FEMA Floodway Study, Shadow Effect
FEMA Floodway Study, Shadow Effect
(OP)
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
Thanks.
Kelsy





RE: FEMA Floodway Study, Shadow Effect
RE: FEMA Floodway Study, Shadow Effect
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.
RE: FEMA Floodway Study, Shadow Effect
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.
RE: FEMA Floodway Study, Shadow Effect
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.
RE: FEMA Floodway Study, Shadow Effect
RE: FEMA Floodway Study, Shadow Effect
You might consider also posting your question at the HEC-RAS support forum.
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"If you are a licensed user of HEC-RAS 3.1 from BOSS International, this is a free upgrade for you."
RE: FEMA Floodway Study, Shadow Effect
RE: FEMA Floodway Study, Shadow Effect
Russ Faust
Civil Engineer
Salem, OR
rwf7437@attbi.com
Have you had difficultuy with FEMA's new DFIRM viewer fmit.exe ? I sure have.
RE: FEMA Floodway Study, Shadow Effect
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.
RE: FEMA Floodway Study, Shadow Effect
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