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Griffiths: This equation is recommended by the author for use in rigid gravel-bed channels with the condition that 1 <= R/d50 <= 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.

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.

n N-value.
R Hydraulic radius in meters.
d50 Bed material diameter corresponding to that 50 percent of the material is finer, unit in meters
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