Coastal Landscapes

What physical factors influence coastal landscape systems?

Introduction

Coastal landscape systems are dynamic environments shaped by the interaction of various physical factors. Understanding these influences is key to analysing the processes that form and transform coastal landscapes for OCR A-level Geography students. This guide explores the potential influences of winds, waves, tides, geology, global ocean currents and other factors on coastal systems.

1. Winds

Winds play a pivotal role in shaping coastal landscapes as they drive wave formation and influence sediment transport.

Key Influences of Wind:

  1. Speed:
    • The strength of the wind determines the energy of the waves it generates. Stronger winds produce larger, more powerful waves that can erode coastlines more effectively.
  2. Direction:
    • The direction of the prevailing wind affects the angle at which waves approach the shore. This, in turn, influences longshore drift and the movement of sediment along the coastline.
  3. Frequency:
    • Frequent winds in a consistent direction (dominant winds) have a greater cumulative effect on coastal landscapes compared to occasional winds from variable directions.

Example:

  • The south-westerly winds in the UK generate powerful waves that shape the Atlantic-facing coastlines through erosion and sediment transport.

2. Waves

Waves are the primary energy transfer agents in coastal systems, influencing erosion, transport, and deposition.

Wave Energy

Waves possess two types of energy:

  1. Potential Energy: This results from the position of the wave above the trough. The height difference between the crest and trough is proportional to the potential energy.
  2. Kinetic Energy: This comes from the motion of water particles within the wave, which move in circular orbits. Importantly, the wave itself does not move water forward in bulk; instead, it imparts a circular motion to individual water molecules. This motion diminishes with depth.

How Waves Transfer Energy Without Moving Water

  • In deep water, the energy of the wave moves forward, but the water molecules themselves follow near-circular paths, returning to almost the same position after the wave passes.
  • As waves approach shallow water, these circular orbits become more elliptical due to interaction with the seabed, leading to the breaking of waves and energy transfer to the coast.

Calculating Wave Energy

The amount of energy in a wave in deep water is approximated by the formula:

Wave energy can be calculated using the following formula:

P = H²T

Where:

  • P = Power of the wave (in kilowatts per metre of wave front)
  • H = Wave height (in metres)
  • T = Time interval between wave crests, also known as the wave period (in seconds)

Explanation of Variables:

  1. Wave Height (H):
    • The vertical distance between the wave crest and the trough.
    • Power increases with the square of the wave height, meaning even small increases in height result in a significant rise in energy.
  2. Wave Period (T):
    • The time it takes for two successive wave crests to pass a fixed point.
    • Longer wave periods mean the wave carries more energy over a greater duration.
  3. Power (P):
    • Represents the energy carried by the wave per metre of wave front.
    • Measured in kilowatts (kW).

Example Calculation:

If a wave has a height (H) of 2 metres and a period (T) of 10 seconds:

P = 2² x 10

Substitute the values:

P = 4 x 10 – 40kW per metre of wave front.

Significance:

  • This formula shows that wave height and wave period are critical factors in determining wave power.
  • High-energy waves with greater heights and longer periods have a larger impact on coastal processes such as erosion and sediment transport.
  • It is particularly useful in coastal engineering for estimating potential wave energy for renewable energy projects.

How Waves Transfer Energy Without Moving Water

  • In deep water, the energy of the wave moves forward, but the water molecules themselves follow near-circular paths, returning to almost the same position after the wave passes.
  • As waves approach shallow water, these circular orbits become more elliptical due to interaction with the seabed, leading to the breaking of waves and energy transfer to the coast.

Wave Formation:

  • Waves are formed by wind blowing over the surface of the sea. The size and energy of waves depend on:
    • Wind Speed: Faster winds generate more energy.
    • Duration: Longer-lasting winds produce larger waves.
    • Fetch: The distance over which the wind blows without interruption.

Wave Development:

  • As waves travel, their energy increases. In deep water, they move as circular oscillations of water, but as they approach shallow water, their height increases, and they begin to break.

Breaking Waves:

As waves enter shallow water, their behaviour undergoes significant changes. Shallow water is typically defined as having a depth equal to half the wave’s wavelength. At this depth, the deepest water molecules, which move in circular orbits, come into contact with the seafloor. Friction between the seafloor and the water affects the waves’ speed, direction, and shape.

