Where is the BFE for AO Zone Depth 1 foot?

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kwilson
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Where is the BFE for AO Zone Depth 1 foot?

Postby kwilson » Wed Mar 10, 2021 3:07 pm

Where is the Base Flood Elevation for an AO Zone with a depth of 1 foot?

In general, these zones are found in cities with low lying areas where the idea is that in a 100 year storm the water will rise to a depth of 1 foot. Years ago, i listened to a seminar put on by some experts in this area and they stated that the governing elevation would typically be based on the HTOC which is the highest Top of Curb on the street adjacent to the particular lot. Of course, if there is no curb, then they suggested using the centerline of the road. Then add one foot to the HTOC or centerline of road and that is the BFE.

How are you other surveyors handling this?

Also, while not a building designer, we are called upon to prepare Elevation Certificates prior to and during construction. From my experience, City and County Building departments will require that the finish floor of new buildings be 1.00 feet higher than the BFE which would then be 2.00 feet higher than HTOC or centerline.

What has been your experience in this area?

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Jim Frame
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby Jim Frame » Thu Mar 11, 2021 9:24 pm

I did one in an AO zone in 2017. I was told by the county to use the highest adjacent grade to the proposed house as the starting point, then add a foot to get to BFE, then add a foot to meet county standard for design floor. No problem, I did that, set a bench mark on the curb adjacent to the house, and gave the owner a bench mark diagram along with the pre-construction Elevation Certificate.

I didn't hear anything further until last summer. It turns out that the owner built the house himself and ignored the bench mark, just graded a flat spot and poured his slab. He didn't call me out for a pre-pour certificate, and I guess he didn't call the county for a pre-pour inspection, he just poured the slab and built the house. When he applied for an occupancy permit last summer the county wanted a post-construction Elevation Certificate, so I got called back. That's when we learned that the guy built the slab 1.3 feet below the design floor elevation.

Many go-rounds with the county later (by the owner, not me) a civil engineer is brought in. He had me run 8 cross-sections on the nearby flooding source, most of them between 1500' and 2000' long, so he could do a localized study. Fortunately for the owner, the resulting BFE is about 1.2 feet below the FF, which means he just barely made it. He *may* have to elevate his HVAC condenser pad, as it's a tenth or so low.

I'd say that he dodged a bullet, but what he really did was shoot himself in the foot.
Jim Frame
Frame Surveying & Mapping
609 A Street
Davis, CA 95616
[url]framesurveying.com[/url]

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LA Stevens
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby LA Stevens » Fri Mar 12, 2021 8:07 am

My past reading on the subject is you need to figure out the flooding source. If you have a 1 foot of sheet flow flooding running perpendicular to your contours and you can demonstrate that your lowest adjacent grade to the structure is above the 1 foot of sheet flow flooding, it can be removed from the SFHA. I would still recommend to the owner to have flood insurance, but it will be much less if the structure is outside the SFHA.

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Jim Frame
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby Jim Frame » Fri Mar 12, 2021 12:27 pm

Update: The PE corrected a typo in his report, lowering the BFE by a foot. I got the email just before submitting to the county, so was able to fix everything before sending it off. It looks like my hapless client is finally going to get an occupancy certificate.
Jim Frame
Frame Surveying & Mapping
609 A Street
Davis, CA 95616
[url]framesurveying.com[/url]

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LA Stevens
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby LA Stevens » Fri Mar 12, 2021 3:27 pm

Sometimes the clients get lucky!

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PLS7393
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby PLS7393 » Mon Mar 22, 2021 9:54 am

To establish a Base Flood Elevation (BFE) in a Zone AO (Depth 1'), there are three options depending on the SFHA.
It is all outlined and defined in the FEMA MT-1 Technical Guidance (May 2016). See Section 4.7, pg 27

When the SFHA is contained in the street, getting a LOMA is a much easier process. If the street is not completely in the flood way, there is no mathematical way to get the structure removed calculating the BFE by average ground elevations around the structure + 1'.
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Jim Frame
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby Jim Frame » Mon Mar 22, 2021 1:30 pm

To establish a Base Flood Elevation (BFE) in a Zone AO (Depth 1'), there are three options depending on the SFHA.
It is all outlined and defined in the FEMA MT-1 Technical Guidance (May 2016). See Section 4.7, pg 27


I don't think you read far enough into the document -- check out page 29 and the HAG option, which is what the county initially used in my client's situation.
Jim Frame
Frame Surveying & Mapping
609 A Street
Davis, CA 95616
[url]framesurveying.com[/url]

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PLS7393
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby PLS7393 » Mon Mar 22, 2021 3:18 pm

Jim Frame wrote:I don't think you read far enough into the document -- check out page 29 and the HAG option, which is what the county initially used in my client's situation.


Jim,
As I have discussed some issues with FEMA, this option #3 is when there is no constructed street to assist with the floodway.
furthermore, in the second to last paragraph on pg. 29, per 44 CFR 60.3(c)(7), how many structures have you seen with the lowest floor (crawlspace or basement) constructed (elevated) above the HAG? Step up to the crawlspace? Doesn't happen in or around the Bay Area where I have been successful in obtaining well over 100 LOMA's from FEMA, and mostly in Zone AO.

This may be an issue in the Central Valley that I have not seen.
Keith Nofield, Professional Land Surveying
PLS 7393

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Jim Frame
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Re: Where is the BFE for AO Zone Depth 1 foot?

Postby Jim Frame » Tue Mar 23, 2021 7:10 am

This may be an issue in the Central Valley that I have not seen.


We don't have a lot of AO-zoned areas around here, so this situation was unusual, at least in my experience. But we also don't build many houses on raised foundations, let alone with basements, in flood zones anymore. They're almost all slab construction (top of slab being the lowest floor), all of which are above the HAG.
Jim Frame
Frame Surveying & Mapping
609 A Street
Davis, CA 95616
[url]framesurveying.com[/url]


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