The Effects of Armoring Process on Flow Resistance in Coarse-Bed Rivers

Friction factor as an index for flow resistance depends on grain size and bed forms in coarse-bed rivers. A wide range of particle sizes located in these rivers causes development of the armor layer and cluster structures, which have great effects on friction factor. In this study, coupling effects between the armoring process and the friction factor due to an armor layer have been investigated. Therefore, considering logarithmic velocity distribution characteristics, the total friction factor was determined. Then, grain friction factor was determined by the Keulegan and the Shields parameter methods and results were compared. Finally, form friction factor was determined by subtracting the grain friction factor from the total one. Analysis of results showed that the friction factor is rather independent of grain-size distribution of the material forming the bed and the major controlling parameter is the slope of the energy grade line. These unexpected results are due to the coupling of the friction factor with the incipient motion problem and the rearrangement of the grains of the coarsest fraction in the armor layer. Also, the results showed that making and development of the armor layer accompanied with clusters of particles caused an increase in the relative roughness of the bed and a reduction in the relative submergence resulting an augmentation in flow resistance and bed stability. Field observations demonstrated that rivers could change the critical Shields stress due to making equal mobility of coarse and fine sediments.