Thresholds of upper-stage plane bed regime for intense bed load in open channel

Anotace: Results are discussed of laboratory experiments on criteria determining the transition between the regime of dunes and the upper stage plane bed (UPB) regime and the transition between the UPB regime and the regime of wavy flow. The experiments were carried for 3 fractions of plastic material and two fractions of glass beads in a broad range of flow conditions (different discharges of water and solids and longitudinal bed slopes) in a tilting flume. The experiments reveal that, contrary to expectations, a constant value of the Shields parameter is not an appropriate criterion for the transition between the dune regime and the UPB regime. Instead, the criterion seems to be well represented by a constant value of the longitudinal bed slope. The transition between the UPB regime and the wavy regime is found at a constant value of the densimetric Froude number. Relatively high values of Froude number at the threshold are due primarily to the presence of the collisional transport layer which protects the bed from an influence of undulating water surface.