AO Zone

[kwilson]

About 12-15 years ago I went to a seminar on Elevation Certificates. It was taught by a couple of guys who seemed to have worked for or with FEMA at one time. They even had a nice notebook as a handout. I will get their names on this post later. They taught that in an AO Zone, the Base Flood Elevation (BFE) should be determined by adding the depth of the zone to the highest top of curb in the adjacent street.

A local city is saying that the BFE in an AO zone is the Highest Adjacent Grade (HAG) plus the depth of the zone.

So for example, if the HAG is 100.0 and the AO flood depth is 1 then the BFE is 101.0 If the finish floor is 101.0 or less, then its underwater in a 100 year storm.

What are other jurisdictions doing in this regard?

[Jim Frame]

A local city is saying that the BFE in an AO zone is the Highest Adjacent Grade (HAG) plus the depth of the zone.

Same in my home county.

[LA Stevens]

What the City official is saying doesn't make sense.

You need to understand the source of flooding, the drainage pattern for the area, and the existing elevations, and then provide that information on an application for a letter of map amendment. FEMA has accepted many applications and removed buildings from the SFHA after demonstrating the lowest adjacent grade of the building was more than x amount of feet above the sheet flow flooding (ie: 2').

[Jim Frame]

What the City official is saying doesn't make sense.

It does in the right context. I agree with Larry that a formal drainage study is the best way to resolve the BFE question, but those don't come cheap. The "1 foot about HAG" is the low-cost way that some floodplain administrators allow.

A few years ago I got (had?) to witness this up close. A client bought a lot in a rural subdivision on which he planned to build his home, and the lot was shown in an AO zone. The county told him they'd approve his building permit if he sited the slab 1' above the existing HAG, so he retained me to provide an Elevation Certificate on that basis. I established a site BM, took the necessary shots, and delivered the pre-construction EC and a bench mark diagram. I explained that he needed to make sure his contractor use the BM to ensure that the slab was at least 1' above BFE.

3 years later (!) he asked me to provide an as-built EC so he could get his occupancy permit. The measurements showed that the slab was 0.3' above BFE.

It turned out that he had built the house himself. He ignored the BM, graded a spot level and built his slab on it. The county refused to issue an occupancy permit.

Another year passed and then a hydrologic engineer got involved. He had me run 8 cross-sections -- some as long as 2,000' -- through the nearby slough and the area surrounding the site in order to model the flow. His conclusion was that the slab was almost 2 feet above the modeled BFE. Using the new BFE, I provided another EC and the owner got his occupancy permit. But my fee for the cross-sections alone was 1.5 times what had already been spent on ECs, and that doesn't count the engineering fee.

He could have saved a bunch of money if he'd gone with the 1-foot-above-HAG approach and paid attention to the bench mark.

[PLS7393]

A local city is saying that the BFE in an AO zone is the Highest Adjacent Grade (HAG) plus the depth of the zone.

Ken,
The equation you are referencing is not a FEMA requirement, but it is the Building Code. I know it doesn't make sense, as the City of San Jose is the only agency that I work with that follows this under the Building Code, not the FEMA requirements. Designers need to show the AC Unit pedestal being 2.0' above ground in AO (Depth 2'), and it looks like crap with custom homes I've been associated with.

IMHO, the building code should be revised, but good luck with that.