Big Tides
The bay looks starkly different when the tide is out and the vast intertidal flats are exposed. © Kandy Curran
THE TIDES of Roebuck Bay are caused by gravitational pull by the moon and sun; 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 available in Broome.
Roebuck Bay has semi-diurnal tides – two tides of similar amplitude occur each day. © Kandy Curran
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 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 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 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.