Marine Systems

Coastal processes

Coasts are shaped by the sea and the action of waves. Waves act in different ways, through the processes of erosion, transportation and deposition.

Coasts Waves Erosion Transportation and deposition

1- Coasts

Coastline in Pembroke shire, Wales

A coast is found where the land meets the sea. Coasts undergo change due to coastal processes and (as with rivers) there are three main processes at work:

erosion transportation, and deposition

Each of these processes involves the power of the sea and the effect of waves that are carried to shore. However, other factors also change and shape our coasts:

2- WavesThe power of waves is one of the most significant

forces of coastal change. Waves are created by wind blowing over the surface of the sea. As the wind blows over the surface of the sea, friction is created - producing a swell in the water. The energy of the wind causes water particles to rotate inside the swell. This moves the wave forward.

The size and energy of a wave is influenced by: The length of time that the wind has been blowing The strength of the wind How far the wave has traveled (called the fetch)

Waves can be destructive or constructive.

1. Destructive waves: operate in storm conditions are created from big, strong waves when the

wind is strong and has been blowing for a long time

occur when wave energy is high and the wave has traveled for a long time

tend to remove material from the coast and associated with erosion

backwash is stronger than the swash.

2. Constructive waves: operate in calm weather are less powerful waves break on the shore and tend to deposit

material, building up beaches are responsible for transporting material . swash is stronger than the backwash.

Waves: Powerful destructive waves are associated with erosion (left) and constructive waves are calmer and involved with transport and deposition

When a wave breaks, water is washed up the beach: this is called the Swash. Then the water runs back down the beach: this is called the Backwash. With a constructive wave, the swash is stronger than the backwash. With a destructive wave, the backwash is stronger than the swash.

Spilling Breaker

Powerful SwashWeak Backwash

Constructive Wave

Crashing breaker

Weak swash

Powerful Backwash

Destructive Wave

3- ErosionThe sea changes and shapes the coastal landscape.

Erosion is the wearing away and breaking up of rock and beach material found along the coast. Destructive waves will erode the coastline in the following four ways.

The constant force of waves crashing on the shore damages it. This is called hydraulic action.

Waves bring with them bits of rock and sand. These help to grind down cliffs. This is called abrasion or corrasion.

Waves cause rocks and pebbles on the shore to smash into each other and break down. This is called attrition.

Acids contained in sea water will slowly dissolve certain types of rock. This is called corrosion or solution.

4- Transportation and deposition

Waves tend to approach the coast at an angle; this is because of the direction of the prevailing wind. This causes the waves to break on the beach at an angle. The swash of the waves carries material up the beach at an angle. The backwash then flows back to the sea in a straight line. This movement of material is called transportation.

On many coasts the combined effect of continually repeating swash and backwash is to transport material sideways along the coast. This movement of material along coasts is called longshore drift.

When the sea loses energy, it drops its load of sand, rock particles and pebbles, that it has been carrying. This is called deposition. Deposition happens when the swash is stronger than the backwash.

Deposition is likely to occur when: Waves enter an area of shallow water Waves enter a sheltered area There is little wind

Coastal features Coastal features are caused either by the

processes of erosion (the wearing away of rocks) or deposition (movement of eroded material by the sea to a new location).

Two types of waves break on a coastline. Destructive waves are powerful waves that attack the coast, causing erosion and transportation of material (e.g. sand from a beach). Constructive waves have less energy. Instead of eroding they deposit material, so building beaches.

1. Coastal features created by erosion2. Coastal features created by deposition

Coastal features created by erosion

Along a coastline there are often many features created by erosion. The most common of these are:  

1.Cliffs 2.Headlands and Bays3.Caves 4.Arches 5.Stacks 6.Stumps

1. Cliffs  One of the most common features of the

coastline in Britain and around the world are cliffs. Cliffs are shaped through a combination of erosion and weathering. The weather attacks the cliff top. The waves attack the cliff foot, causing a wave-cut notch at the bottom.

Soft rock erodes easily and creates gently sloping cliffs. Hard rock is more resistant and erodes slowly and creates steep cliffs.  

Seven Sisters chalk cliffs on the East Sussex coast

2. Headlands and baysAnother group of features shaped by erosion are

headlands and bays. Headlands are formed when the sea attacks a section of coast consisting of alternating bands of hard and soft rock.

 The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant hard rock, such as chalk. This leaves a section of land jutting out into the sea; this is called a headland. The areas where the soft rock has eroded away, next to the headland, are called bays.

Headland and bay: Myrtos Bay, Kefalonia

Coasts where the geology alternates between strata (or bands) of hard rocks and soft rocks is called a discordant coastline. Discordant coastlines will have alternating headlands and bays. Concordant coastline is where the rock remains the same along the coastline. Concordant coastlines tend to have less bays and headlands.

Along the coastline of the Isle of Purbeck in Dorset (south coast of England), there are both discordant and concordant coasts. The discordant coast has been formed into Studland Bay (soft rock), Ballard Point (hard rock), Swanage Bay (soft rock) and Durlston Head (hard rock). After Durlston Head the rock remains hard. This concordant coast has less features.  

Erosion can create caves, arches and stacks along a headland. Again weathering can also help to create these landforms.  

3- Caves occur when the waves force their way into cracks in the cliff face. The water contains sand and other materials that help to grind away at the rock until the cracks become a cave.

4- If the cave is formed in a headland, it may eventually break through forming an arch.

