I am preparing an Elevation Certificate on a building that sits in a backwater of the river floodplain. Can anyone explain to me the proper procedure of determining the BFE for a structure in this situation? Can anyone also reference the FEMA publication or rule that establishes the proper procedure. I have received contradictory advice regarding this from the authorities.
I can see 4 ways of computing the BFE:
1. The building is shown between BFE lines at the same elevation of 4539.0 so the BFE should be 4539.0
2. (B) The building is perpendicular to the FIS at 4539.8 so the BFE should be 4539.8
3. (A) The point of land where the 1% line crosses over to the backwater is at 4539.7 on the FIS so the BFE should be 4539.7
4. (C) The building is 20% of the distance upstream between cross-sections X at 4539.3 and Y at 4540.4 so the BFE should be 4539.5
Can you tell me what method is the correct one?
I have a picture but I am not sure how to display it.
Thanks, of course.
Floodplain Management - Principles and Current Practices
From Chapter 6 – Utilizing Information from Flood Hazard Studies
Determining Stream Distance
Flood profile sheets in the Flood Insurance Study are used to calculate flood elevations at the development site. The scale on a flood profile is expressed in terms of stream distance in either feet or miles above the mouth of the stream or other reference. Once a site has been located on the flood map (as described above), the next step is to determine the site’s position relative to the flood profile. Figure 6-20 illustrates this procedure. In the example, a site, marked by an “x,” is
located between cross sections C and D.
Find the letter designation for the nearest upstream and downstream cross sections on the Flood Boundary and Floodway Map (FBFM) or the new format Flood Insurance Rate Map (FIRM).
Locate these same cross-section letters on one of the flood profiles in the FIS. To determine the site’s location relative to the profile, use one of the two procedures outlined below, depending on whether the distance is expressed in feet or miles.
Draw a line from the site perpendicular to the center of the stream, as is shown in the Figure 6-20. (Another line has also been drawn from the site to the centerline of the stream at another location. Notice it is not perpendicular to the stream and must not be used in these calculations.)
From the point where the perpendicular line intersects the center of the stream, measure the distance to the nearest cross section—scale the distance along the center of the stream, following all bends and curves. (It would be worthwhile to measure distances to both cross sections to check accuracy.)
In this instance, the site is about 500 feet upstream of cross section C and 250 feet downstream of cross section D. The cross-section lettering system starts downstream, usually with the letter “A” and goes upstream. Cross section A is downstream of cross section B; B is upstream of A, etc.)
In the same manner, site “y” can be located in relation to the flood profile. It is about 600 feet downstream of cross section B and 300 feet upstream of cross section A.
Hope this helps,
DDSM:beer:
Thanks Dan.
In this particular case the structure is isolated from the river by a elevated roadway but is situated within a backwater that is expected to be innundated in a 100 year flood. In a previous CLOMR-F I was instructed by FEMA to determine the BFE at the point where the floodwater would break over the roadway and begin to fill the backwater. I have also received contrary instructions.
In this case the structure is midway between two BFE lines both labeled as 4539.0 and the breakover point is 4539.7. The 4539.0 BFE line that is in the backwater is actually upstream of the 4540.0 BFE line on the main stream and that has me confused about what is really happening in the model.
My structure has a LAG and a TBF of 4540.2 so it is above any of the resulting calculated BFEs I just want to understand the procedure and post the correct number.
Could you create an annotated FIRMett and an annotated FIS profile? Convert them to JPG and upload them here using BeerLeg upload button. A picture would help. I've got a story about a backwater from a major river confusing the floodway width of a smaller 'river' that may be in common.
DDSM:-O
BeerLeg rocks! :beer:
Here is a sketch of the situation: 'A' is the line perpendicular to the FIS at the point of crossing the road
'B' is the line perpendicular to the FIS at the structure
'C' is the line between cross-sections crossing through the structure
Note the FEMA map BFE lines on either side of the structure are both 4539.0
Why not let the FPA determine the BFE?
Who is the FPA? If you mean a local flood plain manager of local govenment I don't think it will happen within the time frame we have been asked to complete this in. We have two CFM on staff and we are all undecided about the proper procedure. On a similar request I have involved both local and state authorities and still do not have a definitive answer on that FEMA issue.
I have reached out to someone I trust associated with FEMA and expect an opinion from that person shortly. I will share it.
The correct answer is 'A' 4539.7
>3. (A) The point of land where the 1% line crosses over to the backwater is at 4539.7
Amazing - a logical, practical result in connection with a government agency.
😉
DDSM
