Morphodynamic modelling of the wilderness breach, Fire Island, New York. Part I: Model set-up and validation

On October 29, 2012, storm surge and large waves produced by Hurricane Sandy resulted in the formation of a breach in eastern Fire Island, NY. The goals of this study are to gain a better understanding of the physical processes that govern breach behavior and to assess whether process-based models can be used to forecast the evolution of future breaches. The wilderness breach grew rapidly in size during the first winter following formation. Growth of the breach was accompanied by the formation of a complex of flood shoals inside Great South Bay, a primary channel that flowed through the eastern part of the flood shoals, and an ebb shoal on the ocean side of the breach. From the summer of 2013 through late 2015, the breach continued to change and evolve, albeit at a much slower pace than in the first year after formation. A hybrid combination of Delft3D and XBeach models is used to hindcast the morphodynamic evolution of the wilderness breach over the first three years after formation. The formation of the breach during Hurricane Sandy is not part of the simulations. Model simulations are initiated with a post-storm topography in which the breach is already present. The models are capable of hindcasting the main morphodynamic changes of the wilderness breach. The spatial patterns, as well as the bulk statistics, such as breach geometry and sediment volume changes, are reasonably well reproduced by the model. The model sheds light on previously unknown processes of breach evolution, especially regarding sediment transport and flow regimes within the breach complex.