Observations of the stability of oscillatory flow above the seabed and of sand ripple formation

The results of two field experiments are described, both of which were carried out at Blackpool Sands, Start Bay. In the first experiment in 1978, observations were made of the near-bed flow, and of the movement of coarse sand on the bed, beneath progressive swell waves in shallow water. In the second experiment in 1980, similar observations were made, but for a bed comprising medium to fine sand, and for a more varied range of wave periods. In addition, a number of observations were made of the formation of ripples on an initially flat sand bed. For the naturally rippled beds, critical conditions for the onset of vortex formation and shedding have been established, and reasonable agreement with previous laboratory results has been found. In particular, it has been shown that vortex formation occurs above the lee slopes of ripples only if the near-bed orbital excursion exceeds the ripple wavelength. Prior to certain experimental runs, the area of the seabed in the vicinity of the bottom rig was flattened by divers, and an (equilibrium) ripple pattern was allowed to develop. The wavelengths of the ripples which formed have been found to be in close agreement with the field results of previous workers. To examine in detail some of the properties of separating flow above a rippled bed, an irrotational standing vortex model is presented.