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floodplain determinations

SGB

Cyburbian
Messages
3,387
Points
25
A question for public sector planners:

How do you handle requests for floodplain determination, i.e. "Can you tell me if this property is in the floodplain?"

Our policy has been to show the person inquiring the applicable FEMA map, and let them make their own determination. This is primarily to avoid liability for making an incorrect determination.

How do others handle such inquiries?
 

Queen B

Cyburbian
Messages
3,179
Points
25
Yes, I have heard the liability issue also but we make determinations anyway. Generally it is because we have to for building permits. If it is really close or if they are defiantely in then we make them get a pre and post survey done.

I am headed for FEMA training next week, it should be interesting.
 

Queen B

Cyburbian
Messages
3,179
Points
25
Just thought of something else that might work for you.
I have directions on how to make a FIRMette off the FEMA website but not sure how to get that information to you.. I'm not real good at this linking thing. I can do an Email attachment.
 

SGB

Cyburbian
Messages
3,387
Points
25
QB -

"Our Great Leader" (tm) has posted a really good explaination at this link on how to add URL links to your posts.

Or, if all else fails, just cut and past the full URL into a message.

I'm sure I'm not the only one interested in seeing the FEMA page.
 

Repo Man

Cyburbian
Messages
2,550
Points
25
We do determinations for people. We always say "according to the FIRM map provided by FEMA" in the letter. If their mortgage lender disagrees we tell them they need an elevation certificate to prove that they are out.

I would be interested in seeing that link also.

Apparently the County where I work is in the process of mapping the floodplain for GIS use. I can't wait to have that so that I am not trying to scale distances off these 20 year old FEMA maps anymore.
 

BKM

Cyburbian
Messages
6,464
Points
29
Our Public Works Department takes the lead on floodplain issues. For a fee, they will provide a determination for a citizen. Not sure there has ever been an issue with liability related to this issue.
 

Tom R

Cyburbian
Messages
2,274
Points
25
FEMA

When I did thees, I would only give a firm (pun intended) answer if there was no doubt about being in or out. If it was on the edge or close I would recommend that the applicant hire an engineer to confirm the FEMA maps on site. If you have streams that are mapped in detail they probably have cross sections which give the various flood elevations. It may be possible to use these to determine the flood elevations on a particular sire based on topographic contours. If it requires extrapolation between two cross sections, let the engineers do it.

I had one "interesting" determination for some upper floor condos that are in a converted factory. The lower levels of the factory were right on a river and were definately within the flood plain. But the upper floors definately were not but the maps being two dimentional couldn't show it. I don't know how it was resolved.
 

Jeff

Cyburbian
Messages
4,161
Points
27
You're dredging in legal hot water with making determinations, unless it is blatantly obvious the property is or is not in the floodplain.

Anything that is close (which is why they are seeking a determination probably) needs to have a boundary survey of the property and soil test probes done to determine the floodline.

The margin of error for FEMA maps is high, and where are you getting boundary info. Unless from a recently completed survey where all pins were found, the lot lines you are using are only close at best.

At my old job, the banks selling the insurance would always send the customer to us to make a determination. We wouldn't. I always referred them back to bank, and said that if you are the ones selling (or not selling) the insurance, you need to hire a qualified consultant to make the call. They agreed.
 

Cardinal

Cyburbian
Messages
10,080
Points
34
Mike D is definitely heading in the right direction here. The FEMA maps are a nice start, but hardly the precision you need to make a determination, or that a builder should have before engineering a development. Run it by the city attorney, but I think you would be safe in saying something to the effect that:

"The 1967 FEMA Flood Plain Map depicts the 100-year flood plain extending over the northwest corner of the property. You will need to have the floodplain delineated by a professional surveyor."

or

"The 1967 FEMA Flood Plain Map does not depict any flood-prone areas on your property. The FEMA maps only delineate, at an approximate scale, the boundaries of flood plains contiguous to rivers and other water bodies. Local ground water and surface water conditions may affect this property and should be assessed by a qualified engineer."

