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Hurricanes: Science and Society
Numerical Models of Storm Surge, Wave, and Coastal Flooding

Numerical models of storm surge, wave, and coastal flooding are needed to 1) provide hindcasting of past hurricanes; 2) provide realtime forecasting of a current hurricane; and 3) assess future flood risks. As such, numerical models play very important roles in the scientific understanding of surge, wave, and flooding dynamics, as well as the preparation, planning, response, and mitigation of hurricane and coastal flood hazards.

Aerial photo of the Mississippi coastline after Hurricane Katrina (2005).  The area was heavily damaged by storm surge.
Aerial photo of a portion of the Mississippi coastline after Hurricane Katrina (2005). The area was heavily damaged by Katrina's storm surge. In the image on the left, one can clearly see a "debris line", which marks the furthest extent of the surge (and everything that was carried with it). The photo on the right shows a zoomed-in image of the area in the red box on the left. Here, one can see the empty slabs from which houses were pushed and the sea of debris which surrounds those houses left standing. Image credit: NOAA

Since Hurricane Katrina, there have been significant advances in numerical modeling of storm surge, wave, and coastal flooding. Presently, there are numerous storm surge models with different model physics and numerics. Although one-dimensional storm surge models are still being used, most storm surge models are two-dimensional or three-dimensional to better represent the physics and the complex bathymetry and topography of coastal domain. While some models still do not include tide and wave action, newer storm surge models do include these variables. Models also differ widely in their computational speed. While some models use a cluster of hundreds of computers to handle high-resolution computations over a large domain, other models are used to provide quick, lower resolution forecasts for a coastal region. Uncertainties of storm surge and coastal flooding calculations come from a variety of sources, including model physics, model domain and grid, model dimensionality, parameterization of model processes (e.g., surge-wave interaction, interaction with topographic features), and input data (model forcing such as wind, precipitation, tide, and open boundary condition; as well as bathymetry and topography data).

One model used to evaluate the threat from storm surge is the SLOSH (Sea, Lake and Overland Surges from Hurricanes) model. It is a two-dimensional computer model run by the National Hurricane Center (NHC) to estimate storm surge heights by taking into account several variables such as the size of a hurricane, its forward speed, and track. The calculations are applied to a specific locale's shoreline, incorporating the area’s unique features (bay and river configurations, bridges, roads, etc.). Emergency managers use the SLOSH-produced inundation map, Maximum of Maximums (MOMs, for each hurricane category), to determine which areas must be evacuated due to storm surge. MOMs provide a worst-case storm surge estimate at a regional level and are therefore important to hurricane evacuation planning. The SLOSH model is generally accurate within plus or minus 20 percent given a perfect track, intensity, and size of the hurricane. For example, if the model calculates a peak 3 m (~10 ft) storm surge for the event, one can expect the observed peak to range from about 2-4 m (~8 -12 ft). Individual SLOSH runs provided in forecast mode are not very useful for evacuation decisions because the track errors can cause huge errors in the forecast storm surge amounts. Because of this, the MOMs are relied upon more today for evacuation decisions. Beginning in 2009, an ensemble of SLOSH runs has been made for U.S. landfalling hurricanes based upon a variety of possible tracks, intensity and size. These allow for a probabilistic approach to storm surge forecasting (e.g., 30% for at least 8 ft of surge) that may become a useful tool for evacuation decision-making by emergency managers.

Many other storm surge models have been developed and are being used for a variety of purposes listed above. After Katrina, the Interagency Performance Evaluation Taskforce (IPET) used the Advanced Circulation (ADCIRC) model, primarily a two-dimensional model, to simulate the storm surge during Katrina, and to produce coastal flood maps with a 1% and 0.2% annual chance of occurrence in a given year for the New Orleans region. ADCIRC is also being used by FEMA to produce Flood Insurance Rate Maps (FIRMs) in several coastal regions.

A two-dimensional and three-dimensional storm surge modeling system (SSMS), Curvilinear-grid Hydrodynamics in 3D (CH3D)-SSMS, has been developed and used for simulating storm surge, wave, and coastal flooding during several hurricanes, including Isabel (2003), Ivan (2004), Wilma (2005), Katrina (2005), and Fay (2008). The modeling system has been used to investigate the effect of various factors (storm intensity, size, forward speed, and angle of approach, as well as wave and tide) on storm surge and coastal flooding. The studies on Hurricane Isabel and Hurricane Ivan showed that the model simulations of surge, wave, and currents are quite accurate, and the effect of wave on surge can be quite significant, with wave-induced surge accounting for up to 25% of the peak surge during both storms. This model is also being used for forecasting and to produce MOMs and FIRMs in various coastal domains, including Pinellas County, Florida, and Southwest Florida.

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