Spatial and temporal structures of suspension and transport over megaripples on the shore face

Abstract

Megaripples of 0.10-0.15 m height and 0.75-0.9 m wavelength occurred under swell wave activity on a macrotidal beach in southern Cornwall, England. Megaripples are relatively common features on the shore face in wave-dominated environments, although their occurrence is not well predicted by bed form models; of the four bed form models examined [Clifton, 1976; Mogridge et al., 1994; Nielsen, 1981; Wiberg and Harris, 1994], only that of Clifton [1976] predicted the formation of megaripples. The average suspended sand concentration profiles (measured using acoustic backscatter) differed considerably, in absolute magnitude and vertical gradient, depending on whether the measurements were made over the crest or trough of the bed form. The temporal structure of the suspension also varied with position relative to the megaripple. Utilizing the motion of the bed form beneath the instruments, patterns of suspension over the megaripple were calculated as a function of the phase of the wave. Sand transport was computed from the cospectra of the suspended concentration with an electromagnetic current meter at 10 cm from the seabed. Transport was dominantly at the infragravity frequencies and was consistently directed offshore, while the transport associated with the swell waves was smaller and varied with position over the megaripple; the motion of the megaripple itself was toward the shore. Copyright 1999 by the American Geophysical Union.