We stand with Jamaica 🇯🇲
Coastal Processes
Longshore Drift, Refraction, Erosion, Transportation, Deposition
Author:
Syed Ali
Edu Level: Unit1
Date: Sep 6 2025 - 4:40 AM
⏱️Read Time: 3 min
Major Coastal Flows
Two major processes act along coastlines:
- Wave Refraction
- Longshore Drift
Wave Refraction
Wave refraction is the bending of waves caused by variations in water depth.
- When part of a wave enters shallow water, it slows down, while the section still in deeper water continues moving faster.
- This speed difference makes the wave bend.
Examples:
- Along a straight shoreline, if waves approach at an angle, the section in shallow water slows, allowing the deeper section to catch up. This causes the crest to realign and become nearly parallel to the shore.
- Around a circular island, waves approaching from one direction curve around the island so that they strike beaches almost parallel from all sides.
Longshore Drift
Longshore drift occurs when waves approach the coast at an angle.
- The swash carries material up the beach at the same angle as the incoming wave.
- The backwash, pulled by gravity, drags sediment back down at a 90° angle to the shoreline.
- This creates a zig-zag movement of sand and shingle along the coast.
Sediment Sorting:
- Coarser, heavier material is usually deposited updrift.
- Finer, lighter particles are carried further downdrift.
Coastal Erosion
Coastal erosion is the wearing away of cliffs, beaches, and dunes by waves, tides, currents, and runoff. It can involve both the loss of material and the redistribution of sediments.
Main Types of Erosion:
- Hydraulic Action – Waves force air into cracks in rocks, building pressure until pieces break away.
- Attrition – Rock fragments carried by waves collide, breaking into smaller, smoother, rounder pieces.
- Corrasion (Abrasion) – Waves hurl rock fragments against the cliff face, wearing it down like sandpaper.
- Corrosion (Solution) – Acidic seawater dissolves certain rocks (e.g., limestone).
- Wave Pounding – The sheer force of steep waves hitting cliffs creates shockwaves.
- Sub-aerial Processes – Weathering from rain, surface runoff, and wind weakens cliffs, leading to soil creep, landslides, or slumping.
Factors Influencing Coastal Erosion
- Rock Geology – Softer rocks (e.g., sandstone, siltstone) erode faster than harder rocks (e.g., granite, basalt). Joints, fractures, and bedding planes increase vulnerability. Rock dip (vertical/horizontal layering) also influences collapse rates.
- Wave Breaking Point – Waves release maximum energy when breaking directly against cliffs. Energy is reduced if they collapse offshore.
- Wave Steepness – Steeper waves nearshore are more erosive than low-energy swell waves formed far out at sea.
- Sea Depth, Fetch, and Coastline Shape – Steep beaches create powerful breakers. Long fetches give waves more time to build energy. Headlands focus energy due to refraction.
- Beach Material Supply – Sediment can protect cliffs by absorbing wave energy, but removal of material increases erosion.
- Beach Morphology – Steep coasts produce destructive plunging waves, while gentle slopes create constructive spilling waves.
- Human Activity – Construction, quarrying, or removing beach material weakens coasts and accelerates erosion.
Coastal Transportation
Sediment transported by longshore drift moves through processes similar to river transport:
- Traction – Large rocks rolled along the seabed.
- Saltation – Medium particles bounced along the seabed.
- Suspension – Fine particles carried in the water.
- Solution – Minerals dissolved in seawater.
Coastal Deposition
Deposition occurs when waves lose energy and can no longer transport material.
- The swash moves sediment up the beach at an angle, while the backwash pulls it down at 90°.
- Over time, material shifts along the shore, shaped by prevailing wind direction.
- Heaviest material is deposited first, while lighter sediments travel further.
- Deposition builds up beaches and can create distinctive coastal landforms such as spits, bars, and tombolos.
Read Next
Limestone
Go to Content →Coastal Landforms
Go to Content →Waves
Go to Content →Drainage Patterns
Go to Content →
