Nearshore water motion and mean flows in a multiple parallel bar system

Abstract

Measurements of surface waves and nearshore water motion were made with wave staffs and electromagnetic currentmeters across the straight outer bars of a multiple parallel bar system at Wasaga Beach, Georgian Bay, Ontario. Three sets of experiments were carried out between 1983 and 1985, and this paper examines the characteristics of spatial and temporal variations in incident waves and mean flows. Incident wave spectra during storms are typical of fetch-limited conditions, with a prominent peak in the wind wave frequency and a sharp cut-off towards lower frequencies. There is little evidence of significant energy at infragravity frequencies. Mean flows perpendicular to the shoreline during storms, measured at 0.1 m above the bed, are offshore, with speeds of 0.04−0.10 m s −1 during moderate storms and 0.25 m s −1 during intense fall storms. Longshore currents are relatively weak, and their direction closely follows changes in the incident wind angle about shore perpendicular. There is no evidence of rip cell development, and offshore return flow occurs as an undertow uniformly alongshore, forming a simple, two-dimensional net circulation pattern. The presence of the undertow very close to the bed is an important control on the direction of suspended sediment transport, and thus in maintaining the equilibrium of the nearshore bars.