Dimensionless critical shear stress evaluation from flume experiments using different gravel beds

Flume experiments were conducted using four different gravel beds ( D 50 + 12–39 mm) and a range of marked particles (10–65 mm). The shear stresses were evaluated from friction velocities, when initial movement of marked particles occurred. Two kinds of equations were produced: first for the threshold of initial movement, and second for generalized movement. Equations of the type 0 c + a( D i /D 50 ) b , as proposed by Andrews (1983) are applicable even if the material is relatively well sorted. However, the values of a and b are lower (respectively 0·050 and ‐0·70) for initial movement. Generalized movement requires a higher shear stress ( a + 0·068 and b + ‐0·80). D 90 of the bed material and y 0 (the bed roughness parameter) were also used as reference values in place of D 50 . They produced lower values than in natural streams, mainly owing to the fact that the material used in the flume is better sorted: clusters are less well developed and the bed roughness is lower.