Coastal Case Study - The Dorset Coast
- The geology of the Dorset coast is perfect for both erosional and depositional landforms
- It has bands of sedimentary rock, consisting of soft clay and harder limestone and chalk
- These rocks erode at different rates creating headlands, bays, arches, a long tombolo and more
-
This stretch of coastline forms part of the commonly known Jurassic Coast that stretches for 155km from Exmouth in Devon to Poole in Dorset
How has it changed?
Rock Formation along the Jurassic Coast
| Triassic Period | 250-200 million yrs. ago, rocks were formed in desert conditions - sandstone |
| Jurassic Period | 200-140 million yrs. ago, sea levels were higher and layers of sedimentary rocks formed such as clay and limestone |
| Cretaceous Period | 140-65 million yrs. ago, sea levels fell and rose, depositing more sedimentary layers such as chalk |
| Quaternary Period | 2.6 million years to present, after the last ice age (10,000 yrs. ago) sea levels rose again and the processes of erosion and deposition have created the modern coastline |
Erosional landscape
- Durdle Door is an example of an arch formation
- Wave erosion opened a crack in the tough limestone headland
- It is unusual as it has formed parallel to (along) the coastline
- Further erosion has led to a cave which has developed into an arch in the headland
- Softer rocks behind the limestone have been washed away leaving an eroding line of chalk cliffs by mechanical, chemical and biological weathering
- Lulworth Cove is a small bay that was formed when a gap was eroded in the band of tough limestone
- Lying behind this limestone is a band of soft clay, and this has been scooped out (eroded) to form a bay
- The entrance to the cove is narrow because the harder band of limestone is more resistant to erosion
- The limestone cliffs forming the back wall of the cove are vulnerable to mass movement and sometimes experience small slides and slumps
- Old Harry and his wife sit at the end of The Foreland
- This chalk headland has eroded to form caves, arches and a stack (Old Harry)
- Further erosion has resulted in a stump called Old Harry's Wife
- Chemical weathering and erosion have gradually eroded these features
- Biological weathering, through surface vegetation on the headland, is also weakening the rock
Main features along the Dorset Coast, UK
Map showing main features of the Dorset Coast, UK
- Swanage sits on two beach bays called Studland Bay and Swanage Bay
- The cliffs behind the bays are areas of soft sandstone and clay
- Between the two bays is The Foreland, a headland of harder chalk
- Longshore drift affects the bay carrying material (mainly gravel) from the south to the north of the beach
- Erosion is the dominant process in the bay with the depositional beach losing material year on year
Depositional landscape
- Chesil Beach is an 18-mile-long pebble/shingle barrier beach, that has rolled into a tombolo (spit that joins an island to the mainland) and stretches north-west from Portland to West Bay.
- Formed through the process of longshore drift, it joins the Isle of Portland to the mainland
- There is a shallow salt water lagoon called The Fleet Lagoon that separates the beach from the mainland
- Studland Bay has four miles of sandy beaches within sheltered waters and backed by sand dunes
- Studland’s dunes are unusual because:
- Sand only began to be deposited about 500 yrs. ago
- A freshwater lake has formed in-land called the Little Sea
- The dunes are made of acidic sand that has a low shell content
- This acidity means that the dunes will be colonised by dune heather and not grass
Exam Tip
- Make sure you can name four distinctive landforms from your studied example
- For each landform:
- The geological time period it dates to
- The type of rock or rocks it is made from
- State if it was formed through erosion or deposition
Impacts of the 2014 Valentine’s Day storm
- On 14 February 2014, a large storm battered the coastline with winds of up to 80mph and storm waves of more than 30ft high
- Roofs were lost from buildings, power cuts, overturned lorries and landslides
- Hundreds of people were evacuated
- Sea defences were breached and huge waves threw rocks from Chesil Beach into the streets behind the seafront
- Portland Beach Road was under 4 feet of water from flooding at Hamm Beach
- Parts of Chesil Beach were lost and the 150-million-year-old Pom Pom Rock collapsed
- The main railway line from Plymouth to Exeter was washed away at Dawlish and was closed for over 2 months
- West Bay cliffs retreated by a few metres after the cliffs collapsed
- Despite the storm, Lyme Regis was sufficiently protected by its coastal defences
Effects of climate change on the impacts
- As global sea temperatures increase, water levels rise through melting ice caps and seawater expansion
- A warmer atmosphere leads to more intense and frequent storms, creating powerful destructive waves
- A warmer atmosphere can hold more water vapour, which will lead to higher levels of precipitation and increased rates of weathering and erosion. This will lead to increased cliff instability leading to increased levels of landslips and falls
- Climate change has the ability to impact the whole of the UK's coastline, particularly those areas where the cliffs are made from softer rock and clay or are lying close to sea level and can, therefore, flood
Coastal management
