عنوان مقاله [English]
The Float method is a quick approximation technique for measuring velocity and thus flow rate. Surface velocity obtained by the Float method of correction factor is multiplied by the average velocity in the cross section obtained. This coefficient by the The United States Bureau of Reclamation (USBR) to measure the Float method is presented. Surface velocity coefficients published by USBR are based solely on average water depth. But other hydraulic factors such as longitudinal bed slope, the wall roughness height, the shape of the channel cross section, the location of the float object and ... may also affect the value of the coefficient. In this study the effect of these parameters on the surface velocity coefficient in a rectangular and compound open channel studied and the results are compared with the coefficients published by USBR. The results indicate that these parameters are the effective surface velocity coefficient and considering effect of other parameters in addition to the average water depth, be accuracy of the float method could significantly increase.