LOMA
LOMA
(OP)
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.
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.





RE: LOMA
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).
RE: LOMA
RE: LOMA
good luck
RE: LOMA
RE: LOMA
htt
RE: LOMA
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
RE: LOMA
Corection: "First, it is probably NOT within your authority to approve this study, ....
RE: LOMA
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
RE: LOMA
If you are the city's consultant, they will rely upon your recommendation.
RE: LOMA
RE: LOMA
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.
RE: LOMA
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??
RE: LOMA
RE: 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??
RE: LOMA
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
RE: LOMA