Determination of Form Friction Factor of Armored Gravel-Beds Rivers ‎

Friction factor plays a substantial role in hydraulic computations. In coarse-bed rivers, friction ‎factor depends on grain size and bed forms. A wide range of particle sizes and developing ‎armor layer have great effects on friction factor. Therefore, determination of form friction ‎factor of armored river beds has been investigated in this study and, a friction equation was ‎developed based on computing grain friction factor of the total friction factor. Analysis of ‎obtained equation showed that the friction factor is rather independent of grain-size ‎distribution of the material forming the bed and the maximum grain size of this material. The ‎major controlling parameter is the slope of the energy grade line. These results are explained by ‎the coupling of the friction factor with the incipient motion problem and the rearrangement of ‎the grains of the coarsest fraction in the armor coat. To verify the equation, Keulegan method ‎was used which results 80% agreement between two methods‏.‏‎ Also, including the major part of ‎total friction factor to form friction factor (40%) shows the great effects of bed form on flow ‎resistance. The results of the current study showed that making and development of armor ‎layer increase bed roughness and friction factor and decrease average flow velocity causing a ‎reduction in threshold Shields parameter and augmentation in bed stability. Field observations ‎are employed to test the methods.‎