Geography (0460) | Coasts

Hello IGCSE Geographers! Welcome to Coasts!

Coasts are some of the most dynamic and beautiful environments on Earth. They are constantly being changed by the power of the sea and the wind. In this chapter, we will learn about the incredible processes that shape these landscapes—the erosion that creates massive cliffs and the deposition that forms sandy beaches and spits.
Understanding coasts is vital because they are home to unique ecosystems and are often densely populated areas that need protection from powerful natural hazards like storms. Let's dive in!

Section 1: The Work of the Sea and Wind

1.1 Understanding Waves

The sea's energy is delivered to the coast through waves. Waves are categorized based on their energy and effect on the land:

(a) Destructive Waves (Erosion)

• Characteristics: High frequency (many waves per minute), steep and tall.
• Action: They have a powerful backwash (water flowing back down the beach) that is stronger than their swash (water washing up).
• Effect: The strong backwash drags material away from the shore, leading to erosion and destruction of coastal features.

(b) Constructive Waves (Deposition)

• Characteristics: Low frequency, long and gently sloping.
• Action: They have a powerful swash (water washing up the beach) that is stronger than their backwash.
• Effect: The swash carries material up the beach, and the weak backwash leaves it there. This leads to deposition and the building up of beaches.

Quick Tip: Think of a Destructive wave as an aggressive bully, pulling everything away. Think of a Constructive wave as a careful builder, gently placing material down.

1.2 Coastal Processes: Erosion, Transport, and Deposition

The power of the sea shapes the coast through three main activities:

(a) Coastal Erosion (Breaking down the land)

There are four main processes of coastal erosion. The first two are the most effective:

1. Hydraulic Action (A/H): The sheer force of the water crashing against the cliff face. Air and water are forced into cracks, compressing them. When the wave retreats, the pressure is released, causing the crack to widen and eventually shatter the rock.
2. Abrasion (or Corrasion): Waves pick up material (like pebbles and rocks) and throw them against the cliff face, wearing it away like sandpaper.
3. Attrition: Material carried by the waves (the rocks and pebbles themselves) crash into each other, becoming smaller and more rounded over time.
4. Solution (or Corrosion): Chemical weathering where weak acids in the seawater dissolve certain types of rock, such as chalk or limestone.

(b) Coastal Transportation (Moving the material)

Material (sediment) is moved along the coast primarily by a process called Longshore Drift (LSD). This process is essential for building many depositional landforms.

The Longshore Drift Process (Step-by-Step):

1. Waves usually approach the shore at an angle (driven by the prevailing wind).
2. The swash carries sediment diagonally up the beach.
3. The backwash carries the sediment straight back down the beach due to gravity.
4. This zig-zag movement results in the gradual movement of material along the coast in the direction of the prevailing wind.

(c) Coastal Deposition (Dropping the material)

Deposition occurs when the sea or wind loses energy. This often happens in sheltered areas like bays or when the waves switch from being destructive to constructive. The dropped sediment then builds up landforms.

Section 2: Erosional Landforms

Erosional landforms are generally found on coastlines made of hard rock.

2.1 Headlands and Bays

Coasts are often composed of alternating bands of hard and soft rock.

• Bays: Form where the softer rock (like clay) is eroded quickly by the sea, creating sheltered inlets.
• Headlands: Form where the harder, more resistant rock (like granite) sticks out into the sea, as it erodes much slower.

Did you know? Waves focus their energy on the headlands, making erosion there much faster, while waves lose energy and deposit material in the sheltered bays, leading to beaches.

2.2 Cliffs and Wave-Cut Platforms

Cliffs are steep rock faces at the edge of the sea.

Formation Process:

1. The sea attacks the base of the cliff through hydraulic action and abrasion.
2. This action carves out a notch called a wave-cut notch at high-tide level.
3. As the notch gets deeper, the rock above it becomes unstable and collapses due to gravity.
4. The collapsed material is carried away by the waves, leaving a steep cliff face.
5. As the cliff retreats inland, a gently sloping area of rock is left behind at the base, exposed at low tide. This is the wave-cut platform (WCP).

2.3 The Headland Sequence: C-A-S-S

Once a headland forms, the destructive waves continue to attack it, creating a sequence of features.

Memory Aid: C-A-S-S (Caves, Arches, Stacks, Stumps)

1. Caves: Weak points (joints or faults) in the headland are attacked by hydraulic action and abrasion, leading to the formation of a sea cave.
2. Arches: If two caves on either side of the headland erode back towards each other, they meet, creating a hole right through the headland, forming a sea arch.
3. Stacks: The roof of the arch is continually attacked by weathering and erosion. Eventually, the roof collapses, leaving an isolated column of rock standing away from the headland. This is a stack (e.g., The Twelve Apostles in Australia).
4. Stumps: The stack continues to be eroded at its base until it collapses, leaving a small, low-lying rocky platform or column that is often only visible at low tide. This is a stump.

Section 3: Depositional Landforms

Depositional features form where wave energy is low or where material accumulates due to Longshore Drift.

3.1 Beaches

A beach is an accumulation of material (sand, shingle, pebbles) between the high and low water marks.

• Formation: Formed by constructive waves depositing material, usually in sheltered areas like bays where wave energy drops.
• Sand vs. Shingle: Sandy beaches are usually gently sloping because the small particle size allows the water to soak in quickly. Shingle beaches are often steeper because the larger particles allow the strong backwash to pull the material down.

3.2 Spits

A spit is a long, narrow ridge of sand or shingle that extends from the land out into the sea or across an estuary.

