Friction Factor for Gravel-Bed Channel with High Boulder Concentration

Abstract

This paper reports the results of an experimental investigation on flow resistance law for a gravel-bed channel having bed arrangements characterized by different values of the number of boulders. The dependence is studied of a parameter, proposed for describing the gravel-bed surfaces, on the boulder concentration and on the ratio between the median diameter of the coarser particles and the median diameter of the bed layer. Flume measurements carried out for the hydraulic condition of transition and large-scale roughness (1.5 ≤ h/d50 ≤ 6.9) show that the intercept of the semilogarithmic flow resistance law depends on the ratio and on boulder concentration. For concentration values less than 50%, a relationship is developed between the intercept and proposed parameter; for higher concentration values, the intercept is assumed constant and equal to −1.5. Further differences between the experimentally determined values of the friction-factor parameter and values calculated using the semi-logarithmic expression for flow resistance are explained in terms of the ratio between the Shields parameter and its critical value.