BFE Interpolation

Note that the Eastern floodplain does not have a profile baseline. Check the FIS for a profile for this tributary. Follow this tributary downstream to the first lettered section and determine if your site is in a 'backwater' flood. Verify your results with local FPA.

Hope this helps
DDSM

I'd pass this one off to the community flood pain manager, not really clear where that 113 contour intersects.

How severe would it be if you used 111.5 compared to using 111.0? Sometimes the number isn't really going to change things all that much when you're talking six inches. This doesn't appear to be a super flat area where a tenth or two greatly expands the affected area.

Holy Cow, post: 415821, member: 50 wrote: How severe would it be if you used 111.5 compared to using 111.0? Sometimes the number isn't really going to change things all that much when you're talking six inches. This doesn't appear to be a super flat area where a tenth or two greatly expands the affected area.

About fifteen years ago my Dad bought a house near a canal. The builder had an ecert done and all was well. Then a letter came informing Dad the BFE was being amended due to some changes downstream. Instead of 1.6 above he was now 0.1 below. I got to work checking things.
I tied the onsite bench to the RM. I ran one closed loop with my buddy on the rod and another in reverse wuth me on the rod. All measure ups and downs were done independently by both if us. Lo and behold he was clear by 0.3 and qualified for a LOMA.
I reviewed everything with the other Surveyor and we agreed. His comment was, 'I always round everything conservative. He saw it as s business decision to avoid liability. I saw it as 1800 a year in premiums for a guy on a fixed income...

That's precisely why I was inquiring. For an existing structure, it could be a big deal. If it's an all new structure and they want to make sure they build it high enough, you can help them out in the long run by guesstimating on the high side. Never go lower.

Holy Cow, post: 415835, member: 50 wrote: That's precisely why I was inquiring. For an existing structure, it could be a big deal. If it's an all new structure and they want to make sure they build it high enough, you can help them out in the long run by guesstimating on the high side. Never go lower.

Good point Cow and you're probably right, but given the picture that would be extrapolation and not interpolation, in CA I can't and won't play Civil on my LS. :p

Too many times there is no real help on answering the question in the original post. There is no cross-section nearby because it is the very top of the drainage area. The local flood plain manager probably has no better information than what you have. When 111 is the highest number provided, it is a given that 112 is too high or it would be shown if other information is shown in one foot changes. You can't compare what's happening in the other drainage course as it is a completely different situation. Continuing education courses on this are inconsistent. One I attended several years ago said there was no interpolation. He said it is the higher number until you hit the lower number. That does not agree with the flood studies, however. I suppose it was his way of saying to always use a higher number. Other courses push interpolation. Much of the boundary limit setting is based on interpolation of topographic data with five-foot or 10-foot intervals. That's a joke in this case as the maximum increased value is less than one foot.

The bottom line is that either you drop the project or make up a number that you can live with.

I have been told during many seminars with FEMA over 20 years, that one should not interpolate the contours shown on a FIRM panel. A surveyor should be using the FIS Profile. That made quite a bit of sense, after looking at the profile and finding out in many instances is not a straight grade. I also have found the contours do not always match up with the FIS profile. It is also good to check the FIS so you understand the flooding source for the area.

I quit attending seminars on FEMA regs long ago. The information being given was so grossly inconsistent I couldn't stand it. I found that searching on line was the best way to find FEMA directorate policy letters and tech bulletins. If it's confusing I call the map service center, tell them the titles of what I'm looking at and we walk through it.
Mr. Cow (whom I greatly respect) has given some good business advice. I would take it one further: determine a repeatable number that you can support, or don't give a number.

Thanks for the response guys. I only bring this up as I did several last year and one of the closest ones (Quite literally at the fill line along his foundation along the lowest adjacent grade was the BFE, just 0.5' lower than his walk in basement elev.) came back from FEMA with an INTERPOLATED elevation that now put his basement AT the interpolated BFE. I had always been told through seminars, and as some have mentioned here, NOT to interpolate.

My thought on the one I show at the start of the thread is to take a reasonable measurement to the next drainage 112 contour and end up with something around 111.5 as holy cow suggested. Then see where the actual elevations fall on the structure.

Big insurance is involved with the underwriting on policies for the homeowner and flood insurance is now big money. I can see what they are trying to do to squeeze as much out of owners as possible. I have another one for a local homeowner that has been paying flood insurance for years but was just informed by her broker that they are raising her premium by 25%.

John

johnhls, post: 416072, member: 3240 wrote: I can see what they are trying to do to squeeze as much out of owners as possible. I have another one for a local homeowner that has been paying flood insurance for years but was just informed by her broker that they are raising her premium by 25%.

What they're trying to do is bring the rates up -- way too slowly, in my opinion -- to the actual cost of providing the insurance. The taxpayers have been subsidizing flood insurance premiums for decades, but Katrina brought the cost of doing this into sharper focus. Congress has slowed the process because the Representatives don't like angering their constituents, but there's no good reason for the taxpayers to bail out (pun intended) people who own property in flood zones.

I'd support an exception for a one-time partial subsidy that includes abandoning the flood-prone improvements in areas not historically subject to flooding but that are now in flood zones due to external forces (e.g. climate change). But FEMA has been paying flood damage claims multiple times on some of these properties, yet the insurance premiums remain ridiculously low. That's neither reasonable nor sustainable.

Just went to the annual meeting with our state coordinator for FEMA. They are raising the rates 5-18% over the next few years. He stated that FEMA is 24 billion in debt due to Sandy and Katrina. Now they are readjusting the rates and also the methodology for determining the rate. What gets me is that the only thing they did after Katrina was move the the line... What was once the 2% line is now the 1% line in our area. Eventually they'll see the amount of LOMAs submitted and change the elevations.

No doubt the impact Katrina and Sandy had on the population and the public debt was the impetus for the recent changes. Outside of our community, I have read and heard many good investigative reports on the companies now involved in, essentially a bailout, to erase those debts. Frankly, I had no idea of the outside money to be made by companies that were hired to oversee and underwrite policies.

My skeptical self could see the dots being connected between the clients mortgage company requests and the reports of big profits. I live in eastern MA and see the impact every winter storm has on our coast and the homeowners who have been bailed out more than once. Yes, they do have the biggest increase in premiums which only a certain subset can afford, but it does seem to unnecessarily burden the average inland homeowner with a sump pump in the basement.

If only it was the EPA.

You also have the issue of enforcement by the local floodplain administrators. After the huge flood here last August, FEMA helped the local communities perform damage assessments because they were shorthanded. In my city alone, nearly 50% of the structures (total structures in the city, not just the ones that flooded) received substantial damage determination letters and 90% of those were below BFE. So you would think that would thousands of houses and buildings would be either A) torn down and rebuilt higher or B) elevated. Clearly I overlooked option C - get a lowball bid from a contractor that reduces the damage estimate to 49% of the value and appeal the substantial damage determination. It's up to the community to review the appeal and revise the determination, not FEMA. I've read several quotes from these communities stating they don't have time to verify the accuracy of these bids, so they accept them on face value. So unless they get busted by FEMA during an audit, thousands of buildings that received substantial damage last year (and likely flooded at least once before that) will just get gutted, rebuilt, and be ready to flood again. I certainly empathize for the thousands of people that flooded for the first time, but rebuilding at the same elevation doesn't make a lot of sense. And, as long as these homeowners don't let their policy lapse, they'll continue to get cheap rates, unless FEMA changes their tune.