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1. Coastal Processes

The coastal system constantly acts upon the coastline, shaping it through processes such as erosion, deposition and sub-aerial processes. These three processes are often confused, but they are different and have different consequences on the shape and formation of the coastline.

Term Definition and TypesErosion Active, moving processes that lead to the wearing

away of rock. The four main types of coastal erosion are:

Hydraulic action, where air in cracks in the cliff are compressed as the wave pounds them in.

Abrasion, where particles of sediment smash against the surface of the cliff, breaking more of it off.

Solution, where soluble rocks like limestone are dissolved in the slightly acidic water

Attrition, where particles of already eroded rock smash against each other, breaking them into smaller, rounded pieces.

Deposition The process by which sediment is dropped due to the water having insufficient energy to transport it.

Sub-aerial processes

Refer to all the processes that are not linked with the action of the sea. They can be classed into three categories:

Weathering, this weakens the cliff and makes it more vulnerable to erosion. For example, freeze thaw.

Mass movement, which is the movement of material downhill due to gravity, including landslides and slumping.

Throughflow and surface run-off over the cliff face. In times of especially heavy rain, this process can saturate the cliff, making it heavier and more vulnerable to mass movement.

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Coastal Landforms

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Soft Roc

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The rate at which a coastline is eroded depends on several factors:

1. The presence and width of a beach. Beaches protect coastlines from erosion by slowing waves down, so a large beach with a long distance between the low and high tide marks will offer more protection. The beach may also cause the waves to break earlier.

2. The aspect. This is the direction the coastline faces. If the coast faces the dominant wind and wave direction, it is clear that rate of erosion will be higher.

3. The fetch of the waves. Waves with a longer fetch (distance over which they have travelled) are generally higher and steeper, so will have more energy and will cause more erosion.

4. The rock type. Hard rocks, like granite, are more resistant, so will erode slower than soft rocks, like boulder clay.

2. Types of Coastline

Coastlines can generally be sorted into two categories – concordant and discordant. Although any one part of the coastline may not fit exactly into these categories due to the complex nature of geology, it is helpful to think of it as simply as possible.

A concordant coastline (right) is one which has bands of alternating hard and soft rock running parallel to the coastline.

A discordant coastline (left) is one which has bands of alternating hard and soft rock running perpendicular (at right angles) to the coastline.

A good way to remember which way around they go is to think that coves form on concordant coastlines (explained later).

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3. Types of Wave

There are two main types of wave – constructive, and destructive.

Constructive waves (right) are responsible for the construction of depositional features. They can be identified by their low, long appearance and low frequency – typically 6 to 8 per minute. Their swash is weaker than their backwash, so they deposit sediment.

Destructive waves (left) are responsible for forming erosive landforms. They are steep, high waves with a higher frequency – about 10-14 per minute. They have a more powerful backwash than swash, so erode sediment.

4. Headlands and Bays

Headlands and bays are a very common landform, found on discordant coastlines.

1. The prevailing wind and waves erode the cliff.2. The soft rock is less resistant to the erosion, so is worn away

quicker, this forms a bay.3. The hard rock is more resistant, so remains sticking out into the sea

further – a headland4. Since the waves going into the bay have less energy due to

refraction, sediment is likely to be deposited in the bay to form a beach.

5. Over time, due to refraction, the hard rock will be eroded, causing it to retreat back into the bay.

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Wind and WavesSEAHard Rock

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5. Wave-Cut Notches and Wave-Cut Platforms

1. The foot of the cliff is eroded by the action of the sea, undercutting it.

2. A wave-cut notch (indentation into the foot of the cliff) is formed. The overlying rock is weathered and weakened.

3. The overlying rock gives in to the force of gravity and collapses. The rock falls into the notch and forms a flat platform as it is eroded. Over time this process will lead to the retreat of the cliff.

6. Coves

Coves are a less common coastal landform. They are found on concordant coastlines.

1. A fault in the hard rock (caused by sub aerial processes) suffers from high intensity erosion over time.

2. The fault grows larger and larger until it breaks through to the band of soft rock.

3. The soft rock becomes eroded much quicker than the hard rock, so a circular shape cove is formed.

4. Since the cove is so sheltered, waves are likely to be constructive within the cove, so a beach may be built up over time.

