Floodway/Floodplain simple question 3

[nutbutter]

sup people,
i have a very simple question that i should probably know the answer to. i recently switched to land development after 3 years in environmental engineering (primarily remediation). anyway, i'm wondering what the difference between a 100-year floodplain and a floodway are. can anyone explain that to me?
i have a line in a drawing that we received and i'm not certain exactly what it is. my boss said that there is huge difference between a floodway and a floodplain and then he failed to explain. he's a pretty poor boss, inasmuch as i can't learn anything from him because he's socially inept and always in a horrible mood. i put off going to talk to him because he's always in such an unpleasant mood. he seems like a prime candidate for going postal. he fits the profile to a tee. if we were in a post office i'd probably come to work packin'.

anyway, if anyone could enlighten me on the difference between floodplains and floodways i would be very grateful. thanks,

stoddard

 

[sam74]

As it has been explained to me: The floodplain which is a boundary on either side of a channel will show you the approximate location of a 100 yr storm event based upon existing topography. The floodway is a boundary on either side of a channel which will show you the boundary of a 100 yr flood where the existing topography within the floodplain has been filled enough to cause one foot in rise of water surface elevation in the channel as compared to the floodplain elevation.

 

[ccor]

sam74 has provided a good pragmatic definition. From a regulatory standpoint no fill may be discharged into the floodway, you may however, be able to redefine the floodway. In the floodway fringe (the delineated floodplain outside the floodway) fill may be able to be placed, consult your floodplain administrator. Also, note that if the floodway has already been defined, they may not have used the entire 1.0 rise allowed.

 

[nutbutter]

Hmmmmmmm.........sam74, that definition is a bit difficult to understand. I'm not sure exactly what you mean. Can anyone explain it better?

I'm not really concerned about placing fill in the floodplain or doing any earthwork at all. I'm just curious as to what the difference is between the floodway and the floodplain. I know what the floodplain is.

My understanding from your definition is that if I were to fill the floodway fringes (floodplain) to an extent that will raise the 100-year WSE 1' then that new floodplain boundary is the floodway? It doesn't really make sense to me. Someone put in the simplest terms possible please.

peace,
stoddard

 

[DCunningham]

Actually, I can see why you're confused. sam74's definitions are solid, but they're backwards. The floodway represents the channel limits during a 100-year storm. The floodplain is the extent of the flood if you were to raise the surface of that floodway one foot. You can think of it as if the floodplain contains a bit of a safety factor, where the floodway does not.

ccor is right about the fill. Floodway = No. Floodplain = Maybe.

Also, although I can't say for your area, in Pennsylvania most areas don't have delineated floodways, so the boundaries are taken as 50' from the normal channel limits.

 

[ccor]

Think of the floodway as the limits of encroachment on the floodplain. It is the area that must be kept free of fill and obstructions to convey the base flood (100-year event) without raising flooding elevations by more than 1-foot. It is the primary conveyance area.

 

[sam74]

After a double check I'll stand by my original definitions. Here is a link to a document that I obtained previously from a post on another forum.

http://www.pdhonline.org/courses/h110/h110content.pdf

See page 3 for a pictorial explanation of your question. This is a pretty good reference and should probably answer any of your basic knowledge questions on the subject.

I believe this link came from a post on the storm/flood engineering forum also on this website. This would have probably been a more appropriate location to post your question although I realize manuvering to all forums in the site is a bit difficult when you have just began exploring it.

 

[nutbutter]

Thanks DCunningham! That was a perfect explanation and it made the definition very clear. I understand what you other guys were getting at now, but it was a little unclear at first. Makes sense. I know about the requirements involved with filling the floodplain and all that.
Peace,
Stoddard

 

[nutbutter]

Great link, Sam!

Peace,
Stodard

 

[TerryScan]

DCunningham

Your explanation seems entirely backwards. Sam's is correct and is explained well in his linked document.

A floodway is NOT the channel limits during a 100-yr storm. It is a ficticious limits of channel that could convey the 100 year flood, at an elevation 1 foot higher, if one filled the rest of the floodplain.

There are varying local regulations on the possibility of filling in a floodway. Some allow fill in the floodway as long as one shows: no rise...no more than 0.1' rise...etc.

 

[nutbutter]

The key to understanding the difference is the notion of "ficticious". The floodway is a fictitious line that would be delineated by the stream channel only if you filled the entire floodplain (fringes) to an extent that raised the 100-year WSE 1 foot. The floodplain is an actual line that could be measured/observed.

It's a pretty difficult concept to explain to someone. The link above that sam74 supplied is the best at explaining the difference.

Peace,
Stoddardvilla

 

[DCunningham]

Now I'm the ass. I asked this very question at work a few months back and got the answer that I posted above. Thank you all for setting me straight.