AQA Geography A-level 3.1.3: Coastal Systems and Landscapes Notes

The Coastal System and Sediment Cells

  • Definition of the Coastal System: The coast is classified as an open system. This means it receives energy and material inputs from outside the system (terrestrial, atmospheric, or oceanic sources) and transfers outputs (material and energy) away from the coast into other systems, such as the rock, water, and carbon cycles.
  • Sediment Cells:     - Coasts are divided into horizontal sections known as sediment cells.     - These are typically bordered by prominent headlands which act as natural boundaries.     - Within a sediment cell, the movement of sediment is largely self-contained.     - Dynamic Equilibrium: The flows of sediment within a cell act in a state of dynamic equilibrium. This balance can be upset by human intervention over the long term or by natural variations (e.g., storms) in the short term.     - Each major sediment cell is further divided into smaller subcells.

Components of the Coastal System

  • Inputs (Material and Energy):     - Marine: Waves, tides, and salt spray.     - Atmosphere: The sun, variations in air pressure, wind speed, and wind direction.     - Humans: Pollution, recreational activities, settlement development, and coastal defenses.
  • Outputs (Material and Energy):     - Ocean currents.     - Rip tides.     - Sediment transfer (out of the cell).     - Evaporation.
  • Stores and Sinks:     - Depositional and erosional features that hold sediment: Beaches, sand dunes, spits, bars, tombolos, headlands, bays, nearshore sediment, cliffs, wave-cut notches, wave-cut platforms, caves, arches, stacks, stumps, salt marshes, tidal flats, and offshore bands/bars.
  • Transfers and Flows (Processes Linking Elements):     - Wind-blown sand.     - Mass-movement processes.     - Longshore Drift.     - Weathering.     - Erosion: Including hydraulic action, corrosion, attrition, and abrasion.     - Transportation: Including bedload, suspension, traction, and solution.     - Deposition: Including gravity settling and flocculation.
  • Energy Sources: The power driving coastal transfers and flows comes from wind, gravitational forces, and flowing water.

Sediment Sources and Budgets

  • Primary Sources of Sediment:     - Rivers: These account for the vast majority of sediment in the coastal zone. Sediment is often deposited in estuaries, which are brackish (mixed salt and fresh water) environments and vital wildlife habitats.     - Cliff Erosion: Crucial in areas with unconsolidated (weakly bound) cliffs. Erosion frequency usually peaks in winter months due to increased storm activity.     - Wind: Acts as an energy source that blows sand along or up a beach. This is vital for sand dune formation and occurs significantly in glacial or desert coastal environments.     - Glaciers: When ice masses flow into the ocean and calve (break off), they deposit sediment previously stored within the ice.     - Offshore: Waves, tides, and currents erode offshore sediment sinks (like offshore bars) and transport this material onto the beach, aiding beach build-up.     - Longshore Drift: Prevailing winds alter wave direction, allowing sediment to be moved along the coast. This serves as an output for one section and an input for another.
  • Sediment Budgets: This involves the use of data on inputs, outputs, stores, and transfers to assess the net gain or loss of sediment within a cell. While the system ideally operates in dynamic equilibrium where inputs equal outputs, human actions and natural variations often disrupt this state.

The Littoral Zone

  • Definition: The littoral zone is the area of land stretching from the cliffs or dunes on the coast to the offshore area that lies beyond the influence of the waves.
  • Dynamic Nature: The zone changes constantly due to:     - Short-term factors: Tides and storm surges.     - Long-term factors: Changes in sea level and human intervention.

Coastal Energy: Wave Formation and Factors

  • The Sun: The primary source of energy for all natural systems.
  • Wave Formation Process:     - Wind moves across the water surface, causing frictional drag.     - This drag creates ripples and waves, initiating a circular orbital motion of water particles.     - As the water becomes shallower near the coast, the orbit of particles becomes more elliptical.     - This leads to horizontal movement; wave height increases, while wavelength (distance between waves) and wave velocity decrease.     - Water backs up behind the wave until it breaks (collapses) and surges up the beach.
  • Factors Affecting Wave Energy:     - Strength of the Wind: Related to the pressure gradient between two areas; larger gradients cause stronger winds and thus more powerful waves.     - Duration of the Wind: The longer the wind is active, the more energy the waves can accumulate.     - Size of the Fetch: The fetch is the distance over which the wind blows. A larger fetch results in more powerful waves.

