The latest models resolve the flow of water or waves in two horizontal directions, and they are referred to as 2-D models. Simpler, 1-D models resolve flow or waves in one direction. Each has advantages and disadvantages. For now, FEMA uses both types of models. It uses 2-D wave models in the ocean and in bays, and a 1-D model-Wave Height Analysis for Flood Insurance Studies (WHAFIS) - over land.
On a large scale, 2-D wave models offer certain advantages over simplified wave analyses-the 2-D models better capture the physics of wave propagation, account for waves changing direction in response to bathymetric features.
However, the 2-D models still have some unresolved technical and computational issues related to overland wave propagation:
- The 2-D wave models do not have the resolution required to capture small details and changes in topography and land use/land cover. Grid cells are usually hundreds of feet across, and physical features must be smoothed or averaged over each grid cell. Using smaller grid cells on land necessitates using smaller time steps in the model and drives up the time and cost required to complete the analysis.
- The 2-D models lack a robust method for calculating wave dissipation effects (of vegetation, buildings and other obstructions) comparable to the procedures developed for FEMA's 1-D (WHAFIS) method.
WHAFIS captures the key aspects of waves moving along a specified transect over land and has been used successfully applied to many flood studies and flood map revisions.
The use of WHAFIS on land does not eliminate the use of 2-D wave models seaward of the shoreline. 2-D wave models are still used by modern flood studies to transform waves from deep water to the shoreline, at which point the 2-D wave characterization can be used as an input to WHAFIS. WHAFIS will be eventually be replaced as 2-D models become able to handle topography and development characteristics on a finer scale; but until that time, FEMA will continue to use WHAFIS.
Please also note that Letters of Map Revision (LOMRs) that are based on better topography or changed site conditions can now be carried out with relatively little effort and at modest cost using WHAFIS. Use of a 2-D overland wave model would make these map revisions more challenging and expensive. At some point in the future, when 2-D models overcome the technical issues mentioned above, and as their use becomes more widespread, flood studies and flood map revisions using the 2-D overland wave models will become feasible.