Experimental Investigation of Effective Factors on the Velocity Index for Calculating Discharge in Open Channels

Due to the measurement problems related to direct methods for estimating discharge, indirect methods based on the concept of surface velocity have been recently developed. In the present research according to the limited number of studies and the lack of appropriate relations between river geometric and hydraulic parameters with velocity index, the effect of dimensionless parameters including dimensionless Manning roughness coefficient, relative depth, Froud number and bed slope on the velocity index has been experimentally investigated. The results showed that by increasing the parameters of relative depth, dimensionless Manning roughness coefficient and Froude number the velocity index decreases. Meanwhile the influence of the bed slope is not clear. The mean value of the velocity index in rectangular channels was 0.92 and the its average error was estimated to be about 4.9%. Also, the analysis of the analytical models of the vertical velocity distribution showed that in the case of a smooth bed, the power law model with 4.5% average error and in the case of the bed with metal mesh, the logarithmic law with 10% average error have the best estimate of the velocity index.