Tides, waves, currents and coastal processes (QCE Marine Science Unit 4)
“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”
Tides are the rise and fall of the sea caused by the gravitational pull of the Moon and Sun. Waves are generated by wind transferring energy to the sea surface. Currents are driven by wind, density differences and the Earth's rotation. Together these move sediment, nutrients and pollutants along coasts through longshore drift, building beaches by accretion in some places and stripping them by erosion in others.
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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.
Spring tides (largest range) happen at full and new moon when the Sun and Moon line up and their gravitational pulls add together. Neap tides (smallest range) happen at the first and third quarter moons when the Sun and Moon are at right angles and their pulls partly cancel. "Spring" refers to the tide springing up, not the season.
Set the scene and identify the agent
A Gold Coast beach loses much of its sand during a winter storm sequence, then slowly rebuilds over the following months. The agent of change is wave energy moving sediment via longshore drift and the swash-backwash cycle.
Explain the erosion phase
During storms, large high-energy waves with strong backwash carry more sand offshore and along the coast than is supplied, so the beach erodes rapidly, and a higher storm surge lets waves reach further up the profile.
Explain the accretion phase
In calmer conditions, gentler waves with constructive swash push sand back onshore and longshore drift delivers sediment from updrift, so supply exceeds removal and the beach accretes, rebuilding the berm.
Link to management
Conclude that because net longshore drift moves sand persistently in one direction, managers install groynes and a sand-bypassing system to maintain the beach, illustrating how understanding water movement underpins coastal management decisions.
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.
- Saying tides are caused only by the Moon
- The Sun also contributes; spring and neap tides come from the Sun and Moon combining or opposing.
- Confusing spring tides with the season
- Spring tides happen twice a month at full and new moon, not in spring.
- Thinking the water travels with a wave
- In open water the energy moves forward but the water circles in place; net transport happens only when waves break near shore.
- Forgetting Coriolis
- The Earth's rotation deflects currents and helps organise the gyres; leaving it out loses marks on current questions.
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.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice4 marksExplain the difference between spring tides and neap tides in terms of the relative positions of the Sun, Moon and Earth.Show worked answer →
Four marks: each tide type with the alignment that causes it.
Spring tides (2 marks). Occur at new and full moon, when the Sun, Moon and Earth are aligned so the gravitational pulls of the Sun and Moon add together, producing the largest tidal range.
Neap tides (2 marks). Occur at the first and third quarter moons, when the Sun and Moon are at right angles relative to Earth so their pulls partly cancel, producing the smallest tidal range.
Markers reward the alignment (syzygy) for springs, the right-angle configuration for neaps, and linking each to the resulting tidal range, not to the season.
Practice6 marksExplain the process of longshore drift, and analyse how it shapes a coastline through erosion and accretion, using a named Australian example.Show worked answer →
Six marks: the process, then erosion and accretion analysed with an example.
- Longshore drift (2 marks)
- Waves approach the beach at an angle, so swash carries sand up the beach at that angle and gravity pulls the backwash straight down, moving sand along the coast in a zig-zag.
- Erosion and accretion (2 marks)
- Where waves remove more sediment than is supplied (often updrift of a barrier or during storms) the shore erodes; where sediment is supplied faster than it is removed, accretion builds spits, bars and growing beaches.
- Named example (2 marks)
- On the Gold Coast, strong northward longshore drift moves large volumes of sand, so groynes and a sand-bypassing system are used to manage erosion and maintain beaches; storms such as the 2022 east-coast events stripped beaches rapidly.
Markers reward the swash-backwash mechanism, the erosion-accretion balance, and a specific managed example.