Category 4 or 5 Hurricane Rainfall vs. 100 Storm 1

[jjpengr]

My company is currently working on the design for a hospital that is literally within a couple hundred yards of the ocean. We have established the limits of the 100-year floodplain and ensured that the hospital finished floor elevation is several feet above. However, the question has been raised about a correlation between the rainfall created by a category 4 or 5 hurricane and a 100-yr storm event. Is there a correlation? If not, is there any data to show the amount of rainfall generated by a category 4 or 5 hurricane?

 

[sam74]

How did you establish what the 100 yr floodplain was? Was it an existing FEMA floodstudy? If so in the study it should say what methods were used to compute the info. If your not using one and there is one I think I'd compare it. You should be able to pull such a study of their website. It seems like I once briefly looked at a coastal area FEMA study and I thought it mentioned huricanes in the calculation method. I could be wrong though.

 

[jgailla]

If you are in a coastal zone, the storm surge will probably govern for the floodplain.
Flooding caused by rainfall is separate from flooding caused by a storm surge.
Hurricanes are categorized by wind speed, so there is no real correlation between the category storm and rainfall. A slow moving tropical storm could drop more rain than a category 5 hurricane. However, the hurricane winds will cause a storm surge independent of excessive rainfall.
There is no such thing as a category 5 rainfall event.

 

[DMcGrath]

I agree there is no correlation between hurricane category and rainfall. Here in Richmond, VA about two years ago we had Tropical Storm Gaston that moved in and parked, dumping up to 14" of rain in 8 hours in some localized areas. I think I remember hearing this amount was later classified as the 6,000-year storm.

 

[blueoak]

The US Army COE Coastal Engineering Manual will help with surge and runup. These will control if you are near a coast. It's pretty easy to 10 feet of surge when you have no fetch limitations. I am a little surprised that a key piece of infrastructure is okay with a 100-yr event. I only worry about my mortgage, but that is still about a 30% chance in 30 years. Reading about New Orleans and living through a few floods, just remember that basements ALWAYS flood. I think they had the generators in the basement in New Orleans.

As for major precipitation, the NWS has a curve of maximum precipitation (worldwide) for different durations, if you have an HMR from them it most likely includes hurricanes in the analysis.

 

[SlideRuleEra]

I agree with jgailla and others that storm surge that close to the ocean will overwhelm any rainfall effects. Surge is very localized and depends not only on the storm characteristics, but also coastal geography and exactly were the eye crosses the coastline. On the "strong" side of the hurricane eye wall (northeast quadrant for a storm impacting the east and gulf coasts of the US) Category 4 & 5 storm surges have been well over 20 feet. Move to the opposite side of the eye wall and storm surge may be minimal, say 2 or 3 feet, or even "negative" (wind blowing water away from shore, making it look like and extremely low tide).

Another good reference is the "Coastal Construction Manual, FEMA 55". It is not available on the web but can be ordered. Here is a link for information

http://www.fema.gov/pdf/plan/prevent/nhp/nhp_fema55.pdf

I have experienced several, including the eye of a Cat 4 storm.

http://www.SlideRuleEra.net