THE EFFECT OF SEA-BREEZE ON BEACH MORPHOLOGY, SURF ZONE HYDRODYNAMICSAND SEDIMENT RESUSPENSION

Beach morphodynamic processes were investigated over part of a sea bre eze cycle on a microtidal, low wave energy sandy beach in southwestern Australia. Prior to the onset of the sea breeze, offshore winds with speeds less than 5 m/s prevailed. During the sea breeze, alongshore wi nds with speeds higher than 10 m/s were experienced. The sea breeze in duced pronounced changes to the nearshore morphodynamics which were si milar to that of a storm event: (1) root mean square wave height incre ased from 0.3 to 0.5 m; (2) zero-upcrossing wave period decreased from 8 to 4 s; (3) mean cross-shore flows reached velocities of 0.2 m/s di rected offshore; and (4) the longshore current increased in strength f rom 0.05 to 1.0 m/s. Before the sea breeze, sediment resuspension typi cally occurred during isolated high-wave events associated with the pa ssage of wave groups. Flux coupling between the wave-oscillatory curre nts and the suspended sediment (Jaffe, B.E., Sternberg, R.W., Sallenge r, A.H., 1984. The role of suspended sediment in shore-normal beach pr ofile changes. Proc. 19th Int. Conf. Coastal Engineering, ASCE, pp. 19 83-1996) induced net onshore suspended sediment transport, resulting i n beachface accretion and a steepening of the foreshore profile. In co ntrast, during the sea breeze, sediment resuspension was almost contin uous. The suspended sediment load increased six-fold and, as a consequ ence, the longshore suspended sediment transport rate increased by a f actor of 100. During the sea breeze, cross-shore transport was directe d offshore and primarily associated with mean offshore flows. This res ulted in erosion of the beachface and deposition in the surf zone and, consequently, a flattening of the beach profile. The suspended sedime nt load was strongly related to the Shields parameter defined for the combined action of waves and currents. Investigation of the suspended sediment profiles indicated that the degree of vertical mixing, parame terised by the sediment diffusion coefficient, increased with distance from the bed. In addition, the sediment diffusion coefficient exhibit ed a large temporal variation, ranging between 0.005 and 0.018 m(2)/s. The variability in the sediment diffusion coefficient is possibly due to changes in the sea bed morphology. (C) 1998 Elsevier Science B.V. All rights reserved.