- Any coastal management aims to protect the environment but mostly people from the impacts of erosion and flooding
- Not all coastal areas can be protected or managed as there are economic constraints
- Two types of management can be used:
- Soft engineering that works with natural processes
- Hard engineering that works against natural processes
Hard engineering
- Hard engineering involves building some form of sea defence, usually from concrete, wood or rock
- Structures are expensive to build and need to be maintained
- Defences work against the power of the waves
- Each type of defence has its strengths and weaknesses
- Protecting one area can impact regions further along the coast, which results in faster erosion and flooding
- Hard engineering is used when settlements and expensive installations (power stations etc.) are at risk - the economic benefit is greater than the costs of build
Hard Engineered Defences
| Strategy | Description | Advantages | Disadvantages |
| Sea Wall |
A wall, usually concrete, and curved outwards to deflect the power of the waves |
Most effective at preventing both erosion and flooding (if the wall is high enough) |
Very expensive to build and maintain It can be damaged if the material is not maintained in front of the wall Restricts access to the beach Unsightly to look at |
| Groynes |
Wood, rock or steel piling built at right angles to the shore, which traps beach material being moved by longshore drift |
Slows down beach erosion Creates wider beaches |
Stops material moving down the coast where the material may have been building up and protecting the base of a cliff elsewhere Starves other beaches of sand. Wood groynes need maintenance to prevent wood rot Makes walking along the shoreline difficult |
| Rip-rap |
Large boulders are piled up to protect a stretch of coast |
Cheaper method of construction Works to absorb wave energy from the base of cliffs and sea walls |
Boulders can be eroded or dislodged during heavy storms |
| Gabions |
Wire cages filled with stone, concrete, sand etc |
The cheapest form of coastal defence Cages absorb wave energy Can be stacked at the base of a sea wall or cliffs |
Wire cages can break and they need to be securely tied down Not as efficient as other coastal defences |
| Revetments |
Sloping wooden or concrete fence with an open plank structure |
Work to break the force of the waves Traps beach material behind them Set at the base of cliffs or in front of the sea wall Cheaper than sea walls but not as effective |
Not effective in stormy conditions Can make beach inaccessible for people Regular maintenance is necessary Visually unattractive |
| Off-shore barriers |
Large concrete blocks, rocks and boulders are sunk offshore to alter wave direction and dissipate wave energy |
Effective at breaking wave energy before reaching the shore Beach material is built up Low maintenance Maintains natural beach appearance |
Expensive to build Can be removed in heavy storms Can be unattractive Prevents surfing and sailing |
Soft engineering
- Soft engineering works with natural processes rather than against them
- Usually cheaper and does not damage the appearance of the coast
- Considered to be a more sustainable approach to coastal protection
- However, they are not as effective as hard engineering methods
Soft Engineered Defences
| Strategy | Description | Advantages | Disadvantages |
| Beach replenishment | Pumping or dumping sand and shingle back onto a beach to replace eroded material |
Beaches absorb wave energy Widenbeachfrontnt |
Has to be repeated regularly which is expensive Can impact sediment transportation down the coast |
| Fencing, hedging, and replacing vegetation |
Helps to stabilise sand dunes or beaches Reduces wind erosion |
Cheap method to protect against flooding and erosion |
Hard to protect larger areas of coastline cliffs |
| Cliff regrading | The angle of a cliff is reduced to reduce mass movement |
Prevents sudden loss of large sections of cliff Regrading can also slow down wave cut notching at the base of cliffs as wave energy is slowed |
Does not stop cliff erosion |
| Managed retreat | Existing coastal defences are abandoned allowing the sea to flood inland until it reaches higher land or a new line of defences |
No expensive construction costs Creates new habitats such as salt marshes |
Disruptive to people where land and homes are lost. The cost of relocation can be expensive Compensation to people and businesses may not be paid |
- There are conflicting views about using a particular type of engineering for coastal defence
- Most coastal managers aim to use a range of methods depending on the value of what is being protected
- This method is known as Integrated Coastal Zone Management (ICZM)
- ICMZ aims to use a combination of methods to best reflect all stakeholder needs
- Management of coastal regions is performed by identifying coastal cells
- This breaks a long coastline into manageable sections and helps identify two related risks:
- The risk of erosion and land retreat
- The risk of flooding
- Identification allows resources to be allocated effectively to reduce the impacts of these risks
- The 'cost-benefit' is easier to calculate using coastal cells
- Shoreline Management Plans (SMP) set out an approach to managing a coastline from flooding and erosional risk
- The plans aim to reduce the risk to people, settlements, agricultural land and natural environments (salt marshes etc.)