Here's an interesting one. We have a property adjacent to, and downstream of a dam. While the buildings are outside of the floodplain, in the event of a catastrophic failure of the dam, some buildings would be impacted. Consider that it has been calculated that a wall of water 500 feet tall would sweep through the Grand Canyon if Glen Canyon Dam failed. Would this be a "floodplain?" I do not know if that issue has been addressed anywhere.
 

Tom R

Cyburbian
Messages
2,274
Points
25
dam

Michael Stumpf said:
Mike D is definitely heading in the right direction here. The FEMA maps are a nice start, but hardly the precision you need to make a determination, or that a builder should have before engineering a development. Run it by the city attorney, but I think you would be safe in saying something to the effect that:

"The 1967 FEMA Flood Plain Map depicts the 100-year flood plain extending over the northwest corner of the property. You will need to have the floodplain delineated by a professional surveyor."

or

"The 1967 FEMA Flood Plain Map does not depict any flood-prone areas on your property. The FEMA maps only delineate, at an approximate scale, the boundaries of flood plains contiguous to rivers and other water bodies. Local ground water and surface water conditions may affect this property and should be assessed by a qualified engineer."

Here's an interesting one. We have a property adjacent to, and downstream of a dam. While the buildings are outside of the floodplain, in the event of a catastrophic failure of the dam, some buildings would be impacted. Consider that it has been calculated that a wall of water 500 feet tall would sweep through the Grand Canyon if Glen Canyon Dam failed. Would this be a "floodplain?" I do not know if that issue has been addressed anywhere.
Someone, maybe the Corps of Engineers, has probably done a study of a catastrophic breach of the dam and what would be flooded down stream. If it hasn't been done, it should be. Also, I believe the design standard for dam construction is the 500 year storm event. That is why the 500 year flood is on most FEMA maps.
 

Cardinal

Cyburbian
Messages
10,080
Points
34
Re: dam

Tom R said:


Someone, maybe the Corps of Engineers, has probably done a study of a catastrophic breach of the dam and what would be flooded down stream. If it hasn't been done, it should be. Also, I believe the design standard for dam construction is the 500 year storm event. That is why the 500 year flood is on most FEMA maps.
True, the Corps will do analysis before any new dam is constructed. But no analysis was ever done for Glen Canyon Dam or many others of the same magnitude and era of construction, until long after they were built. The same is true of our two local dams, originally built around 1850. The FEMA maps, in our case, do not show the area that would be impacted. We only mapped it during a structural/functional analysis of the dam last year.
 

Tom R

Cyburbian
Messages
2,274
Points
25
Re: Re: dam

Michael Stumpf said:


True, the Corps will do analysis before any new dam is constructed. But no analysis was ever done for Glen Canyon Dam or many others of the same magnitude and era of construction, until long after they were built. The same is true of our two local dams, originally built around 1850. The FEMA maps, in our case, do not show the area that would be impacted. We only mapped it during a structural/functional analysis of the dam last year.
Scary, ain't it!
 

Cardinal

Cyburbian
Messages
10,080
Points
34
Re: Re: Re: dam

Tom R said:


Scary, ain't it!
For the Grand Canyon, it certainly is scary. Think of the big dams as terrorist targets and what their failure could mean. Last I heard, nobody has done a full analysis of Hoover Dam, and certainly none has been done on the ability of it to withstand the wall of water that would hit it if Glen Canyon dam failed upstream. I have no idea about the status of Grand Coulee Dam or the other big ones.

For us, the problem is pretty minor. If Tripp Lake Dam fails, only a couple properties are impacted by getting a foot or two of water at most.
 

Queen B

Cyburbian
Messages
3,179
Points
25
Although, not on a large scale, I was involved in a difficult decision process. Small community lake with a dam going bad. They as a community did not have money to repair the dam. They made arrangements with a nearby city to supply them water. Then at public action they sold the lake. The kicker is that the only buildable area was below the dam. To allow them to build or not. When I left my position there, they were still trying to decide what they wanted to do. I had been strongly encouraging them not to take that option.
 

Chris

Member
Messages
19
Points
1
sorry - what's FEMA?

Else:
try a DEM and a probability distribution based on your HHF data
 
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