Initiation of motion in overland flow

This article reports the results of an experimental study concerning the initial movement of sediment in overland flow. Experiments were carried out in a small flume using water and quartz materials of various grain sizes. The comparison of our experimental results with classic criteria showed that the Shields-curve and the criterion of Yang can also be used for turbulent overland flow. Generally, higher shear stresses are necessary to provoke bed-instability under laminar flow conditions, especially at higher shear-velocity particle Reynolds numbers. A relative roughness effect also exists in laminar flow. Contrary to the observations by Savat on fixed beds, laminar flow does not show an ‘uncommon selectivity’, when loose, homogeneous beds are considered.