What gives a coastline its energy, and why does the balance of constructive and destructive forces vary from place to place?
Explain how waves are generated and how wave type, tides and sediment supply determine the energy and behaviour of a coastline
A focused answer to the H2 Geography outcome on coastal energy. Wave generation and fetch, constructive and destructive waves, tides and tidal range, and how the energy budget governs erosion and deposition.
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What this dot point is asking
SEAB wants you to explain how waves get their energy, to distinguish constructive from destructive waves, and to show how tides and sediment supply shape the energy budget that governs whether a coast erodes or builds. The central insight is that a coastline is an energy system: inputs (wave and tidal energy) act on available material (sediment), and the balance decides the landform outcome.
The answer
Wave generation
Waves form when wind blows over the sea, exerting frictional drag that transfers energy to the surface and sets water particles into circular orbital motion. The energy carried depends on three factors:
- Wind speed (stronger wind, more energy);
- Duration (the longer it blows, the more energy transferred);
- Fetch, the distance of open water over which the wind blows (a longer fetch builds larger waves).
As a wave enters shallow water its base is slowed by friction with the bed; the wave steepens, the crest overtakes the base and it breaks, releasing energy onto the shore as swash (up the beach) and backwash (back down).
Constructive and destructive waves
- Constructive waves are low, long and infrequent (about 6 to 8 per minute), with strong swash and weak backwash. Net sediment moves up the beach, so they build it. They occur in calmer conditions and gentle gradients.
- Destructive waves are high, steep and frequent (about 10 to 14 per minute), with weak swash and strong backwash. Net sediment is dragged seaward, so they erode the beach. They occur in storms and steeper profiles.
The same beach can switch between the two as conditions change, which is why beach profiles vary seasonally.
Tides
Tides are the periodic rise and fall of sea level caused by the gravitational pull of the Moon and, less so, the Sun, which raise tidal bulges that the Earth rotates beneath. The tidal range (difference between high and low water) sets the vertical zone over which waves operate:
- A macrotidal (large) range spreads energy over a wide intertidal zone and exposes extensive mudflats.
- A microtidal (small) range concentrates wave attack in a narrow band.
Tides also generate currents that transport sediment along and across the shore.
The sediment budget
A coast behaves according to the balance between energy input and sediment supply. Where rivers, eroding cliffs or offshore banks deliver abundant sediment, deposition can dominate and beaches grow; where supply is starved, the same waves cause net erosion. Thinking of the coast as a sediment cell, with sources, transport and sinks, ties the whole system together.
Examples in context
Example 1. Singapore's sheltered, low-energy coast. Singapore sits in the relatively enclosed waters of the Strait of Malacca and Singapore Strait, with short fetch and a modest tidal range of around two to three metres. Wave energy is generally low, so much of the natural coast favoured deposition and mangrove growth before reclamation, illustrating how limited fetch produces a low-energy regime quite unlike open ocean coasts.
Example 2. High-energy Atlantic coasts. Coasts facing the open Atlantic, such as southwest England or western Ireland, receive swell generated over thousands of kilometres of fetch, giving powerful destructive waves in winter storms. The contrast with Singapore shows how fetch and exposure, not local wind alone, set a coast's energy and therefore its dominant processes.
Try this
Q1. Name the three factors that determine the energy of a wave. [3 marks]
- Cue. Wind speed, the duration the wind blows, and the fetch (the distance of open water over which the wind acts).
Q2. Explain why constructive waves build a beach while destructive waves erode it. [3 marks]
- Cue. Constructive waves have a strong swash and weak backwash, so net sediment moves up the beach; destructive waves have a weak swash and strong backwash, so net sediment is dragged seaward.
Q3. Explain how a large tidal range affects where wave energy acts on a coast. [2 marks]
- Cue. A large (macrotidal) range spreads wave attack over a wide intertidal zone as the waterline moves far between tides, exposing extensive mudflats rather than concentrating energy in one narrow band.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksExplain how waves are generated and distinguish between constructive and destructive waves.Show worked answer →
Argument: waves are generated by wind transferring energy to the sea surface, and the resulting waves behave as constructive or destructive depending mainly on their steepness and the relative strength of swash and backwash.
Generation to explain: wind blowing over the sea exerts frictional drag on the surface, transferring energy that sets water particles into a circular orbital motion. The energy of the resulting waves depends on three factors, the wind speed, the duration the wind blows, and the fetch (the distance of open water over which it blows). A long fetch and strong, sustained wind produce high-energy waves.
Constructive waves: low, long waves of low frequency (around six to eight per minute) with a strong swash and weak backwash, so net sediment is moved up the beach, building it. They are associated with gentle gradients and calmer conditions.
Destructive waves: high, steep waves of high frequency (around ten to fourteen per minute) with a weak swash and strong backwash, so net sediment is dragged seaward, eroding the beach. They are associated with storms and steeper profiles.
Markers reward the wind-friction-orbital generation, the fetch-duration-strength controls, and a precise swash-versus-backwash contrast for the two wave types.
Original8 marksExplain how tides and sediment supply influence the energy and behaviour of a coastline.Show worked answer →
Argument: tides control where and for how long wave energy acts on the shore, while sediment supply determines whether that energy produces erosion or deposition.
Tides: the gravitational pull of the Moon and Sun raises tidal bulges, giving high and low water. The tidal range, the difference between them, sets the vertical zone over which waves operate. A large (macrotidal) range spreads energy over a wide intertidal zone and exposes extensive mudflats; a small (microtidal) range concentrates wave attack in a narrow band. Tides also drive currents that transport sediment.
Sediment supply: where rivers, cliff erosion or offshore sources deliver abundant sediment, deposition can dominate even under moderate energy, building beaches and spits; where supply is starved, the same waves cause net erosion. The balance between energy input and sediment availability sets the coast's behaviour.
Markers reward the gravitational origin of tides, the role of tidal range in spreading or concentrating energy, and the sediment-budget idea linking supply to erosion or deposition.
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