FEMA Floodway Study, Shadow Effect

[kausland]

Do you know of any special FEMA exclusions for building a structure (e.g. garage) in a flooday, if it is in the 'shadow' of an existing structure? I have heard that there may be such exclusions, which would allow me to perform a simple 'no-rise' study in lieu of a backwater analysis.

Thanks.
Kelsy

 

[RWF7437]

I don't know of any exclusions in the rules but you could go ahead an do the "no rise" analysis and see if it demonstrates, that is, see if it convinces you, that the "shadow effect" exists. If you you're not convinced yourself, you probably can't convince FEMA.

 

[kausland]

I thought I'd post the answer I found to my own question for any of you who may be interested...

I spoke with a hydrologist from FEMA Region X. If a structure is to be built within the "shadow" of an existing building, no study would be required because the shadow is considered ineffective conveyance. The shadow is defined by a 4:1 ratio - for every 4 ft downstream, the shadow is 1 ft less in width (on each side).

This was the opinion of our FEMA hydrologist - the position of other regions may vary.

 

[cimmeron]

There is a "shadow" of a building because any building in the flood plain acts as a barrier, much as a berm would, it reduces the velocity and the ability of the channel to carry water. For the flow consideration, the building would act as a "shadow" in that the channel flow would already be slowed. However, there is also the question of flood plain volume. Generaly you cannot reduce the existing volume of any flood plain storage area. A building would reduce the available volume and would affect the flood plain. The loss could be offset by removing dirt from the flood plain equal to the volume displaced by the building.

You must check with the local zoning offices as they probaly have certain rules on this matter. Normally, they require a backwater analysis by an engineer.

 

[RWF7437]

Many doubts about this arise !

First, Cimmeron is not correct in saying that you cannot reduce the storage volume in a floodplain. It is only in a floodway that you must demonstrate no rise, under FEMA's rules. You may fill or build elevated structures in the floodplain fringe, outside the established floodway.

Second, I'd be very wary of using some 4:1 shadow effect number given to me by a hydrologist at FEMA. If such a number is correct, which I doubt, it should be documented in FEMA's regulations or in their guidance documents. If it is a real rule it should be as true in Region I as it is in Region X.

Third, the last person I would ask such a question of would be the local planner or zone admistrator. While these folks are nice people and often helpful they are rarely trained in hydrology or hydraulics and do not ususally have anyone on staff who is.

Just my experience after 40 years in the flood plain "business".

Be carful Kausland.

 

[kausland]

I appreciate the feedback. Our FEMA Region X hydrologist did some searching and found some documentation regarding the 'shadow effect'. It's on it's way to me via snail mail, so once I receive it, I'll be sure to post my findings.

 

[RWF7437]

Hello Kausland,

You might consider also posting your question at the HEC-RAS support forum.

"Fellow Modelers:

"HEC-RAS 3.1.1 was recently released by Hydrologic Engineering Center. Version 3.1.1 is primarily an update of Version 3.1 to fix reported problems. All known bugs (at the time of this posting) are fixed with this new version.

"You can download HEC-RAS at:

http://www.bossintl.com/products/do.../HEC-RAS.html#0

"If you are a licensed user of HEC-RAS 3.1 from BOSS International, this is a free upgrade for you."

 

[kausland]

I have FEMA 'guidance document' on the hydraulic shadow effect. If you post your email address, I will pass it on.

 

[RWF7437]

Thank you Kausland. I'd be very interested in seeing the FEMA guidance on this. You can email it, or a link to it if it is available on line.

Russ Faust
Civil Engineer
Salem, OR
rwf7437@attbi.com

Have you had difficultuy with FEMA's new DFIRM viewer fmit.exe ? I sure have.

 

[RWF7437]

Thanks Kausland for the FEMA guidance. I've read it with interest and, as far as it goes, it is helpful. It does not, however, answer the question of how to model the reach at the obstruction and immediately upstream of it. For example, I would think some ineffective flow area adjacent to the obstruction would be advisable. Also there is an abrubt transition upstream from the obstruction as the flow must change direction quickly.

I found some useful clues on this in Hydraulics of Open Channel Flow by Sergio Montes, ASCE Press, page 107 in the paperback version. Take a look at it if you have a copy.

 

[waterguy34]

Kausland,

I would appreciate a copy of the FEMA guidance document for the 'shadow effect'.

Could you send me a copy at
a.klemp@gremmerassociates.com

Thanks