EFFECTS OF CROSS-SECTIONAL GEOMETRY OF PRISMATIC REACHES ON THE MANNING COEFFICIENTS

The resistance coefficient is needed in designing a channel and in determining the characteristics of both man-made and natural channels. Many factors affect the value of the coefficient; some of them have been reported. The present research gives emphasis on the effects of cross-sectional geometry on the resistance coefficient in the uniform-flow formula of Manning by modelling physically the uniform flow in the laboratory. Nine model-prismatic reaches are used in the present investigation. They consist of two kinds of cross-sectional geometry, i.e. rectangular shape (three reaches) and semi-circular shape (six reaches). Three kinds of homogeneous surface are considered, they are PVC, gutter carpet and sandy carpet. The longitudinal slope of each reach can be adjusted. The slope has 10 variations from 0.0025 to 0.025 m/m with the step of 0.0025 m/m. The flow rate is determined by measuring the water volume and the time taken. This is done five times to get the discharge average. The flow depths are measured at five points to get the flow depth average. Such measurements are performed for each slope variation of the considered reach. The experimental results show that the reaches with the same cross-sectional geometry even though they are different in size give ‘nearly' the same resistance coefficient. However, the reaches with the different cross-section give the different coefficient even though they have the same surface. The reaches with sharp-curvature geometry (e.g. rectangular) give higher value of the coefficient than those with smooth one (e.g. semi-circular). In the future, the selection of the cross-sectional geometry should be considered in designing a channel.