Characteristics and effects of constructive and destructive waves

Waves play a crucial role in shaping coastal landscapes through processes of erosion and deposition. They are broadly classified into two types: constructive and destructive, each with distinct characteristics and impacts on the coastline.

Features of constructive waves

• Depositional nature - Often referred to as 'spilling' or 'swell' waves, these waves build up beaches by depositing sediment.
• Wave structure - Characterised by a long wavelength and low height, which allows for a gentler impact on the shore.
• Frequency and period - Operate at a low frequency of about 6-8 waves per minute, with a high period of one wave every 8-10 seconds.
• Swash dominance - The forward movement of water (swash) is stronger than the return flow (backwash), leading to sediment accumulation on the beach.

Features of destructive waves

• Erosional nature - Known as 'surging', 'storm', or 'plunging' waves, these waves erode coastal features by removing sediment.
• Wave structure - Have a short wavelength and high height, resulting in a more forceful impact on the coast.
• Frequency and period - Occur at a high frequency of around 10-12 waves per minute, with a low period of one wave every 5-6 seconds.
• Backwash dominance - The return flow (backwash) is stronger than the swash, pulling sediment away from the shore.

The role of tides in shaping coastal landscapes

Tides are the regular rise and fall of sea levels caused by the gravitational forces exerted by the moon and the sun on the oceans. They significantly influence coastal processes and landforms.

Impacts of tides on coastal environments

• Tidal range - The difference between high tide and low tide determines the vertical extent over which erosion, deposition, weathering, and biological activity occur along the coast.
• Geographical variations - Tides are more pronounced in bays and funnel-shaped coastlines, where water is concentrated, amplifying their effects.
• Scouring effect - The movement of tides can remove debris from coastal areas, reshaping beaches and exposing underlying structures to further erosion or weathering.

Sources of sediment supply to coastal areas

Sediment is a fundamental component of coastal landscapes, contributing to the formation of beaches, bars, and other features. It originates from various natural processes and locations.

Origins of coastal sediment

• Mass movements - Large-scale events such as landslides deliver substantial material to the coast, which can bury beaches and offer temporary protection to cliffs.
• River transport - Rivers carry fine-grained materials like silts, clays, and sands to the coastline, adding to sediment budgets.
• Periglacial processes - Frost-shattered shingle from cold environments contributes to beach formation.
• Cliff erosion - Waves erode cliffs, producing sediment that helps build beaches and may shield the cliff base from further erosion.
• Marine transport - The sea moves sediments onshore, forming features like offshore bars and beaches.
• Wind action - Wind erodes and transports fine sand, contributing to dune formation and beach replenishment.
• Volcanic activity - Eruptions produce dust and ash, which can settle on coastlines and become part of beach sediment.

Subaerial and wave processes affecting cliffs and coastlines

Coastal landscapes are shaped by a combination of subaerial (land-based) and wave-driven processes. These mechanisms work together to erode, weather, and alter cliffs and shorelines.

Subaerial processes on cliff faces

• Salt weathering - Sodium and magnesium compounds in seawater expand within rock joints and cracks, weakening the structure over time.
• Freeze-thaw weathering - Water trapped in rocks freezes and expands, fracturing jointed rocks and contributing to cliff degradation.
• Biological weathering - Organisms such as molluscs, sponges, and urchins break down rock material, particularly significant in low-energy coastal environments.
• Surface run-off and rain wash - Water flowing over cliff surfaces erodes material, while weathering by wind and frost further degrades exposed rock.
• Mass movement - Includes soil creep, landslides, and slumps, which displace large volumes of material down cliff faces.

Wave-driven processes at the coast

• Abrasion (corrasion) - The wearing away of cliffs by rocks and sand hurled against them by waves.
• Hydraulic action (wave pounding) - Shock waves from waves striking cliffs can exert pressures up to 30 tonnes per square metre, causing significant erosion.
• Hydraulic pressure - Trapped air in rock crevices is compressed by wave impact and suddenly released, fracturing the rock.
• Solution - Carbonic acid in seawater dissolves minerals like limestone, gradually eroding coastal features.
• Corrosion - Salt crystallisation disintegrates weaker rock layers, while blue-green algae contribute to rock breakdown.
• Attrition - Broken materials in the water are worn down into smaller, more rounded particles through constant collision.
• Currents - Generated by waves and tides, these move sediment and influence erosion patterns along the coast.
• Biotic factors - Burrowing and browsing organisms further erode coastal materials.
• Human activity - Can increase run-off and erosion, though sea defences may mitigate wave impact on cliffs.

The process of littoral drift and its impact on sediment movement

Littoral drift, also known as longshore drift, is a key process in the movement of sediment along coastlines, driven by wave action and prevailing winds.

Mechanics of littoral drift

• Wave angle influence - Waves approach the shore at an angle due to prevailing winds, causing the swash to push sediment up the beach diagonally.
• Backwash movement - The backwash pulls water and sediment back down the beach along the steepest slope, usually perpendicular to the shoreline.
• Net sediment transport - This zigzag motion of swash and backwash results in a net movement of sediment along the coast, known as longshore drift.