Help determining "n" values of stream channels 3

[Scott367]

I got into a heated debate with a municipal engineer about the "n" value of a stream channel adjacent to a site I was designing. He was being very conservative and it was painting an ugly picture hydraulically. Determining "n" values seems very subjective and I couln't find anything to rebut his position. I found some examples at the USGS website but they were drainage areas of 50 square miles plus. The drainage area of this channel was only 750 acres and the USGS examples didn't help much. Anyone know of any source to help in determining the "n" value?

 

[francesca]

In a large storm event, most of the flow will be on the overbanks, not in the channel. HEC-RAS varies the n-value across the cross-section, making a composite n-value for each x-section that is frequently higher than the channel n-value, particularly with wooded overbanks.

 

[psmart]

There are a number of good references for Manning's values including:
* Open Channel Hydraulics by Ven Te Chow, 1959
* Handbook of Hydraulics by Brater, King, Lindell, & Wei, 1996
* HEC-RAS Documentation

As francesca said, the value will probably vary with depth, so you will have to use one of several techniques to determine a composite value. (Horton, Pavlovskii, Lotter, etc.) But some routing procedures subdivide the flow across the channel, rather than attempting to determine a single representative Manning's value.

 

[blueoak]

USGS Paper 2339 is a fabulous work. A pretty good method though is to vary your roughness to see how it impacts your site. If you vary it within reasonable values and you have problems I wouldn't be so quick to ignore them.

What values are you looking at?

 

[RWF7437]

You can find illustrated examples of "n" values at:

http://wwwrcamnl.wr.usgs.gov

Do try different values as suggested by Blueoak to see how the n value affects the calculated depth of flow.

good luck

 

[Scott367]

My manual from Intelesolve Hydrograph says Earth channel straight with some grass has an "n" value of .026. That is what I was using. He required me to use .035. Even with his artifically high "n" the flows were contained within the channel at the 100 yr storm. The problem it was causing me was the high water elevation was controling where the bottom of my detention was. I believe the numbers he was forcing me to use were creating false high water numbers. I didn't know how to rebut his position on the "n" value.

Keep in mind this all came about because he wanted the basin above the 100 yr flood plain. The FEMA map said the project was well upstream of the 100 yr flood plain and I thought that was enough in itself.

 

[francesca]

I was always told that a good rule-of-thumb is that "bank high" is equivalent to a 1.5 year storm. Obviously that's something that in practise will change from stream to stream and from location on a stream to location on a stream, but I would be highly sceptical of a 750-ac basin stream not bursting its banks in a 100-year storm.

In a 100-year storm event you're likely to get tree limbs down and other debris in the channel. I would be inclined to use the higher number for the sake of being conservative.

As regards FEMA maps, in the area I worked, the FEMA maps were developed in 1974. The area had undergone significant development since then and they were relatively worthless; good for a ballpark estimate, but not so good for setting finished floor elevations. If your Manning's n-value is being problematic with regards to the bottom of your detention basin, then the guy was right, you're not far enough upstream of the 100-year floodplain. If you were, it wouldn't matter what n-value you picked, your detention basin would be unaffected.

 

[sam74]

Here is a site that has measured mannings n values that go with pictures of the channel.

http://manningsn.sdsu.edu/

 

[blueoak]

Francesca, Nicely said.

Scott367,
When I did small channel design we typically used 2 n values (and of course looked at the sensitivity of our model). First was a cleaned channel which typically was about 0.02 to 0.025 and then a roughened channel of anywhere from 0.035 to 0.05 depending on how well established we thought it would be to how much maintenance we expected. I am most comfortable with 0.025 really only for soon after initial construction if it will be used to define a high water line.

The only reason we even looked at the smaller n value was to make sure that the channel was stable until we could get vegetation established.

You have to look at the fact that a clean smooth straight channel with no vegetation starts off with a base n of about 0.015 to 0.016 just for soil to water interface(USDA-ARS Agricultural Handbook Number 667). For storm based flows rather than a continual feed of a canal it would be more consistent in a general sense to see 0.035 used, but of course this is site specific. (0.035 doesn't seem high to me)

As you consider your n value you may wish to consider maintenance. Is it only once a year? Weeds only take a few weeks to establish and effect flow parameters. If it will be maintained and mowed regularly a lower n value may be more appropriate.

 

[queque]

A simple approach in engaging a reviewer is to conduct a sensitivity analysis. Use different n values and plot them against Q.
Many times, the channel, if incised, will contain the flows with minor changes in elevation (Supercritical flow).
Overflow is a different problem, with n vlues increasing as a function of slope. The flatter the slope, the greater the n value. The change in results will still be minimal.