- There are four approaches available for coastal management, with differing costs and consequences:
- Hold the line
- Long term approach and the most costly
- Build and maintain coastal defences so the current position of the shoreline remains the same
- Hard engineering is the most dominant method used with soft engineering used to support
- Advance the line
- Build new defences to extend the existing shoreline
- Involves land reclamation
- Hard and soft engineering is used
- Managed realignment
- Coastline is allowed to move naturally
- Processes are monitored and directed when and where necessary
- Most natural approach to coastal defence
- Mostly soft engineering with some hard engineering to support
- Do nothing
- Cheapest method, but most controversial of the options
- The coast is allowed to erode and retreat landward
- No investment is made in protecting the coastline or defending against flooding, regardless of any previous intervention
- Hold the line
- Decisions about which approach to apply are complex and depend on:
- Economic value of the resources that would be protected, e.g. land, homes etc
- Engineering solutions - it might not be possible to 'hold the line' for moving landforms such as spits, or unstable cliffs
- Cultural and ecological value of land - historic sites and areas of unusual diversity
- Community pressure - local campaigns to protect the region
- Social value of communities - long-standing, historic communities
Jurassic Coast and SMP
- As areas of the Dorset coast are being eroded, properties and infrastructure are at risk
- Landslides and rockfalls put people at risk
- Coastal management strategies along the Dorset coastline to prevent erosion have impacted the landscape and caused changes to the natural environment
- New timber groynes were installed on Swanage Beach in 2005-6
- These have reduced the loss of beach material
- However, beaches further along the coast are becoming narrower and subject to more erosion (due to reduced ability to absorb wave energy)
- Timber groynes have been replaced in Poole and Bournemouth in 2021
- Concrete sea walls are in place along most of Swanage beachfront
- They are recurved and reflect waves back out to sea preventing erosion to the base of the cliff
- However, this creates a strong backwash which removes sand from the beach and leads to erosion under the wall
- As natural erosion has been halted, natural beach replenishment has stopped, reducing the overall beach levels
- Beach replenishment
- To create a wider beach in parts of upper Swanage Bay, sand and shingle were dredged from the sea bed at Poole Harbour in the winter of 2005-6
- This slowed wave energy, which reduced erosion and helped protect properties and the cliffs
- Although successful, the cost was £5 million and needs to be repeated approximately every 20 years
Human activity impacts the landscape
- Industry and tourism affect the landscape along the Jurassic coast
- As the coastline is a major tourist attraction, footpaths are worn down as people repeatedly walk along them
- Vegetation along the chalk cliff tops is trampled and worn away, exposing the soil and rock to weathering and erosion and increasing cliff instability
- Portland and West Chesil Beach are quarried for limestone and used in construction
- Quarries expose vast areas of rock to weathering and erosion
- Up until the 1960's Chesil Beach's shingle was extracted commercially and also used in construction
- The shingle was removed so quickly, that natural processes couldn't replenish it and the landform was damaged
