Sediment is primarily transported along a coastline by wave action, a process known as longshore drift.
Here's a more detailed explanation:
Sediment transport along a coastline, predominantly sand, occurs due to the combined effects of wave action and the angle at which waves approach the shore. This process, called longshore drift, involves two key stages:
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Swash: Waves rarely approach the coastline directly head-on. Instead, they usually approach at an angle. The swash is the movement of water and sediment up the beach face. Because of the angled approach of the wave, the swash carries sediment up the beach at an angle.
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Backwash: The backwash is the return flow of water back down the beach face due to gravity. This backwash travels perpendicular (at a right angle) to the shoreline, pulling sediment straight back down the beach.
This angled swash and perpendicular backwash result in a zigzag movement of sediment along the coast. Over time, immense amounts of sediment can be transported considerable distances by this process.
Here's a summary in table form:
| Process | Description | Direction |
|---|---|---|
| Swash | Movement of water and sediment up the beach face. | At an angle, determined by the prevailing wind and wave direction. |
| Backwash | Return flow of water and sediment down the beach face. | Perpendicular (at a right angle) to the shoreline. |
| Longshore Drift | Net movement of sediment along the coastline due to the combined swash and backwash. | Parallel to the coastline, in the direction determined by the angle of wave approach (the 'drift' direction). |
Furthermore, longshore currents, which are currents that run parallel to the shoreline within the surf zone, also contribute to sediment transport. These currents are generated by the angled approach of waves and can carry significant amounts of sediment along the coast.
Features like groynes (coastal defense structures built perpendicular to the shoreline) clearly demonstrate the effect of longshore drift. Sand accumulates on the updrift side of the groyne, while the downdrift side is often eroded, illustrating the interruption of the sediment transport process.
In conclusion, sediment is transported along a coastline primarily through the process of longshore drift, driven by the angled approach of waves resulting in a zigzag movement of sediment up and down the beach. Longshore currents also play a role.
