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FEMA AE Zone BFE 3' below flood waters.
Posted by Daniel S. McCabe on January 7, 2014 at 3:12 pmThis is a first for me, I have new construction in an AE zone where I no that the flood waters have gotten, at least, 3′ above the BFE.
The client knows as well and is going up in the air, so no big deal, but could you imagine building a house and then getting 3′ of water in it?D.Wheat replied 10 years, 8 months ago 6 Members · 7 Replies -
7 Replies
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We had quite the floods here in April, much more than a 500 year event. Had a client want us to set a benchmark on his property at BFE so he could rebuild. We found a good location to set a BM. We shot the ground and deduced we had to cut about 2′ to get to the BFE. We spent a lot of time checking math, benchmarks, etc, because the client said he had about 3′ of water over the ground where we were going to set the BM.
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How old are the FEMA maps in that Area? I’ve seen newly issued map change the BFE by as much as 9 feet.
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That has been happening for a long time in some places.
Usually man made changes are the cause of it.
Erosion and sediment buildup into a flow area will cause a rise in waters.
In this area it is usually caused by beaver and/or driftwood blocking drainage areas and culverts.
0.02
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This is a valuable lesson and discussion. Remember that the BFE is for a 1% chance flood. However, hydrology is not an exact science, even with empirical data and the most detailed of analysis by well qualified individuals. There is still a big margin of error in evaluating rainfall data, basin size and configuration, time of concentration(s), infiltration rate(s), and a host of other sometimes not subtle influences such as groundwater fluctuations, rain on snow events, etc. etc. etc.
Bottom line, is that the target flood for the stated BFE is almost impossible to nail down. BFE’s can and should be refined over time. However often a BFE is initially derived as a theoretical derivation from somewhat limited data, which is either not truthed and revised on subsequent empirical observations, nor sometimes correctly evaluated by properly trained individuals.
Most important is recognizing that a BFE is simply a benchmark of risk. The NFIP and insurance industry recognizes this by substantial reduction of rates for how much finish floor construction is placed above the BFE.
Recently I assisted an individual who was preparing to build in an area next to the Sacramento River. Most of the homes in the area were built on slabs. Many of these finish slabs were only built a few tenths above the BFE. Even as well documented and somewhat controlled the flows on the Sacramento River are, I explained much of what I discussed here and above with the individual. He subsequently did additional investigation by searching out individuals in the insurance industry who were knowledgeable about the NFIP. Consequently, I received a call from him, thanking me for pointing him in the right direction. The local agency was fine with him building a tenth or 2 above the BFE. However, after his looking into the matter, he found he was much better off elevating several feet above the BFE. Not only in annual flood insurance rates, but in peace of mind and understanding that the 1% benchmark is not as fixed of an elevation as he originally perceived.
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The maps were issued in 2010, the flood happened in 2012 after hurricane Isaac.
The old zone was A and the BFE in that area was around 12.0′.
In this case it does not really matter, the owner is going to be around 14′, but I did let my FPA know what I was finding and he, basically, told me that we have to hold the map.
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