Using Nature to Increase Coastal Resilience: Implicit and Explicit Wave Attenuation Modeling at varying Spatial Scales

Coastal communities, especially along the mid-Atlantic and the Gulf of Mexico, are at a high risk of damages due to impacts from waves and storm surge during extreme events. In the last decade, natural and nature-based features (NNBF) have gained popularity as a mean to mitigate such coastal hazards. Yet our understanding of the specific risk reduction functions of NNBF is still limited, making it challenging to compare their benefits to more traditional coastal resilience approaches. Some of the difficulties in understanding NNBF flood defense functions derived from the fact that each site has unique vegetation and elevation characteristics, thus hampering the extrapolation of local NNBF studies from one site to a larger region. Furthermore, by extrapolating to a regional scale, communities could easily make decisions about whether to implement NNBF over traditional coastal resilience approaches. Studies that have evaluated NNBF potential to attenuate waves by implementing numerical models rely on two different approaches to accurately represent the wave dissipation by NNBF: (1) implicitly, with bottom friction from land cover datasets, such as the National Landcover Dataset (NLCD) and the National Wetlands Inventory (NWI); or (2) explicitly, incorporating vegetation characteristics. In order to provide guidance on extrapolating local NNBF studies to a regional scale, we investigated different wave dissipation formulations comparing local (i.e. XBeach) and regional (i.e. ADCIRC+SWAN) scale numerical models. We then compared the models' calculated wave heights at field sites where we measured wave heights from Hurricane events throughout a marsh cross-section. Both the local and regional scale models were able to recreate the observed dissipation rates; however, the implicit approach was unable to with a 40% difference to the observations. Comparing the results of the explicit approach for the local to the regional scale models, the accuracy increased at finer resolutions, such that the local scale model outperformed the regional. However, even at the regional scale, the explicit model provides useful information on the potential of NNBF to attenuate waves, which can aid decision makers regarding whether to implement more NNBF alternatives over traditional coastal resilience approaches.