Increase of Manning equation accuracy in partially filled cicular cross sections by new definition of hydraulic radius

Accurate estimation of mean velocity and discharge of open channels is one of important goals in hydraulic knowledge. Application of Manning equation is one of the methods with abundant use for mean velocity calculation. Application of this equation in several cross sections such as circular section is associated with error. In this study, effective hydraulic radius is defined based on cross section shape in Cartesian coordinates. In this definition, the effect of water surface roughness is applied in calculations with respect to a weight factor greater than one. By application of effective hydraulic radius in Manning equation for circular cross section a good accordance was observed between calculated and existed experimental results. Root square means error of relative computational velocity and discharge from the conventional and effective hydraulic radius toward experimental data are 0.14904, 0.08125 and 0.01113, 0.00042, respectively. Results showed that application of effective hydraulic radius has a significant effect on increasing the accuracy of Manning equation compared with conventional hydraulic radius.