Manning roughness coefficients for Grass lined creeks 2

[jboucher50]

I am looking for a source of Manning Roughness coefficients for open channel creeks. I have the Chaudry text book which gives some great examples of coefficients along with photos, unfortunately, the creek I am working on is much smaller that the examples. If it helps I can send a photo of the creek for you to get a visual. Basically it is 1.5m deep, 5m top width and lined on both sides with thick grass. Any help is appreciated, thanks in advance. Jeremiah

 

[Bayou]

Typically for grasslined channels, Manning's values tend to run 0.30 - 0.45. This depends on the channel condition. Lower values for channels that have been improved or cleared. The fact that the grass isn't mowed regularly does not matter in most ditches.
If there are no trees or bushes in the ditch, the standard in my area is to use 0.35.

 

[gibfrog]

Check out this US GOV link (FHWA). The third to the last download in the HYDRAULIC REPORT section (the third section) WSP 2339 deals with the procedures to select a manning factor for a natural channel. Cover is a significant factor, but not the only factor.

http://www.fhwa.dot.gov/bridge/hydpub.htm

In real practice, what your juridsiction will accept is may be more important than what the best judgement and federal guidelines suggest unless you desire to educate your local bureaucrats. (The bureaucracy prefers conservative data that leads to overdesign to make up for existing inadequacies in the stormwater conveyance system - let private development fund the correction of public problems) Of course, if you are performing a study and will calibrate your model to existing data, that is another story.

Finially, Bayou's recommendation is fine.

 

[cvg]

you may also want to investigate the sensativity of your stream to a change in N value. How much does varying the N value affect the resultant flow velocity, depth, sediment transport, scour, flow regime etc. Also, check around and see what is usual and customary in your area.

SCS (NRCS) has a method which is widely used which considers several factors. For example:

1. Basic N value for channel in earth 0.20
2. Modification for vegetation (grass) 0.005
3. Modification for channel irregularity (avg) 0.005
4. Modification for Obstructions (minor) 0.005
5. Modification for Channel Alignment (avg) 0.002

N = N1+N2+N3+N4+N5
N = 0.037

Chow (1959) would indicate something between 0.030 to 0.050
depending on the alignment, bed and channel condition etc. You would need to quantify the following:
rocks, pools, shoals, ineffective flow areas, timber or underbrush, flow stage, channel alignment etc.

Also, if you have a high water mark and if you can correlate this water mark with a known flow in the channel, you could then back calculate the N value to verify your assumption.

Another thing to consider is that the channel has a "movable" bed. In other words, during a flood, the channel cross section may change shape. Weeds and grass may be washed away and sediment transport may deepen the channel or form sand bars. The channel may migrate laterally. Is the N value during the flood event the same as it is on a sunny day? Chuck
cgopperton@stantec.com

http://www.stantec.com/

 

[glennd]

Another thing that may be worth considering is the effect of the water flow on the long grass. The force of the water in the channel can "push over" the grass so it lays flat on the bed and banks of the channel effectively reducing the manning's n value. For a well grassed channel i would still use the figures in the order of 0.03 to 0.035. The lowest you could possible expect for a channel of that size if it has been mowed would be 0.0275. I have seen lower n values on larger channels.

You should do a sensitivity check to see what the different n values mean to your water profile or capacity values to see what effect they may have on back water profiles, headlosses at structures or possible overtopping.

 

[sgwpe1]

"n" varies, depending on the length and stiffness of the grass and depth of the water. And the depth of the water will vary depending on the length of the grass. Sounds like a conundrum, Grasshopper, no? Just kidding.

Anyway, you should design or analyze a grass channel using the SCS methodology which considers vegetal retardance at two limit conditions:
1- When the grass is short, especially if it is mowed. The n will be lower e.g., D or even E vegetal retardance. Often this condition will result in maximum velocity and the channel section shape and slope need to be controlled to maintain velocity below erosion-causing levels.

2- At the other end is maximum vegetal retardance condition, where the grass is longest without bending. This results in the highest n and lowest velocity. The channel section must be large enough to convey the discharge.

The parabolic cross section seems to handle this well. You can design a channel within a channel and you get stable hydraulic radii. Also it controls meander.

SCS n graphs show the variance over conditions in the grass channel. If erosion in the center is a problem, a stone strip down the center could solve that.

Good luck...You could email me at whna@fast.net (my office)for source of these graphs.
Scott

 

[johnhan76]

North American Green has a good software program on their website that can calculate the "n" for you. Go to

http://www.nagreen.com

 

[LHA]

Manning's "n" in grass will swing anywhere from .03 for a full flowing stream to over 0.2 for a small driveway swale. Manufacturer's software will only give a composite n, with one of their linings under it.

To vary n with slope (S) and hyraulic radius (R) iteratively:

Per page 50-60 of FWHA HEC15, the equation: is n = R^(1/6) / [X + 19.97log{R^(1.4) S^(0.4)}, where X varies w/ retardance class.
Retardance X
A 15.8
B 23.0
C 30.2
D 34.6
E 37.7

As always, check with the reviewer and specifications before you invest your client's money. If the reviewer and/or specifications has a predefined "n" in mind, you've just saved a lot of time. In PA, local conservation districts require the above approach, and it is far more conservative than any predefined n.

 

[bltseattle]

The FHWA HEC-15 is for design of channel linings. In that reference are charts to provide Manning's n for grass lined channels. You can download it from this link.

http://www.fhwa.dot.gov/bridge/hec15SI.pdf

 

[bleugene]

I have all these charts and hydrology programs for Manning's n in open channels and I use them alot, but I really want a formula, an equation with a vegetal retardance variable, that I can input myself. I want to lose the vagueness/interpretation of the charts and avoid placing complete trust in the unseen software gremlins.

 

[minedrainageguy]

I have been working on the exact same issue as bleugene (needing to compute manning's 'n' for open channel flow), I want the formula that was used to generate the nomographs that the NRCS provides in tp-61 (last updated in 1966 when they converted to SI units) I used spreadsheets for designing ditches and waste time playing around with nomographs. It seems like the NRCS nomographs have been around so long they are accepted as gospel with no basis for there calculations. I want the equation! Thanks!

 

[LHA]

As I had previously given on 20 Jan 04 6:52, the FHWA formula above has always worked well with my spreadsheets. I've never had it challenged, especially since I cite HEC15.

To vary n with slope (S) and hyraulic radius (R) iteratively:

Per page 50-60 of FWHA HEC15, the equation: is n = R^(1/6) / [X + 19.97log{R^(1.4) S^(0.4)}, where X varies w/ retardance class.
Retardance X
A 15.8
B 23.0
C 30.2
D 34.6
E 37.7


Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[minedrainageguy]

Thank you Iha!!! The HEC 15 reference is exactly what I am looking for!!! YOU HAVE MADE MY WEEK!