Classification of Wave Types

  • Constructive Waves:     - Formation: Created by weather systems in the open ocean.     - Wavelength: Long.     - Frequency: 6−96-9 per minute.     - Characteristics: Low waves that surge up the beach.     - Swash/Backwash: Strong swash, weak backwash.     - Effect: Builds up/creates the beach; typically found on gently sloped beaches.
  • Destructive Waves:     - Formation: Created by localized storm events with strong winds close to the coast.     - Wavelength: Short.     - Frequency: 11−1611-16 per minute.     - Characteristics: High waves that plunge onto the beach.     - Swash/Backwash: Weak swash, strong backwash.     - Effect: Removes material from/decreases the beach size; typically found on steeply sloped beaches.
  • Temporal and Environmental Variations:     - Constructive waves usually dominate in summer; destructive waves dominate in winter.     - Storms can cause constructive waves to turn destructive.     - Climate change may increase UK storm frequency, affecting wave dominance.     - Coastal management interventions can also alter wave types.

Coastal Feedback Loops: Beaches and Waves

  • Negative Feedback Mechanism:     - Constructive waves cause deposition, making the beach profile steeper.     - A steeper beach profile then favors the formation of destructive waves.     - Destructive waves erode the beach, reducing the profile (making it more gentle).     - A more gentle profile leads back to the formation of constructive waves.     - Result: Beaches are generally gentler in summer (fewer storms, more constructive) and steeper in winter (more storms, destructive dominance).

Tides, Currents, and Coastal Energy

  • Tides:     - Driven by the energy of Gravity.     - Tidal Range: The height difference between high and low tides. This range is usually largest in channels such as river estuaries.     - Spring Tide: Occurs when the sun and moon are in alignment; their gravitational forces combine to create the highest high tides, lowest low tides, and the largest possible tidal range.     - Neap Tide: Occurs when the sun and moon are perpendicular to each other; their gravitational forces work against each other, creating the smallest tidal range (lowest high tide, highest low tide).
  • Rip Currents:     - Powerful underwater currents near the shoreline.     - Formed when plunging waves cause a buildup of water at the top of the beach; the backwash is forced under the surface due to resistance from incoming breaking waves.     - These act as an energy source and can cause significant sediment output from the beach area.
  • Coastline Categorization:     - High-Energy Coastlines: Characterized by powerful waves and large fetches. Features include rocky headlands and frequent destructive waves. Erosion rates typically exceed deposition rates.     - Low-Energy Coastlines: Characterized by less powerful waves in sheltered areas where constructive waves prevail. Features include sandy areas and depositional landforms. Deposition rates exceed erosion rates.

Wave Refraction

  • Mechanism: On uneven coastlines, waves turn and lose energy as they bend around headlands.
  • Energy Distribution: Wave energy becomes concentrated on headlands (creating erosive features) but is dissipated in bays (creating depositional features like beaches).
  • Negative Feedback of Refraction:     - Differential rock strength leads to headlands (resistant rock) and bays (unconsolidated rock).     - Refraction increases erosion on headlands but reduces it in bays because the beach protects the coastline behind.     - Eventually, headlands are worn away, which then increases the potential for erosion in the bays again.

