Initiation of Ripples by Artificial Disturbances

The experiments described show that sediment ripples may be produced on flat channel beds by artificial disturbances at lower flows than those at which ripples form naturally. The artificial disturbances used consisted of vertical thin plates set normal to the flow. These plates produced separation eddies, and the motion of grains at the flow reattachment point was observed. The development of a ripple train downstream of the disturbance is related to the bed shear stress measured upstream of the disturbance. The initiation of ripples in these circumstances is also related to the height of the disturbance, the water depth, and grain diameter, in addition to the normal parameters of Reynolds' Number and Shields' Mobility Number. Quartz grains of 0.137-mm, 0.495-mm and 2.0-mm median diameter were used.