Coastal transport moves loose material along and up and down the shore, and deposition drops it where the water loses energy. A question may give you a map or photograph of a spit, bar or beach and ask you to explain how transport built it.
This lesson follows explaining wave action through a diagram and is the second step in coasts and marine processes.
How does material travel along a coast?
Material moves in four ways: traction (rolled along the bed), saltation (bounced in hops), suspension (carried within the water) and solution (dissolved). Along a beach, the main movement is longshore drift.
- Waves approach the beach at an angle, set by the wind.
- Swash carries sand and pebbles up the beach at that angle.
- Backwash drags them straight back down the steepest slope.
- Repeated again and again, the material makes a zigzag path along the shore.
How does drift build a spit?
Where the coast bends sharply or meets a river mouth, the drifting material keeps going straight and meets open water. It is deposited as a ridge that grows out from the land.
The end is the tip, and the sheltered area behind it can collect fine mud and form salt marsh. A bar forms when the ridge reaches across a bay and closes it.
Worked example
All figures are invented for practice. Five pebbles are marked and placed on a beach at low tide. A day later they are found to have moved along the shore by 3 m, 5 m, 4 m, 6 m and 2 m.
Step 1, total distance: 3 + 5 + 4 + 6 + 2 = 20 m.
Step 2, mean: 20 ÷ 5 = 4 m per day.
Step 3, direction: the waves approach from the south-west onto a south-facing coast. Swash runs up toward the north-east and backwash runs straight down toward the south, so the net movement is eastward.
Step 4, link to the landform: at a sharp bend in the coast, drift carries material east past the end of the beach and deposits it in sheltered water, so a spit grows out in that direction.
A spit measured at 300 m in one survey and 420 m ten years later has grown 120 m, which is 120 ÷ 10 = 12 m per year on average. The words “on average” matter, because the real growth may be uneven.
The mistake to watch for
Weak answer: “The waves push the sand along the beach, so a spit forms.”
This leaves out the angle of the swash and the straight backwash, which are the reason the movement is sideways.
The fix is to include both halves of the cycle: “Swash moves material up at an angle, backwash returns it straight down, so material is moved along the coast.”
A second slip is to give the direction of drift from the wrong side. Draw the wave arrow first, then the swash, then the backwash, and read the net direction from the diagram.
Check yourself
1. Tracer pebbles move 1 m, 3 m and 5 m in a day (invented). Find the mean.
Show answer
1 + 3 + 5 = 9, and 9 ÷ 3 = 3 m.
2. Waves strike a west-facing coast from the north-west. Which way does longshore drift move material?
Show answer
Swash runs up toward the south-east, and backwash runs back toward the west, so the net movement is southward.
3. Why can a bar trap a lagoon?
Show answer
The ridge grows across a bay and joins land at both ends, so the water behind it is cut off from the open sea.
Where this leads next
The next lesson moves from natural landforms to people’s response, in comparing a hard and a soft management mechanism. The statistics and distribution explorer can summarise tracer data.
If your explanations of landforms stay at the level of names, our teachers in online one-to-one Geography tuition can practise the chain of cause and effect on your own diagrams.