Systems and processes

Coastal Systems and Processes: The Engines of Change (9635, 3.1.3.2)

Hello Geographers! Welcome to the fascinating world of coasts. This chapter, "Systems and Processes," is crucial because it explains how our spectacular coastal landscapes—from towering cliffs to sandy beaches—actually form and change. Think of the coast as a massive, complicated machine. To understand the output (the beaches and cliffs), we need to understand the inputs (energy and sediment) and the inner workings (the processes).

Don't worry if some of the terms sound technical! We'll break them down into simple steps and use plenty of analogies to make sure everything clicks.

1. Sources of Energy in Coastal Environments

The coast is an open system, which means energy is constantly flowing in and out. The main sources of energy driving coastal change are winds, waves, currents, and tides.

Winds

Winds are the primary driver, as they create waves. Stronger, faster winds blowing over a larger area of open water (the fetch) create bigger, more powerful waves.

Waves: The Coast's Sculptors

Waves are the most important process because they do all the heavy lifting (or eroding!). We categorize waves into two main types based on their impact:

A. Constructive Waves (Low Energy Coasts)

• Action: Swash (water moving up the beach) is stronger than backwash (water draining back).
• Outcome: They deposit sediment, building up the beach gently.
• Analogy: Think of a careful construction worker gently placing materials down.
• Associated Coasts: Low energy coasts, often found in sheltered bays.

B. Destructive Waves (High Energy Coasts)

• Action: Backwash is much stronger than swash.
• Outcome: They erode the coast, transporting sediment away, creating steep beach profiles.
• Analogy: Think of a bulldozer aggressively scraping material away.
• Associated Coasts: High energy coasts, common along exposed headlands and coastlines facing large fetches (like the open ocean).

Currents and Tides

Currents: These are the permanent or semi-permanent movements of water in the seas and oceans. They help transport sediment along the coast.

Tides: These are the regular, predictable rises and falls of sea level caused by the gravitational pull of the moon and sun.

• Tidal range (the difference between high and low tide) affects where processes happen. A large tidal range spreads erosion over a wider area, while a small range concentrates it on a narrow band.

2. Sediment: Sources, Cells, and Budgets

The entire coastal landscape relies on the supply and management of sand, pebbles, and mud, collectively known as sediment.

Sediment Sources

Where does coastal sediment come from? It has several origins:

• Fluvial (Rivers): Rivers are the most important source globally, dumping sediment into the sea.
• Marine (Offshore): Sediment is moved from the seabed towards the shore by waves and currents.
• Erosion (Cliffs): Material eroded directly from coastal cliffs and landforms.
• Aeolian (Wind): Wind blows sand from inland areas or exposed beaches (especially important for sand dunes).

Sediment Cells and Budgets

Geographers use a systems approach to manage coastlines:

• Sediment Cell: This is a self-contained stretch of coastline, usually bordered by major headlands or deep-water areas, where sediment movement is largely confined. There are 11 major sediment cells around the coast of England and Wales, for example.
• Sediment Budget: This concept treats the sediment cell like a bank account.

Inputs (Income): Sources like river discharge, cliff erosion, or offshore sediment transfers.

Outputs (Expenditure): Losses due to deposition in sinks (like sand dunes), or transfers offshore or deeper into estuaries.

If Inputs > Outputs, the coast will experience net deposition (beaches grow).

If Outputs > Inputs, the coast will experience net erosion (beaches shrink, cliffs retreat).

3. Geomorphological Processes: The Big Picture

Geomorphology is the study of landforms. Along the coast, four major groups of processes are always at work:

1. Weathering: The breaking down of rock in situ (in its place) by chemical, physical, or biological means.
2. Mass Movement: The movement of material down a slope (like a cliff face) under the influence of gravity (e.g., slumps, slides).
3. Erosion: The wearing away and removal of material by a moving force (like waves or wind).
4. Transportation: The carrying of eroded material by waves and currents.
5. Deposition: The dropping of material when the energy of the wave or current decreases.

4. Distinctively Coastal Processes: Marine Erosion

The sea erodes the coast in six main ways. These are crucial terms you must know!