Marine Erosion Processes

  • Corrasion: The sea picks up sand and pebbles from sediment sinks and hurls them against cliffs, particularly at high tide.
  • Abrasion: Sediment moved along the shoreline is worn down over time.
  • Hydraulic Action: Waves crashing onto rock force air into cracks. High pressure causes cracks to widen.
  • Cavitation: A subset of hydraulic action where bubbles in the water implode under high pressure, creating tiny, erosive water jets.
  • Corrosion (Solution): Mildly acidic seawater dissolves alkaline rocks such as limestone.
  • Wave Quarrying: Breaking waves hit the cliff face with enough force to pull away rock or remove weathered fragments.
  • Factors Affecting Erosion Rate:     - Wave energy and type.     - Beach size and presence (larger beaches protect cliffs).     - Activity of subaerial processes.     - Rock faults and overall rock lithology.

Coastal Transportation and Deposition

  • Transportation Processes:     - Traction: Large, heavy sediment rolls along the seabed, pushed by currents.     - Saltation: Smaller sediment bounces along the seabed.     - Suspension: Small sediment is carried within the water flow.     - Solution: Dissolved material is carried in the water.
  • Longshore Drift (LSD):     - Waves approach the beach at an angle (determined by prevailing wind).     - Swash pushes sediment up the beach at that angle.     - Backwash carries sediment straight back down to the sea due to gravity.     - Over time, sediment migrates along the length of the beach.
  • Deposition Processes:     - Occurs when sediment is too heavy or waves lose energy.     - Gravity Settling: Heavier material drops out of the water column.     - Flocculation: Fine particles (like clay) clump together due to chemical reactions in saline water, becoming heavy enough to sink.     - High-energy coasts deposit large rocks and shingle; low-energy coasts deposit smaller sediment.

Weathering and Feedbacks

  • Definition: The breakdown of rocks in situ over time, providing material inputs to the littoral zone.
  • Weathering Feedbacks:     - Positive Feedback: If weathered rock is removed from the cliff base faster than it is produced, more rock is exposed, promoting further weathering and erosion (providing more rocks for saltation/abrasion).     - Negative Feedback: If weathered debris builds up at the cliff base (removal is slower than weathering), it protects the cliff foot from erosion/weathering by reducing the exposed surface area.
  • Mechanical (Physical) Weathering:     - Freeze-thaw (Frost-Shattering): Water freezes in cracks, expands, and forces cracks to develop.     - Salt Crystallisation: Evaporating seawater leaves salt crystals that grow and widen cracks; salt can also corrode ferrous rock.     - Wetting and Drying: Some rocks (e.g., clay) expand when wet and contract when dry, causing them to break.
  • Chemical Weathering:     - Carbonation: Rainwater absorbs CO2CO_2 to form weak carbonic acid, which reacts with limestone to form soluble calcium bicarbonate.     - Oxidation: Minerals (especially iron) react with oxygen, increasing in volume and causing the rock to crack.     - Solution: Minerals like rock salt are directly dissolved.
  • Biological Weathering:     - Plant Roots: Roots grow into cracks and exert pressure.     - Birds: Species like Puffins dig burrows that weaken cliffs.     - Rock Boring: Clams secrete chemicals that dissolve rock.     - Seaweed Acids: Kelp contains sulphuric acid which dissolves minerals.     - Decaying Vegetation: Water flowing through decaying plants becomes acidic.

Mass Movement

  • Definition: The movement of material downhill under the influence of gravity, categorized into creeps, flows, slides, and falls.
  • Specific Types:     - Soil Creep: Slow downhill movement of particles that rise/fall due to wetting/freezing; creates shallow terracettes.     - Solifluction: Specific to periglacial environments; top layers thaw in summer and flow over deeper frozen layers.     - Mudflows: High water content reduces friction, causing soil and mud to flow rapidly over bedrock/clay; creates a significant threat to life.     - Rockfall: Occurs on steep cliffs ( > 40^\circ or vertical) due to mechanical weathering; creates scree slopes at the base.     - Landslide: Heavy rain reduces friction, causing an intact block of rock to move quickly down a flat slope.     - Landslip or Slump: Occurs on curved slopes; land collapses under its own weight due to increased Pore Water Pressure (PWP), creating a terraced look.     - Runoff: Water flowing over the surface erodes the cliff face and carries sediment/pollution into the littoral zone.
  • Vulnerability: Colder climates favor mechanical weathering; warmer climates favor chemical weathering.

