Has anyone done one of these?
From what I read online FEMA will calculate BFE provided sections upstream and downstream of the structure provided in graphic and tabular form.
Which form is used?
Is there a online tutorial or would some be willing to share one of theirs?
Thanks.
> Has anyone done one of these?
> From what I read online FEMA will calculate BFE provided sections upstream and downstream of the structure provided in graphic and tabular form.
> Which form is used?
> Is there a online tutorial or would some be willing to share one of theirs?
> Thanks.
I've done about 4 of these in the past few years but was not aware that FEMA would calculate the BFE. We used our in house engineers and provided them with the appropriate cross section/topo data to calculate the BFE. Then you simply plug in the BFE on your elevation certificate as you normally would and include the engineered report in your submittal. Haven't had any issues yet.
A few cross sections, put them in HECRAS with a flow and you have an elevation.
USGS regression flows work fine. Certainly a PE job (though it's not rocket science).
> A few cross sections, put them in HECRAS with a flow and you have an elevation.
> USGS regression flows work fine. Certainly a PE job (though it's not rocket science).
Then I suppose you don't mind when you hear a PE say that boundary surveying is easy. All you need is a handheld gps and a couple of coordinates.
Both have high liabilities if not done accurately with sound methodology. Both have numerous considerations far too complicated to state a generalized procedure in a single sentence.
We run cross sections, send to PE, he sends back report with a BFE, and then we send in the info and have the structure removed.
I hesitate to get FEMA to send a BFE since so many of them are way off to begin with, maybe in other parts of the country it will work.
You need the cross sections sent to you in a report, and the engineer needs to explain his methods.
> Has anyone done one of these?
> From what I read online FEMA will calculate BFE provided sections upstream and downstream of the structure provided in graphic and tabular form.
> Which form is used?
> Is there a online tutorial or would some be willing to share one of theirs?
> Thanks.
I have a time or 2 or more.
It depends on the locale specifics, the "approximate methodology" already developed, the office you're dealing with and what you are really trying to achieve.
Tabular form is a station/offset/elev depiction of each node of each cross section, allowing for easy data entry into the various modeling programs. Excel or similar spreadsheet works good.
I'm unaware of a tutorial. Like I said before, it kind of depends on the locale specifics on how best to approach it and what your particular FEMA office will be looking for. Broad alluvium v narrow gulch, small value rural structure v high dollar commercial, and so on are all important aspects of how to proceed.
And most of all, be fully aware of what the BFE is based on. I highly recommend a competent hydrologic/hydraulics engineer be involved. You want to be able to give your client an understanding of what his/her flood risk really is. Rating risk for flood insurance is one thing. Informing your client of the true risk of flood frequency and depth is unfortunately not always viewed as one and the same by those trying to get a quickie dickie BFE. Also unfortunate is that some may not fully understand what they may receive from FEMA as to whether it is simply a BFE for insurance risk rating, or if it is a confident value which you can educate your customer as to the true inherent risks.
I note you have the title "cfm" I also note that that title does not have anything to do with having an engineer's understanding of hydrology or hydraulics. Not being mean, just wanting to be clear.
Moe,
you use the procedure I highly recommend.
Umm what...
I am a PE, LSIT with utmost respect for both professions (have done multiple Zone A, No BFE LOMAs)
Take a drink and chill, its almost the weekend.
> > Has anyone done one of these?
> > From what I read online FEMA will calculate BFE provided sections upstream and downstream of the structure provided in graphic and tabular form.
> > Which form is used?
> > Is there a online tutorial or would some be willing to share one of theirs?
> > Thanks.
>
>
> I'm unaware of a tutorial.
DDSM
Forgot to mention, FEMA publishes the "ZONE A MANUAL" for those wanting to figure it out. Its a little dated since it refers to HEC2 and their Quick program, but it should give good background on how the BFE elevation is determined.
FEMA person on phone told me to leave the BFE slot BLANK, and say, "No BFE available
locally" and they will assign one.
I have seen the LOMA form that they sent the client later and a BEF is shown on the form and it is one tjhey came up with.
> Umm what...
> Take a drink and chill, its almost the weekend.
or one of these?
Oh I'm pretty chill. I'm also a PE and LS who has developed a fair share of hydrologic and hydraulic models. There are some situations where getting FEMA to derive a BFE for Zone A is apprpriate. There are many that it is not and which would fall far short of the necessary due diligence one should apply.
Here's one that I'm dealing with now: a subdivision was built on fill circa 1990. The engineering firm developed BFEs and got LOMR-Fs for the lots on the other side of a 200-foot wide greenbelt from my client's property, which was a remainder parcel and is now apartments. The local agency doesn't have any BFEs more current than those developed in the 1990.
The flooding source is a nearby drainage channel that runs parallel to the tier of lots leading up to the greenbelt; the area is pretty flat, so the BFEs as they approach the greenbelt descend by a hundredth or two per lot.
FEMA has remapped a couple of times since the '90s, and the current FIRM shows the Zone A line (no BFE) cutting through the building on my client's property. My client is pursuing a LOMR-F.
The original BFEs were NGVD29, which are shown in parentheses. (The conversion to NAVD88 is pretty consistent in the area at about 2.55 feet, which is the VERTCON value.) I'm tempted to extrapolate the BFE across the greenbelt. Any thoughts on whether or not FEMA is likely to see that as reasonable?
