Equations
Griffiths: This equation is recommended by the author for use in rigid gravel-bed channels with the condition that 1 <= R/d
50 <= 200. 84 data points obtained from 72 reaches on 46 New Zeland gravel-bed rivers were selected for use in this study. The author also derived an equation for mobile gravel-bed rivers, but did not recommend its use due to inferior statistics.
Reference:
Griffiths, G.A., 1981, Flow Resistance in Coarse Gravel Bed Rivers, Journal of the Hydraulics Division, American Society of Civil Engineers, vol. 107, no. HY7, pp. 899-918.