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

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. Me garipples are relatively common features on the shore face in wave-dominate d environments, although their occurrence is not well predicted by bed form models; of the four bed form models examined [Clifton, 1976; Mogridge et n l., 1994; Nielsen, 1981; Wiberg and Harris, 1994], only that of Clifton [19 76] predicted the formation of megaripples. The average suspended sand conc entration profiles (measured using acoustic backscatter) differed considera bly, in absolute magnitude and vertical gradient, depending on whether the measurements were made over the crest or trough of the bed form. The tempor al structure of the suspension also varied with position relative to the me garipple. Utilizing the motion of the bed form beneath the instruments, pat terns of suspension over the megaripple were calculated as a function of th e 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 w as consistently directed offshore, while the transport associated with the swell waves was smaller and varied with position over the megaripple; the m otion of the megaripple itself was toward the shore.