2 thoughts. First is you'll be accepting the liability of the quality of work of the firm that developed the BFE. It is notable that the firm's study is not adopted by the local agency or FEMA even though the data was accepted for the LOMAs. Its your decision if you have confidence in their numbers. FEMA likely will not contest your converted values.
The bigger question is your extrapolation method and whether it is a sound engineering determination of the hydraulic grade line. It is an engineering decision in CA Jim and you will be putting your CA LS license at risk if Ric or whomever replaces Ray catches wind that you're practcing civil engineering without a license. There is a huge liability in determining BFEs and it is important you understand how to correctly determine the hydraulic grade line and have reviewed the methodology the other firm used that you'll be hanging your hat on.
That said, yes FEMA will likely accept your proposed methodology.
Will it stand up in a civil action or before BPELSG?
> 2 thoughts. First is you'll be accepting the liability of the quality of work of the firm that developed the BFE. It is notable that the firm's study is not adopted by the local agency or FEMA even though the data was accepted for the LOMAs.
The local agency did adopt the BFEs for the Aspen 2 subdivision lots shown, so that's not an issue. As a further complication, though, the city's floodplain manager sent email to my client stating: "For your engineer/surveyor’s information: the Base Flood Elevation on recent submissions for a LOMR-F in the Aspen 2 subdivision have been 51.8-ft (NAVD-88)." I've asked for clarification, because either that's an NGVD29 number, or it's NAVD88 a long ways from my client's site.
> The bigger question is your extrapolation method and whether it is a sound engineering determination of the hydraulic grade line. It is an engineering decision in CA
This is where things get kind of murky for me. This area is so nearly flat that the flooding is almost lacustrine rather than riverine, so the encroachment upon the practice of engineering by extrapolating a series of competently engineered grades over 200 feet would seem to be de minimis.
Perhaps what I should do is submit the extrapolation concept to the city and suggest that they consider using it to determine a BFE. That should keep me out of trouble with Ric. 🙂
I have not done one of these in several years, but the way it used to work was you had to submit a LOMA first. FEMA engineers would determine if they would accept the BFE you came up with using approximate methods. If not, they would then agree to determine one. They would hire a firm in the area who was already a contractor for FEMA to perform the cross sections.
Our local flood manager requested BFE's from FEMA and never got anywhere that way. The areas were not high enough priority.
Part of the legislative intent is to not make this process economically unviable, hence the allowance for "approximate" methods. In reality all methods are "approximate". I had only one where FEMA decided they needed cross sections and to come up with their own. When the 1% flood came, it was right on the stakes I had set and 200 feet from the FEMA determination (client took pictures). The same river an engineering study (expert PE's) was litigated because the flood was higher than projected. Lots of liability because two different studies, both done correctly, can always come up with different numbers. There is no one correct answer.
In any event, the client should always be told in the contract and the finished product that flood insurance is recommended and they are likely to be flooded if they are close enough to the zone that an EC or LOMA is needed.
Obviously there is a lot to performing an engineering study and documenting correct procedures for liability purposes, and reasonable results. These are required for new, major projects so the taxpayer does not get stuck with the bill for profit making ventures, and to ensure public works projects get full scrutiny and safety measures up front. But existing conditions for individual houses and such should not be stuck with tens of thousands of dollars in costs by having to choose between a full blown private engineering study or paying thousands a year in flood insurance.
Thanks for the replies.
This is where I found the info about FEMA determining the BFE it's from:
www.watershedmanagement.vt.gov/rivers/docs/nfip/rv_azonesurvey.pdf
I Goggled it after the Local FPM said one had be done the area.
Regions labeled as “Zone A” on your community’s floodplain map indicate those areas where FEMA has designated the approximate floodplain, but has not done the detailed river studies necessary to establish a base flood elevation.
If your parcel is less than 5 acres in size, FEMA will calculate the base flood elevation for you IF you provide the appropriate survey information with your LOMA application:
- The surveyor must establish a cross-section of the stream. This cross-section should begin at the upstream edge of your structure, and should extend perpendicular to the stream. The cross-section should extend on the opposite bank of the stream to the point where the relative ground elevation on the opposite bank equals or exceeds the ground elevation of your structure.
- Several survey points should be established along this cross-section. A survey point should be included at every point where there is a significant break in slope or change in topography along the cross-section. Elevation and distance along the cross-section should be measured for each survey point. Elevation can be measured using an “assumed datum”, where the surveyor establishes an arbitrary elevation at a nearby landmark and records relative elevations compared to that point. Distance along the cross-section can be made starting from the corner of your structure.
- All distance and elevation measurements should be recorded in a table. The surveyor should include a scaled diagram of the cross-section that includes each of the survey points (see “cross-section view” above). An “overhead view” of the cross-section (including the survey points) that shows the home in relation to the stream should also be included.
- Include all other information relevant to the survey. You should include several photos of the banks on either side of the stream. Knowing the type of ground cover nearby is necessary for FEMA to model the base flood elevation.
- Your LOMA application should also include the ground elevation at the lowest point around your foundation (also called the “lowest adjacent grade”), using the assumed datum.
This survey information should allow FEMA to calculate a base flood elevation for your site, and compare that information to the lowest adjacent grade.