Geography (9696) | Characteristics and formation of coastal landforms

Welcome to Coastal Environments! (9696 Advanced Physical Geography Option)

Hello! You're diving into one of the most dynamic and exciting topics in Geography: Coastal Environments.
Coasts are constantly being shaped by powerful natural forces, making them a fantastic natural laboratory for physical geography.

Don't worry if some of the terminology seems technical. We will break down the processes of waves, erosion, transport, and deposition step-by-step. By the end of these notes, you'll understand exactly why a coastline looks the way it does—whether it's dominated by towering cliffs or flat, sandy beaches!

Section 8.1: Coastal Processes – The Dynamics

1. Wave Generation and Characteristics

Waves are the primary drivers of coastal change. To understand the coast, we first need to understand the waves hitting it.

Key Factors Determining Wave Energy:

• Fetch: This is the distance over which the wind has blown uninterrupted across the water surface.
  

  Analogy: Imagine blowing across a cup of tea. If you blow for a long distance (long fetch), you create big ripples (high energy). If you blow only a short distance (short fetch), the ripples are small (low energy).

• Wind Speed: The faster the wind, the larger and more energetic the waves.
• Duration: The length of time the wind has been blowing.

Wave Classification: High Energy vs. Low Energy

Waves are classified based on their behavior, specifically the relationship between swash (water rushing up the beach) and backwash (water flowing back down the beach).

A. High Energy (Destructive) Waves:

• Generated by distant storms and long fetch.
• Characterised by high frequency (many waves per minute) and steep profiles.
• The backwash is stronger than the swash (Backwash > Swash).
• Result: They drag material off the beach, leading to erosion.
• Look out for: Steep beach profiles and pebbles/shingles.

B. Low Energy (Constructive) Waves:

• Generated by local winds and short fetch.
• Characterised by low frequency and gentle, spilling profiles.
• The swash is stronger than the backwash (Swash > Backwash).
• Result: They push material up the beach, leading to deposition and beach building.
• Look out for: Gentle beach profiles and fine sand.

Wave Refraction

Refraction means bending. When a wave approaches an irregularly shaped coastline (like a bay next to a headland), it slows down unevenly.

The Process:

1. Waves approach the coast.
2. Friction with the seabed causes the part of the wave closest to the shore (e.g., near a headland) to slow down first.
3. The parts of the wave in deeper water (e.g., approaching a bay) continue at their original speed.
4. This causes the wave to bend and concentrate its energy on the headland, while spreading energy out and reducing its intensity in the bays.

Impact: Wave refraction explains why headlands are often eroded aggressively, while bays accumulate sediment and are protected.

2. Marine Erosion Processes

Marine erosion is the breakdown and removal of rock/sediment by the sea. There are five main types:

• 1. Hydraulic Action:
  

  This is the sheer power of the water itself. When a wave hits a cliff, the force of the water compresses air in cracks and joints. When the wave retreats, the trapped air expands explosively, weakening and enlarging the cracks. It's like a tiny pneumatic drill!

• 2. Cavitation:
  

  A specific and powerful type of Hydraulic Action, usually restricted to very high-energy zones. It occurs when rapidly moving water in small spaces (like a wave-cut notch) creates microscopic air bubbles that then collapse/implode violently, generating immense shockwaves that erode the rock.

• 3. Corrasion (or Abrasion):
  

  This is the 'sandpaper effect.' Loose material (pebbles, sand) picked up by the waves is hurled against the cliff face, wearing it down. This is the most effective form of erosion during storms.

• 4. Solution:
  

  This is chemical erosion. Water dissolves soluble rock types, especially limestone (which is vulnerable to mild carbonic acid in rainwater/seawater). The material is carried away in solution.

• 5. Attrition:
  

  This process affects the load (sediment) itself, not the cliff. Rock fragments carried by the waves smash into each other, becoming smaller, smoother, and more rounded over time. (Analogy: Like rocks tumbling in a giant washing machine until they become smooth pebbles.)

Coping with Complexity: The 'SACCHA' Mnemonic

To remember the five main erosion processes, try to remember the acronym SACCHA:
Solution, Attrition, Corrasion/Abrasion, Cavitation, Hydraulic Action.

3. Sub-Aerial Processes: Working Above the Waves

Marine erosion is confined to the foreshore. However, coasts are also attacked from above by processes known as sub-aerial processes. These weaken the cliff, making it more vulnerable to marine attack.

