Tides are caused by the combined gravity of the Sun and Moon
pulling on the water in the oceans.
When the Sun, Moon and Earth form a line in space the Sun's gravity reinforces
the Moon's gravity to produce the strongest gravitational pull.
This is why the highest (spring) tides occur around the time of the new and full
Moons.
When the Moon is at first quarter or third quarter, the Sun and Moon are at 90° to
each other and the forces due to the Sun partially cancel out those of the Moon.
This is the time of the smallest (neap) tides.
The shape of the ocean floor and the depth of the ocean change
these ideal tidal patterns.
Tides thus vary greatly from one site to another.
But how come the tides are so VERY big at King Sound near Derby?
Oceanographers
call these tides "macro" tides.
Macro tides are much larger than the normal high tides caused by
the action of the Sun and Moon, because they are also influenced
by the shape of the local coast.
King Sound, where Derby is situated, is an extension of the wide,
shallow north-west continental shelf.
It is shaped like a huge funnel
with a shelving bottom.
When the water comes in from deeper areas,
it is constricted by the shallow continental shelf.
The incoming waters of the Sound are squeezed into the narrow
V leading into the mouth of the Fitzroy River.
This causes the water to pile up, the tide is therefore amplified
as it moves shoreward.
Tides at Broome are very high, but the tides are even higher
in King Sound, because this area is shaped like a huge funnel.
Here, the water is forced up the narrowing Sound, causing these
massive tides and creating the King of all Australian tides at
Derby.
Tides are higher when the sun is more directly overhead at any
location.
In Derby this is between the Equinoxes and the Southern Solstice (October to
May).
Derby's highest astronomical tide is predicted at 11.8 metres.
This is the highest at any port in Australia and one of the highest
in the world.
Following are the nine largest macrotidal systems globally listed
in decreasing order of maximum tidal range.
1. Bay of Fundy (Canada) 16m
2. Mont St Michel (France) 15m
3. Severn Estuary (UK) 14.6m
4. Puerto Gallegos (Argentina) 13.2m
5. St Helier (France) 12.3m
6. Bhavnagar (India) 12.2m
7. Yampi Sound (Australia) 11.9m
8. King Sound (Australia) 11.5m
9. Broome (Australia) 10.5m
Why brown water?
The movement of huge volumes of water every 12 hours means that the fine silt
from the Fitzroy River estuary remains in suspension giving the water a light
brown colour.
Microscopic plants and animals in the water attract fish into the area as the
tide comes in.
The tides are semi diurnal usually giving two high tides and two
low tides in a 24-hour period.
The extremely muddy water seen off Derby wharf is caused by the
powerful tidal currents stirring up the huge load of sedimentary
mud washed into King Sound from the Fitzroy River during the Wet.
During neap (small) tides, the lower tidal currents don't stir
up the mud so much and so we sometimes see "blue water" at
the Derby wharf.
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