Manning's n values 1

[212mm]

Hello all - looking for reaction to the use of a Manning's 'n' value of 0.001 (yes, the decimal is in the right spot) to characterize the roughness of the surface of floodplain pools to be constructed in conjunction with a stream corridor restoration project. The weighted value of the pools with adjacent extensive riparian plantings (0.08 proposed) is 0.04 (or less) for the whole cross-section.

Beyond me how this value is physically possible in an engineered section, let alone a naturalized watercourse. Interested in other practitioners' reactions.

Thanks.

 

[cvg]

i think you already know the answer

 

[beej67]

Glass and polished brass tubing are .010, ten times the number you've quoted.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[TerryScan]

Is it even appropriate to apply a surface roughness value for hydraulics to the surface of a pool? A pool is not a fixed surface that supplemental fluid will flow accross. The fluids will interact. I would think that pools would be incorporated into the overall n-value for the floodplain.

You may find the USGS paper: "Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains" helpful:

http://www.fhwa.dot.gov/bridge/wsp2339.pdf

In addition to methodology, this document contains photographs of various flood plains with associated known n-values. One may be able to find an example that is similar to your application.

 

[Drew08]

Unless you have an impenetrable boundary, which I doubt, at the surface of the flood pools, there will be significant 3 dimensional flow paths within the flood pools that increase energy loss significantly.

 

[77JQX]

What question are you trying to answer? Why would you apply a n-value to the surface of a pool?

 

[froude]

If you were to use something that low for pools, then would you use something that low for perennial streams that always have water in them? Or ephemeral streams that have isolated reaches of ponding?

 

[beej67]

There's this pesky thing called the "no slip condition." Water doesn't flow on top of a pool. Water flows in the pool. A pool is a big, wide, slow, section that water flows through. You don't model a pool as having no volume and no friction, you model a pool as having a depth, and friction at the bottom of that depth.

I'm guessing here that you're trying to model numerous small pools that you're putting into a flood plain that would ordinarily be stagnant, but aren't during a flood. (?) Presuming that's the case, you should cut more sections, show the depth in the pool, and presume a roughness for the bottom of the pool. The fact that water moves easier over the pool will be modeled by the increased conveyance of that section, since the pool's area is counted in the cross section.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[212mm]

Thanks, everyone, for your responses. Just to clarify, I am not proposing the use of 0.001 for a pool surface - it's something I've seen in a report that I found unbelievable and your responses confirm my reaction to this number. I agree there can be no separation between the pool water and the flood flow - there will be mixing, recirculation, etc. Perhaps the only case where a lower value (but not 0.001) might be used is if the pool surface were frozen - but that is not the case that sets the regulatory flood line.

 

[jgailla]

Is it possible that the 0.001 value is used to fill in a value on a model which should not necessarily have an n-value? For a pond or series of ponds, I would generally assume that Q in equals Q out instantaneously. I could see being forced into using a model approach in which the computer demanded a value which was not really applicable as in this case where n=0.001 is used to model instantaneous discharge. Of course, as noted above, there is no real physical meaning to n=0.001.

 

[beej67]

I would use .010 for ice.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com