Decadal Hindcast of Hydrodynamics and Barrier Island Morphology

In the northern Gulf of Mexico, storm surge poses a major risk due to the low-lying topography and the high frequency and intensity of tropical cyclones. As climate change increases sea surface temperatures and sea levels, storm surge magnitude is expected to increase nonlinearly as a result of tropical cyclone intensification, sea level rise, and changes in land use/land cover. When barrier islands experience storm surge, they are subject to erosion, flattening, and overtopping, all of which reduce their flood protection benefits to the mainland. This work is part of a larger USGS-funded project that aims to provide land management recommendations by simulating barrier island morphology, accounting for tropical cyclone-driven storm surge and fair-weather wave and tide action. Hindcasts were conducted for 11 tropical storms that made landfall within 200km of Dauphin and Petit Bois Islands (Alabama and Mississippi) between 2005-2020 using ADCIRC+SWAN. Following each storm event, storm-induced barrier island morphological changes and the islands’ natural recovery are simulated using XBEACH and EDGR models. The XBeach/EDGR output were then interpolated onto the mesh used in the subsequent ADCIRC+SWAN simulation. In addition to running each simulation on the XBeach-updated DEM, all storms were also simulated using a Post-Ivan (2004) DEM, and the most up-to-date LiDAR data available at the time of the storm. To determine offshore model accuracy, the simulated water level and wave variables for each DEM were compared with observed data at NOAA tide gauges and NDBC buoys. The simulated hydrodynamic variables were also compared at 44 nearshore points for the three DEMs to determine the effect of slight variations in DEMs on nearshore simulated outputs. The three DEMs were also compared to one another along cross-shore transects, with a particular focus on the differences between XBeach-derived and observed LiDAR DEMs at corresponding time steps. By studying the reciprocal relationship between nearshore hydrodynamics and barrier island morphology, the error and uncertainty of tropical-storm driven morphological change can be estimated. The methods evaluated in this project have broad implications in land management, hydrodynamic modelling, climate forecasting, and risk assessment. Simulating morphological change following extreme events can help land managers determine best practices and prioritize vulnerable locations, improving efficiency and cost-benefit ratios. Determining the relationship between hydrodynamic and morphological error can help practitioners include explicit uncertainty measures in studies that do not directly examine morphological change. Similar error and uncertainty quantification methods can be applied to forecasted morphological changes and can account for sea level rise, TC intensification, and other climate change-associated uncertainties. A combination of these methods can be used to determine risk to infrastructure, ecosystems, and businesses under various storm intensities in both current and future climate and morphologic conditions.