Geography (9696) | Coastal environments

🌊 Welcome to Coastal Environments (9696 Advanced Physical Geography)

Hello Geographers! This chapter is all about one of the most dynamic and beautiful environments on Earth: the coast. Coasts are where the land, sea, and air interact, creating a constantly changing landscape. Because they are so heavily populated and used by humans, understanding the processes here—from powerful waves to delicate coral—is crucial for effective management and future sustainability. Let’s dive in!

8.1 Coastal Processes: The Forces at Work

The coast is a high-energy system driven primarily by waves, currents, tides, and weather. We need to understand the 'inputs' (like wave energy) that drive the 'throughputs' (like erosion and transport) to create the 'outputs' (landforms).

Wave Generation and Characteristics

Waves are the single most important factor shaping a coast.

Fetch: This is the distance of open water over which the wind blows. The longer the fetch, the more energy the wind transfers to the water, resulting in larger, more powerful waves. (Think of blowing air across a swimming pool vs. blowing air across the Atlantic Ocean!)

Wave Energy: Related directly to fetch, wind speed, and duration.

Wave Refraction: Don't worry, this isn't physics! Refraction simply means waves bend as they approach the shore. Waves slow down when they encounter shallow water (like approaching a headland), causing the wave energy to become concentrated, leading to higher rates of erosion on the headland. In bays, the energy is spread out, encouraging deposition.

Breaking Waves: Swash and Backwash

When a wave breaks, the water rushes up the beach (Swash) and then drains back down (Backwash). The relationship between swash and backwash defines whether a wave is high or low energy.

• High Energy Waves (Destructive Waves): These waves are tall and frequent. The backwash is significantly stronger than the swash, pulling sediment off the beach and causing erosion. They are common during storms or winter.
• Low Energy Waves (Constructive Waves): These waves are low in height and less frequent. The swash is stronger than the backwash, pushing material up the beach and causing deposition (building the beach). They are common in calm conditions or summer.

Marine Erosion Processes

The sea erodes the coastline in four main ways (plus one accessory process).

• 1. Hydraulic Action: The sheer force of water hitting cracks in the rock, compressing the air inside. As the wave retreats, the air expands explosively, weakening the rock.
• 2. Cavitation: Less common, this occurs when air bubbles in the water collapse rapidly near the rock surface, creating a shockwave that chips away at the rock.
• 3. Corrasion (Abrasion): Rock fragments and sediment carried by the waves are hurled against the cliff face, wearing it down like sandpaper.
• 4. Attrition: This is the process where the sediment particles themselves collide with each other, becoming smaller, smoother, and rounder. (This doesn't erode the cliff directly, but supplies finer sediment.)
• 5. Solution (Corrosion): Chemical erosion where acids in the seawater (often carbonic acid) dissolve soluble rock types like chalk or limestone.

Memory Aid: Remember the four main hitters with the acronym: How Can Coasts Abrade? (Hydraulic, Cavitation, Corrasion, Attrition).

Sub-aerial Processes: The Land Attacks Back

The cliff face is attacked not only by the sea but also by processes operating above the water line (sub-aerial). These processes weaken the cliff structure, making it easier for marine erosion to remove the material.

• Weathering: The breakdown of rock in situ (in place). This includes chemical weathering (like solution or oxidation) and physical/mechanical weathering (like freeze-thaw or salt crystal growth).
• Mass Movement: The downslope movement of material under the influence of gravity, such as landslides, slumping (rotational slips), and rockfalls. Often triggered by heavy rain soaking the weathered material.

Marine Transportation and Deposition

Once material is eroded, it is moved along the coast.

Sediment Sources: Where does the coastal material come from? Cliffs, rivers (often the largest source), offshore glacial deposits, or even biological materials (like shells or coral).

Sediment Cells: The coast is divided into self-contained stretches called sediment cells. Each cell operates independently, meaning sediment is rarely transferred between cells. Understanding these boundaries is essential for coastal management.

Longshore Drift (LSD): The principal mechanism of transport.

Step-by-Step LSD:
1. Waves approach the shore at an angle (driven by prevailing wind).
2. The swash carries sediment up the beach at this angle.
3. The backwash carries the sediment straight back down due to gravity.
4. This results in a zig-zag movement, gradually transporting material along the coast.

Deposition: Occurs when the energy of the moving water or wind is no longer sufficient to carry the load. Deposition is common in sheltered bays or areas protected by headlands.

8.2 Characteristics and Formation of Coastal Landforms

The interaction between marine processes (erosional/depositional) and sub-aerial processes creates distinct landforms.

