MANNING ROUGHNESS COEFFICIENT FOR GRASS-LINED CHANNEL

Abstract

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 types of vegetation were used in this study, namely Napier grass and Cattail grass, and the impact of each type on flow resistance in an open channel (laboratory flume) was examined. The laboratory flume is rectangular in cross section, with 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 increasing flow depth for both submerged and unsubmerged conditions, with an increment of 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 effect was reversed, with a decrease in Manning's n of 41% for the same increase in degree of submergence; this is attributed to the physical characteristics of Cattail grass, which has no branching stems and leaves. For low density of Napier grass, Manning's n decreased with increasing Reynolds number, Re, for both submerged and unsubmerged vegetation; however, for higher density, this phenomenon occurred only for submerged vegetation, while for unsubmerged vegetation Manning's n increased with increasing Re. For Cattail grass, Manning's n decreased with increasing Re for all densities and both flow conditions. A linear relationship was found between Manning's n and grass density for both submerged and unsubmerged flow conditions, with coefficients of determination ranging from 0.93 to 0.96. When density increased from 20 veg/m² to 40 veg/m², Manning's n increased by 35% for Napier grass and 25% for Cattail grass. The degree of increment in Manning's n with grass density was found to depend on flow depth, vegetation type, and vegetation arrangement.