How coastal recession, sub-aerial processes, eustatic and isostatic sea-level change and storm surges alter coastlines over time.

What this dot point is asking

Edexcel wants you to explain how and why coastlines retreat and change. You need the controls on coastal recession and how it is measured, the sub-aerial processes of weathering and mass movement, the difference between eustatic and isostatic sea-level change, the emergent and submergent landforms each produces, and the causes of storm surges.

Coastal recession and sub-aerial processes

Recession rates depend on lithology (weak boulder clay retreats far faster than chalk), structure (seaward-dipping beds and joints fail easily), fetch (the long North Sea fetch raises wave energy on the east coast) and sub-aerial slumping, where rain saturates clay cliffs until they fail by rotational slip. Geographers measure recession from historic maps, repeat LiDAR surveys and erosion pins driven into the cliff, expressing the result in metres per year.

Sub-aerial processes act on the cliff face from above the waterline. Weathering is mechanical (freeze-thaw, salt crystallisation), chemical (carbonation, solution) and biological (root and burrow action). Mass movement then moves the loosened material: rockfall dominates on hard, jointed cliffs, while rotational slumping dominates on saturated clays, leaving curved scars and slumped lobes at the base. These processes deliver sediment to the sea and steepen retreat.

Eustatic and isostatic sea-level change

Sea level relative to the land changes through two distinct mechanisms, and exam answers must keep them separate.

The two combine into relative sea-level change. Where relative sea level has fallen, emergent landforms appear: raised beaches sit above the modern shore and fossil (relict) cliffs stand inland, as along parts of the Scottish coast lifted by rebound. Where relative sea level has risen, submergent landforms form as the sea drowns the land: rias are flooded river valleys, fjords are drowned glacial troughs, and Dalmatian coasts are drowned valleys lying parallel to the coast.

Recession rates and storm surges

Quantifying recession lets geographers compare coasts and forecast loss.

Storm surges are short-term rises in sea level. Low pressure lets the sea bulge upward, strong onshore winds pile water against the coast, funnelling in narrowing basins such as the southern North Sea concentrates the surge, and a spring tide adds to the height. Where these coincide, water can rise several metres above the predicted tide and overtop defences.

Examples in context

Example 1: Holderness, Yorkshire. The 60 km Holderness coast of soft glacial boulder clay retreats at around 1.8 m per year, the fastest in Europe, losing villages over centuries. At Mappleton, groynes built in 1991 trapped sediment but starved the cliffs downdrift, accelerating recession there, a clear example of human interference shifting the problem along the coast.

Example 2: North Sea storm surges. The 2013 North Sea surge of 5 December drove water more than 2 m above predicted levels down the east coast, flooding Boston and triggering cliff falls at Happisburgh, Norfolk. It echoed the catastrophic 1953 North Sea floods that killed over 300 in England, while subsidence on the Ganges-Brahmaputra delta leaves Bangladesh acutely exposed to surges riding on rising relative sea level.

Try this

Q1. A cliff retreated 36 m in 20 years. Calculate the mean recession rate and explain one limitation of using it. [4 marks]

• Cue. $36 \div 20 = 1.8$ m yr$^{-1}$; the limitation is that recession is episodic, so the mean hides storm-driven loss in individual years.

Q2. Explain how a storm surge can cause coastal flooding. [4 marks]

• Cue. Combine low pressure raising the sea, strong onshore winds piling water, funnelling in narrowing basins and a coincident spring tide overtopping defences.