• Weathering: Breakdown of rock in situ (in place).
  • Physical (Mechanical) Weathering: E.g., Freeze-thaw (water freezes in cracks, expands by 9%, forcing rock apart) or Salt Crystal Growth (salts dry out, crystalise, and expand, putting pressure on porous rock).
  • Chemical Weathering: E.g., Carbonation (acid rain reacting with limestone).
• Mass Movement: Downslope movement of material under gravity.
  • This includes rapid events like rock falls (common on steep, jointed cliffs) and slower movements like slides or slumps (common on weak, unconsolidated material like clay, especially when saturated with water).

Key Takeaway: Marine processes attack the cliff base (the notch), while sub-aerial processes weaken the cliff face and top. Both work together to cause retreat.

4. Marine Transportation and Deposition

Sediment Cells

The coast is often viewed as a system of Sediment Cells.

• A sediment cell is a largely closed system (meaning sediment is not usually exchanged with adjacent cells).
• Each cell contains sources (where sediment comes from—e.g., river mouths, eroded cliffs), transfers (longshore drift), and sinks (where sediment is deposited—e.g., deltas, deep water).
• Example: In England and Wales, the coastline is divided into 11 primary sediment cells, helping coastal managers understand where to focus their management efforts.

Longshore Drift (LSD)

This is the main mechanism for moving sediment along the coast.

Step-by-Step LSD:

1. Waves approach the shore at an angle (driven by the prevailing wind).
2. The swash carries sediment up the beach at that angle.
3. The backwash returns the water and sediment straight down the beach, perpendicular to the coastline (due to gravity).
4. This creates a continuous zig-zag movement, transporting material in the direction of the prevailing wind.

Load Transport Mechanisms

Just like in a river (Fluvial Geomorphology), coastal sediment is moved in four ways:

• Traction: Large pebbles/boulders rolled along the seabed.
• Saltation: Medium material (sand/gravel) bounced or hopped along the seabed.
• Suspension: Fine material (silt/clay) held within the water column.
• Solution: Dissolved chemicals carried in the water (invisible load).

Deposition: Occurs when the energy of the waves and currents drops (e.g., in sheltered bays, estuaries, or when high-energy destructive waves change to low-energy constructive waves).

Section 8.2: Characteristics and Formation of Coastal Landforms

1. Erosional Landforms: Cliffs and Platforms

Erosional landforms are dominant along high-energy coastlines (like parts of the UK Atlantic coast).

Cliffs and Wave-Cut Platforms

The creation of these features is a continuous cycle:

1. The sea attacks the base of the cliff (mostly via hydraulic action and abrasion).
2. A hollow known as a wave-cut notch forms at the high-water line.
3. As the notch deepens, the material above it becomes unsupported and collapses (often aided by sub-aerial processes like weathering and mass movement).
4. The collapsed material is removed by wave action, and the cliff retreats.
5. A gently sloping, rocky surface is left exposed at the cliff base at low tide. This is the wave-cut platform.
6. The platform's width is limited (usually only a few hundred metres) because as it gets wider, waves lose energy crossing it, eventually reducing the erosion at the cliff base.

Caves, Arches, Stacks, and Stumps (CASS)

These features typically form along headlands (where wave energy is concentrated due to refraction).

Step-by-Step CASS Formation:

1. Cracks/Joints: Waves exploit structural weaknesses (faults or joints) in the rock.
2. Caves: Erosion (hydraulic action and abrasion) enlarges the cracks into a cave.
3. Arch: If the cave penetrates right through the headland, or if two caves erode back-to-back, a natural arch is formed.
4. Stack: Continued weathering and erosion attack the arch's roof, causing it to collapse. This leaves an isolated pillar of rock, the stack. (Example: Old Harry Rocks, England).
5. Stump: The stack is attacked at its base, eventually collapsing to form a small, low-lying rocky outcrop visible only at low tide, known as a stump.

2. Depositional Landforms: The Builders

Depositional landforms are found on low-energy coastlines or where sheltered bays allow sediment to accumulate.

Beaches (Profile and Plan)

• Beach Profile (Cross-Section):
  • Berms: Ridges of sand/shingle built by constructive waves at high tide levels.
  • Cusps: Semi-circular depressions or indentations formed by wave swash and backwash (sometimes related to specific wave patterns).
• Beach Plan (Overall Shape):
  • Swash-aligned beaches: Waves hit the coast head-on (parallel to the shore). Little LSD occurs. Bays often have these.
  • Drift-aligned beaches: Waves approach the coast at an angle. Strong LSD occurs, moving material along the coast. Spits are often formed on these beaches.

