Tide-modified beaches

Tide-modified beaches occur predominately in northern Australia where higher tide ranges and lower waves result in the spring tide range being three to ten times greater than the average breaker wave height (see distribution in beach typology). They consist of three types.

7. Reflective plus low tide terrace

Reflective plus low tide terrace beaches are the most common tide-modified beach occurring predominantly across northern Australia where they are exposed to short period waves averaging 0.45 m in height, with tide range averaging up to 10 times the wave height (i.e. 4.5 m). These are characterised by a relatively steep cusped reflective high tide beach, usually composed of medium sand (0.45 mm). The beach face slopes to low tide where it abruptly grades into a low gradient, usually finer sand low tide terrace, which can extend tens of metres seaward. At high tide waves pass unbroken over the terrace and only break on reaching the high tide beach, similar to the reflective tide-dominated beach. As the tide falls, waves begin to increasingly break across the terrace and at low tide break on the outer edge producing a wide, shallow surf zone across the terrace. If rips are present, they will cut a channel across the terrace and are only active at low tide.

Reflective + low tide terrace (+rips) Tide-modified beach conceptual model shown at low tide with the steep cusped high tide beach face, 100 m wide low tide terrace cut by one small rip.

Figure 13. Reflective + low tide terrace (+rips) Tide-modified beach conceptual model shown at low tide with the steep cusped high tide beach face, 100 m wide low tide terrace cut by one small rip.

A steep high tide beach fronted by a wide sandy terrace at low tide, at North Harbour beach, Mackay, Queensland, where the tide range is up to 8 m (photo: A D Short).

Figure 14. A steep high tide beach fronted by a wide sandy terrace at low tide, at North Harbour beach, Mackay, Queensland, where the tide range is up to 8 m (photo: A D Short).

8. Reflective plus low tide rips

Reflective plus low tide rips beaches are the highest energy of the tide-affected northern Australian beaches exposed to waves averaging 0.7 m, together with medium sand and tides averaging 2.5 m. At high tide the waves pass over the bar without breaking until the beach face, where they usually maintain a relatively steep beach with cusps. As the tide falls, waves begin breaking on the bar and at low tide there is sufficient time and wave energy to generate the rips and scour rip channels, which have an averaging spacing of 140 m. These are similar to the transverse bar and rip systems, but only operate at low tide, begin covered by up to a few metres of water at high tide.

Reflective plus low tide rips Tide-modified beach conceptual model shown at low tide with a steep cusped high tide beach, a 100-200 m wide intertidal zone with a low sand ridge, and outer low tide zone cut by regularly spaced rip channels and currents, which only flow around low tide.

Figure 15. Reflective plus low tide rips Tide-modified beach conceptual model shown at low tide with a steep cusped high tide beach, a 100-200 m wide intertidal zone with a low sand ridge, and outer low tide zone cut by regularly spaced rip channels and currents, which only flow around low tide.

View of Nine Mile Beach, central Queensland showing the high tide beach (left), a 100 m wide intertidal zone with a low ridge and runnel, and low tide surf zone dominated by a strong rip feeder channel and rip, highlighted by purple dye (photo: A D Short).

Figure 16. View of Nine Mile Beach, central Queensland showing the high tide beach (left), a 100 m wide intertidal zone with a low ridge and runnel, and low tide surf zone dominated by a strong rip feeder channel and rip, highlighted by purple dye (photo: A D Short).

9. Ultradissipative

Ultradissipative beaches occur across northern Australia in higher energy (waves averaging 0.6 m high) tide-modified locations, where the beaches are also composed of fine sand. They are characterised by a very wide (200-400 m) intertidal zone, with a low to moderate gradient high tide beach and a very low gradient to almost horizontal low tide beach. Because of the low beach gradient waves break across a relatively wide, shallow surf zone as a series of spilling breakers which continually dissipate the wave energy, hence the name ‘ultradissipative’. The fine sand and shifting breaker zone act to plane down the beach, while the continuously shifting breaker zone precluding the formation of bars and rips, which require a more stationary surf zone.

Ultradissipative Tide-modified beach conceptual model showing the cusped high tide beach and 300 m plus wide low gradient intertidal beach.

Figure 17. Ultradissipative Tide-modified beach conceptual model showing the cusped high tide beach and 300 m plus wide low gradient intertidal beach.

Ultradissipative beach at Stanage Bay, central Queensland, showing the wide low gradient essentially featureless intertidal zone. (Photo: A D Short).

Figure 18. Ultradissipative beach at Stanage Bay, central Queensland, showing the wide low gradient essentially featureless intertidal zone. (Photo: A D Short).

Wave-dominated beaches | Tide-dominated beaches | Beaches plus rock/reef flats


Author

Andrew Short - University of Sydney

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