Isle of Purbeck case study

Looked on these websites for certain climate change projections: https://earth.gov/sealevel/about-sea-level-change/future-sea-level/the-basics/ [&] https://sealevel.nasa.gov/ipcc-ar6-sea-level-projection-tool?type=global&data_layer=warming

Where is the Isle of Purbeck and why is it significant for coastal systems?

It is on the south coast of Dorset, part of the Jurassic Coast World Heritage Site, offering a compact area containing both concordant and discordant coastlines, a full range of erosional and depositional landforms, and clear links between geology, processes, and landforms.

What are the main sediment inputs to the Purbeck coastal system?

Marine erosion of chalk and limestone cliffs (e.g., Ballard Down), reworked offshore sediment, limited fluvial input, and sediment transported by longshore drift from the English Channel.

What are the key energy inputs?

Prevailing SW winds, high‑energy destructive waves in winter, constructive waves in summer, and a tidal range of ~2m.

What is the dominant direction of longshore drift and why?

South to north, driven by prevailing SW winds generating oblique wave approach along Studland and Swanage Bays.

Which erosional processes dominate Purbeck’s headlands?

Hydraulic action, abrasion, solution (especially in chalk), and wave quarrying exploiting joints and bedding planes.

What mass movement processes occur and where?

Slumping on Wealden Beds (clays) due to saturation; rockfalls on chalk cliffs after freeze‑thaw weathering.

What are the main outputs of the Purbeck coastal system?

Sediment deposition forming beaches, dunes, spits, and the removal of sediment offshore during storms; erosional landforms such as stacks, stumps, and coves.

How does geology create a concordant coastline at Lulworth?

Hard Portland limestone runs parallel to the coast, protecting softer Wealden Beds behind. Once breached, rapid erosion of clays forms a circular cove with a narrow limestone entrance.

How does discordant geology shape Swanage Bay?

Alternating bands of soft clay/sandstone and hard chalk/limestone run perpendicular to the coast, producing bays (Swanage) and headlands (Ballard Point, Durlston Head) through differential erosion.

Explain the formation of Old Harry Rocks.

Marine erosion exploits joints in chalk = caves then arches which then collapse to create a stack (Old Harry) then a stump. Chalk’s jointed structure accelerates erosion despite its hardness.

Why is Lulworth Cove a classic example of structural control?

A breach in resistant limestone allowed rapid erosion of softer clays behind, forming a circular cove. Stair Hole shows the early stage of this sequence.

How did Durdle Door form?

Wave action exploited vertical joints in Portland limestone, creating an arch. Continued erosion will eventually form a stack.

What is Chesil Beach and how did it form?

An 18‑mile tombolo connecting Portland to the mainland, formed by longshore drift from the west and reworking of glacial sediments. The Fleet lagoon lies behind it.

Describe the dune succession at Studland.

Embryo dunes then foredunes then yellow dunes then grey dunes then mature heathland. Formed by onshore winds and stabilised by marram grass.

How does seasonal wave climate affect Purbeck’s beaches?

Summer constructive waves build beaches; winter destructive waves remove sediment, deepen nearshore profiles, and create offshore bars.

What hard engineering exists at Swanage and what are its impacts?

Sea walls, groynes, and rock armour. They protect the town but cause sediment starvation down‑drift, increase scouring, and require high maintenance.

What soft engineering is used at Studland Bay?

Dune stabilisation (marram planting, boardwalks), zoning, and limited managed retreat. Effective but vulnerable to storms and tourism pressure.

How is visitor pressure managed at Lulworth cove?

Footpath reinforcement, education boards, and restricted access to fragile areas.

How is climate change expected to affect Purbeck?

1. Sea‑level rise leads to increased cliff retreat, especially in soft Wealden Beds.

2. More storms leads to dune erosion at Studland.

3. Saltwater intrusion into The Fleet.

4. Higher maintenance costs for defences at Swanage.

Why is Purbeck an excellent case study for systems thinking?

It shows clear feedback loops between geology, wave climate, sediment transport, and human intervention. It also demonstrates how coastal systems respond to disturbance (storms, sea‑level rise, tourism).

