Flood Zone Mapping Accuracy

I would think Haities would freeze over before FEMA admitted any margin of error in ANY of their mappping products. In writing, anyway.

Here's the web address to the FEMA manual for Zone A's that you might find useful. http://www.fema.gov/media-library/assets/documents/7273?id=2215

Cy

> I would think Haities would freeze over before FEMA admitted any margin of error in ANY of their mappping products. In writing, anyway.

I agree with that opinion.B-)

BTW, did you mean HADES?:-O

If it is Zone A (No BFE) then all bets are off. I know some that are 200 feet off from reality. The lines must have been put on there by buttonpushers.

A straight Zone A delineation is a little better than no delineation at all. Not much, but, a little. First, there is absolutely no good way to consistently guess as to what elevation may have been used in any fairly short stretch. The best we have tried to use is to determine where the line crosses a road. But, even then, one cannot get very far away from there and feel somewhat comfortable using that standard. The USGS maps around here have 10-foot intervals as standard. Sometimes the Zone A delineation will appear to be following or paralleling one of those lines. The problem, obviously, is that 9.5 feet up or 9.5 feet down can happen with no recoginition of that hump or swale. One can clearly see how the water line may deviate a long distance from the line on the Zone A map in either direction.

A few days ago I was chatting with a local farmer and former county commissioner about his new neighbors. We had cut out a tract of 10 acres from a 160 about one-quarter mile from his home. I happened to mention that we had to move the tract a couple hundred feet to the west of where the buyer and seller had originally agreed to put it. This was because of a Zone A slicing across the field. Within a stretch of 600 feet along where the edge of Zone A was supposed to be you could get elevation differences in excess of five feet. The fellow I was chatting with has lived across the road for more than 40 years. His comment was "What flood zone? I've never seen water anywhere but in the channel."

> I have a project where the subject parcel is in two flood zones, A and unshaded X. The FIRM delineates the limits of the zone A area. My client is proposing a new house, approximately 120' outside of the zone A designation, based upon overlaying the zone A limits using relatively precise aerial data.

It seems to me that if the proposed construction is demonstrably outside of Zone A, then the project isn't subject to floodplain requirements. Unless the Zone A boundary is defined somewhere other than on the FIRM, then the FIRM is the authoritative source. This wouldn't be the same as the BFE contours, which are subordinate to the flooding source data.

So my question for the experts is: Are the zone boundaries defined by a document other than the FIRM?

>
> "What flood zone? I've never seen water anywhere but in the channel."

I have seen places like this also. On the other end of the spectrum, I've seen places that have a foot of water standing on them (outside the channel) most of the year that are not flood hazard areas according to FEMA.

Can't you fill out a LOMA application (eg MT-EZ) and seek a "Out as shown" determination from FEMA? Let them officially make the call. Provide as much information as possible.

"Are the zone boundaries defined by a document other than the FIRM?"

Absolutely not! The maps are the definitive, regardless of ground elevation or anything else...

My recomendation is to meet with the flood administrator in your area and take a look at the profile maps. The profile maps control the BFE's. Let he/she make the call in writing as to where the flood zone line is located.

Find out from the same person what is the BFE in the "A" zone and advise your client to build at least one foot above that BFE just in case there is an issue in the future.

Since late 1970s, I have performed 50 or more flood insurance studies for several COE Districts, FIA and later FEMA. The detailed work was performed using new aerial mapping for the flood plain, along with field run channel sections as required and field located bridge, etc. structures. These are now generally the AE zones.

The Zone A was for areas determined by agencies "not to be studied in detail" and usually just a copy of the original Flood Hazard maps. The delineation on the Flood Hazard maps was usually following flood plain soils types from the SCS soil maps. Sometimes an extrapolated depth from historical records and gaging stations where available, etc. As expected, horizontal limits are not good enough to locate structures or anything else.

> Absolutely not! The maps are the definitive, regardless of ground elevation or anything else...

Agreed. The map is as etched in stone similarly to a call to the centerline of a road, or other topographic feature in a legal description.

The problems start when the cost of the flood study far exceeds the improvement costs, and FEMA knows that. Pretty soon the entire US will be in Flood Zone A, by virtue of their map as legislated by whoever draws the lines. Then every mortgage will require an elevation certificate.... we'll be rich :-/

I wouldn't build anywhere close to a flood zone of any kind. You don't know what the next revision will show, and haven't they ended grandfathering?