Erosional Landforms

These features develop primarily on coasts composed of hard, resistant rock where destructive waves dominate (high energy environment).

• Cliffs and Wave-Cut Platforms:

  1. Marine erosion concentrates at the base of the cliff, forming a wave-cut notch (an undercut section).
  2. As the notch deepens, the rock above collapses under gravity, causing the cliff to retreat.
  3. A gently sloping, smooth rock surface (the wave-cut platform) is left behind at the base of the retreating cliff. This platform is typically only exposed at low tide.

• Caves, Arches, and Stacks:

  These develop on headlands where weaknesses (joints or faults) exist.
  1. Erosion attacks the weak points, creating a cave.
  2. If caves form on both sides of a headland, they may eventually meet, forming an arch.
  3. Further erosion and weathering weaken the arch roof until it collapses.
  4. The isolated pillar of rock left standing is called a stack.
  5. The remnant small stump is called a stump (eventually eroded to a wave-cut platform level).

Depositional Landforms

These features develop where constructive waves and sheltered locations encourage deposition (low energy environment).

• Beaches: Accumulations of sand or shingle (pebbles).
  • Beach Profile (Cross Section): Gently sloped beaches (usually sand) often have berms (small ridges built by constructive waves). Steep beaches (usually shingle) absorb wave energy quickly.
  • Beach Plan (Map View): Swash-aligned beaches form perpendicular to the incoming waves, resulting in very little longshore drift. Drift-aligned beaches form parallel to the direction of longshore drift, often resulting in features like spits.
• Spits and Tombolos:
  • Spit: A long, narrow ridge of sand or shingle attached to the land at one end. It forms where longshore drift carries sediment across the mouth of a river or estuary. The end of the spit often curves inwards (hooked end) due to wave refraction or secondary winds.
  • Tombolo: A spit that connects an offshore island or stack to the mainland.
  • Simple vs. Compound Spits: Simple spits are relatively straight. Compound spits have multiple recurved ends, showing different growth phases.
• Offshore Bars and Barrier Beaches:
  • Offshore Bars: Ridges of sediment submerged in the nearshore zone, formed by destructive wave action.
  • Barrier Beaches (or Barrier Islands): Long, narrow, low-lying islands of sand or sediment running parallel to the coast, separated from the mainland by a lagoon or marsh.
• Coastal Dunes: Mounds of sand built by wind action (aeolian processes) that form behind the beach. Require a large source of sand, strong onshore winds, and pioneer vegetation (like marram grass) to stabilise them.
• Tidal Sedimentation in Estuaries, Coastal Saltmarshes, and Mangroves:

  In sheltered, low-energy tidal areas, fine sediment (silt/mud) is deposited.
  - Saltmarshes form in temperate (cooler) climates where salt-tolerant grass (halophytes) traps sediment.
  - Mangroves form in tropical (warmer) climates, where tree roots stabilise the mud, providing critical habitats and natural coastal defence.

The Role of Sea Level Change

Changes in global sea level (Eustatic) or local land level (Isostatic) dramatically influence coastal landforms.

• Rias and Fjords (Submergent Coasts): These are valleys drowned by rising sea levels. Rias are submerged river valleys (e.g., in SW England). Fjords are submerged glaciated valleys (U-shaped, deep, e.g., Norway).
• Raised Beaches and Relict Cliffs (Emergent Coasts): These occur where the land has risen (isostatic recovery) or sea levels have fallen, leaving old beach materials and former wave-cut cliffs located above the current sea level.

8.3 Coral Reefs

Coral reefs are one of the most productive and fragile coastal environments, critical for biodiversity and coastal protection.

Characteristics, Distribution, and Formation

Coral is a living organism, a small polyp that secretes calcium carbonate (limestone) to build its skeleton. Thousands of these skeletons combine to form a reef.

• Distribution: Primarily found in tropical and subtropical waters (between 30°N and 30°S).

Conditions Required for Coral Growth (The Coral Recipe)

Coral is very sensitive and needs specific conditions:

1. Temperature: Warm water (18°C minimum, optimally 20–28°C). Too cold or too hot kills the symbiotic algae (zooxanthellae) that live in the coral, leading to bleaching.
2. Light: Shallow, clear water (less than 25m deep) to allow photosynthesis by zooxanthellae.
3. Salinity: Normal seawater salinity (32–42 ppt). Coral cannot tolerate fresh water (like near major river mouths).
4. Clean Water: Low sediment/turbidity. High sediment blocks light and clogs the polyps.
5. Oxygenated Water: Requires constant wave action/currents.
6. Substrate: A hard, fixed base (often volcanic rock) for the polyps to colonise.

