Abstract

Beach morphology, wave-current dynamics, or sediment attributes such as grain-size changes and sorting processes intrinsically characterize each other. The extreme cases of the morphological spectrum include reflective beaches which are steep and devoid of inshore topographic expression or surf-zone circulation, and dissipative beaches which are flat and have inshore barred topography with attendant longshore currents and rips. For a study of four beaches in Broken Bay, Australia, representing these morphological extremes, four facts become obvious. First, reflective beaches have rapid variaiton in foreshore grain size alongshore (mean settling velocity changes greater than 0.3 log 10 cm/sec/km, grain-size changes greater than 0.2 mm/km) whereas dissipative beaches have weaker variation (mean settling velocity changes approximate 0.1 log 10 cm/sec/km, grain-size changes less than 0.1 mm/km). Second, the pattern on reflective foreshores remains fixed over time whereas the pattern on dissipative foreshores varies to the point that size gradients can reverse direction within one year. Third, this temporal variation reflects the directional width of the incoming wave spectra and not changes in the level of wave power. Reflective beaches have waves approaching almost shore-normal because of the nullification of deep-water variation by wave refraction or by the sheltering effects of headlands. Wave approach on dissipative beaches varies daily, and the beach planform and sediment must respond over time to this variation. Fourth, grain size changes fit one of two postulates. The "energy" postulate states that grain size and sorting increase along a foreshore as energy increases. The "drift" postulate states that grain size increases downdrift. The "energy" postulate is more appropriate on reflective beaches whereas the "drift" postulate accounts for the difference in the magnitude of grain-size changes alongshore between reflective and dissipative beaches. On reflective foreshores, coarse material is naturally concentration at the base of the swash zone because of gravity effects or shear sorting in backwash. Larger grains here move faster alongshore than finer ones near the top of the foreshore because longshore currents at the base of the swash are stronger than the zigzag motion of swash drift further up the beach face. This drift sorting produces strong longshore size gradings on the foreshore. This effect is negated on dissipative beaches by swash bore transport of coarse grains upslope, by dilution with inshore sediment that has undergone preferential movement of fines alongshore in suspension, and by locally reversed longshore currents near rips.

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