Determination of Friction Factor over a Pool Reach by Double-average Method

Bed-form characteristics have been investigated because of the close relation to hydraulic friction. Pool is one of the most important bed forms in coarse-bed rivers. The form friction factor should be considered in determining many river engineering projects including stable channel design, the scour depth along bank protection and at bridges piers. This study was conducted in the coarse-bed, Saymareh river, in western Iran. Two reaches, with Froude number less than 1.0, were selected to investigate the contribution of the double-average method in determining friction factor, one with pool bed form, range of Reynolds numberfrom 1.325×106 to 2×106, and the other, plane bed with range of Reynolds number from 5.75×105 to 6.22×105. In this study, the friction factor of the selected pool was calculated and then to compare the results with the one of the selected plane bed. The average flow velocity, shear velocity, shear stress and friction factor of the selected four cross sections were investigated and determined In each reach. The results revealed that the pool in a gravel-bed river affects considerably friction factor and the double-average method worked well for applying the law of the wall. The form friction, which contributed 82% to total friction factor in the selected pool, showed the necessity of considering bed form effect in river engineering projects.