Coastal systems, marine and sub-aerial processes, and the erosional and depositional landforms they create.

What this dot point is asking

WJEC wants you to treat the coast as an open system, identify the marine and sub-aerial processes operating on it, explain sediment transport within a sediment cell, and account for the main erosional and depositional landforms with located examples. Strong answers move beyond listing landforms to sequencing the processes that produce them and grounding each in a named place such as the Pembrokeshire coast.

The answer

The coast as a system

The coast is an open system: inputs are wave, wind and tidal energy plus sediment from rivers, cliffs and offshore; stores are beaches, dunes and spits; transfers move sediment along the coast; and outputs lose sediment offshore or inland. Wave energy depends on fetch (the open distance of water over which wind blows), wind speed and duration. The long south-westerly fetch across the Atlantic gives the Welsh coast high-energy waves, which is why the Pembrokeshire cliffs are so heavily eroded.

Marine and sub-aerial processes

Marine erosion includes hydraulic action (the force of water and compressed air in joints), abrasion (sediment hurled at the cliff), attrition (sediment grinding smaller and rounder) and solution (carbonate rock such as Carboniferous limestone dissolved by slightly acidic seawater). Sub-aerial processes act on the cliff face above the waves: weathering (freeze-thaw, biological and chemical) and mass movement (rockfall, slumping, mudflow). These weaken and retreat cliffs even without wave attack, which is why slumping is common on the soft glacial-till cliffs of the Holderness coast in Yorkshire.

Transport and the sediment cell

Sediment moves by longshore drift: swash carries material up the beach at the angle of wave approach and backwash returns it straight down the steepest gradient under gravity, producing net movement along the coast in the direction of the dominant wind. Within a sediment cell, sources (eroding cliffs, rivers, offshore banks) feed transfers (longshore drift, tidal currents) that supply sinks (beaches, spits, offshore bars). Understanding the cell explains why protecting one beach can starve the next.

Erosional and depositional landforms

On a discordant coast of alternating resistant and weak rock, headlands and bays form by differential erosion: weaker clays and sands erode into bays while resistant rock stands out as headlands. A headland develops a wave-cut notch and platform, then a sequence of cave, arch, stack and stump along lines of weakness such as faults and joints. Depositional landforms include beaches, a spit (where longshore drift extends sediment across a river mouth, often hooked by wave refraction, with salt marsh behind), a bar (a spit linking two headlands), and a tombolo (a bar linking an island to the mainland, as at Chesil Beach connecting to the Isle of Portland).

Examples in context

Example 1. Headland-and-bay erosion at Pembrokeshire (Wales). The Pembrokeshire Coast, the UK's only coastal national park, is cut into folded Carboniferous limestone and Old Red Sandstone. Differential erosion of weaker and stronger bands has produced a textbook discordant coastline: resistant headlands such as St Govan's Head stand between eroded bays, and the limestone cliffs near Castlemartin display the full erosional sequence of caves, arches (the Green Bridge of Wales), stacks (Elegug Stacks) and blowholes (the Bosherston Mere). The high Atlantic fetch supplies the wave energy, and the varied geology controls where erosion concentrates, making it the classic Welsh case study for an exam.

Example 2. Spit formation and managed loss at Spurn Head (Humber, England). Spurn Head is a $5.5$ km curved sand-and-shingle spit at the mouth of the Humber estuary, fed by longshore drift carrying sediment southward from the rapidly eroding Holderness cliffs. Wave refraction at the spit's end has produced a recurved hook, and salt marsh and mudflats have accumulated in the sheltered water behind. Spurn shows the direct link between an eroding source (Holderness) and a depositional sink (the spit): the same drift that destroys the till cliffs builds the spit. After a tidal surge breached it in 2013, the spit became a tidal island, illustrating how dynamic and vulnerable depositional landforms are.

Try this

Q1. Define the term sediment cell. [2 marks]

• Cue. A self-contained coastal stretch, bounded by headlands, where sediment movement (sources, transfers and sinks) is largely closed off from neighbouring cells.

Q2. Explain how longshore drift transports sediment along a coast. [3 marks]

• Cue. Swash moves material up the beach at the angle of the dominant wave approach; backwash returns it straight down under gravity; net movement is along the coast in the direction of the dominant wind.