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Stage 1: Before you start

Stage 1: Waves | Wave transport | Coastal deposition | Beaches | Questions to investigate

What are low energy coasts?

• stretches of the coastline where waves are not powerful
• often the rate of deposition exceeds the rate of erosion
• landforms include beaches and spits

Waves

Waves are created by the action of wind blowing over the surface of the sea. Wave energy depends on

• wind strength
• wind duration (how long the wind is blowing)
• water depth
• the fetch of the wave

Fetch is the maximum distance of open sea a wave can travel over.

The highest part of a wave is the crest and the lowest point is the trough. The difference between crest and trough is the wave height.

Out at sea, water moves in a circular motion as each wave passes. The water does not move across the ocean. Only the energy of the wave moves. But when a wave approaches land, friction with the sea bed makes the base of the wave travel more slowly than the top of the wave. The top of the wave will eventually topple over, and the wave will break.

When a wave breaks, water washes forward onto the sea-shore. This part of the wave is called the swash. The swash transfers energy up the beach. The water that returns down the beach is called the backwash. The backwash returns energy down the beach.

Constructive waves and destructive waves

There are two types of wave: constructive waves and destructive waves.

• Constructive waves have limited energy. Most of this is used by the swash to transport material up the beach. Constructive waves are lower and less frequent (under 13 waves per minute). They have a strong swash that pushes material up the beach.
• Destructive waves have much more energy. Most of this is used by the backwash to transport material back down the beach. Destructive waves are higher and more frequent (about 15 waves per minute). They have a strong backwash that scours the beach and pulls material down the beach.

Beach angle affects the strength of the backwash. If the beach is gentle, little water percolates into the sand. Instead it will wash down the beach and destroy the swash from the next wave. If the beach is steep, more water percolates into the sand and so the backwash is weak.

Wave refraction

The direction in which a wave moves may be altered by the shape of the coastline. Waves travel faster in deeper water. If, for example, a wave is approaching a coast at an angle, it will bend round, as the side of the wave nearer to the coast travels more slowly. This is because the side nearer the coast loses more energy to friction, as the sea is shallower. The shape of the waves is also affected by headlands and bays.

Wave refraction where waves are diagonal to the coast	Wave refraction in an indented coastline	Waves can be refracted if they pass through a narrow channel

Wave transport

Waves transport material in the same ways as rivers transport material. Once rocks have been eroded, the end product is moved away by the force of the waves. There are four ways in which material can be moved.

• Traction - the largest rocks are rolled along the sea bed by the force of the waves.
• Saltation - sand and pebbles bounce along the sea bed.
• Suspension - silt and clay are supported within the water column.
• Solution - minerals dissolved in water are carried in the water flow.

The load is the total amount of material carried by a wave.

Waves need energy to carry material. The competence of a wave is the maximum size of particle that the wave can transport. Waves need more energy to carry larger particles. Only the waves with the highest energy can transport rocks and boulders. The weakest waves can only transport sand and clay.

Material can be transported longshore or onshore. Longshore movement is parallel to the coast. Onshore movement is up and down the beach.

The direction of sediment movment is affected by currents.

• Longshore currents - transport material by longshore drift. Waves approach the beach at an oblique angle. Refraction of waves around a headland raises the water surface at headlands, from where currents flow to areas of lower surface elevation in bays.
• Rip currents - when waves move in a direction perpendicular to a beach, the backwash is confined to evenly spaced zones. They are capable of transporting sediment seawards.

Longshore drift

Longshore drift is the movement of particles parallel to the coast. The rate of longshore drift is greatest when waves approach a beach at an angle. The swash, produced by breaking waves, moves diagonally up the beach at the same angle as the wave. In contrast, the backwash moves down the beach perpendicular to the shoreline.

If the wind drives waves from one direction more frequently than others, longshore drift will also move in one direction more frequently than others. If wind direction, and hence wave direction, are more variable, longshore drift may move in several directions. The rate and direction of longshore drift can be investigated with tracer pebbles.

Coastal deposition

 Where does the material transported by waves come from? There are several sources of sediment at the coast:

• sediment deposited by the waves
• sediment produced by mass movement
• sediment deposited by rivers entering the sea
• sediment deposited by human activity

Sediment deposited by the waves has been eroded and transported from elsewhere. Deposition occurs when the waves lose energy and can no longer transport such a large load. As wave energy falls, wave competence falls and the largest particles are deposited first. Wave deposits are rounded by attrition and sorted by particle size

Beaches

Beaches are found in the intertidal zone, i.e. between low and high tide levels. Beach sediment is composed of sand, shingle, pebbles, boulders and mud. Beach sediment comes from eroded headlands and cliffs, river deposition and erosion of other beaches. Material is sorted up and down the beach by the swash and backwash.

The intertidal zone

There are four different zones of the shore.

• Offshore zone - out at sea beyond the influence of the waves
• Nearshore zone - between the breakpoint bar (the zone of breaking waves) and the low tide mark
• Foreshore zone - between the low tide mark and the high tide mark, also called the inter-tidal or surf zone
• Backshore - above the high tide mark and usually above the influence of the waves

The limit of normal high tide may be marked by a berm. This is a 1-3m ridge of shingle and sand deposited by the swash but not removed by the backwash. On the landward side of the berm, the beach slopes downwards. A marsh may develop in this depression. A series of berms can be left by the retreating tide.

Material carried by the waves, including seaweed and litter, are left at the strand line which corresponds to the level of high tide.

Cusps are semi-circular patterns of sand and shingle found in combination on some beaches where the waves move perpendicular to the shore. They are formed by powerful swash when waves break onto the beach. The swash breaks into two components. Larger particles are deposited. The returning backwash re-combines and scours the centre of the cusp.

Beaches are often covered in sand ripples. These are small berms produced by individual waves. They may also be produced by water currents. There are two main currents.

How does the gradient of the beach change during the year?

The gradient of the beach changes during the year. The beach is steeper in summer than winter. This is because constructive (lower energy) waves are more common in summer, but destructive (higher energy) waves are more common in winter.

The strong swash of a constructive wave deposits the largest material at the top of the beach. As the upper beach builds up, the backwash becomes even weaker because a greater proportion of the water drains away by percolation, rather than running down the beach.

The weak swash of a destructive wave deposits material at the base of the beach. It cannot advance further up the beach because it is destroyed by the backwash from the previous breaking wave.

How does sediment size affect beach gradient?

The largest particles (large pebbles and rocks) is found at the top of the beach. Only the largest and highest-energy waves, carrying the greatest load and with the greatest competence, reach this part of the beach. The smallest material (sand) is found at the base of the beach. The top of the beach is called the storm beach because only storm waves and spring tides reach it.

Beach gradient and particle size
Particle size	Beach gradient (degrees)
Cobbles	24
Pebbles	17
Granules	11
Very coarse sand	9
Coarse sand	7
Medium sand	5
Fine sand	3
Very fine sand	1

Questions to investigate

A field investigation of a beach can involve a number of working hypotheses, such as

• Beach material will increase in the direction of longshore drift
• Pebbles will become smaller in the direction of longshore drift
• Pebbles will become rounder in the direction of longshore drift
• Pebbles will become smoother in the direction of longshore drift
• Pebbles will become more uniform (less varied) in the direction of longshore drift

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