During hurricane season (1 June through 30 November) Royal Haskoning also supports the
US Army Corps of Engineers (USACE) with the assessment of expected maximum water levels and wave heights in Louisiana. In 2008 Royal Haskoning developed and introduced the use of the Hurricane Surge Atlas. In addition Royal Haskoning staff is now part of the crisis team that acts if a hurricane threatens.
In the United States the
National Hurricane Center (NHC) monitors all storm developments on the Atlantic Ocean during Hurricane season. After a tropical storm develops, the NHC issues forecasts of air pressure, storm size (diameter), forward speed, wind speeds and the expected storm track every 6 hours. 24 to 36 hours prior to landfall, the
National Weather Service (NWS) uses these parameters in their SLOSH (Sea, Lake and Overland Surges from Hurricanes) computer model to forecast maximum water levels. Besides SLOSH the USACE also uses the results of a modified the
ADCIRC (ADvanced CIRCulation) model to forecast surge levels.. To speed up processing time a lighter version (ADCIRC Lite) of the model is used which has a decreased spatial resolution. The numeric model computes water levels based on predicted wind speeds, air pressure, bathymetry and surface roughness of an area. This information is used to make operational decisions on USACE structures.
Hurricane Surge Atlas
The Hurricane Surge atlas was developed by using more than 300 different “Hypothetical” Hurricanes that make landfall in the State of Louisiana. The presented water levels in this atlas are the results from existing model runs done with the high resolution ADCIRC SL15 computation model. The water levels also include the wave-setup. The wave characteristics (wave height and wave period) are calculated with a different model (STWAVE). The big advantage of the Hurricane Surge Atlas is that it allows for a very quick first estimate of maximum surge levels, wave heights if the forecasted track is matched to one of the 300 storms in The Hurricane Surge Atlas.
Quick and Simple
Two ArcGIS tools help the staff in the emergency centre to quickly and efficiently produce maps with maximum water levels and wave heights. Based on the latest Advisory of the NHC a hurricane is selected from the atlas that most resembles the forecasted hurricane. The location of Landfall, minimum air pressure, size (diameter) and intensity (wind speeds) are the most important selection criteria. A second tool was developed to generate a hydrograph on-the-fly. This is important to help decide at what point in time to close a gate or open a gate. During the storm, the tool also ”makes it possible to compare the atlas hydrographs to real-time observed hydrographs.
Evaluation
During hurricanes Gustav and Ike in 2008 a first version of The Hurricane Surge Atlas was used. The evaluation after the hurricane events showed that The Hurricane Surge Atlas was very valuable. In some cases the atlas provided even better estimates of the maximum water levels than the live model run results from SLOSH and ADCIRC Lite. An explanation for this could be the fact that that the computer models that run during a hurricane event make use of a relative coarse calculation grid to reduce computation time. The information from the hurricane surge atlas is based on results produced by model runs done on a very detailed calculation grid. Due to the complex nature of the Louisiana coast a very high level of detail is required to produce accurate surge level estimates.
Future developments
If a hurricane is still far from landfall the forecasts are still very uncertain and may vary a lot from advisory to advisory. Two days before landfall Hurricane Gustav was predicted to become a hurricane category 4 or 5 but in reality turned out to make landfall as a weak category 3 hurricane. In order to take such uncertainties into account a methodology is being developed to select multiple storms. Based on maximum water levels in the selected set of storms it would subsequently be possible to calculate the standard deviation for example. In this way it is possible to quickly visualize and analyze best/worst case scenarios and provide the user a confidence level with the predicted surge levels.
In addition to this improvement it is planned to use The Hurricane Surge Atlas to identify sites with possible damages to levees. By spatially combining the maximum water levels, wave heights from the atlas with current levee and floodwall heights it would be possible to evaluate the chance that a typical stretch of levee is overtopped. The results from this quick scan could than be used to plan levee damage assessments in the wake of a storm.