Transition of the Coastal and Estuarine Storm Tide Model to an Operational Model for Forecasting Storm Surges

The coastal and estuarine storm tide (CEST) model is being tested to meet the National Hurricane Center (NHC)’s operational needs for storm surge forecasting. The CEST model has been developed to run on conformal grids without modification to the numerical algorithms, which are the grids used by the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model. SLOSH is the operational storm surge model used by NHC for planning, forecasting, and disaster response. In the past two years, 30 SLOSH basins have been converted into corresponding CEST grids and tested by performing simulations using hypothetical hurricane tracks provided by NHC. There are about 10,000-70,000 hypothetical hurricanes with varying intensities, forward speeds, and incoming directions for each grid. The maximum envelope of water (MEOW) and maximum of MEOWs (MOMs) were generated using CEST to compare with the SLOSH results in the same grids.

The CEST model was robust for most cases without further adjustment, and produced maximum MEOWs and MOMs heights comparable to those created by the SLOSH model. However, the maximum inundation extents from CEST are smaller than the extents from SLOSH. We found that the difference in the treatment of the overland bottom friction due to the variation of land cover was the major factor to cause the different MOMs from SLOSH and CEST. In CEST, the effects of land cover on bottom friction are considered by introducing varying Manning’s coefficients based on the national land cover dataset, while the land cover effect is not considered in SLOSH. A slip coefficient is used to adjust the surge height and inland extent in the SLOSH model. This difference has been examined by simulating Hurricane Rita (2005) in both CEST and SLOSH along the Louisiana coastline. The comparison of computed storm surges with field measurements indicated that SLOSH overestimated the magnitude and extent of overland flooding in the near flat coastal areas, while CEST produced the pattern matching with the observations. The slip coefficient in the SLOSH model was adjusted and successfully generated an inland inundation envelope similar to CEST. The storm surge unit at NHC has adjusted the computation of overland flooding on the new Huston basin based on this finding and is examining this adjustment for other locations.