Investigation on the Effect of Gravel Particles Shape on Manning's Roughness Coefficient in Mountain Rivers

Flow resistance equations are a classical component of river hydraulic analysis, required for such applications as flood routing, prediction of flow depths and velocities in the design of floods conveyance structures, channel flood capacity estimation and the indirect estimation of flood discharges by the slope-area technique. In this research the effect of particle shape and bed slope of channel on Manning’s roughness coefficient have been investigated experimentally. To achieve this aim, two types of gravels (natural and crushed shapes) with three average gravel sizes (3.8, 4.66 and 6.53 cm), four bed slopes (0.004, 0.006, 0.008 and 0.01 percent) were used under different hydraulic conditions. The results showed that as the gravel size and bed slope increases or the relative submergence decreases, the Manning’s roughness coefficient (n) increases. Moreover, the results revealed that the average value of Manning’s roughness coefficient (n) of crushed gravels were by 2.7, 3.7, 3.8 and 5.9 percent more than natural gravels in the bed slopes of 0.004, 0.006, 0.008 and 0.01 percent, respectively. Also, the difference between the values of Manning’s roughness coefficients (n) for crushed and natural particles increased by decreasing relative submergence and increasing bed slope. In other words, the effect of particle shape on Manning’s roughness coefficient is applicable in steep slopes and low relative submergences (large-scale roughness).