Introduction of Wave Set-up Effects and Mass Flux to the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) Model
Donald Slinn, Shaun Kline
Hurricanes wreak havoc on the lives and infrastructure of coastal communities. Storm surge, a local rise in sea level elevations, is perhaps the most devastating element of these tropical cyclones. Storm surge depends on the tidal stage, barometric pressure, Coriolis effects, wind stress, and wave forcing, as well as the local bathymetry. In the past, many storm surge numerical models, such as Sea, Lake, and Overland Surges from Hurricanes (SLOSH) (Jelesnianski et al, 1992), neglect wave forcing components to conserve computational efficiency. This omission would surely be preferred when wave forcing is not significant. However, numerous situations could necessitate the inclusion of waves' effects to more correctly model the surge both spatially and temporally. In its effort to characterize the combined effects of hurricane hazards (hurricane wind, storm surge, and waves) for use in developing structural design criteria for coastal structures, the Building and Fire Research Laboratory (BFRL) of the National Institute of Standards and Technology (NIST) in collaboration with the Meteorological Development Laboratory (MDL) and the Oceanic and Atmospheric Research (OAR) of the National Oceanic and Atmospheric Administration (NOAA) has developed a methodology that incorporates hurricane science, hydrology, probabilistic methods, and structural engineering needs for use in developing site specific, risk-based design criteria for coastal structures subjected to the above hurricane hazards (Phan et al, 2007). This early effort utilizes program SLOSH for hydrodynamic simulations without consideration of wave effects. Recognizing that wave set-up and mass flux might have a significant influence on total storm surge levels, the BFRL/NIST then collaborated with the National Hurricane Center (NHC) of NOAA to provide funding (NIST Grant 60NANB8D8103) and technical guidance to the University of Florida/Department of Civil and Coastal Engineering) for the incorporation of a wave model into the SLOSH model to extend SLOSH capability. The result of this effort is described in this report. We chose two wave forcing components, set-up from wave stresses and mass flux transport, to incorporate into the SLOSH storm surge model through a two-way coupling methodology. Our aim is to better understand the relative contribution of each effect and their relationship to both storm strength and bathymetry. To this end, we conduct numerous tests of different forcing variations: wind-stress only, wind and wave stresses, and wind and wave stresses with mass flux transport. These options were simulated on three hurricanes and two SLOSH basins. The storms range in intensity between a Category 1 (34 m/s) and Hurricane Andrew, a Category 5 storm (74 m/s). Our two basins were chosen for bathymetric contrast: Tampa Bay, a shallow and gentle shelf, and Miami, which has a steeper and deeper shelf. Wave stresses and mass transports were obtained using the Simulating Waves Nearshore (SWAN) third-generation wave model with time dependent water level and wind inputs from the SLOSH wind-stress-only test. We determine that the impact of wave set-up and mass flux to storm surge levels varies between locations even for the same storm in the same basin proving that the interaction between the wind and wave forcing components is indeed very complex. On average, however, the addition of the wave set-up and mass flux raises the maximum storm surge levels 10 to 30 percent, although isolated positions experience increases well above 100 percent. We also look at the effects of the wave forcing on specific points and overland inundation. We find that the largest increase to overland inundation occurs for the shallow basin, a result that most likely indicates a lack of proper grid resolution in the Miami basin. Coarse resolution in the breaking zone results in wave stress from SWAN to be minimized during the interpolation process. Further, w
and Kline, S.
Introduction of Wave Set-up Effects and Mass Flux to the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) Model, Grant/Contract Reports (NISTGCR), National Institute of Standards and Technology, Gaithersburg, MD
(Accessed September 30, 2023)