Manning roughness coefficients for Grass lined creeks
Manning roughness coefficients for Grass lined creeks
(OP)
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





RE: Manning roughness coefficients for Grass lined creeks
If there are no trees or bushes in the ditch, the standard in my area is to use 0.35.
RE: Manning roughness coefficients for Grass lined creeks
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.
RE: Manning roughness coefficients for Grass lined creeks
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/
RE: Manning roughness coefficients for Grass lined creeks
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.
RE: Manning roughness coefficients for Grass lined creeks
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
RE: Manning roughness coefficients for Grass lined creeks
RE: Manning roughness coefficients for Grass lined creeks
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.
RE: Manning roughness coefficients for Grass lined creeks
http://www.fhwa.dot.gov/bridge/hec15SI.pdf
RE: Manning roughness coefficients for Grass lined creeks
RE: Manning roughness coefficients for Grass lined creeks
RE: Manning roughness coefficients for Grass lined creeks
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
RE: Manning roughness coefficients for Grass lined creeks