Study of Sediment Ripple Formation

Laboratory results are used to show the variation of the bed friction factor with the formation of ripples and dunes under flowing water. The flow pattern in the lee of a ripple is compared with the wake at an abrupt expansion. For the latter, the flow pattern and results of turbulence measurements are shown in graphical form. Velocity, pressure, and shear stress measurements on a fixed ripple form are examined. An explanation is proposed for the mechanism of the formation of the regular ripple pattern. It is reasoned that the increased agitation and sediment entrainment where the interface, between the main flow and the wake, meets the boundary is responsible for the upstream sloping face of the ripple. The regular pattern will persist only as long as the wake is dissipated before it reaches the next ripple crest. The ripple height is determined when the surface drag over the ripple crest is on the point of exceeding the value it had for the same flow over the flat sand bed.