seagrass can play an important role in the evolution of the entire barrier‐marsh‐bay system. While previous work has investigated the evolution of shallow coastal bay, back‐barrier marsh, and barrier‐island subsystems in isolation (e.g., Carr et al, , , ; Mariotti & Fagherazzi, ; Moore et al, ) or considered the effects of connections to a single adjacent subsystem (e.g., Brenner et al, ; Carr et al, ; Lauzon et al, ; Mariotti & Carr, ; Mariotti & Fagherazzi, ; Walters et al, ), no study has previously examined the coupled dynamics of these subsystems all together. Here we develop an integrated barrier‐marsh‐bay system model—herein named GEOMBEST++Seagrass—by incorporating seagrass dynamics into GEOMBEST++ from Lauzon et al ().…”