In this paper a methodology of shear stress determination and the main results of experimental investigation on the beginning of grain’s movement of sandy materials are presented. Six kinds of alluvial sand with mean grain diameter d₅₀ = 0,20–1,7 mm and different uniformity and regularity of granulation ware used in experiments. The measurements, in steady uniform flow conditions in laboratory flume with rectangular cross-section and sandy bottom were carried out at variable water depths and bottom slopes. Investigations confirmed influence of granulation irregularity of sandy bed material on dimensionless critical shear stress. Based on measurements carried out in range Re_* = 3–55 were determined expressions for sand-grain height of flat bottom surface in conditions of beginning grain’s movement, with consideration of uniformity and regularity of granulation.

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