Longshore drift

Follow one grain along the beach

Pick one grain of sand at the water's edge. The wave coming towards it is not square on to the beach: the wind has driven it in from one side, so it arrives on a slant.

Each wave takes the grain up the beach on a slant and drops it straight back down. Watch where it ends up.

Predict the direction

Choose before you read on. Getting it wrong here is the fastest way to remember the rule.

A straight beach runs from west to east, with the sea to the north of it. The waves that reach it most often come in from the north-west. Which way will the beach material travel, and why?

Getting in the way of the drift

tap a part ↓

The dashed line is where the water's edge would run if the drift passed along this coast uninterrupted. The sand reaches past it where material has been held back, and falls short of it where the beach has been starved.

The drift runs left to right. Turn the dashed line off and on, and tap a part to see what it does.

What you need to know

• Longshore drift is the movement of sediment along a coast, not up and down a single beach.
• It happens because waves arrive at an angle to the shoreline while the backwash drains straight back down the slope under gravity.
• The direction of drift is set largely by the angle at which the waves approach, which is related to the dominant fetch.
• In England, the general direction of transport is southwards on the eastern coast and eastwards on the Channel coast.
• Groynes, revetments and walls interrupt this movement and change the coastal sediment balance.

The big picture

Waves that reach a shore at an angle run up the beach on a slant, and gravity then drains the water straight back down. Each wave leaves sand and shingle a short distance further along the shore than it started, and the sum of all those small steps is longshore drift — the slow travel of sediment along a coast.

⚠ Watch out

Saying the sea carries the sand sideways, as if a current ran along the coast by itself. The along-coast movement comes from the waves: each runs up the beach at an angle and drains straight back down. Change where the waves come from and the drift changes too.

Memory hook

Up at an angle, back straight down. That little triangle is one step along the coast.

Check yourself

Draw a beach with waves arriving at an angle from the left. Trace one grain's path through three waves, then say why it is not where it began and what would reverse it.