Resistance to Reversing Flows Over Movable Beds

Configuration of a movable bed changes periodically as the velocity of the reversing flow over it varies in time. Experiments are carried out with bed materials of various densities. It is seen that the ripples generated on the bed surface are essentially similar to those of unidirectional flow, but their shapes are reversed as to conform with the flow direction. The resistance of such a bed depends on grain Froude number and grain Reynolds number. The importance of viscosity in determining the resistance is emphasized. The resistance function at the instant of peak velocity is determined for the conditions of movable-bed tidal models. Ripple friction factor is found not to depend on the depth and the period of the flow in the ranges used in the experiments. Relationships determining the resistance of a movable bed for tidal model conditions are presented.