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Response of sand ripples to change in oscillatory flow

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Abstract

Ripples take time to evolve to a new equilibrium state in response to a change in wave-generated oscillatory flow. The paper presents results from flow tunnel experiments designed to examine oscillatory flow transient ripple processes under controlled, full-scale laboratory conditions. The experiments include study of the growth of ripples from flat bed and the evolution of existing ripples to new equilibrium ripples in response to a step change in the flow. In general, ripples evolve through a combination of two main processes: (i) from a flat bed or from a bed consisting of ripples that are smaller than the equilibrium ripples through a combination of ‘slide’ and ‘merge’; (ii) from a bed consisting of ripples that are larger than the equilibrium ripples through a combination of ‘split’ and ‘merge’. The experimental results show that equilibrium ripple geometry is independent of initial bed morphology while the time to reach equilibrium is largely independent of the initial bed and the equilibrium ripple size. The time to reach equilibrium depends strongly on the mobility number, and a new empirical equation relating mobility number and the number of flow cycles to equilibrium is proposed. This equation is combined with a simple exponential function for ripple height growth or decay to produce a new empirical model for ripple height evolution, which gives a reasonably good overall agreement with the measurements. The model is based on experiments involving one sediment size only and further work is needed to develop the model for other sand sizes.

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