Stability of an erodible bed of lead granulates at low wall shear stress

The stabily of a smooth, flat bed composed of lead granulates is studied by the evolution of artificial depressions exposed to low wall shear stresses. It was found that the main instability parameter is the depth of the initial disturbance, it has to be larger than 40 viscous units; the second important parameter is the diameter of the depression which has to be larger than 200 viscous units for the system to become unstable and to form a ripplelike bedform. It has been proven that beds with a depletion are more stable than those with a hump of the same size. The ripple formation process is described until it becomes unstable itself.