> 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.
