LOMA 5

[CivilTyro]

Hi everyone,
I am reviewing a LOMA study. West half of the proposed site (residental, almost 450 acres)is in Zone A (special flood hazard area inundated by 100 yr flood where no base flood elevations determined) and the east half is in Zone C (minimal flooding).
The engineer is proposing to remove zone A of the west half since his HEC-RAS study states the water surface elevations are less than a foot for the cross sections he studied. He analyzed six cross sections but two of them are actually have water surface elevations exceeding a foot (Critical W.S.-Min. Channel Elev.).
the flows on the propoerty range from 4000 cfs upstream to nearly 13000 cfs downstream. The flow is mostly sheet flow and has low velocities.

My question is, Should I not approve this study since some cross sections show WSE of more than a foot Or approve since engineer studied the cross sections and found out the base flood elevations for cross sections even though some exceed 1 foot?
I guess another question that comes out of it is, what happens with Zone A delineation after the study show flow depths of more than a feet?? Is it still Zona A or something else??

Need an answer ASAP. Thank you in advance.

 

[francesca]

I'm sorry but I'm having trouble interpreting what you're saying. Over 1' depth is not sheet flow. 13,000 cfs or even 4,000 cfs would not sheet flow.

What legal implications are there to your "approving" the study? Do you then assume the legal responsibility if the study is flawed and the homes flood? If so, is the study up to FEMA standards? If so, why has it not been submitted to FEMA for a FIRM revision?

6 cross-sections for 450 acres does not seem like a very rigorous HEC-RAS model to me; it takes 2 or 3 cross-sections downstream of the study limit for the calculation to settle which means you have 3 cross-sections representing 450 acres, giving me even less confidence (unless the stream is perfectly straight and perfectly regular).

 

[cvg]

I would agree with Francesca and FEMA will not approve if the modeling suggests flooding is greater than a foot. By the way, this sounds like a CLOMR for a "proposed" site. You would need as-built conditions for a LOMR. sounds like you may need to either build up the site or increase the size of your channel

 

[RWF7437]

Agreeing with all of the above responses; I'd suggest you contact the FEMA Office for your area (Region) and find out who their "local" study contractor is. Explain that you have doubts about the adequacy of the study and ask for their help and guidance. You may also ask if you can submit the study to their local contractor for review. There will be a cost associated with this but that cost, for even a cursory review, should be borne by the person who submitted the study to you.

good luck

 

[RWF7437]

http://www.fema.gov/about/contact/regions.shtm

 

[RWF7437]

An example;

http://www.azwater.gov/dwr/content/...gment_Docs/SS1-97_Technical_Documentation.PDF

 

[RWF7437]

"My question is, Should I not approve this study since some cross sections show WSE of more than a foot Or approve since engineer studied the cross sections and found out the base flood elevations for cross sections even though some exceed 1 foot?"

First, it is probably NIT within your authority to approve this study, even if you're willing to. That authority resides at FEMA and at the City and County within which the study is performed. All of these entities must agree to accept the study before it becomes "the law".

Second, it appears you may be misunderstanding "depth of flooding". The depth of flooding at all points within the floodplain is NOT the water surface elevation minus the lowest elevation in the stream. That is the depth of flooding only at the stream. The depth of flooding at any other point within the floodplain is the water surface elevation minus the ground elevation at the point in question. Also, it is good to remember that computed water surface elevations are usually considered accurate within plus or minus 1 foot.

good luck

 

[RWF7437]

"First, it is probably NIT within your authority to approve this study, ..."

Corection: "First, it is probably NOT within your authority to approve this study, ....

