Validation of the double-averaging velocity profile in decelerating flows over coarse-bed rivers

A clear understanding of the interaction of bed-form and flow characteristics in natural streams is essential for a variety of ecological and river engineering projects such as river rehabilitation, pollution control, and stable channel design. The present study deals with the logarithmic layer of double-averaged (in time and space) streamwise velocity profiles in non-uniform flows, decelerating flow, in coarse-bed Rivers. The streamwise velocity profiles were obtained from field measurements conducted in the Haraz and Rostam Abad Rivers. Velocity measurements were collected at 200 Hz and 120 seconds by using ADV. Both riverbeds composed of coarse gravel and cobble, with relative submergences ( ) from 4.4 to 10.4. Due to the limit application of the double-averaging method in Iranian Rivers, the validity of this method was investigated by using the logarithmic law and the boundary layer characteristic method. From the observations, the flow may be divided into two different layers: the inner layer (logarithmic layer) and the outer layer. In the inner layer ( ) the double-averaged velocity profiles confirmed the logarithmic law, however, the velocity data deviated from the logarithmic law in the outer region without a particular pattern. The results of this study can help a better estimation of hydraulic parameters in coarse-bed streams, decreasing the cost of hydraulic plans in rivers.