A sandwave field in the Outer Thames Estuary,Great Britain

A method of presentation, in which seabed profiles are scale-corrected and replotted in three-dimensional form, has been developed to aid the study of complex areas of the seabed. For sandwave research the continuity of crests can be studied, and the positional relationship of individual features compared. By plotting successive surveys of the same area, changes can be detected and significant movement measured.In the sandwave field at Long Sand Head, Thames Estuary, the sandwaves can be grouped in morphological zones, depending upon their amplitude, wavelength and cross-sectional asymmetry. Despite marked asymmetry, which suggests opposing directions of movement in different zones, little progressive movement occurs, but rather a “flexing” of sinuous crest lines results in displacement of up to 25 m in a year.The study has been supported by side-scan sonar surveys, which emphasize the complexity of the sandwaves within the field and the presence of less stable dune bed forms (American Society of Civil Engineers, 1966), which often lie at considerable angles to the major crests, suggesting a secondary flow regime.