Erosional Landforms

  • Caves, Arches, Stacks, and Stumps Sequence:     - Energy concentrates at headlands.     - Joints are eroded into caves or blowholes.     - Erosion pierces through the headland to form an arch.     - The arch roof collapses (due to lack of support), leaving a stack.     - Erosion and weathering reduce the stack to a stump.
  • Cliff Profiles:     - Steep Cliffs: Strong rock, missing beach, long fetch, high-energy waves.     - Gentle Cliffs: Weak rock, prone to slumping, large beach (dissipates energy), low-energy waves.     - Rate of Retreat: Highest in areas with unconsolidated rock and sands.
  • Wave-cut Notch and Platform:     - Erosion at the high-tide line creates a wave-cut notch.     - Overhanging cliff becomes unstable and collapses.     - A platform of the cliff base remains, forming a wave-cut platform.     - Negative Feedback: As the platform lengthens, waves lose energy before reaching the cliff, slowing the rate of erosion.

Depositional Landforms

  • Beaches:     - Stretch from the low tide to high tide line.     - Accretion occurs in summer (constructive waves); Excavation occurs in winter (destructive waves).     - types: Swash-aligned (waves approach perpendicular, limited LSD) and Drift-aligned (waves approach at an angle, significant LSD, sediment travels along the coast).     - Sediment size is larger at the top (winter storm deposits) and near cliffs (angular scree).
  • Spits:     - Material is deposited in line with the coast where it changes direction.     - Recurved spit: Ends curve due to wind change or wave refraction.     - Compound spit: Multiple recurved ends.     - Sheltered areas behind spits often become mudflats or saltmarshes.
  • Barrier Beaches and Bars:     - Form when a spit or beach joins two headlands, trapping a brackish lagoon.     - Can be formed by contemporary processes or post-glacial rising sea levels.     - If detached from the mainland, it becomes a barrier island.
  • Tombolos: A beach or bar connecting the mainland to an offshore island (e.g., due to wave refraction reducing velocity).
  • Offshore Bars: Submerged regions of sand where wave energy is too low to carry material further inshore. They absorb wave energy and can act as sediment inputs when waves pick material back up from them.

Coastal Vegetation and Succession

  • Functions: Roots bind soil; plants provide a protective submerged layer; vegetation reduces wind speed at the surface to prevent wind erosion.
  • Sand Dunes Succession:     - Wind blows dry sand landwards (large tidal ranges help sand dry).     - Pioneer Species: Resistance plants like sea rocket bind sand.     - Marram Grass: Decaying organic matter adds nutrients/humus, allowing marram grass to grow. Marram is flexible, adapted to reduce transpiration, has roots up to 3 m3\,m deep, and tolerates temperatures up to 60∘C60^\circ C.     - Climatic Climax: The final stage where trees colonize the area.
  • Estuarine Mudflats and Saltmarshes:     - Form in estuaries where water velocity is low or behind sheltered spits.     - Flocculation leads to mud accumulation above high tide.     - Pioneer plants colonize the transition zone; sections eventually rise to form a meadow and reach climatic climax with tree colonization.

Sea Level Change

  • Isostatic Change (Local):     - Land rises or falls relative to the sea.     - Isostatic Recovery: Land rebounds/rises after ice caps melt.     - Isostatic Subsidence: Land sinks under the weight of ice (or due to tectonic activity).     - Example: In the 2004 Indian Ocean earthquake, the city of Banda Aceh sank by 0.5 m0.5\,m.
  • Eustatic Change (Global):     - Affects global sea levels due to changes in water volume.     - Thermal Expansion: Warmer water expands in volume.     - Glacial Cycles: In the last ice age, sea levels were over 100 m100\,m lower because water was stored as ice caps; melting caused a global rise.

