Longshore Drift

Longshore drift is the term used to define the movement of sediments and particles along a coast's shore.

Longshore currents, more commonly referred to as longshore drift, is the zig-zag movement of sediment (predominantly sand) along a coast, near the shoreline.

Dominant swell patterns in a region usually result in waves approaching a coastline in predictable and consistent directions and angles. These waves carry sediment, like sand, up onto the beach in the swash. In the backwash, the waves also carry some of this sediment back down again too.

When swell patterns result in waves approaching the shore at an angle, the swash of the waves deposits sand and other materials on and along the beach in the same direction. The backwash of the waves, moves and deposits the sediment straight back down off the beach, at right angles to the shore. This creates a zig-zag movement of sediments along the beach.

1. Direction of longshore drift
2. Zig-zag movement of sediments along the beach
3. Incoming waves approach shore on a dominant angle due to regional swell patterns

Longshore drift can cause erosional or depositional effects on a beach or coastline. This largely depends on weather conditions.

For example, calm winds and small, weak waves would result in the process of longshore drift having a depositional effect on a beach or coastline. As the waves gently roll in, they carry and deposit sand and other materials onto the beach. The larger, heavier materials - like pebbles, shells and coral - are deposited earlier on, closer to the water's edge, as the force of the swash loses momentum up a beach. Smaller and lighter materials - like sand or buoyant objects - are deposited further up the shore, where the swash loses most of its momentum and just before it turns into backwash returning to the sea.

In contrast, where waves are strong, the swash and backwash is often powerful enough to drag or carry away sediments from the shore. This results in coastal erosion. This can be seen in erosion that occurred at Surfers Paradise on the Gold Coast after powerful and damaging waves hit the beach during storm conditions.

As a result, the continual actions of waves are always changing the shape of the beach and coastline. Cities that are known for their beach attractions often rely on good weather conditions to help maintain the health of their beaches.

Click the button below to open up an activity that will allow you to use 'drawing' and 'text' tools to annotate the process of longshore drift at a beach of your choice on a map.