Coastal landforms: erosional and depositional features, vegetation-related systems

Coastal Environments – A‑Level Geography (9696)

1. Introduction

Coastal zones are the dynamic interface between land and sea. Their form and function are controlled by four interacting factors:

• Wave energy – generation, height, period, direction and refraction.
• Tidal range – determines the vertical zone of influence.
• Sediment supply & transport – source, grain‑size, long‑shore drift and offshore currents.
• Biological factors – vegetation, mangroves, coral reefs and other organisms that modify energy and trap sediment.

Understanding these controls allows us to explain coastal landforms, evaluate erosion‑deposition processes and design appropriate management strategies.

2. Wave Generation & Characteristics

• Wind‑sea generation – Waves are produced when wind blows over a fetch for a given duration at a certain speed.
  • Fetch (km)
  • Wind speed (m s⁻¹)
  • Duration (hours)
• Wave height (H) and period (T) – control the energy carried by a wave.
  Wave energy (E) ≈ (ρ g H²)/8 (ρ = water density, g = 9.81 m s⁻²).
• Wave refraction – Bending of waves around headlands concentrates energy on protruding points and reduces it in bays.
• Wave breaking – Occurs when the orbital motion of water reaches the seabed (shallow water).
  • Spilling – gentle slope; energy is dissipated gradually.
  • Plunging – steep slope; large, abrupt release of energy (typical on cliffs).
• Resultant drift vector (RDV) – The vector sum of incident and reflected wave directions; indicates the net direction of long‑shore drift.

3. Marine Erosion Processes

4. Sub‑Aerial (Above‑Water) Erosion Processes

• Freeze‑thaw (frost shattering) – Water enters cracks, freezes, expands and widens them.
• Salt‑crystal growth – Evaporation of sea spray leaves salt crystals that pry rock apart.
• Mass movement – Slides, falls and slumps on over‑steepened cliff faces, often triggered by wave undercutting.

5. Marine Transport & Deposition

• Modes of sediment transport
  • Traction – Rolling of coarse gravels along the seabed.
  • Saltation – Hopping of sand grains.
  • Suspension – Fine silt and clay remain in the water column.
• Long‑shore drift – Sediment moves parallel to the coast when waves approach at an angle. Direction is given by the RDV.
• Sediment cells – A stretch of coast where sediment input, transport and output are roughly balanced; useful for management planning.

6. Classification of Coastal Landforms

6.1. By Rock Structure

6.2. Erosional Landforms (High‑energy zones)

• Cliff – Steep rock face produced by headward erosion.
• Wave‑cut platform – Flat base of a cliff formed by abrasion and solution.
• Sea‑cave – Hollowed out of a headland by hydraulic action and solution.
• Sea‑arch – Formed when two caves meet.
• Sea‑stack – Isolated column left after arch collapse.
• Blowhole – Vertical shaft that releases compressed air and spray.

6.3. Depositional Landforms (Low‑ to moderate‑energy zones)

• Beach – Accumulation of sand or shingle parallel to the shoreline.
• Bar – Linear ridge of sediment that may close a bay or estuary mouth (e.g., Chesil Bar).
• Spit – Narrow sand extension projecting from the coast into open water (e.g., Spurn Head, England).
• Barrier island – Offshore sand ridge protecting a lagoon (e.g., The Barbados Barrier).
• Tombolo – Sand bridge linking an island to the mainland (e.g., St Michael’s Tombolo, Cornwall).
• Dune system – Wind‑blown sand accumulations behind the beach, often vegetated.
• Salt‑marsh – Low‑lying, vegetated intertidal zone that traps fine sediment.
• Mangrove swamp – Tropical intertidal forest with prop roots that stabilise sediments.
• Coral reef – Calcium‑carbonate structures built by living corals; can be fringing, barrier or atoll (e.g., Great Barrier Reef).

7. Vegetation‑Related Coastal Systems

Plants and other organisms modify coastal processes by trapping sediment, reducing wind speed and dissipating wave energy.

7.1. Dune Vegetation

• Marram grass (Ammophila arenaria) – Deep rhizomes bind sand and promote dune accretion.
• Sea oats (Uniola paniculata) – Dominant on Atlantic dunes; stabilise fore‑dunes.
• Function: reduces wind shear, traps aeolian sand, creates a protective barrier for inland areas.

7.2. Salt‑Marsh Vegetation

• Spartina spp. (cordgrass) – Tolerates regular inundation; traps fine sediments and builds vertical accretion.
• Salicornia spp. (glasswort) – Halophytic succulent that raises marsh surface.
• Function: buffers wave energy, provides habitat, and contributes to land‑building.

7.3. Mangrove Systems (Tropical)

• Typical species: Rhizophora spp. (prop roots), Avicennia spp. (pneumatophores).
• Complex aerial and prop roots trap suspended sediment and dissipate wave energy.
• Critical for coastline protection in low‑lying tropical regions and for biodiversity.

7.4. Coral‑Reef Systems

• Built from calcium‑carbonate skeletons of scleractinian corals.
• Act as natural breakwaters, reducing wave energy reaching the shore.
• Require clear, warm water and adequate sunlight; highly sensitive to climate change and ocean acidification.

8. Linking Landforms & Vegetation – Analytical Skills Required by the Syllabus

• Interpretation of wave‑energy diagrams – Identify high‑energy zones (predict erosional landforms) and low‑energy zones (predict depositional landforms).
• Construction of sediment‑budget tables – Compare sediment inputs (river delivery, long‑shore drift, offshore sources) with outputs (offshore transport, erosion).
• Evaluation of vegetation effectiveness – Use case‑study data (e.g., dune‑planting at Skegness, salt‑marsh restoration on the Thames Estuary) to assess changes in sediment capture rates and shoreline stability.
• Cause‑effect chains – Explain how a change in one factor (e.g., reduced wave energy due to a breakwater) influences landform development and subsequent vegetation succession.

9. Management of Coastal Areas

9.1. Hard Engineering

• Sea walls – Provide immediate protection to infrastructure but may increase erosion downstream (downdrift scouring).
• Groynes – Trap long‑shore drift sediment, build up beaches locally; can starve adjacent downdrift sections.
• Breakwaters & offshore reefs – Reduce wave energy; may cause sediment accumulation on the leeward side and alter natural drift patterns.

9.2. Soft Engineering

• Beach nourishment – Adds sand to eroding beaches; requires periodic replenishment and monitoring of sediment compatibility.
• Dune restoration – Planting marram grass, installing sand fences and using brushwood to encourage dune formation.
• Managed realignment – Allows controlled retreat, creating new intertidal habitats (e.g., salt‑marshes) that act as natural buffers.

9.3. Integrated Coastal Zone Management (ICZM)

• Combines scientific monitoring, stakeholder involvement and adaptive management.
• Balances economic, environmental and social objectives.
• Examples: UK “Coastal Management Plan”, New Zealand “Coastal Policy Statement”, and the EU “Integrated Coastal Management Directive”.

10. Syllabus Review – Actionable Checklist (Requirement 8)

11. Summary

Coastal environments result from a balance between wave‑driven erosional forces and sediment‑driven depositional processes. The intensity and distribution of these forces are governed by wave generation, tidal range, rock structure and sediment supply. Vegetation – whether dune grasses, salt‑marsh halophytes, mangroves or coral reefs – plays a crucial role in stabilising sediments and attenuating energy. Effective management therefore requires a clear understanding of the physical processes, the ecological contributions of vegetation, and the trade‑offs between hard and soft engineering within an Integrated Coastal Zone Management framework.