U.S. Geological Survey simulations of hydrodynamics and morphodynamics during Hurricane Michael (2018)

The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST v3.8; Warner and others, 2010; Warner and others, 2019) modeling system was used to simulate ocean circulation, water levels, waves, and sediment transport that occurred during Hurricane Michael (2018). Simulations were performed with coupled and concurrent ocean and wave models simulated on a series of refined, cascading grids. They were first performed on an oceanic scale grid for the entire Gulf of America, and then further downscaled to focus on the resulting inundation, dune overtopping, and barrier island breaching during the storm along both Mexico Beach, FL, and at Cape San Blas, FL. The largest scale grid covered the entire US east coast and Gulf of America (5km resolution), and a subsequent grid (GOMSAB, 2km resolution) downscaled to cover Florida. Results on the GOMSAB grid were analyzed to investigate coastal ocean-scale dynamics and circulation, and then to further drive coastal scale grids to resolve the Florida Great Bend region (Mexi and Apala, each at 200m resolution). Finally, these results further drove grids to cover Mexico Beach (MexB, 10m alongshore and varied from 7 m to 3m in the cross-shore) and Cape San Blas (CSB, 5m resolution). Several simulations on the MexB and CSB grids were performed to investigate the impacts of infragravity waves on total water level at Mexico Beach and on effects of vegetation (included or excluded) on the breaching processes at Cape San Blas.
 
Surface atmospheric forcings were obtained from the Coupled Ocean/Atmosphere Mesoscale Prediction System for Tropical Cyclones (COAMPS‐TC; Doyle and others, 2012), developed by the U.S. Naval Research Laboratory. COAMPS‐TC is a non‐hydrostatic, convection‐permitting model. The simulation was conducted by the US Navy with a 4‐km resolution storm‐following nest that was periodically re-initialized using a modified Rankine wind vortex. The track and intensity were nudged to fit to the Best Track from the National Hurricane Center (NHC; Beven and others, 2018). Results included surface winds, pressure, relative humidity, and air temperature.
 
Simulations were performed on the GOMSAB, Mexi, and Apala grids for a time period from 1200hr October 7, 2018 to 0000hr on October 12, 2018 UTC. These results were used to investigate the coastal current, waves, and inundation water levels. The MexB grid was simulated for a 14 hour period to cover the initial runup and the maximum inundation of Mexico Beach from 0600 to 2000 on October 10, 2018. This simulation was used to investigate the impacts of infragravity waves and coastal erosion at Mexico Beach. The CSB grid was simulated from October 10 at 0600 to October 11 at 0000. Four simulations on the CSB grid: 1) no morphology and no vegetation, 2) no morphology and yes vegetation, 3) yes morphology and no vegetation, and 4) yes morphology and yes vegetation. The first 2 provided an understanding of how the vegetation altered the bottom stress, and the last 2 allow that variation of bottom stress to evolve the morphology. The sediment on the seafloor was initialized with a uniform 10m thick distribution. These simulations were limited to one layer with a sand grain size 0.125mm, erosion rate of 2.0e-4 kg/m2/s, critical erosion stress of 0.14 N/m2, and a settling velocity of 8.7 mm/s.
 

References cited:
 
Beven, J., Berg, R., and Hagen, A., 2018, National Hurricane Center Tropical Cyclone Report AL 142018, Hurricane Michael 2018, https://www.nhc.noaa.gov/data/tcr/AL142018_Michael.pdf.
 
Doyle, J. D., Jin, Y., Hodur, R. M., Chen, S., Jin, H., Moskaitis, J., Reinecke, A., Black, P., Cummings, J., Hendricks, E., Holt, T., Liou, C.-S, Peng, M., Reynolds, C., Sashegyi, K., Schmidt, J., and Wang, S., 2012, Real‐time tropical cyclone prediction using COAMPS‐TC: Advances in Geosciences, Vol. 28, pp. 15–28, https://doi.org/10.1142/9789814405683_0002.

Warner, J.C., Armstrong, B., He, R., and Zambon, J.B., 2010, Development of a coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system: Ocean Modelling, v. 35, issue 3, p. 230-244, https://doi.org/10.1016/j.ocemod.2010.07.010.
 
Warner, J.C., Ganju, N.K., Sherwood, C.R., Kalra, T.S., Aretxabaleta, A., He, R., Zambon, J., and Kumar, N., 2019, Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System: U.S. Geological Survey Software Release, 23 April 2019, https://doi.org/10.5066/P9NQUAOW.