Development in floodplain w/ no floodway 2

[MattMurawski]

Looking for experience/advice on hydraulic evaluation of a proposed house in a floodplain with no floodway.

A client that wants to build a house in a floodplain where no floodway has been developed. The local Zoning Administrator references the NFIP regs and says that, in the absence of an established floodway, the developer must show that the house and "all existing and anticipated development" will not raise the baseflood by more than a foot. Showing that the house will not raise the flood level by a foot would be simple with a HEC-RAS model. The "anticipated development" is what is problematic.

A standard encroachment analysis would work, but it may well put the client's proposed house in the floodway. Do any of you have experience with or thoughts on addressing the "anticipated development" part of the regulation other than via an encroachment analysis? Thanks.

 

[RWF7437]

Sounds like you need to establish the floodway. As I recall, there are at least 5 ways to do this, all of which require an encroachment analysis. If, as you say, it is likely the house would end up being in the floodway then your clients should probably seek another site for their house. Without knowing more, it isn't possible to answer your questions regarding anticipated development. That development could take many forms in many locations.

good luck

 

[Rye1]

Hey Matt,
I would suggest reading the city's/town's/county's floodplain regulations. It is often the case that the floodplain administrator for smaller local governments wear several hats and rarely have a grasp of the complexities of the NFIP rules. If you find this to be the case, then you have to be very, very tactful in guiding the person to the light. But, if you get some friction, remember you still have to play by the local community’s “interpretations” of their ordinance – or be prepared to go to court.

From your original post, it sounds like you are trying to develop in an “unnumbered A” zone. This area has not been studied by detail H&H modeling. FEMA has a publication for developing in these areas. Generally, in an unnumbered A zone, you have to establish a FFE and build to that elevation. FEMA has a program (it’s crap-don’t use) called Quick-2 they try to use. I would probably set up a simple HEC-RAS model and run to establish the BFE and then add 2’ to get the FFE.

I don’t understand why you would need to do an encroachment analysis or flood study. In my area we only have to do encroachment analysis or flood studies if we encroach into the FEMA floodway – in an unnumbered A zone there isn’t a floodway.

 

[MattMurawski]

Thanks for the input. This is an unusual case in that there is a published BFE (and resulting floodplain limits) based on high water marks, but no detailed H&H. The local floodplain administrator is more knowledgeable than most, but is deferring to the State (who, by State law, must sign off before a local NFIP administrator can issue a permit). The State person is taking the position (backed up by FEMA) that the absence of a delineated floodway does NOT mean a floodway doesn't exist. On the contrary, their position is that the entire floodplain should be treated as floodway until an applicant provides information to rebut that presumption.

So, I can set up a HEC-RAS model and do a encroachment analysis (which is the conventional way of accounting for anticipated development) to delineate the floodway. But I don't think my client will like the answer. Thus I'm looking for the experience of others in alternate approaches (if they exist) to addressing anticipated future development.

 

[Rye1]

Yep, that helps out with the picture. You have an unnumbered A zone and all of the regulators are going to take the easy way out and require you to do a detailed flood study. Knowing what is required is often half of the battle with regulators.

I do belive you are or the floodplain administrator is putting to much meaning on "anticipated future development". In my office (I'm a gov't regulator), we require at least three models for every flood study: Duplicate, Corrected Effective, and Proposed. The proposed model contains your proposed project - the project you will build if your permit is granted.

FEMA and the NFIP rules have problems with future anticipated development. When a new detailed flood study is created for the purpose of creating new FIRM panels, they look at what is there then. Manning's n values are based on what is present at the time not 10-15 years down the road. Maybe not smart but more defendable in court.

So, after said all that, I think you are going to have to develope HEC-RAS models for each condition and also delineate the floodway with an encroachment analysis.

Quite a bit of work and will definetly at some cost to the project.

 

[RWF7437]

OR,

There are many nice houseboats on the market.

good luck

 

[queque]

Rye1.
We have a floodway that we want to modify through channelization. The HEC-RAS analyses indicates that is possible. Are you referring to a 1 ft or 2 ft encroachment analyses?
I thought the requirement was 1 ft.
Duplicate is a FEMA requirement right?

What is Corrected Effective?
A revision based on elevations, new flow data , both?

Proposed -- modified with encroachment?

 

[Rye1]

queque,
If you have an existing floodway, and you want to change the floodway with a proposed channel, you will probably be required to go through the CLOMR/LOMR process.

The minimum FEMA/NFIP required floodway encroachment surcharge is 1'. Your community may adopt more stringent stantards such as a 0.5', 0.1', or even a 0.0' surcharge.

Our process for floodstudies basically follows the FEMA process with a few minor differences:
We require at least three models:

Duplicate - this is the existing model. The basic idea for this model is to see what you started with.

Corrected effective - the existing model updated to current conditions.

Proposed - the corrected effective model with your proposed conditions.

If you go through the CLOMR/LOMR process you may have to add additional models such as a Duplicate effective - which would be like an existing conditions HEC-2 model that was updated to a HEC-RAS model with any existing conditions updates.

 

[queque]

Rye1
What are the typical fees for a LOMR with these 4 possible computer runs/interpretations/analyses.
A estimated range of fees is adequate.

 

[RWF7437]

QueQue,

You should really start a new thread.

 

[Rye1]

queque,
The permit fee for a CLOMR is about $4,000 and another $4,000 for a LOMR. Depending upon the type it is a little more or a little less.

These are only the fees paid to FEMA...

 

[GRANITE80]

Matt -

My question, is it at all possible to build outside the floodway? I'd be cautious about avoiding a HEC-RAS encroachment study because your client won't like the results. I am by no means a lefty environmentalist, but there are good reasons why development is not allowed in floodways unless "there is no other alternative".

Just using your judgment based on the 100-year BFE marks and site topography, it sounds like you have a good idea that the homesite will be in the actual "floodway" and will be in the conveyance of flood waters.

Many homeowners are ignorant to the hazards of living along a waterway and I feel it's in part our job to educate them on the process. I like to mark the BFE on a tree to show clients before they build, so they get an understanding of what will be underwater in a 100-year event. I've also declined projects because I've been ethically opposed to the development too near the river or in a riperian zone.

Just my thoughts on the matter. Good luck!

 

[MattMurawski]

GRANITE80--
All very good points. I too am in no rush to find a way to get a house permitted in an unsafe place, and I like your idea of physically marking the BFE.

Were this client interested in putting there house at the outer limits of the floodplain, the ethical/safety issues are less distracting, but regs still require showing that "all anticipated development" will not raise the BFE by more than a foot.

A previous poster indicated that in his experience, just the proposed development itself was typically considered (as opposed to the more encompassing "all anticipated development"). The local and State regulators in this case aren't satisfied with the narrower view. So, an encroachment analysis appears the be the most defensible approach (should the project move forward). Still strikes me as an awfully large burden to place on a single landowner.

Thanks for all the feedback.