Need some regulatory guidance

[djstrick110]

Hello, all. One of my clients, a local municipality, is in the process of developing a new floodplain management ordinance. They have requested that we provide them with some recommendations as to best practices within Zone A riverine floodplains. They want to include in the ordinance a rule pertaining to floodway preservation and approximation within Zone A. I have several years of experience in floodplain management and modeling myself, but I have never run into a regulation or ordinance with guidelines for approximating the floodway in a Zone A. In that situation, I'd normally recommend that a HEC-RAS model and LOMR be performed.

I've heard general rules of thumb such as "5x the top width of the stream," etc., but this seems awfully general given the asymetrical nature of a floodway and the fact that its size doesn't necessarily relate to the size of the channel.

Can anyone point me towards any REGULATORY (preferably Federal, but state or local examples will be appreciated as well) guidance that speaks to floodway preservation within a Zone A floodplain?

Thanks in advance!

 

[gbam]

It has been a while since I have dealt with FEMA but; without looking this up, I thought by the definition of Zone A there is no "defined" Floodway. A Floodway is the extent to which the water will rise 1-foot by "squeezing" the floodplain. Is this not a detailed anaysis approach (AE) not A?

 

[froude]

I believe gbam is correct; there is no regulatory floodway associated with a Zone A because Zone A is considered to be unstudied. To determine the floodway extents, one would have to basically do a detailed study. If this were LOMR'd, it would in all likelihood become a Zone AE or similar.

Your client could be referring to "floodway" as a general term for "channel," but their reference to Zone A makes me think that they probably were thinking on a regulatory basis.

Is it possible that your client is interested in floodplain approximations in Zone A? If so, I have heard rumors, but never verified, that some people used to map Zone A floodplains based on Water Surface Elevations determined using Manning's Equation/basic Normal Depth calculations. I guess this could be a rough estimation (very rough), but again, I never fully bought or confirmed this.

If it's okay, I would like to add to djstrick's question by asking how the Zone A floodplains shown on FIRM maps were/are delineated.

 

[djstrick110]

Thanks to you both! I understand that there is no floodway associated with Zone A, and yes, if a LOMR were submitted for a Zone A stream it would become Zone AE with established BFEs, floodway, etc.

Froude, what you mentioned about my client being interested in floodplain approximations is exactly what I'm after. They want to include an item in a new ordinance for preserving a "floodway" within Zone A should any development occur in such a floodplain. I'm just curious if anyone out there has seen any examples along these lines. For instance, another municipality in the state requires that 5x the top width of the stream be preserved on either side of the stream. I have no idea how they arrived at the "5x the top width" stipulation, but again, I was just curious if anyone new of any regulatory guidance on this.

 

[froude]

I think I follow you now. Sorry for above, certainly wasn't trying to restate the obvious. Perhaps this document could help? Other than this, I have no clue! Good luck, in any case!

It's a FEMA document entitled "Managing Floodplain Development in Approximate Zone A Areas"

http://www.fema.gov/library/viewRecord.do?id=2215

 

[gbam]

froude I have heard the same rumor of normal depth and that the discharge values were also approximated using either regression or envelope curves. Either way the Qs were rougher than a detailed study.

djstrick - could your client be confusing floodplain with floodway? I get that, on occaision, from some of the reviewers on our projects. I typically inform them of the difference.

 

[cfj104]

Not sure if this helps you or not, but I read an interesting article in Stormwater magazine a month or so ago that addressed Riparian Buffers and their effectiveness for widths. It cited some actual studies and results of width on environmental impact reductions.
Not specifically related to floodplains, but maybe that is what your township is looking to accomplish?