Spits and Tombolos

• Spits (Simple and Compound):
  • A spit is a long, narrow ridge of sand or shingle extending out into the sea from the land, usually across an estuary or indentation.
  • Formed when Longshore Drift (LSD) carries sediment past a sudden change in coastline direction.
  • The end often curves inwards (a recurved end) due to secondary wind/wave directions or wave refraction.
  • A simple spit has one main recurved end; a compound spit has multiple recurved ends, showing phases of growth. (Example: Hurst Castle Spit, UK).
  • The sheltered area behind the spit often develops into a saltmarsh.
• Tombolos: A beach or bar that connects an island to the mainland (or two islands together).
  (Analogy: Like an umbilical cord connecting two land masses.) (Example: Chesil Beach sometimes behaves like a Tombolo connecting the Isle of Portland to the mainland).

Offshore Bars and Barrier Beaches

• Offshore Bar: A submerged ridge of sand/shingle lying parallel to the coast, often created by destructive waves scouring material from the beach and depositing it just offshore.
• Barrier Beach/Island: An elevated, extensive area of sand/shingle parallel to the mainland coast, separated from the mainland by a shallow lagoon or marsh. These are often relics of Post-Glacial sea level rise (sea level change is vital here!).

Tidal Sedimentation Landforms (Estuaries)

These landforms are found in sheltered, low-energy environments, often estuaries, where fresh river water meets salt water, causing fine sediment (mud/silt) to be deposited.

• Coastal Dunes: Sand deposited by the sea is blown inland and shaped by wind. Stabilised by specialised plants (pioneer species) like Marram Grass, which build the dune profile over time.
• Coastal Saltmarshes: Vegetated areas of mud/silt in sheltered areas (like behind a spit or in an estuary). They are submerged during high tide. Pioneer plant species trap fine sediment, gradually raising the land level (a process called succession).
• Mangroves: Tropical equivalents of saltmarshes, found in low-energy, tidal areas in tropical climates. The specialised root systems of mangrove trees trap sediment and protect the coast from erosion.

Key Takeaway: Erosional landforms (CASS) need high energy and strong, resistant rock. Depositional landforms (Spits, Beaches) need low energy and a good supply of sediment (a healthy sediment cell).

3. The Role of Sea Level Change in Landform Formation

The position of the coastline and the characteristics of the landforms we see today are fundamentally controlled by sea level change over geographical time.

• During the last Ice Age (Pleistocene), huge amounts of water were locked up in ice sheets (Eustatic Fall), exposing continental shelves.
• Since the Ice Age ended (Holocene), sea levels have generally risen (Eustatic Rise), submerging former landscapes.

Landforms created/affected by Relative Sea Level Change:

• Barrier Beaches: Many are thought to have formed as sea levels rose, pushing existing beach ridges inland and isolating them from the mainland.
• Estuaries, Saltmarshes, and Mangroves: These form in coastal areas that are being slowly submerged. The gradual rise of sea level provides the necessary shallow, intertidal zone for mudflats and subsequent plant succession to take place and build vertically.
• Wave-Cut Platforms: The current elevation and exposure of platforms are directly linked to the current Mean Sea Level (MSL) line. Significant sea-level rise would drown existing platforms, while a fall would expose them and prevent wave action from cutting further.

Brief Connections to Other Coastal Topics

Coral Reefs (8.3)

Coral reefs are massive coastal structures built by living organisms (polyps). They are highly specialized depositional landforms.

• Conditions Required for Coral Growth:
  • Warm water (Min. 18°C, ideally 20°C–25°C).
  • Shallow water (Max. 50m depth—they need sunlight for the algae they host).
  • Clear, sediment-free water (sediment smothers polyps).
  • Moderate salinity.
• Types: Fringing Reefs (close to shore), Barrier Reefs (separated from shore by a lagoon, Example: Great Barrier Reef), and Atolls (circular reefs surrounding a central lagoon).

Sustainable Management (8.4)

Managing coastlines requires balancing human demands (homes, tourism, ports) with natural processes (erosion, deposition, sea level rise).

• Hard Engineering: Using man-made, fixed structures to stop coastal processes (e.g., sea walls, groynes, rock armour). These are expensive, often ugly, and can cause damage further down the coast by interrupting LSD.
• Soft Engineering: Working with nature (e.g., beach nourishment/replenishment, dune stabilisation, managed retreat). These methods are often cheaper, more sustainable, and blend better with the landscape.

The study of coastal landforms is essential to coastal management, as you must understand *how* the landform was created before you decide *how* to protect it!