Coasts

protest bannerLandforms of coastal erosion

Landforms of coastal erosion include cliffs, wave-cut platforms, arches, stacks, headlands and coves (amongst others). The existence of many of these features depends upon the maintenance of a vertical cliff-face through an on-going cycle of undercutting, collapse and retreat.

When waves break at the foot of a rock face, a wave-cut notch is created by marine erosion processes. Over time, a section of rock is removed from the bottom of the cliff face, leading ultimately to the collapse of the un-supported section above. Every time this process is repeated, a new near-vertical cliff-face is created.

Without marine erosion operating in this way, sub-aerial processes (normal atmospheric weathering processes such as carbonation or hydrolysis) would degrade the cliff face, creating a gentle slope where the upper element has been worn away by weathering while fallen material collects at the bottom of the cliff. Vegetation can colonies such slopes over time, leading to greater stability.

Wherever land has risen from the sea in the past as a result of isostatic processes (when land subsides, rises or tilts), degraded cliffs can often be seen that are no longer reached by the sea. Instead of boasting a dramatic near-vertical profile, gentle vegetated slopes are apparent on account of marine erosion no longer attacking and undermining the landform. Plenty of these abandoned (or relict) cliffs can be seen in the north-west of Scotland.

How will climate change impact on erosional processes and landforms in British waters?

Waves erode via a number of processes. These are wave pounding, hydraulic action, abrasion and corrosion. The effectiveness of the first three of these depends upon wave energy. With climate change, stormier weather could increase rates of marine erosion and the UK has already been reported as becoming twice as stormy in the past 50 years! According to new Hadley Centre research, climate change has modified the North Atlantic oscillation and forced deep depressions that used to hit Ireland further south, causing more of them to pass over southern parts of the UK where there are many cliffed coastlines (for instance, the world-famous Jurassic Coastline).

In addition, climate change is likely to bring about significant eustatic rises in sea level, initially because of thermal expansion of sea water and later due to the increased volume of melt-water as a result of glaciers melting. In the event of major land-based ice sheets in Greenland and Antarctica melting, sea levels could rise by many tens of metres - although this is not a change anyone alive today is likely to live to see. The impact that rising sea-levels will have on cliff erosion depends upon existing conditions and also on the operation of isostatic processes (where the land is rising or sinking independently or any change in ocean water levels). In general:

  • Areas where cliffs are currently subject to undercutting and collapse will continue to retreat, although the position of the wave-cut notch and the level and gradient of the wave-cut platform may begin to change over time as waves will attack the cliff face at a higher point.
  • Abandoned or relict cliffs may find themselves subject to retreat once more as sea levels may rise to a point where waves can reach the bottom of the cliff face.
  • In northern Scotland, isostatic changes may continue to take place at the same rate as eustatic rises occur. There will therefore be no net change in the observed sea level and little change may be witnessed in coastal morphology.
  • Rates of erosion and retreat will increase in any area that appears to be experiencing a higher frequency of stormy weather, especially in areas of softer clay.

Student Practice Question:

Explain why rates of coastal erosion vary.

A large part of the discussion will look at spatial variations in rates of erosion, linked to factors such as rock type and structure, wave energy and the presence of defences. However, a good response will also consider temporal changes and the impact that climate change can have on whether or not waves are reaching the base of a cliffed coastline (as sea-levels rise or fall).