What are the main weaknesses of current management?

Hard engineering disrupts sediment cells; soft engineering requires constant upkeep; climate change may exceed current defence capacity.

Prevailing wind direction?

South‑west.

Longshore drift direction?

South to north.

Type of coastline at Lulworth?

Concordant

Type of coastline at Swanage?

Discordant

What is Old Harry made of?

Chalk

Key southfacing (concordant) landforms

Lulworth Cove, Stair Hole, Durdle door, Man O’War Bay, Mupe Bay, Worbarrow bay

Key southfacing (concordant) rock types

Limestone, clays, chalks

Key southfacing (concordant) processes

Rapid erosion, structural control

Key east facing (discordant) landforms

Studland Bay, Shell Bay, Swanage Bay, Ballard Point, Old Harry Rocks, Durlston Head

Key east facing (discordant) rock types

Alternating chalk, clay, sandstone, limestone

Key east facing (discordant) processes

Differential erosion, longshore drift

Key south-west/west facing landforms

Kimmeridge Bay, Chapman’s pool, St Aldhelms Head, Chesil Beach

Key south-west/west facing rock type

Clays, shales, limestone, shingle

Key south-west/west facing processes

Slumping, wave-cut platforms, longshore drift

What is the typical winter significant wave height along Purbeck’s exposed south coast?

1.5m—2.5m

How high can storm waves reach at Durlston Head?

Over 4m

What is the tidal range along the Purbeck coast?

1.8–2.2 m.

What is the approximate Atlantic fetch influencing Purbeck?

Up to 3,000 km.

What is the erosion rate of Portland limestone?

0.1–0.3 cm/year.

What is the erosion rate of Wealden Beds (clays)?

1–2 m/year.

What is the erosion rate of Swanage Bay’s clay cliffs?

0.5–1 m/year.

How fast are the chalk cliffs at Ballard Down retreating?

0.1–0.2 m/year.

What is the estimated retreat rate of Old Harry Rocks?

0.1–0.3 m/year, with episodic collapses.

What is the dominant direction of longshore drift at Studland and Swanage?

South to north.

What is the typical sand grain size at Studland?

0.2–0.5 mm.

How long is Chesil Beach?

29 km (18 miles).

What is the height of the Chesil Beach ridge?

Up to 14 m.

What is the shingle size at the Portland end of Chesil Beach?

Up to 10 cm diameter.

How wide is Lulworth Cove?

~400 m

How wide is the limestone entrance to Lulworth cove?

~80m.

How many visitors does Lulworth receive annually?

~500,000.

How tall is the arch at Durdle Door?

~30 m.

How wide is the arch opening at Durdle Door?

~20 m.

How tall is Old Harry (the stack)?

20–25 m.

How far offshore is Old Harry — the stack?

~50 m.

How tall can the dunes at Studland get?

Up to 20 m.

How wide is the dune system inland? (Stundland Dunes)

500–800 m.

How many visitors does Studland receive annually?

1.1–1.5 million.

How much dune retreat can occur in severe storm years?(Studland Dunes)

2–3 m.

How wide is the wave‑cut platform at low tide? (Kimmeridge Bay)

Up to 500 m.

What is the erosion rate of Kimmeridge’s Clay?

1–1.5 m/year.

How much sand was added in the 2005 beach nourishment? (Swanage Bay)

90,000 m³

How many groynes are installed at Swanage?

18 timber groynes.

What was the cost of the 2005–06 coastal defence scheme?

£2.2 million.

How long is the sea wall? (Swanage Bay)

~1.8 km.

How long is the boardwalk system? (Studland management)

Over 1 km.

How much does dune fencing cost annually? (Studland management)

£20,000+.

How many visitors can Studland receive on peak summer days?

Up to 5,000 per day.

What is the projected sea‑level rise by 2100 (medium emissions or middle of the road)? [according to IPCC]

Roughly 0.29–0.70 m.

How much might storm frequency increase by 2100?

10–20%.

What is the projected dune retreat at Studland under high‑end scenarios?

Up to 5 m/year.

How much of Swanage Beach could be lost by 2100 without intervention?

50–100%.