Variations in tidal amplitude are caused by the differing gratitional forces exerted by the sun and the moon at various stages of the lunar cycle. At both the full and new moons the sun and the moon are in line, so that their combined gravity pulls the seawater to the sides of the globe, causing spring tides. At the quarter moons, the gravitational pull is lessened, causing neap tides. On a Kimberley spring tide, the difference between a high and a low tide can be as much as 14.1m as measured in Collier Bay near Yule Entrance (-0.4m – 13.7m range). The highest tides of the year occur a day or two after the full or new moon nearest to the equinoxes (in March and September).
Kimberley tides are known as macromareal, a term for tides that differ by more than 4m between high and low tides. The large tidal amplitude of the Kimberley coast occurs because of the shallow shelving nature of the underlying coastal structure.
The vertical rise and fall of the tides also creates ‘tidal currents’, the ebb and flow of water into and out of bays and estuaries. Water flowing in is called a flood current, and water flowing out an ebb current.
The majority of beaches on the Kimberley coast are tide dominated; the spring tide range can be 10 to 50 times the average breaker wave height. This causes a relatively steep high tide beach, with shallower, shelving sand flats or mud flats, sometimes fringed at the high tide mark by mangroves, averaging 300m wide.
See article by Dr Andrew Short.