Tide-modified beaches occur predominately in northern Australia where the spring tide range is ten to fifty times greater than the average breaker wave height (see distribution in beach typology). They consist of a low high tide beach fronted increasingly by inter- to low-tide tidal flats, the latter grading with lower energy into true tidal flats.
10. Reflective plus ridged sand flats
Reflective plus ridged sand flats is the highest energy of the tide-dominated beaches, occurring where waves average 0.5 m and tides average 4.5 m. They are characterised by a relatively steep, occasionally cusped high tide beach, which abruptly grades into a very low gradient sandy intertidal zone, covered by regularly spaced low amplitude (5-10 cm), shore parallel sand ridges. In northern Australia the intertidal flats average 600 m in width and have on average seven ridges with a mean spacing of 80 m. The flats can range from 50 to 5000 m in width, with up to 22 ridges. They are most prevalent around Cape York Peninsula and also occur in the South Australian gulfs and south Tasmania bays.
Beach plus sand flats is the most common beach type in northern Australia particularly in the Kimberley and Territory while they also occur in the South Australian gulfs and bays. They are similar to the ridged sand flats, except waves are lower (mean=0.26 m) and tides higher (mean=5 m). These conditions produce a relatively small, steep high tide beach, which grades abruptly into intertidal sand flats that average 300 m width (range 10-3000 m). The sand flats are low and featureless apart from small wave ripples, indicating wave energy is still sufficiency high to imprint itself upon the flats, but not high enough to form sand ridges.
Beach plus tidal sand flats differ from the sand flats in that they receive lower waves (mean=0.16 m) though similar tides (mean=5 m). They usually have a small, steep reflective coarse-grained high tide beach, fronted by intertidal sand flats averaging 350 m width (range 50-2500 m). The tidal energy is sufficiently high for the tidal currents to imprint themselves on the tidal flats, and in some locations for mangroves to colonise the upper intertidal zone. Many of these flats grade from inner sand flats to outer mud flats, with the sand averaging 300 m wide and the mud extending out on average to 500 m.
13. Beach plus tidal mud flats
Beach plus tidal mud flats occur in similar wave regimes to the tidal sand flats, with waves also averaging 0.16 m, but with tides averaging to 8 m or more. In addition they are usually located near a river mouth which to supplies the mud to the shoreline. They usually have a low narrow high tide beach composed of coarse shelly sand, which grades often very abruptly into wide, very low gradient intertidal mud flats, with mangroves often colonising the upper intertidal. The flats average 500 m in width, ranging from 50-2000 m. Most of these beaches occur in the Kimberley and Northern Territory.
Figure 23. Beach plus tidal sand/mud flats Tide-dominated beaches conceptual model The sandy high tide beach may be fronted by sand, sand then mud or pure mud flats which may contain tidal drainage features including tidal channels.
Andrew Short – University of Sydney