Case study: Cable Beach, Western Australia
Cable Beach at Broome is a wide, 16-kilometre long beach, nearly flat, steepening, and concave on the landward (high tide) end. At low tide the wide, exposed, intertidal beach is marked by changing patterns of low ridges or bars, separated by slight troughs or runnels.
Wave, wind, and tide
Wave energy is low to medium, with wave heights generally less than a metre. However, a great deal of tidal energy affects this beach: the 9.5-metre spring tides (macrotidal) produce an intertidal zone of several hundred metres and generate longshore tidal currents strong enough to entrain sand.
Beach and dune materials
The beach sands are composed of both organic and mineral materials, consisting of about 70% calcareous and 30% quartz. The sands are fine, which is commonly associated with low-angle beach profiles.
Wright et al. (1982) provided a detailed description and analysis of the morphodynamics at Cable Beach. Surf processes were dominant only in the top of the profile, where low energy reflective conditions prevailed. These conditions occurred at high tide, where very low wave energy, following shoaling, led to a relatively steep, short profile. The constant variation of water level through the large tidal range suppressed the development of resonant surf phenomena and did not lead to nearshore currents, rips or waves moving along the shore through the surf zone. Rhythmic topography, with regular variation from storm to fine weather profiles and a series of intermediate states, did not occur. However, erosion occurred at the back of the beach during the surge conditions of very large storms. The mid to low intertidal part of the profile was characterised by shoaling waves, becoming rapidly attenuated across the wide shallow water with many low bars. It was in this section of the beach that tidal longshore currents transported sand, although with a low annual flux.
Wright et al. noted that the change, which occurred with variation of energy showed very little depth of scour. It seems likely that this beach profile is in equilibrium with normal tidal change and wave energy.
Gains and losses at Cable Beach do not show a clear trend. Tidal currents parallel to the shore combined with the turbulence of shoaling waves to entrain some sands, but the beach appeared stable.