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This review focuses on recent advances in process-based numerical models of the impact of extreme storms on sandy coasts. Driven by larger-scale models of meteorology and hydrodynamics, these models simulate morphodynamics across the Sallenger storm-impact scale, including swash,collision, overwash, and inundation. Models are becoming both wider (as more processes are added) and deeper (as detailed physics replaces earlier parameterizations). Algorithms for wave-induced flows and sediment transport under shoaling waves are among the recent developments. Community and open-source models have become the norm. Observations of initial conditions (topography, land cover, and sediment characteristics) have become more detailed, and improvements in tropical cyclone and wave models provide forcing (winds, waves, surge, and upland flow) that is better resolved and more accurate, yielding commensurate improvements in model skill. We foresee that future storm-impact models will increasingly resolve individual waves, apply data assimilation, and be used in ensemble modeling modes to predict uncertainties.
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Supplemental Video 1: Computer simulation of waves, sand transport, and morphology changes during Hurricane Matthew at Matanzas, FL. Cross-shore velocity (red-green colors), pre-storm topography and bathymetry (gray lines), and current topography and bathymetry (black lines) at three cross-barrier transects: northern, breach, and southern. The transects are indicated in white on the map of the pre-storm topography and bathymetry in the top right panel. Time series of the water level gradient across the barrier and of the offshore significant wave height, with the current time marked in red, are on the bottom right. Note that the negative water level gradient indicates that ocean water levels were lower than back-barrier water levels. The water level gradient and the significant wave height (gray line) were filtered to remove infragravity and sea-swell wave group signals. At the beginning of the storm, typical cross-shore circulation patterns emerged, with onshore currents at the surface and offshore currents (undertow) at the bed. As offshore waves increased, wave action on the beach face and at the dune toe lead to erosion and transport of sediment offshore. At the peak of offshore waves, overwash of the now vulnerable dune crest occurred, with transport of sediment from the dune to the back-barrier. Dunes were lowered further. Transitions from collision to overwash storm regimes (Sallenger, 2000) occurred at different times due to spatial variability in dune crest height and water levels. Water levels increased in the back-barrier due to phase lags between the storm surge in the coastal ocean and in the back-barrier waterway, and due to overwash. Ultimately, the water level gradient across the barrier drove a flow that breached the barrier. Sediment was transported into the coastal ocean, where it formed a small deposit. Supplemental Video 2: Computer simulations of morphology change on the Bolivar Peninsula, TX during Hurricane Ike. The three animations show the impact of changing bottom roughness in a Delft3D-4 simulation. The upper left model animation uses a low, spatially uniform roughness (Manning's n = 0.02) and shows large morphodynamic changes, including a large breach of the Peninsula. The upper right model animation uses a higher, spatially uniform roughness (Manning's n = 0.03) in which there is less erosion and accretion than the first simulation, but still causes two breaches of the Peninsula. The lower left model animation uses a spatially variable bottom roughness determined from the National Land Cover Database. This model generates greater erosion on the sandy landward (upper) side of the peninsula, but much smaller changes on the ocean (lower) side, where vegetation increased the bottom roughness. A breach did not occur during this variable-roughness simulation. Supplemental Video 3: Computer simulation of the waves, water levels, currents and subsequent breaching of an uninhabited section of the Fire Island barrier island off the coast of Bellport, NY, during hurricane Sandy.
During the hurricane event, the water level (shown in blue) increases on the Atlantic Ocean side of the barrier (earth tones) due to the wind blowing over the water surface. At the same time, infragravity or long period waves (shown as ripples on the water surface) increase in intensity. Together, the wind and waves drive an alongshore current (white arrows) to the right (West). The waves erode the dunes on the barrier islands, lowering the dune tops. During the peak of the storm, the dunes have eroded so much, and the water level and waves have increased so much that the island is overtopped, and water can be seen flowing over the island (white arrows). These currents further scour the initial gaps into larger breaches. The eroded sediment is deposited on the landward side (front side of the animation) as so-called overwash fans. As the storm passes the water levels recede and the breaches (one of which is still open today) in the dunes and the flattening of the island is visible. The simulation was made using the open-source model XBeach (xbeach.org). The results presented here have been published by Van Der Lugt et al., 2019.