Types of Coral Reefs (Darwin's Theory)

Charles Darwin hypothesised that all reefs transition through these stages, following a subsiding (sinking) volcanic island.

1. Fringing Reefs: Reefs that grow immediately adjacent to the coastline, usually in shallow water. No lagoon separates the reef from the shore. (Initial stage.)
2. Barrier Reefs: Reefs separated from the mainland by a wide, deep lagoon. The Great Barrier Reef is the famous example.
3. Atolls: Circular or ring-shaped reefs enclosing a central lagoon, formed after the volcanic island they fringed has completely subsided beneath the water.

Threats to Coral Reefs

Coral reefs face significant threats, both natural and human-induced:

• Global Warming: Causes sea surface temperatures to rise, leading to coral bleaching (mass expulsion of zooxanthellae).
• Sea-Level Rise: If the rate of rise is too fast, the coral cannot grow upwards quickly enough to stay in the sunlight zone.
• Pollution: Sewage and agricultural runoff increase nutrients (eutrophication), leading to algal blooms that smother the coral.
• Physical Damage: Destructive fishing methods (like dynamite fishing), irresponsible tourism (anchors, touching), and dredging.

Possible Management Strategies:
* Establishment of Marine Protected Areas (MPAs).
* Stricter regulations on coastal development and sewage treatment.
* Banning destructive fishing practices.
* International efforts to reduce greenhouse gas emissions (addressing the root cause).

8.4 Sustainable Management of Coasts (Case Study Focus)

Coastal management aims to protect communities and valuable land from coastal erosion and flooding while maintaining the natural balance of the coastal system (sustainable management). This usually involves choosing between hard and soft engineering approaches.

Hard Engineering Solutions

These methods involve building rigid structures, usually made of concrete or rock, designed to absorb or reflect wave energy.

• Groynes: Wooden or rock structures built perpendicular to the coast. They trap sediment moving via longshore drift, widening the beach on the updrift side (good protection) but often causing severe erosion on the downdrift side (the "terminal groyne syndrome").
• Sea Walls: Concrete barriers running parallel to the coast. Highly effective at preventing erosion and flooding but are very expensive and often increase erosion at the base of the wall (by reflecting wave energy).
• Rock Armour (Revetments/Rip Rap): Large boulders placed at the base of a cliff. They absorb wave energy and are cheaper than sea walls, but are visually intrusive and can make the beach inaccessible.

Common Problem with Hard Engineering: They tend to be unsustainable because they interfere with natural sediment movement, often shifting the problem down the coast to neighbouring areas.

Soft Engineering Solutions

These methods work with nature, using natural materials and processes to protect the coast, often blending into the landscape better.

• Beach Nourishment (Replenishment): Sand is dredged from offshore and added to the existing beach to make it wider. This dissipates wave energy further from the coast. (Example: Miami Beach, USA). This is temporary and expensive but aesthetically pleasing.
• Dune Regeneration/Stabilisation: Planting vegetation (like Marram grass) and fencing off dunes to encourage natural sediment trapping and stabilisation. Dunes act as a flexible, natural buffer against storms.
• Managed Retreat (Coastal Realignment): Allowing the sea to flood low-value land (e.g., farmland) in a controlled way, creating new saltmarsh or wetland areas. These areas act as natural buffers, protecting higher-value land further inland.

The Challenge of Sustainable Management (Case Study Preparation)

When studying a case study (e.g., a specific stretch of coastline like Lyme Regis or Happisburgh), you must evaluate the attempted solutions and the problems of sustainable management.

Key Evaluation Points:

1. Economic Cost: Is the protection cost-effective relative to the land value being protected?
2. Environmental Impact: Does the solution damage natural habitats (like saltmarshes or mudflats)? (Hard engineering is often more damaging).
3. Social Impact: Does the solution look good? Does it affect beach access or tourism? (Sea walls often fail here).
4. Holistic Approach: Does the solution consider the entire sediment cell? (Sustainable management requires looking beyond the immediate area).
5. Viewpoints: Whose perspective matters? Local residents, landowners, tourists, environmentalists, and local authorities often have conflicting interests.

Did you know? The UK government often uses a policy called "No Active Intervention" for low-value coastal stretches. This means they intentionally let the natural processes continue, which is sometimes the most sustainable option, though often unpopular with residents who lose their homes!