Coastal barriers, such as barrier islands, serve as natural buffers between the ocean and the mainland, protecting coastal communities from the destructive forces of storms and sea-level rise. However, these barriers are not immune to the impacts of severe storms, which can result in erosion, sediment transport, and potentially even breaching, leading to the formation of new channels connecting the ocean and inland water bodies. Understanding and predicting the morphologic impact of storms on coastal barriers is crucial for effective coastal management and planning.
How do storms impact coastal barriers?
Storms generate waves and water level setups that can induce powerful overwashing flows across barrier islands. These overwashing flows can have two widely different outcomes. On one hand, they can cause erosion, barrier breaching, and the formation of inlets. On the other hand, the sediments transported by these overwashing flows can also be deposited, forming what is known as washover fans. Predicting which outcome will occur during a storm event has been a challenging task for researchers.
In a recent research article titled “Barrier Breaching Versus Overwash Deposition: Predicting the Morphologic Impact of Storms on Coastal Barriers,” Nienhuis, Heijkers, and Ruessink propose a new analytical theory to estimate the likelihood of barrier breaching and overwash deposition based on storm characteristics, barrier morphology, and dune vegetation. By comparing the sediment volume transported by overwashing flows to the barrier subaerial volume, the researchers suggest that a breach occurs when the transported sediment volume exceeds the available barrier volume.
Can we predict barrier breaching and overwash deposition?
Yes, the newly proposed analytical theory provides a framework for predicting barrier breaching and overwash deposition during storm events. By considering storm characteristics, such as wave conditions and surge height, as well as barrier morphology and dune vegetation, researchers can estimate the volume of sediment likely to be transported by overwashing flows. By comparing this volume to the available barrier volume, they can assess the likelihood of breaching and the formation of washover fans.
The study also tested the analytical theory using two different approaches. First, the researchers conducted simulations using the hydrodynamic and morphodynamic model Delft3D. Second, they analyzed field observations of 21 washover fans and 6 breaches that formed during Hurricane Sandy. The results showed reasonable correspondence between the predictions of the analytical theory and the observed data, suggesting the potential utility of this approach for predicting barrier breaching and overwash deposition.
What factors influence barrier breaching?
The analytical theory developed by Nienhuis, Heijkers, and Ruessink identifies two important controls on barrier breaching: barrier width and storm surge height. Narrow dunes with high storm surge heights are particularly vulnerable to breaching. These findings emphasize the need for considering the specific characteristics of barrier islands when assessing their susceptibility to storm impacts.
How can barrier island models be improved?
The analytical formulations provided in the research article offer a valuable contribution to the improvement of long-term barrier island models. By incorporating the predictions of the analytical theory into existing models, researchers and coastal managers can enhance their understanding of barrier dynamics and better prepare for future storm events. This improved modeling capability can aid in decision-making processes regarding coastal management and resilience strategies.
One potential implication of the research is the importance of vegetation and island elevation in preventing breaching. Vegetation, such as dune grasses, plays a significant role in stabilizing the barrier and reducing the likelihood of erosion. Moreover, higher island elevations can act as a natural barrier against overwashing flows, reducing the risk of breaching. These findings highlight the importance of preserving and restoring coastal vegetation and considering elevation changes in coastal management plans.
What are the alternative controls on overwashing and breaching?
The study points out that the predictions of the analytical theory did not align with observations for developed barrier coasts, where traditional sediment transport equations did not readily apply. This discrepancy suggests the existence of alternative controls on overwashing and breaching in these settings. Further research is needed to identify and understand these alternative controls, which may include human alterations to the coastal environment, such as beach nourishment, artificial seawalls, or other structures that can modify sediment transport processes.
In conclusion, the research by Nienhuis, Heijkers, and Ruessink provides a valuable contribution to our understanding of the morphologic impact of storms on coastal barriers. By developing an analytical theory and validating it through simulations and field observations, the researchers enable better predictions of barrier breaching and overwash deposition. These predictions can inform coastal management strategies, facilitate the design of effective resilience measures, and contribute to the long-term sustainability of coastal regions.
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