1. Hydraulic Action (HA)

This is purely the power of water and air. Waves crash against the cliff face, forcing air into cracks and fissures. When the wave retreats, the pressure is released, causing the air to expand rapidly, which weakens and breaks off fragments of rock.

2. Wave Quarrying (WQ)

Often considered an extreme version of Hydraulic Action, this occurs when powerful, plunging waves hit the cliff face. They scoop out loose rock material, especially at weak points like joints and bedding planes, often creating caves or notches.

3. Corrasion (or Abrasion) (C/A)

This is the "sandpaper effect." Sediment (sand, pebbles, boulders) picked up by the wave is hurled against the cliff or shore platforms, wearing the rock away. The size and velocity of the sediment affect the rate of wear.

4. Cavitation (CA)

This is an advanced process. When water rushes into a tiny crack very quickly, the internal pressure drops, causing pockets of air to vaporise (turn into gas bubbles). As the wave pulls back, these bubbles collapse violently, creating miniature shockwaves that chip away at the rock.

5. Solution (S)

Also known as corrosion. This is a chemical process where weak acids in the seawater (like carbonic acid) dissolve soluble rock minerals (especially limestone). It is most effective on carbonate rocks.

6. Attrition (AT)

This does not erode the cliff, but it reduces the size of the sediment itself. Pebbles, stones, and boulders carried by the waves smash into each other, grinding down, becoming smaller, smoother, and rounder over time.

5. Transportation and Deposition

Once material is eroded, it needs to be moved. The processes of transportation depend heavily on the energy of the waves and currents.

Modes of Coastal Transportation (Like a river, but in the sea!)

When sediment is moved along the seabed or within the water column, it travels in four ways:

1. Traction: The heaviest sediment (boulders and large pebbles) is rolled or dragged along the seabed.
2. Saltation: Medium sediment (smaller pebbles) bounces or hops along the seabed.
3. Suspension: Light sediment (sand and silt) is carried floating within the water column, especially during turbulent conditions.
4. Solution: Dissolved chemicals are carried within the water itself (linked to the erosion process of Solution).

Longshore/Littoral Drift (LSD)

This is the key process responsible for moving vast amounts of sediment along the coastline, forming features like spits and bars.

Step-by-Step LSD:

Step 1: Swash (Input): Waves usually approach the shore at an angle (driven by the prevailing wind direction).

Step 2: Sediment Movement: The swash carries sediment up the beach at this angle.

Step 3: Backwash (Output): Due to gravity, the water drains straight back down the steepest gradient, pulling the sediment with it, perpendicular (at 90 degrees) to the shoreline.

Step 4: Zig-Zag: This constant angled movement up and straight movement down results in a net zig-zag transfer of sediment along the coast.

6. Sub-aerial Processes

These are processes that happen on the land above the sea level, but they are vital because they provide the material for marine processes to act on.

A. Weathering (Breaking Down the Rock)

Weathering weakens the cliff face, making it more vulnerable to erosion and mass movement.

• Mechanical (Physical) Weathering: Includes processes like freeze-thaw (water freezes in cracks, expands, and breaks rock apart) and salt crystallisation (seawater evaporates, leaving salt crystals that grow and exert pressure on rock pores).
• Chemical Weathering: Includes carbonation, where rainwater combines with carbon dioxide to form weak carbonic acid, dissolving rock (especially limestone).
• Biological Weathering: The action of plants (root penetration) or animals (burrowing) that breaks down rock.

B. Mass Movement (Moving the Rock)

Once weathered, gravity takes over. Mass movement involves the downslope movement of rock and soil.

• Slides: Material moves along a straight slip plane (a flat surface).
• Slumps: Material moves along a curved slip plane, resulting in rotational movement, often leaving behind steps or terraces on the cliff face. Slumps are common where permeable rock layers (like sand) sit on impermeable layers (like clay).

C. Runoff

Water flowing over the land surface (usually after heavy rain) can erode and transport loose material down the cliff face and onto the beach, adding to the sediment store.