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Beach

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7. Spits, Bars and Salt Marshes

Spits and bars are formed by the process of longshore drift.

1. The direction of the prevailing winds, and therefore the direction of waves is at an angle to the beach.

2. This causes the swash to be at an angle to the beach. The waves push sediment up the beach at an angle

3. If the waves have a strong backwash, they will pull back sediment down the beach. This occurs at right angles due to gravity.

4. Gradually, sediment is pushed up and down the beach. This causes large volumes of sediment to migrate towards one end.

Spits are formed when the direction of longshore drift pushes sediment into an estuary or a bay.

1. Waves approach the beach at an angle.

2. Longshore drift moves sediment into a river estuary. The sediment forms a spit.

3. The stagnant water behind the spit develops into mudflats and salt marshes.

4. The original coastline is now protected from erosion by a spit and salt marsh.

5. Water in the river channel ends its course by flowing through the estuary.

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Direction of longshore drift

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6. The end of the spit is recurved at the end by occasional changes in the wind and wave direction. The spit continually grows further into the channel.

7. The fastest flow of water from the river is in the middle of the channel. The erosive power of river prevents the spit from reaching the other side of the river mouth. A bar would be formed if the spit reached the other side of the mouth, or bay, with a lagoon behind it.

8. Tombolos 1.1.1.1.1.1.1.1.1.1.1.

The process of longshore drift transports sediment along a beach.2. A spit forms, coming off

of the headland.3. Eventually, as the spit

grows, it joins up with an offshore island, connecting it to the mainland.

9. Caves, Blow Holes, Arches, Stacks and Stumps

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1. A fault or joint occurs naturally in the cliff, or one is formed by sub aerial processes.

2. A blow hole may form if water rushes up the joint, causing high hydraulic pressure, which causes the roof of the cliff to collapse.

3. A sea cave is formed when the area around joints are eroded.4. An arch is formed when a sea cave is eroded through to the other

side of the headland.5. The sides of the arch are eroded, and the force of gravity causes the

arch roof to collapse.6. An isolated stack is formed when the arch roof collapses.7. The top of the stack is eroded and weathered, until it is no longer

exposed at high tide – it is now called a stump.

10. Beaches

Beaches are a depositional feature found almost everywhere along our coastline. They play an important role in protecting our coastal settlements from cliff retreat and for the tourism industry.

1. Constructive waves deposit sediment on the shoreline.2. If the sediment is large, a pebble beach with a steep, narrow profile

will accumulate. If the sediment is small, a sandy beach will form. Sandy beaches normally have a wider, flatter profile. Beaches can have a combination of these sediment sizes and features.

3. Berms (1-2 metre high ridges of sand and pebbles) may form at the high tide mark. They’re formed when coarse material is deposited by the swash.

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4. Runnels are grooves in the beach material that run parallel to the shoreline. They’re formed when the backwash erodes sediment as is drains into the sea.

5. A cusp is a crescent shaped indentation into the shoreline which forms when there is a large tidal range and waves break parallel to the beach.

11. Sand Dunes

1. The wind blows sand particles up the beach.

2. The particles begin to accumulate in berms or on driftwood.

3. Plants like marram grass colonise.4. The roots of the plants support and

stabilise the dune, encouraging more sediment to accumulate.

12. Summary

Landform Erosive or Depositional?

UK Example

Headlands and bays

Erosive Very common – found along almost Britain’s entire coastline.

Wave cut notches and wave cut platforms

Erosive The Eastern end of Exmouth Beach.

Coves Erosive Lulworth Cove, Dorset.

Spits and bars Depositional Spit – Spurn head, Holderness.Bar – Netherfield Lagoons, Nottinghamshire.

Tombolos Depositional St Michael’s Mount, Cornwall.

Caves, blowholes, arches, stacks and stumps

Erosive Cave – Many, scattered around our coastline.Blowhole – Wheal Coates, Cornwall.Arch – Durdle door, Dorset.Stack – Old Harry, Swanage.Stump – Old Harry’s wife, Swanage.

Beaches Depositional Everywhere along the coast! (nearly).

Sand dunes Depositional Exmouth, Devon.

Salt marshes Depositional Dawlish Warren, Devon.

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