Sea, Lake, and Overland Surges from Hurricanes (SLOSH)

Contents

• SLOSH Model
  • Introduction
  • Modeling Approaches
  • Reference Level
  • Surge Inundation
  • Basin Coverage
  • Basin Updates
  • Strengths and Limitations
• SLOSH Display Program

Introduction

The SLOSH model consists of a set of physics equations which are applied to a specific locale's shoreline, incorporating the unique bay and river configurations, water depths, bridges, roads, levees and other physical features.

Modeling Approaches

Deterministic Approach - Forecasts surge based on solving physics equations. This approach uses a single simulation based off of a "perfect" forecast which results in a strong dependence on accurate meteorological input. The location and timing of a hurricane's landfall is crucial in determining which areas will be inundated by the storm surge. Small changes in track, intensity, size, forward speed, and landfall location can have huge impacts on storm surge. At the time emergency managers must make an evacuation decision, the forecast track and intensity of a tropical cyclone are subject to large errors, thus a single simulation of the SLOSH model does not always provide an accurate depiction of the true storm surge vulnerability. Click here to view the National Hurricane Center's (NHC) official annual forecast track error for Atlantic tropical cyclones.

Probabilistic Approach - The Probabilistic Surge (P-Surge) product is the newest addition to a suite of available storm surge products which incorporates statistics of past forecast performances to generate an ensemble of SLOSH runs based on distributions of cross track, along track, intensity, and size errors.

Composite Approach - Predicts surge by running SLOSH several thousand times with hypothetical hurricanes under different storm conditions. The products generated from this approach are the Maximum Envelopes of Water (MEOWs) and the Maximum of MEOWs (MOMs) which are regarded by NHC as the best approach for determining storm surge vulnerability for an area since it takes into account forecast uncertainty. The MEOWs and MOMs play an integral role in emergency management as they form the basis for the development of the nation's evacuation zones.

Reference Level

All water level observations, including SLOSH storm surge heights, are referenced as height above a vertical datum. A vertical datum is an established surface that serves as a reference to measure or model heights and depths. Currently, the SLOSH model utilizes the National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88). All basins are being updated to NAVD88 because NGVD29 is out-dated and no longer being supported. It is important to note that when comparing water level observations (e.g. from tide stations) with other observations, observations with models, or models to other models, one MUST always use consistent vertical datums in order to avoid misinterpreting surge elevations. For more information about datums please refer to the National Geodetic Survey.

Surge Inundation

Storm surge inundation is the term used when referencing storm surge heights as height above ground level. NHC's storm surge forecast public advisory statements now reference storm surge as height above ground level. This means when NHC forecasts storm surge of 20 ft that means 20 ft above ground. Refer to the FAQ section for more information on storm surge inundation.In an effort to help alleviate the confusion inherent in past use of surge above a vertical datum, SLOSH storm surge heights can now be referenced as feet above ground level as a selectable feature in the SLOSH Display Program (SDP).

Basin Coverage

SLOSH has been applied to the entire U.S. Atlantic and Gulf of Mexico coastlines. In addition, coverage extends to Hawaii, Puerto Rico, Virgin Islands, and the Bahamas. The SLOSH model coverage is subdivided into 37 regions or basins. These basins are centered upon particularly susceptible features: inlets, large coastal centers of population, low-lying topography, and ports. An example of a typical computational domain, or basin, is the New Orleans basin.

SLOSH model coverage

Basin Updates

Currently, SLOSH basins are being updated at an average rate of 6 basins per year. Priority for SLOSH basin updates are governed by the Interagency Coordinating Committee on Hurricanes (ICCOH). ICCOH is a tri-agency supported by the U.S. Army Corps of Engineers (USACE), Federal Emergency Management Agency (FEMA), and the National Oceanic and Atmospheric Administration (NOAA). ICCOH manages hazard and post-storm analysis for the Hurricane Evacuation Studies under the National Hurricane Program (NHP). Updates are driven by a number of different factors such as: changes to a basin's topography/bathymetry due to a landfalling hurricane, degree of vulnerability to storm surge, availability of new data, changes to the coast, and addition of engineered flood protection devices (e.g., levees).

Sometimes these updates include higher grid size resolution to improve surge representation, increasing area covered by hypothetical tracks for improved accuracy, conversion to updated vertical datums (NGVD29 to NAVD88), and the latest topography or bathymetric data for better representation of barriers, gaps, passes, and other local features.

Strengths and Limitations

The SLOSH model is computationally efficient, resulting in fast computer runs which makes it an ideal operational system. It is able to resolve flow through barriers, gaps, and passes and models deep passes between bodies of water. It also resolves inland inundation and the overtopping of barrier systems, levees, and roads. It can resolve coastal reflections of surges such as coastally trapped Kelvin waves. Click here to see a Kelvin wave visible in the SLOSH Historical Run for Hurricane Dennis 2005.

However, the SLOSH model does not explicitly model the impacts of waves on top of the surge, it does not account for normal river flow or rain flooding, nor does it explicitly model the astronomical tide (although operational runs can be made with different initial water level anomalies. Future advancements in the SLOSH model will allow for the resolution of some of these limitations.

The SLOSH Display Program (SDP) is software developed as a tool to aid emergency managers in visualizing storm surge vulnerability. The SLOSH model and the SDP are two different tools. The SLOSH model is used by the NHC to forecast storm surge and model storm surge vulnerability; the SDP is the software provided to emergency managers and other users to visualize the data produced by the NHC.

Graphical output from the model displays color-coded storm surge heights for a particular area in either feet above ground level (inundation) or feet above a specific reference level NGVD29 or NAVD88. If using NAVD88 or NGVD29, users will need to know the elevation, relative to the same vertical datum, in order to properly interpret the surge elevations. A sample output of the SLOSH model can be seen here for Hurricane Ike.

The SDP is intended to be used by trained emergency managers, FEMA personnel, and NWS forecasters.

To access the SLOSH Display Web Page and download the program, you will need a username and password, which can be obtained at this website.