Marine processes affecting coastlines

Coastal environments are shaped by dynamic marine processes driven by the energy of waves, tides, and currents. These processes continuously alter the coastline through erosion, transportation, and deposition, forming and changing coastal features.

Key marine processes at work

• Erosion - The wearing away of coastal rocks and sediment by the force of waves, often breaking down cliffs and other structures.
• Transportation - The movement of eroded material along the coast or out to sea via waves, tides, and currents.
• Deposition - The laying down of sediment when the energy of waves or wind decreases, leading to the formation of features like beaches and spits.

Sub-aerial processes shaping coastal landscapes

Beyond marine influences, coastlines are also affected by sub-aerial processes that occur on land. These processes weaken and break down coastal materials, contributing to the overall shaping of the landscape through weathering and mass movement.

Types of sub-aerial processes

• Salt weathering - Salty water enters cracks in rocks during high tide; as it evaporates, salt crystals form and expand, exerting pressure that causes rock fragments to break off.
• Freeze-thaw weathering - In areas with fluctuating temperatures, water seeps into rock joints, freezes, and expands, gradually weakening the rock structure over repeated cycles.
• Wetting and drying - Rocks with clay content swell when wet, creating internal pressure that leads to fragmentation as they dry out.
• Chemical weathering - Rainwater mixed with carbon dioxide forms a mild acid that dissolves certain rocks, such as those containing calcium carbonate, altering their chemical makeup.
• Biological weathering - Roots of plants grow into rock fissures, widening them over time and causing the rock to break apart.
• Mass movement - Material shifts downslope due to gravity, often accelerated by wave undercutting of cliffs, leading to events like landslides, slumping, rockfalls, and mudflows. Gradual movement, such as soil creep, also occurs, especially in unconsolidated materials like clay where heavy rain reduces friction.

Mechanisms of erosion by waves

Waves are a powerful force in eroding coastal landscapes, using various methods to wear down rocks and cliffs. Each mechanism contributes to the breakdown of material, shaping the coastline over time.

Six primary ways waves erode coastlines

• Corrasion (abrasion) - Sediment and rocks carried by waves act like sandpaper, grinding against cliffs and rocks, wearing them down and smoothing their surfaces.
• Hydraulic action - Waves crash into cliffs, compressing air within cracks; this pressure forces pieces of rock to break off.
• Cavitation - As waves retreat, the compressed air expands explosively, applying additional stress to the rock and causing fragments to detach.
• Wave quarrying - The sheer force of waves striking a cliff can dislodge chunks of rock directly.
• Solution (corrosion) - Seawater gradually dissolves soluble rocks, such as limestone or chalk, through chemical reactions.
• Attrition - Rocks and pebbles carried by waves collide with each other, breaking into smaller pieces over time.

Transportation and deposition of coastal sediment

Once material is eroded, it is moved along the coast or out to sea through transportation processes. When the energy carrying this material diminishes, deposition occurs, building up new coastal features.

Methods of sediment transportation

• Solution - Dissolved substances, like certain minerals, are carried within the water, often in slightly acidic conditions.
• Suspension - Fine particles, such as silt or clay, are held within the water column due to turbulent currents, making this the most common method of transport.
• Saltation - Medium-sized particles, like gravel, bounce along the seabed, propelled by the force of water as they are too heavy to remain suspended.
• Traction - Large, heavy materials, such as boulders, are rolled or pushed along the seabed by the water's force.

Processes and causes of deposition

• Marine deposition - Sediment transported by water is laid down when the energy of waves or tides decreases.
• Aeolian deposition - Wind-carried sediment is deposited when wind speed drops or encounters obstacles.
• Factors leading to deposition:
  • Increased sediment load, such as after a cliff collapse, overwhelms the carrying capacity.
  • Reduced energy as water flow slows down in shallow areas or upon reaching land.
  • Increased friction when waves interact with the coastline, reducing their ability to carry material.
  • Turbulence caused by obstacles, which disrupts smooth flow and causes sediment to settle.

The role of longshore drift in sediment movement

Longshore drift, also known as littoral drift, is a key process in the transportation of sediment along coastlines. It results from the interaction between wave direction and the shoreline, moving material in a consistent pattern.

How longshore drift operates

• Swash movement - Waves approach the beach at an angle influenced by the prevailing wind, carrying sediment like pebbles and sand up the shore in this diagonal direction.
• Backwash movement - As the wave recedes, gravity pulls the water and sediment straight back down the beach perpendicular to the shoreline.
• Net sediment shift - When the wind and shoreline are at an angle, this repeated swash and backwash cycle results in a zigzag movement of sediment along the coast, gradually shifting material in the direction of the prevailing wind.