Formation Process:

1. Longshore Drift carries material along the coastline.
2. When the coastline changes direction (e.g., at a river mouth or a sudden bend), the LSD process continues to move sediment straight out into the water.
3. Deposition occurs because the current slows down in the deeper, open water.
4. If the spit is sheltered from strong currents or waves on the landward side (like behind a spit, forming a 'hook'), the water often becomes brackish (slightly salty) and forms a saltmarsh.

3.3 Coastal Sand Dunes

These are accumulations of wind-blown sand found behind the upper part of a beach.

Conditions Required:

• A large supply of sand.
• A strong onshore wind to blow the sand inland.
• Obstacles (like driftwood or rocks) to trap the sand and start the build-up.
• Specialized vegetation (like Marram Grass) to stabilize the dunes.

Dune Succession: As you move further inland, the dunes become older, more stable, and covered by denser vegetation, changing the ecosystem (a process known as succession).

Section 4: Unique Coastal Ecosystems

Some warm coastal areas support incredible living structures: coral reefs and mangrove swamps. These environments require very specific conditions to survive.

4.1 Coral Reefs

Coral reefs are underwater ecosystems built by tiny marine animals called polyps (which secrete calcium carbonate).

Conditions Required for Coral Development:

• Warm Water: Temperatures must be above 18°C, ideally between 20°C and 30°C.
• Shallow Water: Generally less than 30 meters deep, as corals need sunlight for the algae (zooxanthellae) living in their tissues.
• Clean/Clear Water: Corals cannot tolerate high levels of sediment (silt) or pollution, which block sunlight and smother the polyps.
• Salty Water: They need saline (salt) water and die if exposed to fresh water (like near river mouths).

Did you know? Coral reefs support about 25% of all marine life, making them the "rainforests of the sea."

4.2 Mangrove Swamps

Mangroves are salt-tolerant trees and shrubs that grow in sheltered coastal areas in the tropics and subtropics.

Conditions Required for Mangrove Development:

• Tropical/Subtropical Climate: Need high temperatures (no frost).
• Sheltered Coastline: Need protection from strong waves and currents, which would otherwise wash away seedlings.
• Tidal Range: Need a medium tidal range so that their roots are flooded and exposed regularly.
• Muddy/Silty Ground: Grow best in fine sediments found in estuaries or deltas.
• Saline Water: They thrive in brackish (mixed salt and fresh) or salty water.

Section 5: Coastal Opportunities and Hazards

5.1 Coastal Opportunities (Why people live here)

Coasts provide many benefits for human activity, often leading to large settlements developing close to the shore:

• Tourism and Recreation: Beaches, beautiful scenery, and warm climates attract millions of tourists, creating jobs and income.
• Trade and Industry: Flat coastal land is ideal for building ports and harbours for shipping trade.
• Fishing: Coastal waters and mangrove swamps provide rich fishing grounds and nurseries for marine life.
• Energy: Offshore wind and wave power generation is increasingly important.

5.2 Coastal Hazards

Living on the coast means dealing with natural threats:

• Coastal Erosion: The natural loss of land. This is a hazard when homes, infrastructure (like roads), and businesses are built too close to eroding cliffs (e.g., the rapid retreat of soft rock cliffs in Holderness, UK).
• Tropical Storms: These low-pressure systems cause massive hazards, including:
  • High Winds: Cause damage to buildings and transport links.
  • Coastal Flooding (Storm Surge): A sudden rise in sea level caused by the low air pressure and strong winds pushing water towards the coast. This is often the most destructive element.
  • Heavy Rainfall: Causes further inland flooding.

Section 6: Managing the Impacts of Coastal Erosion

To protect valuable land and property from coastal erosion, management strategies are implemented. These can be divided into two categories:

6.1 Hard Engineering (Building resistant structures)

These are structures built to resist the power of the sea, often expensive and visually intrusive.

• Sea Walls: Large concrete barriers built along the shore to absorb and reflect wave energy.
  • Pros: Highly effective at stopping erosion where they are placed.
  • Cons: Very expensive, disrupt the natural environment, and can increase erosion further down the coast (known as terminal groynes syndrome) because they stop sediment moving.
• Groynes: Timber or rock structures built perpendicular (at 90 degrees) to the beach. They trap sediment moved by Longshore Drift, building up the beach on one side.
  • Pros: Effective at building a wide beach, which acts as a natural buffer against waves.
  • Cons: Interrupts LSD, starving the beaches further down the coast, increasing erosion there.
• Rip-Rap (Rock Armour): Piles of large boulders placed at the foot of cliffs or beaches.
  • Pros: Absorbs wave energy well; relatively cheap compared to sea walls.
  • Cons: Looks unnatural; can be dangerous for people accessing the beach.

6.2 Soft Engineering (Working with nature)

These methods aim to enhance natural processes, often being cheaper, more sustainable, and less impactful on the environment.

• Beach Nourishment/Replenishment: Adding large amounts of sand or shingle to an existing beach to make it wider and higher, absorbing more wave energy.
  • Pros: Looks natural; creates a wider beach for tourism.
  • Cons: Requires constant maintenance and repeat costs as the sediment is washed away quickly.
• Dune Restoration: Planting grasses (like Marram Grass) and fencing off dunes to encourage them to grow and stabilize. Dunes provide a natural buffer against the sea.
  • Pros: Very natural and sustainable; cheap.
  • Cons: Takes a long time to establish and is easily damaged by storms or people walking on them.

You've Finished Coasts!

Well done! Coasts are all about processes and sequences. If you can explain step-by-step how a wave-cut platform forms, or how Longshore Drift creates a spit, you're ready to tackle those exam questions! Keep practicing those sequences!