The nature of wave energy and movement

Waves are dynamic movements of energy through water, primarily driven by wind. They play a crucial role in shaping coastlines by eroding, transporting, and depositing materials along the shore.

Factors influencing wave characteristics

• Wind speed and duration - Wave height and length vary according to the speed and length of time a wave has been moving.
• Fetch length - This refers to the expanse of open water over which the wind blows; a longer fetch results in bigger waves due to more energy transfer.
• Wave dimensions - Waves are described by their height (the vertical distance from the lowest point, or trough, to the highest point, or crest) and length (the horizontal distance between two consecutive crests).

Wave behaviour near the coast

• Slowing in shallow water - As waves approach the shore, they encounter shallower depths, which causes them to slow down and become unstable, leading to breaking.
• Swash and backwash - The forward movement of water up the beach after a wave breaks is called swash, while the return flow back to the sea is known as backwash. These movements are essential for material transport along the coast.

The differences between destructive and constructive waves

Waves can be categorised into two types based on their impact on the coastline: destructive and constructive. These types differ in their energy, height, and the resulting coastal processes.

Destructive waves

Destructive waves typically occur during storm conditions and are tall, often several metres high.

Characteristics:

• Feature a weak swash but a powerful backwash, which removes more material from the beach than is deposited.
• Cause significant erosion of the coastline due to their high energy and strong backwash.

Constructive waves

Constructive waves are common in calm weather and are generally less than a metre in height with a more gradual slope.

Characteristics:

• Exhibit a strong swash that carries material up the beach and a weaker backwash, leading to more deposition than removal.
• Contribute to building up coastal landforms by depositing sediments like sand and shingle.

The processes of coastal erosion by the sea

The sea erodes coastlines through a variety of physical and chemical processes. These mechanisms break down rocks and other materials, reshaping the coastal landscape over time.

Mechanisms of coastal erosion

• Hydraulic action - Seawater surges against the land, trapping air and water in cracks within the rock surface. As the wave retreats, the compressed air expands rapidly, exerting pressure that widens cracks and dislodges rock fragments.
• Corrasion (abrasion) - Fragments of rock carried by waves act as abrasives, grinding against cliffs and other coastal features, wearing them down and breaking off additional pieces.
• Attrition - Rock particles transported by waves collide with each other, gradually reducing in size and becoming smoother, eventually forming pebbles, shingle, and sand that may be deposited elsewhere.
• Corrosion - A chemical process where salts and weak acids in seawater dissolve certain types of rock, such as limestone, weakening the coastal structure.
• Wave pounding - The sheer force of waves acts like a battering ram, repeatedly striking the coast and causing mechanical breakdown of rocks through the weight and energy of the water.

The mechanism of longshore drift and material transport

Waves not only erode and deposit material but also transport it along the coastline through a process influenced by the angle of wave approach and prevailing winds.

Understanding longshore drift

• Wave angle - When waves approach the shore at an oblique angle (not perpendicular) due to the direction of the prevailing wind, they create a sideways movement of water and sediment.
• Material movement - Each wave pushes sediment, such as sand and pebbles, a short distance along the beach in the direction of the wave's approach. The backwash then pulls material back at a different angle, resulting in a zigzag pattern of transport.
• Resulting features - This continuous process can lead to the formation of coastal features like spits and bars as sediment accumulates in specific areas over time.