 

[CivilTyro]

Thanks for the responses. My firm is doing the review for a City as a consultant. The City is going to make the decision based on our input.
I do not have lot of experience dealing with floodplain/FEMA issues, but I have learned quite a bit in one day. This is a great site for young engineers like me.
thanks again guys

 

[Rye1]

There should be a MT-1 or MT-2 form in the package somewhere. This is where you will come in to the chain of approval for the submission. On the form, there is a place for the local jurisdictional government to sign. If you agree with the study, you will probably have to recommend that the local floodplain administrator sign off on the form. Once he/she signs the form, the package can then be sent off to FEMA. FEMA has final authority on changing floodplain lines.

If you are the city's consultant, they will rely upon your recommendation.

 

[cvg]

reviewing and recommending for approval a CLOMR / LOMR is serious business. Since it generally has a large impact on a owners ability to develop his land, it can be quite expensive if either the flood analysis has to be redone, flood channels have to be enlarged or even worse yet if the plat gets approved and then FEMA does not approve the eventual LOMR submittal. I hope you have someone looking over your shoulder on this one.

 

[jartgo]

I agree with cvg. This really isn't something you want to rely soley on this website for. While the suggestions and comments here are largely helpful, it's mostly informal (albeit educated) discussions.

At the least you could ask the modelling engineer to go over the results with you, then make a recommendation to someone as to what the next step is. I wouldn't hesitate to ask for assistance.

Having said that, any rise of more than a foot is significant (on various levels) and should be studied further before moving forward.

 

[CivilTyro]

Thanks for the responses. I have mentioned to the engg. that he should apply for CLOMR not LOMR.
This was his reply

"This LOMR application is for the removal of the FEMA Zone A from a portion of the existing site. A Special Flood Hazard Area(Zone A) can be removed from a parcel of land if it can be established that the 100-year depth of flow is less than one foot."

"Thru the use of hydraulic modeling (HEC-RAS), it has been determined that within a large portion of Zone A, covering the existing site, the 100-yr flows are less than one foot in depth and Zone A designation be removed. This will remove the Zone A from these areas prior to the development of the parcel."

What do you guys think?? Is he right?? So LOMR application can also be used fro this purpose?? I thought it for as-built conditions??

 

[RWF7437]

Agreeing with all of the above responses; I'd suggest you contact the FEMA Office for your area (Region) and find out who their "local" study contractor is. Explain that you have doubts about the adequacy of the study and ask for their help and guidance. You may also ask if you can submit the study to their local contractor for review. There will be a cost associated with this but that cost, for even a cursory review, should be borne by the person who submitted the study to you.

 

[CivilTyro]

Thanks RWF7437 and others. I did talk to the local flood control and they said the request should be called LOMA not LOMR. They also said they do not distingiush between LOMR and LOMA anymore, since flood control started looking at them with no difference (This request is to remove ZONE A delineation from the existing site, nothing is built on it yet). Also, MT-2 form on FEMA only has two boxes to check, one for CLOMR and other for LOMR. (Is there a different form for LOMA???)
And LOMR description next to it "A letter from FEMA officially revising the current NFIP map to show the changes to floodplains, regulatory floodway or flood elevations. (See Parts 60&65 of the NFIP Regulations).

So, Is LOMR also for map revisons for the existing non-built site, if you provide hydaulic analysis and show the flood depths are less than one feet in Zone A??

 

[RWF7437]

Seems to me YOU are NOT the person who fills out the form. That should be the Engineer of Record who is proposing the change, should it not ?

As I understand your original question, you are reviewing the work, NOT doing the work. If that is so, let the EOR figure our how to fill out this 52 page (!) form.

Your concern will be to check the EOR's work to see if you agree with it or not.

good luck

 

[indianjoe]

Questions you should be asking...Is there any historical data showing flood elevations in the concerned area? Talk with some of the oldtimers who have lived there a long time. I would like to remind everyone that HEC RAS is not for small watersheds,and at best, all our engineering solutions and number crunching is really not much more that educated guesses. Where peoples lives and property are concerned, you must always be very very careful. Ive been doing engineering for more that 35 years, long before computers, talk to people, it may save you and your companies collective skins.