Big Tides

THE HUGE TIDES of Roebuck Bay are caused by the gravitational pull exerted by the moon and sun, but their magnitude and timing also depend on the topography of ocean basins and coastlines.

In some parts of the world, air-pressure variation and wind effects influence the height of tides. Roebuck Bay lies in a corner of the Indian Ocean where the tidal magnitude is very large by world standards, swamping any wind or air-pressure effects, so tide heights can be predicted accurately with the assistance of tide charts readily available in Broome. It helps that Roebuck Bay has ‘semi-diurnal’ tides in which roughly two tides of similar amplitude occur per day. In some parts of the world tide cycles are much more complex, with varying periods in which there may be only one tide per day, or in which two high water periods may follow one another in very quick succession.

EVERY FORTNIGHT, spring tides occur in Roebuck Bay. They have amplitudes of eight to ten metres, and, as the intertidal zone is so flat, several kilometres of mud are alternatively exposed and covered. There are two high tides every 24 hours, one usually occurring in the middle of the day and one in the middle of the night. Low tides occur around dawn and dusk. Tidal ranges gradually diminish after a spring series, and tides occur about half an hour later each day.

A week after a spring tide series, when the moon appears half-full, tides have diminished to the point that neap tides occur. These have a magnitude of a metre or so, only going out a little way and at high tide only rising to the bases of the beaches and mangroves. The area of exposed intertidal flats at neap low tides is only five to ten per cent of that which is exposed on spring low tides. Unlike spring tides, neap low tides usually occur in the middle of the day and the middle of the night, and during neaps, tides occur one to two hours later each day. After a neap series, tides gradually increase in amplitude until the next spring series occurs a week later.

References: (September 2003), Life along land’s edge Wildlife on the shores of Roebuck Bay, Broome. Rogers, DI. Piersma, T, Lavaleye M, Pearson GB, de Goeij P.