What is longshore drift?

Longshore drift is the net transport of sediment along a coastline, driven by waves approaching at an angle. Swash carries sediment up the beach at the wave angle; backwash carries it straight down, producing a zigzag motion with a net alongshore component.

How do barrier islands form?

Barrier islands are long, low ridges of sand that form offshore, parallel to the coast. They may form from spits that are breached by storms, from sand ridges built by waves and longshore drift, or from submerged coastal dunes as sea level rises. They migrate landward over time.

Waves, Tides & Coastal Processes

The coastline is a battleground between land and sea — constantly rebuilt by waves, tides, and longshore drift.

Intro GeologyUni Year 1

⏱️ About 16 min

Waves, Tides & Coastal Processes — illustration

Illustrative image (AI-generated).

Every wave that breaks on a beach is a geologic event. Over a year, the waves pounding a single stretch of coastline deliver more energy than a nuclear bomb. That energy builds sand spits, moves barrier islands, and grinds cliffs into gravel. The coastline you see today is only a snapshot in an endless tug-of-war between land and sea.

💡

The big idea: Coasts are dynamic environments shaped by wave energy, tidal range, and sediment supply. Waves erode headlands, transport sediment alongshore via longshore drift, and deposit it in beaches, spits, and barrier islands. Tides modulate where waves break and how much of the shore is exposed to erosion. Understanding these processes is essential for predicting coastal change and managing coastal hazards.

🎯 By the end, you'll be able to

Explain how waves form and how wave energy is focused on headlands while bays are sheltered
Describe longshore drift and predict the direction of sediment transport from wave approach angle
Identify coastal depositional landforms: beach, spit, tombolo, barrier island, and lagoon
Contrast erosional coasts (cliffs, sea stacks, wave-cut platforms) with depositional coasts

📎 Helpful to know first

Waves: energy from wind to water

Waves are created by wind blowing over water. The energy they carry depends on wind speed, duration, and the distance over which the wind blows (the fetch). In the open ocean, waves are gentle swells; as they approach shallow water, the bottom drags on the wave, slowing it and causing the wave to grow taller until it breaks.

Waves rarely approach a coastline straight on. They usually arrive at an angle, and this simple fact drives the most important coastal transport process: longshore drift.

Longshore drift: the river of sand

When a wave hits the beach at an angle, the swash (water and sediment rushing up the beach) moves up at that same angle. The backwash (water and sediment draining back) flows straight down under gravity. The result is a net zigzag motion that transports sediment along the coast.

This longshore drift can move enormous volumes of sand — millions of tonnes per year on some coasts. It is why beaches change shape seasonally and why sediment accumulates in certain places while eroding from others.

🔑 Longshore drift follows the wave approach

If waves approach a coastline from the southwest, longshore drift moves sediment northward along the shore. Groynes, jetties, and breakwaters are often built to interrupt this drift — but blocking it in one place starves the beach down-drift, causing erosion there.

Longshore drift: oblique waves drive a zigzag motion of sediment along the beach, creating a net alongshore current.

Erosional coasts: cliffs, arches, and stacks

On coasts with hard rock and high wave energy, erosion dominates:

Wave-cut cliffs: Steep rock faces carved by wave action at their base. As the cliff is undercut, rock above collapses and the cliff retreats inland.
Wave-cut platform: A flat, rocky bench exposed at low tide, formed as the cliff retreats and the base is planed off by wave abrasion.
Sea arches and stacks: Where waves attack headlands along zones of weakness (fractures or bedding planes), they may punch through to form an arch. When the arch roof collapses, an isolated sea stack remains.

Depositional coasts: beaches, spits, and barriers

Where sediment supply is abundant and wave energy is lower, deposition builds:

Beach: A strip of sediment (sand, gravel, or cobbles) along the shoreline, constantly shifting with waves and tides.
Spit: An elongated ridge of sand or gravel extending from the mainland into open water, formed by longshore drift. A baymouth bar closes off a bay.
Tombolo: A spit that connects an offshore island to the mainland.
Barrier island: A long, narrow, low-lying island parallel to the coast, separated from the mainland by a lagoon. Barrier islands migrate landward as sea level rises and storms overwash them.

Erosional and depositional coastal landforms: cliffs, stacks, beaches, spits, and barrier islands.

Tides: the daily rise and fall

Tides are the periodic rise and fall of sea level caused by the gravitational pull of the Moon and Sun. They modulate coastal processes by changing the water level at which waves break:

High tide: Waves reach higher up the beach or cliff, increasing erosion of the upper shore.
Low tide: Wave energy is focused lower, and the intertidal zone is exposed.

Spring tides (highest high tides, lowest low tides) occur when the Sun and Moon align; neap tides (smallest range) occur when they are at right angles.

Check your understanding

1. What drives longshore drift?

Oblique wave approach causes swash to move up at an angle and backwash to flow straight down, creating a net zigzag transport of sediment along the coast.

2. Which coastal feature is formed by wave erosion punching through a headland?

Waves attacking a headland along zones of weakness can erode a hole through it, creating a sea arch. If the arch roof collapses, a sea stack remains.

3. What happens to a down-drift beach when a groyne blocks longshore drift?

Groynes trap sediment on their up-drift side, but the beach down-drift is starved of sediment and erodes — a common unintended consequence of coastal engineering.

✅ Key takeaways

Waves deliver enormous energy to coasts; their power depends on wind speed, duration, and fetch.
Longshore drift transports sediment along the coast in a zigzag pattern driven by oblique wave approach.
Erosional coasts feature cliffs, wave-cut platforms, arches, and sea stacks.
Depositional coasts feature beaches, spits, tombolos, and barrier islands.
Tides modulate where waves break and how much of the shore is exposed to erosion each day.

➡️ Coasts are where terrestrial sediment meets the sea, but much of that sediment continues offshore into deeper water. The ocean floor is not a barren plain — it is blanketed by marine sediments that record climate, biology, and tectonic history in layers kilometres thick.

Want to test yourself on this? Try the Science practice tests →

🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of surface processes.

University courses & departments

Trusted references

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.