G. Masselink et Cb. Pattiaratchi, THE EFFECT OF SEA-BREEZE ON BEACH MORPHOLOGY, SURF ZONE HYDRODYNAMICSAND SEDIMENT RESUSPENSION, Marine geology, 146(1-4), 1998, pp. 115-135
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.
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