no rise analysis flood zone fema

[bdruehl]

never done one... a "no rise analsyis"

would it be worth the learning curve?... must back test with fema data and then model the moved and changed drainage channel the client made without permits to prove that there is no rise in the flood levels upstream (downstream?)... HecRas or other program I suppose. How long does one of these take? i know it would depend on size of area, quality of data, etc... but, any input is helpful. How happy will the client be when we say it will cost... ????

 

[msquared48]

As I understand it, you are checking two things here:

1. You are running a backwater elevation analysis and making sure that the backwater elevations do not change due to any construction.

2. Also, the downstream elevations cannot not increase due to additional outflow capacity from construction.

In either event, flooding would occur as a direct result of your project, which is a no no.

As to the cost, could be very minimal for small projects, or extensive with larger projects interrupting a large percentage of the floodplain. The one I did was very small and able to be verbally justified without calculations.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Rye1]

Do not go into this thing without a good contract and an understanding that a "No-Rise" may not be possible. The best time to plan a No-Rise study is before the project starts. You are being asked to "remedy" a violation.

"would it be worth the learning curve?" It really depends upon your level of comfort with H&H projects. HEC-RAS will probably be the model you use. If there is an existing HEC-RAS model, you'll save a little time.

The first thing I like to do is speak with the local floodplain administrator - if they participate in the NFIP and they have FEMA floodplains then they have to have a floodplain administrator.

As for time, 2-3 months. Cost - most are not cheap but the cost does vary with complexity. There is a considerable amount of liability involved with these projects.

If the channel has been moved, FEMA will require a LOMR within 6 months of project completion.




Robert Billings

http://www.newrivereng.com

 

[francesca]

Let the client know that they have screwed up and whatever your fee is, it's less than having to remove their development. Given that you don't know the process and aren't even sure that HEC-RAS is the tool, I'd say that you should ask the FEMA reviewer for a few references and pass them on to your client.

 

[bdruehl]

yes, indeed... francesca, you were able to gleen that i have not had that much experience with hec-ras in my pro career...however, plenty of modeling torture (including water resources) in my schooling, and there is always a first time for everything, even if it isnt fun! we were contracted to get the grading permit and attempt correcting the violation and the no rise just came up in a plan check..

and as far as removing their development, in fact i have consulted with several firms, and the cost would likely be around 4-6k+... or more! (depnding on the quality of fema data, surveying requirements, etc.) so, im not sure about the removing the development being less than the study argument. it seems to me that a statement of engineering judgement (and its subsequent liability if incorrect) should be enough on a project of this scale... maybe 400 feet of channel in question (for example.. the channel has not been moved signifantly - maybe 50 feet, and has been widened, the excavated soil from the channdel removed from the flood plain, the downstream culvert is undersized according to the community master plan so therefore there is an argument that it will not affect downstream levels, and by similar argument of channel capacity and its"retention/detention" vs its original capacity, neitgher will it affect the upstream...) will try this approach, although when the community developers get an idea that something is requried, they wont accept anything but... a quick scan of the fema requirements states that an explanation of why a no rise analysis isnt required may be accepted in lieu of the actual model results... ofcourse, the actual hydrology is more complicated, but are the models really that precise when we are talkin about a project of this scale, and when the data to begin with is less than desirable?

 

[bdruehl]

oh, and rye1! thanks the for input on the "floodplain administrator"! will explore that avenue for sure. appreciated!

 

[Rye1]

"a quick scan of the fema requirements states that an explanation of why a no rise analysis isnt required"

If you are in a FEMA regulated floodway and you have moved the creek, you are required to submit technical data showing that you have not increased the water surface elevations (CFR 60.3)or you must apply for a CLOMR. If you are doing construction in a FEMA floodway (CFR 60.3(d)), you are required to have a floodland development permit, and either a "no-rise" or a CLOMR.

Please be careful. I'm not trying to be Chicken Little screaming the sky is falling, but I know engineers that did not take the FEMA rules seriously and ended up in front of the Board. I've also been involved in a major lawsuit involving FEMA floodway issues. The lawsuit went all the way to the NC superior court. So, I'm suggesting you do some major CYA on this one.

"it seems to me that a statement of engineering judgment (and its subsequent liability if incorrect) should be enough on a project of this scale" This is not correct. Based on what I've said before, a letter stating that based on engineering judgment the project will not increase WSEs will not be sufficient.


Robert Billings

http://www.newrivereng.com

 

[Rye1]

One more comment...and I'll be quiet.

"but are the models really that precise when we are talkin about a project of this scale"

No they are not. As engineers we know how many assumptions are made with the CNs, Manning's values, Tc's, etc. Does it matter that the models are not that precise - No. FEMA has estabilished a standard for a "No-Rise" study to be less than 0.00 feet. This is not defined in the CFR but is defined in the MT-2 forms and more recent FEMA correspondence. FEMA world does not always match up with reality.

Robert Billings

http://www.newrivereng.com

 

[bdruehl]

thanx for your input rye...MUCH APPRECIATED... i was hoping as msquuared commented that he did once, that a verbal justification might be enough . "the ebngineering certif must be support by technical data OR an explanation of why a hyrdraulic analysis is not required"--- quote diredct from the FEMA documentation. for this project its hard for me to wrap my mind around how creating a wider channel (albeit with a minor course change) with identical inlet and outlet conditions will increase the flood levels anywhere, so a math model that says so seems more than a bit superfluous...but even so, this particular community has serious flooding issues..most downstream, so your advice is well heeded. i have a few bids being drawn up by others....

 

[francesca]

Under some conditions, wider channel = lower velocity head = higher WSE.

 

[bdruehl]

yes that is true... this project has a very flat channel. thanx for your posts.

 

[bdruehl]

after a letter of whining much like the above, the FPA agreed to a compromise which will allow a simple flow analysis/height analysis with no fema data backtesting required. if flow from 100 year flood as dictated by the drainage master plan for the city, and it doesnt increase the flood elevation on the firm map by MORE than a foot over the critical cross section/s!, they will accept. (oh and by the way, if he was to make conditions original by moving his channel to original location, they would have STILL required an analysis due to flood way channel alterations!)... note to self---> dont mess with fema wihtout a permit (or for that matter any insurance related entity)

 

[Rye1]

"note to self---> dont mess with fema wihtout a permit (or for that matter any insurance related entity)"

FEMA has become more aggressive over the last 10 years. Whenever you need to work near a FEMA area, talk to the local FPA and/or the State NFIP coordinator.

Robert Billings

http://www.newrivereng.com