10.1 Erosional coastal landforms

Coastal erosion shapes shorelines through wave action, tides, and environmental factors. Cliffs, platforms, and sea stacks form as waves batter coastlines, with rock type and structure influencing erosion rates and landform development.

Wave energy distribution plays a crucial role in shaping coastal features. High-energy environments create steep cliffs and narrow platforms, while wave refraction concentrates erosion around headlands, forming sea caves and arches.

Coastal Erosion and Landform Development

Wave Action and Environmental Factors

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• Wave action drives coastal erosion with wave energy and frequency directly impacting erosion rates
  • High-energy waves (storm waves) cause more rapid erosion than low-energy waves
  • Wave frequency affects the continuity of erosional processes
• Tidal range influences the vertical extent of wave action and area susceptible to erosion
  • Macrotidal coasts (tidal range >4m) experience erosion over a larger vertical area
  • Microtidal coasts (tidal range <2m) concentrate erosion in a narrower zone
• Climate factors shape weathering rates and erosional event intensity
  • Temperature fluctuations cause thermal expansion and contraction of rocks
  • Precipitation increases chemical weathering and physical erosion
  • Storms amplify wave energy and erosional potential

Geological and Anthropogenic Influences

• Coastal geology determines coastline susceptibility to erosion
  • Rock type impacts resistance (granite more resistant than limestone)
  • Structural features like joints and faults create erosional weak points
• Sea-level changes alter shoreline position and erosional patterns
  • Eustatic changes result from global water volume variations
  • Isostatic changes occur due to local crustal movements
• Human activities modify natural erosional processes
  • Coastal development removes natural buffers (dunes, vegetation)
  • Engineering structures (seawalls, groins) interrupt sediment transport

Formation of Erosional Features

Cliff and Platform Development

• Coastal cliffs form through wave erosion undercutting slope bases
  • Continuous undercutting leads to cliff instability and collapse
  • Cliff retreat rates vary based on rock resistance and wave energy
• Wave-cut platforms extend seaward from cliff bases
  • Formed by continuous wave erosion and cliff retreat
  • Platform width increases as cliffs recede landward
• Formation process progresses over time
  • Cliffs retreat, platforms expand, and overall coastal profile evolves
  • Rate of formation depends on rock resistance, wave energy, and tidal range
• Cliff profiles and platform widths vary with local conditions
  • Steep cliffs often indicate resistant rock or high wave energy
  • Wide platforms suggest prolonged erosion or less resistant rock

Sea Arch and Stack Formation

• Sea arches develop when waves erode through headlands or weak zones
  • Initial formation begins with sea cave development on opposite sides
  • Continued erosion eventually connects caves, forming an arch
• Arch collapse leads to stack or stump formation
  • Isolated sea stacks remain as erosion-resistant rock pillars
  • Further erosion reduces stacks to low-lying stumps
• Feature longevity depends on environmental factors
  • Rock resistance influences the rate of arch and stack erosion
  • Wave energy and storm frequency affect the speed of formation and destruction

Rock Type and Coastal Features

Lithological Influences on Erosion

• Rock type determines resistance to erosion
  • Harder rocks (granite) form resistant headlands
  • Softer rocks (shale) erode into bays
• Chemical composition affects weathering rates
  • Carbonate rocks (limestone) susceptible to chemical dissolution
  • Silicate rocks (quartz) more resistant to chemical weathering
• Differential erosion of varying rock types creates complex landscapes
  • Alternating hard and soft rock layers form crenulated coastlines
  • Resistant rock outcrops become isolated as sea stacks (Old Harry Rocks, UK)

Structural Controls on Coastal Morphology

• Joints, faults, and bedding planes create erosional weak zones
  • Wave action exploits these weaknesses, forming sea caves and arches
  • Orientation of structural features influences the alignment of coastal landforms
• Dip and strike of rock layers affect cliff stability and platform development
  • Seaward-dipping beds often result in more stable cliffs
  • Landward-dipping beds can lead to increased rockfall and cliff retreat
• Geological structure controls feature scale and distribution
  • Fold axes may determine the location of headlands and bays
  • Fault lines can create linear coastal features or influence cliff orientation

Wave Energy and Landform Morphology

Energy Distribution and Landform Shaping

• Higher wave energy environments produce steeper cliffs and narrower platforms
  • Increased erosional force concentrates wave impact at cliff base
  • Examples include exposed Atlantic coastlines (Cliffs of Moher, Ireland)
• Wave approach angle influences erosion direction and intensity
  • Oblique wave approach can create asymmetrical headlands
  • Longshore currents generated by angled waves transport eroded material
• Wave refraction concentrates energy around headlands
  • Accelerated erosion in these areas forms sea caves and arches
  • Examples include Durdle Door in Dorset, UK

Temporal and Spatial Variations in Wave Energy

• Seasonal wave energy variations create erosion and deposition cycles
  • Winter storms often cause increased erosion
  • Summer conditions may allow for temporary sediment accumulation
• Offshore bathymetry modifies wave energy distribution
  • Submarine canyons can focus wave energy on specific coastal sections
  • Offshore islands or reefs may provide protection, reducing erosion rates
• Long-term wave climate changes alter landform equilibrium
  • Climate change-induced sea-level rise may accelerate coastal retreat
  • Changes in storm frequency or intensity can modify existing features
  • Example: Increased erosion rates along vulnerable coastlines (East Anglia, UK)