Laboratory Investigation on the Effect of Height, Density and Arrangement of Artificial Roughness Elements on Manning Roughness Coefficient

Document Type : Research Paper

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Abstract

Determination of flow resistance is an essential parameter for studying and hydraulic analysis
of the open channels. Manning roughness coefficient is often be used for description flow resistance
or relative roughness of a channel or floodplain. This research was focused on the effect of the
height, density and arrangement of the artificial cubic roughness elements on Manning roughness
coefficient. In total 48 experiments were conducted with three values of roughness height
(5,10 and 15 mm), two percentages of roughness density (25% and 50%), two types of roughness
arrangement (regular and staggered patterns), four flow discharge values on rough bed and four
experiments with smooth bed in a laboratory flume. A length of 3.7 meter of the flum bed was
designed as the rough bed according to the experiments plan. The results showed that the influence
of the roughness arrangement on changing the value of Manning roughness coefficient (n) is more
than the influence of the height and density of roughness. Also the mean value of Manning
coefficient increased from 0.009 in smooth bed to 0.0132 in regular pattern and 0.0175 in staggered
pattern. At last in order to compare with the results of previous works, some relationships have been
offered in order to estimate the Darcy-weisbach coefficient (f) on the basis of the roughness density
percentage at the bottom (C), relative submergence ( y ks ), Froude number of flow ( Fr ) and a factor
related to roughness arrangement(€).

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References

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Water and Soil Science
Volume 26, 2-2
September 2016
Pages 67-80

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