FLOOD08739

• County: Statewide
• Community: Statewide
• Basin: Statewide
• Title: Determination of Roughness Coefficients for Streams in Colorado
• Date: 1/1/1985
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

The largest discharge measurements for Cottonwood Creek, South Fork Rio
Grande, and Trout Creek were not used to develop the equation for predicting
Manning's n because of the extreme effects of bank vegetation, as shown in
figure 11, The resulting equation developed for predicting Manning's n for
higher gradient natural channels is:

n = 0.39 s 0.38R-O.16
f

(6)

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I.
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and is graphically depicted in figure 12. The average
mate is 28 percent with a range of -24 to +32 percent.
and deviation of computed from observed value is shown

standard error of esti-
The predicted n value
in table 3.

Barnes' (1967) and Limerinos' (1970) data were used to determine if equa-
tion 6 produced reasonable results and to determine its range of applicabil-
ity. For slopes greater than 0.002 and a hydraulic radius less than 7 ft, the
standard deviation of the percentage differences was 23 percent and ranged
from -44 to +50 percent. An application of this equation is given later in
the report in the section entitled, "Procedure for assigning n values,"

The following limitations need to be observed when using the Manning n
prediction equation as an aid in evaluating channel roughness:

1. The equation is applicable to natural main channels having
relatively stable bed and bank material (cobble- and bou1der-
bed material), for average factors affecting roughness
coefficients.

2. The equation is applicable within a range of slopes from 0.002 to
0.04 and for hydraulic radii from 0.5 to 7 ft. The upper limit
on slope is due to a lack of verification data available for the
slopes of high-gradient streams. Caution needs to be used in
applying available hydraulic equations ,to streams with slopes
greater than 0.04 because the applicability of the equations is
questionable. Results of the regression analyses indicated that
for a hydraulic radius greater than 7 ft, n did not vary
significantly with depth; hence, extrapolation to large flow
depths should not be too much in error as long as the bed and
bank material remain'fairly stable,

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3. The energy loss coefficients due to acceleration or deceleration
of velocity in a contracting reach were assumed to 0 and in an
expanding reach were assumed to be 0.5, Analysis of the data
indicates moderate to severe natural channel expansions caused
large energy losses, and the expansion coefficient needs to be
increased in these reaches.

.

4. The equation is not applicable in reaches of stream affected by
backwater from downstream obstructions.

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