Four quarterly surveys were made of the sand-wave field in 1968–1969. The data on trough to crest height of the sand waves have been expressed as significant wave heights. These show a maximum of 7 m, decreasing to 2 m in the north and towards the coast. Wavelengths are from 200 to 500 m. Variability in the position of the northern and eastern margins of the field is attributed to wave activity. The position of the western boundary to the sand-wave field is due to scarcity of sand and to the existence of a gravel bottom in parts of the deep-water channel. Symmetrical and asymmetrical sand waves, and sand waves with superimposed megaripples are the forms recognised. The maximum height of 7 m is in agreement with predictions using an equation given by Kennedy (1969). Together with northward decrease in height there is decrease in grain size and absence of megaripples on sand waves. The northern boundary to the sand-wave field is caused by decreasing asymmetry of the tidal ellipse. This does not occur in a narrow tongue extending along the coast of Texel where an extension of the sand-wave field is found. The northwards decrease in height is mainly caused by increase in suspension transport of sand. This agrees with a re-evaluation of Bagnold's (1966) suspension criterion which shows that large amounts of sand < 230 μ in diameter will be transported in suspension. Megaripples on sand waves are taken to be large scale analogues of ripples on dunes seen in flumes. The megaripples represent perturbation of bed-load transport rate and the sand waves perturbation of suspended load transport rate. Thus the northwards absence of megaripples on sand waves, decrease in grain size, decrease in height and increase in suspension transport, fit in a coherent dynamical explanation of the characteristics of the sand-wave field. |
