Sediment Transport & Deposition
Longshore (Littoral) Drift is the primary mechanism for lateral sediment movement along a coastline. It is initiated when waves approach the shore at an oblique angle, dictated by the prevailing wind direction.
The Swash carries sediment up the beach at the same oblique angle as the incoming wave. This pushes material both up the shore and further along the coast simultaneously.
The Backwash returns the water to the sea under the influence of gravity. Because gravity acts vertically, the backwash moves sediment straight down the beach at a angle to the shoreline.
The repetition of this swash-backwash cycle creates a zig-zag movement of sediment along the beach. Over time, this can move massive volumes of material miles away from its original source.
Deposition is fundamentally linked to energy reduction. When waves encounter friction with the seabed in shallow water, or when wind speeds drop, the velocity of the water decreases, leading to the settling of sediment.
Coastline Geometry plays a critical role; deposition often occurs in sheltered areas like bays or where the coastline changes direction (e.g., at an estuary). In these locations, wave refraction dissipates energy.
Sediment Sorting occurs during deposition. High-energy environments (exposed headlands) can only deposit large shingle and rocks, while low-energy environments (sheltered bays) allow fine sand and silt to settle.
| Feature | Drift-Aligned Beach | Swash-Aligned Beach |
|---|---|---|
| Wave Orientation | Waves approach at an oblique angle. | Waves approach parallel to the shore. |
| Sediment Movement | Significant lateral transport (Longshore Drift). | Minimal lateral movement; sediment moves up/down. |
| Landforms | Often results in spits and bars. | Usually forms wide, stable beaches in bays. |
| Energy Level | High energy, dynamic systems. | Low energy, sheltered environments. |
Identify the Prevailing Wind: In exam diagrams, always look for the wind or wave arrow first. The direction of longshore drift will follow the direction of the swash, which is aligned with the wind.
Check the Backwash Angle: A common mistake is drawing the backwash at an angle. Always ensure the backwash is perpendicular () to the shoreline, regardless of the swash angle.
Energy-Size Correlation: Remember that larger sediment requires more energy to transport. If a question describes a 'low energy' environment, the answer should involve fine sediments like sand or mud, not boulders.
Spit Formation Logic: When explaining spits, emphasize the change in coastline direction. This causes the waves to lose energy and deposit material into the open water of an estuary.