2.) Use the Cowan method to estimate the Manning n for the channe1 reach describ
ID: 1885207 • Letter: 2
Question
2.) Use the Cowan method to estimate the Manning n for the channe1 reach described below. Reach description: Straight, approximately 660 feet long. Cross section has very little variation in shape: variation in size is moderate, but changes are gradual. Side slopes are fairly regular, but the channel bottom is uneven and irregular. Soils in the channel consist of a yellowish-gray clay along the bottom and light-gray silty clay loam along the banks. Banks are covered with a heavy growth of poplar trees 2 to 3 inches in diameter large willows, and climbing vines during the summer growing season. There is a thick growth of water weed on the channel bottom. At bankfull stage, average depth and top width are about 8.5 and 40 feet, respectively.Explanation / Answer
Cowans method uses cowans table for determining the coefficient based on some parameters:
(i) Basic n:
Usually, the canals are built on earth, basic n can be taken as 0.02
n1=0.02
(ii) For moderate variations, it may be a poor or dredged channel, the n value is taken as 0.010
n2=0.010
(iii) For the channels having shape changes causing occasional shifting of flow from side to side
n3=0.005
(iv) The roughness coefficient for channel obstructions can be neglected here, since it is not specified
n4=0
(v) The roughness coefficient for vegetation for weeds having 2 to 3 in diameter and climbing vines will be having a moderate effect on the roughness coefficient
n5=0.015 (Value can be taken in the range of 0.010-0.020)
(Vi) The meander length is not given, so the effect can be considered as appreciable and the calculations can be made
n=0.15ns
ns=n1+n2+n3+n4+n5
ns=0.02+0.010+0.005+0+0.015
ns=0.185
n=0.15(0.185)
n=0.02775
If the effect is severe, then mannings coefficient may be given by
n=0.30(0.185)
n=0.0555
The mannings value purely is dependent on the type of material used
Note: All the values are taken from cowans tables.1956
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