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MANNING ROUGHNESS COEFFICIENT FOR GRASS-LINED CHANNEL

MANNING ROUGHNESS COEFFICIENT FOR GRASS-LINED CHANNEL,Manal M. Abood,Badronnisa Yusuf,Thamer Ahmed Mohammed,Abdul Halim Ghazali

MANNING ROUGHNESS COEFFICIENT FOR GRASS-LINED CHANNEL  
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A laboratory study has been conducted to analyze the effects of different types of vegetation on the Manning roughness coefficient, n in an open channel, and to develop relationships between the characteristics of the vegetation (density, degree of submergence and distribution) and Manning's, n. Two type of vegetation were used in this study, namely Napier grass and Cattail grass. The impact of each type of vegetation on the resistance of flow in open channel (Laboratory flume) was examined. The laboratory flume is rectangular in cross section and has dimensions of 12 m length, 0.3 m width and 0.3 m height. An area-velocity flow meter was used to measure the mean velocity, and Manning's equation was adopted to determine the value of overall roughness, n. The results show that Manning's, n, for flows with Napier grass increased with the increase in flow depth for both submerged and unsubmerged conditions. The increment in Manning's, n was found to be 282% when the degree of submergence (Y/H) for high grass density increased from 0.5 to 0.875. In the presence of Cattail grass and for high density, the effects were reversed and the decrease in Manning's, n was found to be 41% for same increase in degree of submergence. This is attributed to the physical characteristics of the Cattail grass, which has no branching stems and leaves. For low density of Napier grass, the values of Manning's, n decreased with the incerase of Reynolds number, Re, for both submerged and unsubmerged vegetation. Howerver, for a higher density, this phenomenon only occurred for submerged vegetation. In the case fo unsubmerged vegetation the values of Manning's, n increased with the increase of Re. But for Cattail grass, the values of Manning's, n decreased with increase of Re for all grass densities and for submerged and unsubmerged flow conditions. A linear relationships were found between Manning's, n and grass density for both submerged and unsubmerged flow conditions and the coefficient of determination for the relationships ranges form 0.93 to 0.96. When the density increased form 20 veg/m 2 to 40 veg/m 2 the value of Manning's, n increased by 35% for Napier grass and 25% for
Published in 2006.
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