Abstract: Bed roughness (Ks) is a leading factor in calculations of sand grant resistance, and its accuracy is crucial to the determination of velocity distribution, bed-load movement, and sediment transport rate. Based on the resistance characteristics and a comprehensive consideration of water and sediment conditions, this paper develops a calculation formula of bed roughness using the variables of sediment median grain size, sediment non-uniformity coefficient, hydraulic radius, and bed slope, including more hydraulic factors than those in previous studies. This formula is verified against measured data and its mean square errors are compared with those of the previous methods. The verification reveals that it is applicable to the uniform and non-uniform sediment cases, and the bed roughness calculations based on non-uniform sediment achieve much higher accuracy than those on uniform sediment. The effect of sand waves on its accuracy is smaller than that of sediment transport.

Key words: resistance characteristic, bed roughness dimension, non-uniform sand, uniform sand, formula