As waves drag along the bottom, they slow down. This reduction in speed causes the wavelength to shorten, and the waves begin to bunch together. The base of the wave, being in contact with the seabed, slows more than the crest, causing the wave to steepen as the crest moves ahead of the base. When the water depth becomes less than 1.3 times the wave height, the wave becomes unstable, topples over, and breaks on the shore. At this point, both energy and water move forward significantly for the first time.

  • Wave Steepness:
    • Steeper waves are more likely to break as plunging or surging waves.
    • Gentle waves typically break as spilling waves.
  • Seabed Gradient:
    • Gentle gradients favour spilling waves.
    • Steeper gradients lead to plunging or surging waves.
  • Wave Energy:
    • High-energy waves are more likely to break as plunging or surging waves.
    • Low-energy waves tend to break as spilling waves.
  • Depth of Water:
    • As water depth decreases near the shoreline, waves slow down and break differently depending on the seabed profile.

Breaking waves can be categorised as one of the following:

1. Spilling Waves
  • Characteristics:
    • Occur on gently sloping shorelines.
    • The wave’s crest spills down its front face as it breaks, gradually dissipating energy over a wide area.
  • Wave Energy:
    • Low energy makes these waves ideal for activities like surfing.
    • They cause limited erosion and are more associated with deposition.
  • Associated Features:
    • Formation of wide, gently sloping beaches.
  • Example:
    • Found on sandy beaches with shallow gradients, like many beaches in the UK.
2. Plunging Waves
  • Characteristics:
    • Occur on moderately steep shorelines.
    • The crest curls over and crashes downwards with significant force, creating a “tube” or “pipeline” effect.
  • Wave Energy:
    • High energy, delivering a powerful impact.
    • Associated with significant erosion and turbulent water.
  • Associated Features:
    • Formation of steep beaches.
    • Often associated with strong riptides and surf zones.
  • Example:
    • Common on steep, rocky coastlines or reef breaks.
3. Surging Waves
  • Characteristics:
    • Occur on steep shorelines where the wave does not break entirely but slides up the shore as a surge of water.
    • Minimal breaking as the wave retains most of its energy.
  • Wave Energy:
    • Very high energy, with significant potential to erode the coastline.
    • It can result in dangerous backwash.
  • Associated Features:
    • Development of steep, narrow beaches.
    • It can cause undercutting of cliffs and other coastal landforms.
  • Example:
    • Often found on cliffed coastlines with very steep beach profiles.

Example:

  • The powerful destructive waves along the Holderness Coast contribute to rapid cliff erosion, while constructive waves help build features like Chesil Beach in Dorset.

Constructive and Destructive Waves

Constructive Waves

  • Breaking Style: Spilling waves; break gently over a wide area.
  • Energy: Low energy; smaller wave height.
  • Swash: Strong swash pushes material up the beach, promoting deposition.
  • Backwash: Weak backwash does not remove significant material from the beach.
  • Beach Impact: Build up beaches, creating wide, gently sloping profiles.

Destructive Waves

  • Breaking Style: Plunging waves; break with force, often crashing vertically.
  • Energy: High energy; larger wave height.
  • Swash: Weak swash; limited deposition.
  • Backwash: Strong backwash pulls material down the beach, causing erosion.
  • Beach Impact: Erode beaches, creating steep profiles with limited sediment accumulation.

3. Tides

Tides influence the extent of coastal processes by determining the range over which waves and currents operate.

Tidal Cycles:

  • Tides are caused by the gravitational pull of the Moon and, to a lesser extent, the Sun. A typical tidal cycle includes two high tides and two low tides daily (semi-diurnal tides).
  • The gravitational pull of the Moon attracts water on Earth, pulling it towards the Moon and creating a high tide on the side of Earth facing the Moon.
  • On the opposite side of Earth, a compensatory bulge occurs due to centrifugal forces as Earth and the Moon rotate around their common centre of gravity, resulting in another high tide.
  • Between these two bulges, water levels are lower, creating low tides in these regions.
  • Spring Tides: Occur when the Sun, Moon, and Earth are aligned (during full moon and new moon phases). The gravitational forces of the Sun and Moon combine, creating higher high tides and lower low tides due to the increased gravitational pull.
  • Neap Tides: Occur when the Sun and Moon are at a 90-degree angle relative to Earth (during the first and third quarter moon phases). The gravitational forces partially cancel out, resulting in lower high tides and higher low tides with a reduced tidal range.