5- The arch will gradually become bigger and bigger until it can no longer support the top of the arch. When the arch collapses, it leaves the headland on one side and a stack (a tall column of rock) on the other.

6- Stack:

Original Land surface

Wave-cut platform

stump

stack

Natural Arch

Old cliffline

Coastal features created by deposition

Along a coastline you can find features

created by deposition. These include:

1. Beaches

2. Spits

3. Bars

4. Tombolos

5. Sand dunes

1. BeachesBeaches are one of the most common features of a coastline.

Beaches are made up of eroded material that has been transported from elsewhere and deposited here by the sea.

Constructive waves help to build up beaches. The type of material found on a beach (i.e. sand or shingle) is influenced by the geology of the area and wave energy.

A cross section of a beach is called a beach profile. The ridges often found along a beach are called berms.

The material found along a beach tends to vary in size and type as you move further away from the shoreline (where the waves break on the beach). The smallest material tends to be deposited near the water, while larger material is found nearer to the cliffs at the back of the beach. Large material is deposited here in times of high energy, for example during a storm.

Beautiful beaches around the world are made up of eroded material

2. Spits

Spits are also created through the process of deposition. A spit is an extended stretch of beach material that projects out to sea and is joined to the mainland at one end.

Spits are commonly formed where there is a prevailing wind and where there is a longshore drift. An example of a spit is Spurn Head, found along the Holderness Coast in Humberside.

The development of a spit is shown below:

3. Bars

A bar is a long stretch of beach material (sand or shingle) that joins together two headlands. A lagoon usually forms behind the bar. An example of a sand bar is Slapton Ley in Devon.

4. Tombolos

A tombolo is a stretch of beach material that connects an island to the mainland. An example of a tombolo is Chesil Beach, connecting the Isle of Portland to the mainland of the Dorset coast.

Chesil Beach in Dorset stretches for 18 miles from West Bay to Portland. Lagoons have formed behind the spit.

5. Sand dunesA sand dune is a small hill of sand found at the top of a

beach. The waves do not usually reach this area of the beach. Vegetation may be found on sand dunes and such areas can be important ecosystems. An example of a sand dune ecosystem is found along the back of the beach at Aberffraw on the island of Anglesey, North Wales.

Coastal managementSuccessful management of coastal areas depends on:

1. understanding the differing interests of those who want to use coastal land in different ways, and2. understanding the physical processes impacting on the coast, such as erosion and longshore drift.

Techniques for managing these physical processes can be divided into hard engineering options (such as building sea walls) and soft engineering options (such as beach nourishment and managed retreat).

Interested parties in coastal management Physical management of the coast Case study: the Holderness coast

Interested parties in coastal management

There are many different land uses found in coastal areas - for example, tourism, industry, fishing, trade and transport. This means that there are many different groups of people who have an interest in what happens in coastal areas and how they are managed.

Some of the common-interest groups involved in coastal management issues are:

Local residents Environmental groups Developers Local councils National governments Tourist boards National Parks Authorities, such as the Pembrokeshire

National Park Authority

Each of these interest group may have a different view about what should be done to protect and manage coastal areas. A difference of opinion can cause conflict between interest groups.

There many reasons why groups of people might be concerned about the coast:

erosion is threatening beaches or coastal settlements

people want to develop tourism in the area - or existing tourism is declining

there is a danger of flooding if sea-levels rise there is a problem with sewage and/or pollution.

Physical management of the coast

Physical management of the coast is concerned with natural processes such as erosion and longshore drift. Management techniques fall into two categories: hard engineering and soft engineering.

Some coastal villages need protection from high tide flooding

Hard engineeringHard engineering options tend to be expensive and short-term options. They may also have a high impact on the landscape or environment. The table shows the most common hard engineering solutions.

Rock Armour/ Boulder Barrier

Building Groynes Concrete Sea Wall

Soft engineering options

Soft engineering options are often less expensive than hard engineering options. They are usually also more long-term and sustainable, with less impact on the environment. There are two main types of soft engineering.

1. Beach nourishment This replaces beach or cliff material that has been

removed by erosion or longshore drift. The main advantage is that beaches are a natural

defence against erosion and coastal flooding. Beaches also attract tourists.

While it can be a relatively inexpensive option it requires constant maintenance to keep replacing the beach material as it is washed away.

2. Managed retreat This is where areas of the coast are allowed to

erode and flood naturally. Usually this will be areas considered to be low value.

The advantages are that it encourages the development of beaches (a natural defence) and salt marshes (important for the environment) and cost is low.

While this is a cheap option, it will not be free as people will need to be compensated for loss of buildings and farmland.

Case study: the Holderness coast The Holderness coast is located in the north east of

England. This is one of the most vulnerable coastlines in the world, retreating at a rate of one to two metres a year. There are two causes of the problem.

Strong prevailing winds create a longshore drift that moves material southwards along the coast.

The cliffs are made of soft clay, so they will erode quickly.

The village of Mappleton, perched on the cliff top, has approximately 50 properties. As the cliff is eroded away, the village is under threat.

In 1991, the decision was taken to protect the settlement of Mappleton, along the Holderness coast, south of Hornsea. A coastal management scheme costing £2 million was introduced. This involved two types of hard engineering: placing rock armour along the base of the cliff and building two rock groynes.

The scheme has protected the settlement of Mappleton and the cliffs are no longer at great risk from erosion.

The rock groynes have stopped beach material being moved south from Mappleton along the coast. This has increased erosion south of Mappleton.

This shows that benefits in one area might have a negative effect on another area. This increases conflict between interest groups.