Wave Attenuation of Natural and Nature-Based Features: Comparing Explicit and Implicit Formulations



Miesse, Tyler

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Coastal communities are highly exposed to waves and storm surge during extreme events. One option for protecting coastal communities that has gained popularity during the last decade is the use of natural and nature-based features (NNBF). Data documenting the flood protection capacity of these systems, especially related to the attenuation of wave energy during extreme events, is a key limiting factor to the understanding of NNBF functions as each site has unique characteristics based on vegetation, geology, and historical context. To address the need for more information on the benefits of NNBFs for coastal protection, recent advances in numerical models have incorporated the interaction of the vegetation with hydrodynamics and waves. The current state of the art models represents the vegetation in one of two ways: (1) “explicit” interactions between waves and vegetation through “site-specific” data on the stem height, stem diameter, and the plant density, and (2) “implicit” representation through equivalent bottom roughness length derived from landcover datasets at the National and Regional scale. While the explicit formulation provides higher fidelity in terms of physical processes representation, it requires local information that is typically unavailable for most locations. To evaluate the possible options of wave attenuation this study focuses on two different marshes in the Chesapeake Bay to investigate: 1) the fidelity of the implicit and explicit representation of wave attenuation by vegetation, and 2) the accuracy of numerical models at varying geographical domains and resolutions in calculating the wave attenuation by vegetation. This study compared two different numerical models that can be applied at different geographical scales: a local scale model which is a site-specific domain, and a regional model that has a domain of the United States east coast. Ultimately, this study found that the explicit approach and finer resolution models improved the accuracy of the wave attenuation calculation. When looking at a regional perspective, the implicit approach would provide a 30% underestimation of the wave attenuation at the edge of the marsh, but it will show full attenuation of the wave energy within marsh, when marsh lengths exceed 400m, which would still provide guidance on possible protection strategies.



Wave Attenuation, XBeach, ADCIRC, Salt marshes, Coastal Hazards, Natural and Nature-Based Features