Initiation of Ripples on Flat Sediment Beds

For natural sands under turbulent water flow, ripples are found to form on a flat bed from small deformations caused by the random action of high turbulent velocities close to the bed. The deformations affect the grain movement pattern so as to form a bed disturbance or collection of grains that induce a separation eddy to form. The height of such a disturbance was found to be of the order of two or three grain diameters, and a height-to-length ratio of the order of 1 to 100. Sands with median diameters of 137 micron and 495 micron were used in the experiments. The definition of initial movement of fine sediment established by Grass, was extended by these experiments to define the critical conditions for the initiation of ripples from a flat bed. Additional experiments were made in which the grain Reynolds Number was varied by varying the temperature and thus the viscosity of the water. The results suggested that the described mechanism of ripple formation does not operate for grains larger than 600 micron at normal temperatures.