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How does a beach form?

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Quick answer

A beach forms when waves deposit loose sediment along a shoreline. Rivers and eroding cliffs supply sand and pebbles, and breaking waves carry it up the shore. When the incoming swash is stronger than the outgoing backwash, more material stays than leaves, so sand builds into a sloping beach that keeps shifting with waves and storms.

A beach forms where waves drop loose sediment such as sand, pebbles or shell fragments along the edge of the sea or a lake. Rivers, eroding cliffs and broken shells supply the material, and the balance between waves that build up the shore and waves that strip it away shapes the beach.

Step by step

  1. Rivers, eroding cliffs and broken shells supply loose sand, gravel and fragments to the coast.
  2. Breaking waves push this sediment up the shore in a rush of water called the swash.
  3. The backwash drains down more weakly, leaving some sediment behind on the sloping shore.
  4. Waves striking at an angle move sand along the coast through longshore drift, spreading the beach sideways.
  5. Calm seasons build the beach wider, while storm waves pull sand offshore into bars until calm returns.

Key things to understand

  1. Rivers carry sand and silt to the coast, and waves also grind down nearby cliffs and rocky headlands.
  2. Each breaking wave rushes up the shore as swash, carrying sediment, then drains back as weaker backwash.
  3. Gentle constructive waves have strong swash and weak backwash, so they leave more sediment behind than they remove.
  4. Waves arriving at an angle push sediment sideways along the coast in a zigzag process called longshore drift.
  5. Steep storm waves have powerful backwash that drags sand offshore, which is why beaches shrink in stormy seasons.

Labelled diagram

  1. Sediment source

    Rivers and eroding cliffs supply sand and pebbles to the shore.

  2. Swash

    The breaking wave rushes up the beach carrying sediment forward.

  3. Backwash

    Weaker return flow drains seaward, leaving part of the load behind.

  4. Berm

    A ridge of sand built at the high-water line by repeated swash.

  5. Offshore bar

    Sand dragged out by storm waves, stored until calmer waves return it.

How waves build up a beach profile

Real-world examples

  • Marina Beach in Chennai is a long sandy beach built by waves and longshore drift along the Bay of Bengal.
  • Punalu'u Beach in Hawaii has black sand made from eroded volcanic basalt.
  • Maldivian island beaches are made largely of white sand from broken coral and shells.
  • Chesil Beach in southern England is a long shingle bank of rounded pebbles sorted by wave action.

Common mistake

✗ Myth: A beach is a fixed stretch of sand that stays the same year after year.

✓ Fact: A beach is a moving store of sediment. Sand constantly shifts along the shore and between beach and offshore bars, so its width and slope change with seasons, storms and sediment supply.

People also ask

Where does beach sand come from?
Most beach sand comes from rock broken down on land and carried to the coast by rivers. Waves also erode cliffs and headlands into sand and pebbles. In tropical seas, much sand is ground-up coral, shells and the skeletons of tiny marine organisms, which is why those beaches look white.
Why are beaches disappearing?
Beaches shrink when less sediment arrives than waves remove. Dams trap river sand, sea walls and harbours block longshore drift, and sand mining takes material directly. Rising sea levels and stronger storms add to erosion, so some coasts now pump or truck sand back to rebuild their beaches.
How does longshore drift work?
When waves approach the shore at an angle, the swash carries sand up the beach diagonally. The backwash then runs straight back down the slope under gravity. Repeated thousands of times a day, this zigzag movement shifts sand steadily along the coast in the direction of the prevailing waves.
Why is most beach sand made of quartz?
Quartz is hard and chemically stable, so it survives long journeys down rivers and endless tumbling in the surf. Softer minerals break down or dissolve, leaving quartz grains as the main ingredient of many sandy beaches.
Why are some beaches black or white?
Colour reflects the source rock. Black sand beaches, as in Hawaii, come from eroded volcanic basalt, while bright white beaches in tropical islands are often made of crushed coral and shell fragments.
Why do beaches change between seasons?
Calm-season waves push sand onshore and build a wide beach, while storm or monsoon waves drag sand offshore into bars. Many beaches therefore look narrower and steeper in the stormy season and recover afterwards.
What is the difference between a sandy and a shingle beach?
A shingle beach is made of pebbles rather than sand. Pebbles let backwash soak into the beach instead of flowing back, so shingle beaches are usually steeper than sandy ones.

Part of the guide: Nature & Earth: Living Things and Rocks →