Geography (9635) | Coasts as natural systems

🌊 Coasts as Natural Systems: Your Essential Study Guide 🌊

Hello Geographer! Welcome to the incredibly dynamic world of coasts. If you've ever stood on a beach and watched the waves crash in, you've witnessed a massive natural system in action. This chapter is all about understanding how the coast works—not as a static boundary, but as a busy, energy-driven system where water, land, and sediment constantly interact.

Don't worry if the terminology looks tricky! We will break down these complex ideas (like 'dynamic equilibrium' and 'hydraulic action') into simple, manageable steps. By the end, you'll see the coast through the eyes of a physical geographer!

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1. Coasts as Natural Systems (3.1.3.1)

What is a System?

In Geography, a system is a set of interrelated components (stores) working together to form a whole. The coastal system is an open system, meaning energy and matter can freely enter and leave.

Think of the coast like a savings account that constantly changes:

• Inputs (Income): These are things that bring energy or material into the system.
  • Examples: Energy (winds, waves, tides), Sediment (rivers, offshore currents, cliff erosion).
• Stores / Components (The Account Balance): Where matter or energy is held temporarily.
  • Examples: Beaches, sand dunes, salt marshes, cliff material, nearshore sediment.
• Flows / Transfers (Transactions): The movements of energy or material between stores.
  • Examples: Longshore drift moving sand down the coast, erosion processes (moving material from a store), deposition.
• Outputs (Expenditure): Material or energy leaving the system entirely.
  • Examples: Fine sediment washed out to deep sea, or material blown inland by wind beyond the coastal zone.

Feedback and Dynamic Equilibrium

The system is always trying to balance itself. This balancing act involves feedback loops:

1. Negative Feedback: The Stabiliser

This mechanism reduces or cancels out the effect of a change, helping the system maintain balance.

Example: A severe storm (input) erodes a large amount of beach sand (store). If the erosion makes the beach profile steeper, the steep angle actually causes waves to break earlier and lose energy, slowing down further erosion. This stabilizes the beach profile.

2. Positive Feedback: The Accelerator

This mechanism reinforces or amplifies the effect of a change, driving the system further away from its original state.

Example: A rising sea level (input) submerges an area of salt marsh (store). The submerged marsh cannot trap sediment as effectively, leading to more erosion, which in turn allows the sea to penetrate further and erode even more marsh. (A cycle of rapid destruction).

Dynamic Equilibrium

The coastal system exists in a state of Dynamic Equilibrium. This means the system has a steady state, where inputs and outputs are roughly balanced, but it constantly adjusts to minor changes in energy and sediment. It's stable, but never truly still.

Analogy: Imagine walking on a treadmill. You are moving (energy/flows), but staying in the same relative position (equilibrium).

Landform vs. Landscape

• Landform: An individual, distinct feature created by geomorphological processes. (e.g., a single stack, a cave, a barchan dune).
• Landscape: The overall appearance of an area, formed by the combination of many related landforms. (e.g., a rocky coastline landscape featuring cliffs, caves, arches, and stacks combined).

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2. Energy and Coast Types (3.1.3.2)

Sources of Coastal Energy

The power that drives coastal change comes from several sources:

1. Winds: Winds generate waves and can also transport sediment (especially sand in dunes). The fetch (the distance over water the wind blows) determines the size and power of the waves.

2. Waves: The most important factor in rapid change. Waves are created by wind friction on the sea surface. We classify them into two main types based on their action:

Constructive Waves (The Builders)

• Action: Deposition (building up the beach).
• Movement: Gentle, spilling breaker.
• Swash vs. Backwash: The swash (water moving up the beach) is stronger than the backwash (water draining back).
• Frequency: Low frequency (6-8 waves per minute).
• Outcome: Creates wide, gently sloping beaches.

Destructive Waves (The Destroyers)

• Action: Erosion (wearing down the coast).
• Movement: Powerful, plunging breaker.
• Swash vs. Backwash: The backwash is stronger than the swash, dragging material back down the beach and offshore.
• Frequency: High frequency (10-14 waves per minute).
• Outcome: Creates steep, narrow beaches, often with a storm ridge.

3. Currents and Tides:

• Tides: The regular, periodic rise and fall of sea level, caused by the gravitational pull of the moon and sun. Tides define the vertical zone where coastal processes occur (the intertidal zone).
• Currents: Permanent or seasonal movements of water, often transferring energy and sediment over long distances.

High Energy vs. Low Energy Coasts

Coasts are typically categorised based on the amount of wave energy they receive:

• High Energy Coasts:
  • Characteristics: Exposed coasts facing large fetches (often along the Atlantic Ocean), subject to powerful, destructive waves.
  • Processes Dominate: Erosion and transportation.
  • Landforms: Rocky coastlines with cliffs, stacks, and wave-cut platforms.
  • Example: Much of the Pacific coast of North America or the west coast of Ireland.
• Low Energy Coasts:
  • Characteristics: Sheltered coasts, often in estuaries, bays, or areas with limited fetch, dominated by constructive waves.
  • Processes Dominate: Deposition and sediment accumulation.
  • Landforms: Extensive features like sand dunes, salt marshes, and mudflats.
  • Example: Coasts around the Mediterranean Sea or protected bays.

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3. Coastal Sediment (3.1.3.2)

Sediment Sources, Cells, and Budgets

The material (sand, pebbles, mud) that makes up our beaches is called sediment. It's constantly moving and being tracked within the coastal system.

