Ripple morphology under oscillatory flow: 2. Experiments

Recent large-scale laboratory experiments on the formation of ripples under oscillatory flow are presented. The experiments were performed in the Large Oscillatory Water-Sediment Tunnel (LOWST) at University of Illinois at Urbana-Champaign, using 250 mm silica sand as sediment. The dimensions of the ripples formed under a wide range of flow conditions are compared with some of the existing predictors and with a new predictor presented in a companion paper. For a given near-bed water excursion the size of the ripples is observed to initially decrease with the increase of the maximum orbital velocity, as has been suggested before. However, an abrupt change of the ripple size and the transition to large round-crested ripples is observed when the maximum orbital velocity becomes larger than 0.5 m/s. Above this value the size of these round-crested ripples continuously increased with the increase of the maximum orbital velocity. Additionally, anorbital ripples were never formed despite the long water excursions used in several of our experiments, confirming that anorbital ripples are only formed in fine sands. Finally, the performance of the existing planform geometry predictors and a newly proposed predictor is evaluated using our new experimental data. The results confirm that the bed planform geometry is controlled by the wave Reynolds number and the particle size. The comparison or the new data with previous results from narrow facilities shows that the facility width can restrict the development of bed form three-dimensionality.