How do tides, waves and currents move water, sediment and nutrients, and shape coastlines?
Explain the causes of tides, waves and ocean currents, and describe how water movement transports sediment and nutrients and shapes coastlines through erosion and accretion
A focused answer to the QCE Marine Science Unit 4 dot point on water movement. Explains the gravitational cause of tides, how wind generates waves, what drives ocean currents, and how longshore drift, erosion and accretion shape coastlines, using Queensland and East Australian Current examples.
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What this dot point is asking
QCAA wants you to explain why tides, waves and currents happen, and how they move sediment, nutrients and pollutants and shape the coast. You should link gravitational pull to tides, wind to waves, and these processes to coastal erosion and accretion, using Australian examples such as the East Australian Current.
Tides
Tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon, with a smaller contribution from the Sun. The Moon's gravity pulls ocean water toward it, raising a bulge (high tide) on the side facing the Moon, and inertia raises a second bulge on the far side. As the Earth rotates, most coasts pass through two high and two low tides each day (a semi-diurnal pattern).
- Spring tides (the largest range) occur at new and full moon, when the Sun and Moon align and their pulls add together.
- Neap tides (the smallest range) occur at the quarter moons, when the Sun and Moon pull at right angles and partly cancel.
Tidal range varies hugely around Australia, from under a metre in parts of the Coral Sea to over 10 m at Broome in Western Australia, because coastline shape and ocean-basin resonance amplify or dampen the tide.
Waves
Waves are generated when wind transfers energy to the sea surface by friction. Wave size depends on wind speed, how long the wind blows (duration) and the distance of open water over which it blows (fetch). In open water the wave energy moves forward but the water itself moves in circles and stays roughly in place. As a wave reaches shallow water its base drags on the sea floor, it slows and steepens, and finally breaks, releasing energy that does the work of moving sediment.
Ocean currents
Currents are large-scale flows of water driven by three factors:
- Wind drives surface currents. Global wind belts plus the deflecting effect of the Earth's rotation (the Coriolis effect) organise surface currents into large rotating gyres.
- Density differences (from temperature and salinity) drive the deep thermohaline circulation, the global "conveyor belt" that slowly overturns the whole ocean.
- The Earth's rotation (Coriolis effect) deflects moving water, to the left in the Southern Hemisphere.
The East Australian Current (EAC) is the warm surface current that flows south down the Queensland and New South Wales coast, carrying tropical water (and larvae) southward. It is a key driver of connectivity and is shifting further south as the ocean warms, carrying tropical species into temperate waters.
How water movement shapes coastlines
Water movement is the main agent shaping coasts, by moving sediment.
- Longshore drift. Waves usually approach the beach at an angle, so each wave pushes sand up the beach at an angle (swash) and gravity pulls it straight back down (backwash). The net effect carries sand along the coast in a zig-zag. This transports huge volumes of sand along the Gold Coast, which is why groynes and sand-bypassing systems are built to manage it.
- Erosion. Where waves remove more sediment than is supplied, the shore retreats. Storm waves and cyclones can strip a beach in hours, as happened to Gold Coast beaches during the 2022 east coast storms.
- Accretion. Where sediment is supplied faster than it is removed, the shore builds out, forming spits, sand bars and growing beaches.
Currents and tides also distribute nutrients and pollutants. The same longshore and tidal flows that move sand carry dissolved nutrients, larvae and pollution such as oil or plastic along the coast and out to reefs, which is why a spill or runoff event in one place affects ecosystems far away.
Why this matters
Understanding water movement is the foundation for coastal management. Longshore drift, erosion and accretion decide where beaches grow or vanish and where management like sea walls and sand bypassing is needed, while currents and tides control how nutrients, larvae and pollutants spread, which links directly to the human-impact and management dot points in the rest of Unit 4.