Tidal Range:

  • The difference between high and low tide levels (tidal range) varies by location:
    • Macro-tidal: Areas with a large tidal range (e.g., the Severn Estuary) experience wide zones of coastal interaction.
    • Micro-tidal: Areas with a small tidal range have more limited zones of activity.

Influence of Tides:

  • High Tides: Increase the reach of wave action, allowing erosion and sediment transport further inland.
  • Low Tides: Expose larger areas of the seabed, where deposition and intertidal processes occur.

Example:

  • In estuarine environments, such as the Thames Estuary, tidal flows shape mudflats and salt marshes by depositing fine sediments.

4. Geology

The geological characteristics of a coastline, including lithology and structure, have a significant impact on its formation and resilience.

Lithology (Rock Type):

  • Hard rocks (e.g., granite) are resistant to erosion and often form rugged coastlines with cliffs and headlands.
  • Soft rocks (e.g., clay) erode more easily, creating low-lying coasts with features like bays and estuaries.

Structure (Rock Arrangement):

  • Concordant Coasts: Rock layers run parallel to the shoreline, leading to uniform erosion and features like coves (e.g., Lulworth Cove).
  • Discordant Coasts: Rock layers are perpendicular to the shore, resulting in alternating headlands and bays.

Example:

  • The Dorset Coast demonstrates the influence of geology, with resistant limestone forming headlands like Durdle Door and softer clays forming bays like Swanage Bay.

5. Currents

Ocean currents influence coastal systems by redistributing heat, energy, and sediments.

Types of Currents:

  • Rip Currents: Narrow, fast-moving currents that flow away from the shore, often perpendicular to the coastline. Rip currents pull sediments from the beach and nearshore zone out to deeper water, contributing to the offshore transport of materials. They can create sediment-depleted areas along the beach and contribute to forming offshore bars.
  • Ocean Currents: Large-scale water movements driven by wind, Earth’s rotation, and differences in water temperature and salinity (thermohaline circulation). Ocean currents redistribute sediments over vast distances, moving material along continental shelves and across ocean basins. They shape coastal landforms, influence sediment deposition in deep-sea environments, and contribute to the global sediment cycle.

Influence on Coasts:

  • Warm currents, such as the Gulf Stream, can increase evaporation and storm frequency, affecting erosion and sediment transport.
  • Cold currents, like the Labrador Current, reduce coastal energy levels, influencing deposition.

Example:

  • The Gulf Stream moderates the climate and coastal processes along the western coast of Europe, while the cold Benguela Current contributes to arid conditions along the Namibian coast.

6. Terrestrial Sources

Rivers:

  • Major contributors of sediment to the coastal system, especially in areas with steep gradients.
  • Sediment delivery is often intermittent, typically occurring during flood events.
  • In some regions, rivers contribute up to 80% of coastal sediment.
  • Origin: Erosion of inland areas by water, wind, and ice, combined with sub-aerial processes such as weathering and mass movement.

Wave Erosion:

  • Significant source of sediment, especially from cliffs in high-energy wave environments.
  • Rising sea levels and storm surges increase cliff erosion, particularly of weak cliffs.
  • Contribution: Cliff erosion can provide up to 70% of material to beaches in some locations, though smaller contributions are typical.
  • Sediment can include large rocks and boulders from collapsed cliffs.
  • Longshore Drift moves sediment along the coast, transferring material from one coastal area to adjacent areas.

7. Offshore Sources

  • Marine Deposition:
    • Constructive waves bring sediment from offshore locations and deposit it onshore, adding to the sediment budget.
    • Tides and Currents: Transport sediment from other parts of the coastal system.
  • Aeolian Processes:
    • Wind transports fine sand from exposed sand bars, dunes, and beaches elsewhere along the coast.
    • Wind energy is weaker than water energy, so it can only carry fine-grained material.

8. Human Influence

  • Beach Nourishment:
    • Used to replenish sediment in areas where the sediment budget is in deficit.
    • Methods include:
      • Lorry Transport: Sediment is brought to the beach and spread using bulldozers.
      • Offshore Pumping: Sand and water are pumped onto the beach from offshore sources, with low bunds holding the sediment in place until the water drains away.
    • Purpose: Maintains sediment equilibrium to protect and preserve coastal environments.