Sea ripple formation: the heterogenous sediment case

Ripple formation beneath sea waves is analyzed both by experimental and analytical means when the bottom is made up of a mixture of sands. An oscillatory flow is obtained in a closed duct by the oscillations of two rigidly connected pistons located at the ends of the duct. The amplitude and period of the oscillations can be continuously varied. A fixed tray, located at the bottom of the duct and filled with different types of sediments, allows ripple formation to be observed. The presence of graded sediments is found to have a stabilizing effect and causes longer ripples to appear. Moreover a selective sediment transport is observed and quantified which tends to pile up the coarse grains at ripple crests leaving the fine ones in the troughs. As in the companion paper, the theory is based on a linear stability analysis of a flat sandy bottom subject to an oscillatory flow. Because of the presence of a mixture, a modified version of Exner equation is used and an “hiding” factor should be inserted in the sediment transport rate formula. The flow regime in the bottom boundary layer is assumed to be turbulent. The conditions for ripple appearance are determined along with their wavelengths as they form. Good agreement is found between experimental data and theoretical findings.