Friction factor for gravel-bed channel with high boulder concentration

This paper reports the results of an experimental investigation on flow res istance 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 bould er concentration and on the ratio between the median diameter of the coarse r particles and the median diameter of the bed layer. Flume measurements ca rried out for the hydraulic condition of transition and large-scale roughne ss (1.5 less than or equal to h/d(50) less than or equal to 6.9) show that the intercept of the semilogarithmic flow resistance law depends on the rat io 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 t o -1.5. Further differences between the experimentally determined values of the friction-factor parameter and values calculated using the semi-logarit hmic expression for flow resistance are explained in terms of the ratio bet ween the Shields parameter and its critical value.