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

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 ( s yk ), Froude number of flow ( r F ) and a factor related to roughness arrangement(€). Keyword: Darcy-weisbach coefficient, Open channel, Relative submergence