Experimental study of vortex ripples in full scale sinusoidal and asymmetric flows
Many laboratory studies of vortex ripples have been previously conducted but seldom has the work been conducted at field-scale. Consequently, it has been the purpose of this study to examine vortex ripples in the laboratory at field-scale. A new large oscillatory flow tunnel, the AOFT, constructed to enable the experimental study of vortex ripples at field-scale is described. Ripple geometry experiments carried out in the AOFT are described. The experiments involved a wide range of sand sizes and sinusoidal and asymmetric flows with periods and velocities typical of field-scale conditions. Comparison of measured and predicted ripple geometries leads to the recommendations that the method of Mogridge et al. (1994) be used to predict ripple geometry for field-scale oscillatory flows. Previously suggested criteria for predicting the occurrence of 3-d ripples fail when tested against a wide range of flow and sand conditions. Experiments carried out in the AOFT to obtain detailed concentration measurements above a rippled bed at field-scale are described. The behaviour of suspended and above the rippled bed is described. The level of detail in the concentration measurements, the inclusion of sediment size information and the fact that it is field-scale make this a unique laboratory data set. Despite differences in time-varying concentration behaviour for equivalent sinusoidal and asymmetric flows, and time- and horizontally-averaged behaviour was similar. Field-scale transport experiments performed in the AOFT are described. The measurements showed transport occurring in both the onshore and offshore directions simultaneously. The ripples migrated onshore because of coarse sediment moving as onshore-directed bedload. Net transport was directed offshore because of the large suspended concentrations generated by the onshore vortex.