Why isn't FEMA using the latest two-dimensional wave models to estimate wave effects over land?

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.

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1. Why does FEMA map coastal flood hazards?
2. I have never seen a flood here. Why is this area shown as a high risk flood zone?
3. A recent large storm hit near my house and it didn’t flood; are you sure these maps indicate the current flood risk in my area?
4. What is the difference between a Zone VE and a Zone AE flood hazard designation?
5. Why did FEMA choose the 1-percent standard?
6. How does the preliminary map issuance change my flood insurance rate?
7. Why is FEMA creating new coastal maps when there are so many other public priorities?
8. What different methodologies are used in performing coastal studies and why?
9. Why are there multiple flood studies occurring in my community?
10. Are the flood hazards shown on the Flood Insurance Rate Map based on historic coastal flood events?
11. How accurate are coastal flood study results?
12. How can coastal flood studies be made more accurate?
13. If there is a margin of error in coastal flood studies, how can FEMA be sure that its flood insurance mandatory purchase requirement is not being applied to structures outside the Special Flood Hazard
14. At what scale are coastal flood studies performed?
15. Are the flood study results appealable?
16. Is it possible for community officials or individual property owners to appeal a coastal study when FEMA used supercomputers to obtain the results presented on the Flood Insurance Rate Map (FIRM)?
17. Why isn't FEMA using the latest two-dimensional wave models to estimate wave effects over land?
18. Why is the Base Flood Elevation (BFE) near my property different than the BFE near my neighbor’s property?
19. What is the Limit of Moderate Wave Action (LiMWA)?
20. How is the LiMWA mapped?
21. How should I build in areas near the Limit of Moderate Wave Action (LiMWA)?
22. Does the LiMWA impact flood insurance premiums?
23. What is a Primary Frontal Dune (PFD)? How are the PFD lines drawn?
24. How long does it take to conduct a coastal flood study?
25. Why does a coastal study take so long?
26. What stakeholders are involved in conducting a coastal flood study?
27. Why is the modeling for a coastal study so complex?
28. How long is the appeal process and compliance period?
29. How is FEMA accounting for sea level rise and climate change on the FIRMs? Does sea level rise/climate change affect the FIRMs?
30. How are the FEMA maps different from NOAA’s inundation/evacuation maps?
31. Why isn’t our beach nourishment and/or dune construction project included in the coastal map study analysis and resulting mapping?
32. How do you assure that the analysis includes enough information to depict the flood risk near my home?
33. What is the Joint Probability Method?
34. What is a synthetic storm?
35. Does FEMA include tsunamis on the flood hazard maps?
36. What is the Coastal Barrier Resources System (CBRS)?
37. What is the CBRS digital conversion project?
38. Why shouldn’t the FIRM be used to make CBRS determinations?
39. Why did FEMA remove some survey benchmarks?
40. Why am I in the mapped SFHA if my house is on a dune above the BFE?
41. What are some mitigation actions that coastal communities can take?
42. Can a community require higher BFEs to encourage higher building standards?
43. My homeowners insurance will not cover the additional cost to elevate. How is that fair?
44. Is it considered “new construction” if I replace my home after a disaster event?
45. Can I build on my property in the floodplain?
46. I can’t afford to elevate my home out of the floodplain. Is there any assistance available for me?
47. Why does FEMA promote development in the floodplain by offering flood insurance?
48. Why am I required to purchase flood insurance?
49. When am I required to purchase flood insurance?
50. Who determined that I was required to purchase flood insurance?
51. What information should I bring with me to meet with a flood insurance agent?
52. Can I shop around for flood insurance?
53. Why should I purchase flood insurance?
54. What can I do to get the cost of my flood insurance premium to be as low as possible?
55. Are there any modifications that I can make to my home to reduce my flood insurance rate?
56. Are extensive engineering studies needed in order to obtain a Letter of Map Amendment (LOMA)?