Coastal Evolution and Submergence/Emergence

  • Emergent Landforms: Land rises relative to the sea. includes raised beaches and relic cliffs (with wave-cut notches indicating former erosion).
  • Submergent Landforms: Sea level rises or land sinks.     - Rias: Flooded river valleys/inlets. Deeper at the mouth, shallower inland.     - Fjords: Flooded deep glacial valleys. Deeper in the middle than at the mouth (where the glacier left the valley).     - Dalmatian Coasts: Valleys running parallel to the coast are flooded, leaving islands.
  • Contemporary Statistics:     - 20,00020,000 years ago: Sea levels were 120 m120\,m lower.     - 8,0008,000 years ago: Rise slowed.     - 3,0003,000 years ago: Leveled at current height.     - Since 1880: Increased by 235 mm235\,mm.     - IPCC Prediction by 2100: Rise of 0.3−1.0 m0.3 - 1.0\,m, risking aquifer pollution in atoll islands and inundation of coastal cities.

Risks and Consequences

  • Storm Surges: Caused by low pressure in tropical storms.     - Risk factors: Removal of Mangrove forests (which can handle sea level rise up to 8x current rates) and Global Warming (increasing storm intensity).
  • Socio-Economic Impacts:     - Reduced house prices and land values.     - Destruction of plant successions.     - Increased risk of cliff collapse.     - Global Scale: Over 1 billion people live in coastal flood zones; 50%50\% of the world population lives within 60 km60\,km of the coast; 75%75\% of large cities are coastal.     - Rise in environmental refugees.

Coastal Management Strategies

  • Hard Engineering (Man-made, high impact):     - Groynes: Timber/rock protrusions to trap LSD sediment. Benefits: Builds beach/tourism, cost-effective. Drawbacks: Ugly, starves downdrift areas of sediment.     - Sea Walls: Concrete structures reflecting energy. Benefits: Effective, promenade use. Drawbacks: Expensive, reflects energy elsewhere, ugly.     - Rip Rap (Rock Armour): Large rocks. Benefits: Cost-effective. Drawbacks: Hazard to climb, rocks don't match local geology.     - Revetments: Wooden/concrete ramps. Benefits: Cost-effective. Drawbacks: Maintenance intensive, ugly.     - Offshore Breakwaters: Underwater rock barriers that force waves to break early.
  • Soft Engineering (Work with nature):     - Beach Nourishment: Adding offshore sediment to the beach. Benefits: Natural look, tourism, cost-effective. Drawbacks: Needs constant maintenance, dredging harms habitats.     - Cliff Regrading/Drainage: Reducing cliff angle and draining water. Benefits: Cost-effective. Drawbacks: Sudden falls possible, looks unnatural.     - Dune Stabilisation: Planting marram grass. Benefits: Cost-effective, wildlife value. Drawbacks: Time-consuming planting.     - Marsh Creation (Managed Retreat): Flooding low-lying areas. Benefits: Wildlife habitat. Drawbacks: Loss of agricultural land, necessitates compensation.

Management Evaluation and Policy

  • Cost-Benefit Analysis (CBA): Projects must have tangible/intangible benefits outweighing costs. DEFRA recommends a 1:11:1 ratio for approval.
  • Sustainable Coastal Management: Holistic strategies that manage natural resources, create jobs, educate communities, monitor changes, and consider all stakeholders.
  • Integrated Coastal Zone Management (ICZM): Large-scale integrated strategy across political boundaries. Prioritizes environmental protection over short-term economy and acknowledges that protecting one area may expose another.
  • Shoreline Management Plans (SMPs): Created for each UK sediment cell. Four options per section:     1. Hold the Line (Maintain existing defenses).     2. No Active Intervention.     3. Managed Retreat/Realignment.     4. Advance the Line (Build new defenses seaward).
  • Conflict: Policies create winners (beneficiaries) and losers (those losing property or jobs). Losing social networks and place attachment causes significant distress.
  • Sediment Cell Impacts: sea walls can reflect energy downdrift, increasing erosion elsewhere. Groynes starve downdrift areas of sediment, reducing beach size and exposing cliffs to higher energy waves.