Longshore drift moves sediment along a beach because swash travels at an angle and backwash runs straight down. To trace it, draw the wave direction first, then follow one pebble through each cycle.
This lesson opens coastal processes and management, part of physical geography.
What are swash and backwash?
Swash is the water rushing up the beach after a wave breaks. Backwash is the water returning down the slope.
Swash follows the wave direction, so when waves arrive at an angle, swash climbs the beach at that angle. Backwash follows the steepest slope, which is straight down, at right angles to the shoreline.
A rule for net movement
- Mark the shoreline and the sea side.
- Draw the direction the waves travel.
- Draw the swash arrow up the beach along that direction.
- Draw the backwash arrow straight down the slope.
- Net movement is the sideways shift between the start and the end of one cycle.
Worked example: an original beach
An original beach runs north to south, with the sea to the east. Waves travel from the north-east towards the south-west. A pebble starts at the water’s edge.
Swash carries it up the beach towards the south-west, so it lands south of where it started and higher up the slope. Backwash brings it straight back down, to the east, but it does not return to the starting point, because it is now further south.
Suppose each wave cycle moves the pebble 3 m south. After four cycles, it has moved 4 × 3 = 12 m south. The net direction is southwards, even though the sea is to the east.
The mistake: backwash at the wrong angle
A typical slip is to draw the backwash at the same angle as the swash, returning the pebble along the path it came. The working looks symmetrical, but it cancels the drift, and the diagram would show no movement at all.
| Arrow | Wrong | Right |
|---|---|---|
| Swash | At an angle with the waves | At an angle with the waves |
| Backwash | Back along the same angle | Straight down the slope |
| Net result | No movement along the shore | A zigzag moving south |
A second slip is to say sediment moves “the way the beach faces”. Drift follows the waves, not the coastline’s direction.
Check yourself
An original beach runs west to east, with the sea to the south. Waves travel from the south-west towards the north-east. Give the net direction of sediment movement and describe the path of one pebble in three steps.
Answer
Net movement is eastwards.
Step 1: swash carries the pebble up the beach towards the north-east, so it moves east as it climbs.
Step 2: backwash takes it straight down the slope, to the south, so it does not move back west.
Step 3: the next swash repeats the shift east, so the pebble zigzags east along the shore.
What to study next
Next, sort what this movement builds and removes in distinguishing erosional and depositional landforms. Then test the whole set with the coastal processes practice.
If you want a teacher to check your arrows on your textbook diagrams, see online one-to-one Geography tuition.