Sediment Sources

Where does the coastal material come from?

• Terrestrial: Sediment brought by rivers (fluvial input).
• Fluvial: Runoff from land and glacial meltwater.
• Marine: Sediment brought in by tides or offshore currents.
• Erosion: Material eroded directly from coastal cliffs and shore platforms (the most significant local source on high-energy coasts).

Sediment Cells

A Sediment Cell is a self-contained stretch of coastline, usually bounded by major headlands or river estuaries, within which the movement of sediment is largely confined. These cells are essential for coastal management, as any intervention in one part of the cell will affect other parts.

Did you know? In England and Wales, the coastline is divided into 11 primary sediment cells, each acting as a closed system for sediment transfer.

The Sediment Budget

The sediment budget calculates the balance between the inputs and outputs of sediment within a cell.

Inputs - Outputs = Balance

• Positive Budget: Inputs > Outputs. The coastline is building up (accreting).
• Negative Budget: Outputs > Inputs. The coastline is eroding (retreating).
• Balanced Budget: Inputs ≈ Outputs. The coast is in dynamic equilibrium.

Analogy: If you put $100 into your coastal bank account (input) and only spend $50 (output), you have a positive budget and the beach grows!

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4. The Geomorphological Toolkit (3.1.3.2)

Geomorphological processes are the physical actions that shape the land. At the coast, we have general processes acting everywhere and distinctive processes specific to the sea.

General Geomorphological Processes

• Weathering: The breakdown of rock in situ (in place) by atmospheric or biotic processes.
• Mass Movement: The movement of material down a slope under gravity (e.g., rockfalls or slumps).
• Erosion: The removal of material (often by water or ice).
• Transportation: The movement of eroded material.
• Deposition: The dropping off or settling of transported material.

Distinctive Marine Erosion Processes

These are the unique ways the sea actively destroys the coastline. They are incredibly important for shaping erosional landforms (like cliffs and caves).

Memory Aid: HWAACS

1. Hydraulic Action (H)

The sheer force of water and compressed air. Waves crash into cracks in the rock, trapping air. The immense pressure exerted when the wave retreats causes the trapped air to expand rapidly, shattering the surrounding rock.

This is like smashing a hammer (the wave) onto the rock.

2. Wave Quarrying (W)

When waves hit a cliff face, the force alone dislodges loose blocks of rock and sediment from the cliff face or sea bed. This is distinct from hydraulic action as it focuses on the direct impact and removal of large, loose pieces.

3. Corrasion / Abrasion (A)

The scraping and grinding action of sediment (sand, pebbles, rocks) that the wave is carrying. The load acts like sandpaper, wearing away the cliff base or shore platform.

This is the 'sandpaper effect.'

4. Attrition (A)

Sediment carried by the sea constantly collides with other sediment particles and the sea bed. This causes the rocks to break into smaller, rounder fragments.

This is why beach pebbles are often round and smooth—they have been attritioned.

5. Cavitation (C)

This occurs when collapsing waves cause tiny bubbles (cavities) to form in the water. As these bubbles burst, they release small but highly focused shockwaves, which can contribute to minor rock erosion.

6. Solution / Corrosion (S)

Chemical weathering where acidic sea water (or water spray) dissolves soluble rock types, such as limestone or chalk. The rock material is carried away in solution.

Coastal Transportation and Deposition

Once material is eroded, it is transported along the coast. The methods of transport are similar to those in rivers:

• Traction: Large pebbles and cobbles are rolled along the sea bed.
• Saltation: Smaller, heavier sediment (like coarse sand) bounces along the sea floor.
• Suspension: Very fine sediment (silt and clay) is carried within the water column.
• Solution: Dissolved chemicals are carried in the water (as seen in the erosion process).

Longshore / Littoral Drift

This is the dominant process moving sediment along the coastline. It occurs when waves approach the beach at an angle (driven by prevailing winds).

1. The swash moves sediment up the beach at the same angle as the wave.
2. The backwash carries the sediment straight back down the steepest gradient, due to gravity.
3. This creates a zigzag pattern that constantly shifts sediment along the shore.

Deposition occurs when the energy of the wave or current decreases, meaning the water can no longer carry its load. This is typical in sheltered areas (bays) or where the coastline suddenly changes direction.

Sub-Aerial Weathering, Mass Movement, and Runoff

These are the processes that happen on the land surface, contributing material to the coastal system (often by attacking the cliff face from above).

1. Weathering (Sub-Aerial)

The breakdown of the cliff face:

• Mechanical (Physical): Processes like freeze-thaw (water freezing in cracks and expanding) or salt crystallisation (salt deposits forming crystals that push rock apart) weaken the cliff structure.
• Chemical: Processes like carbonation (affecting limestone) that dissolve rock components.
• Biological: Plant roots growing in cracks or burrowing animals destabilising the cliff.

2. Mass Movement

The sudden or gradual downslope movement of material. This is crucial as it supplies the beach with sediment:

• Example: Slumping (rotational slide), rockfalls, and slides. These are often triggered by heavy rain saturating the cliff material, making it unstable, especially if the base has been undercut by marine erosion.

3. Runoff

Water flowing over the land surface, particularly after heavy rainfall, can erode loose material from the cliff face or slope before it enters the sea. This adds sediment input.