Coastal deposition occurs when waves lose energy and drop the sediment they have been carrying, building new landforms along and across the coastline. Beaches, spits, bars, tombolos and dunes are all depositional features driven largely by longshore drift — the process by which sediment moves in a zigzag pattern along the shore.
What is longshore drift?
Longshore drift is the key process connecting most coastal depositional landforms. Waves approach a beach at the angle of the prevailing wind — rarely head-on. When a wave breaks, it pushes sediment (sand, shingle, pebbles) up the beach at the same angle as the wave — this is called the swash. Gravity then pulls the water and sediment back down the beach at 90° to the shoreline — this is the backwash. The result is a zigzag movement of sediment along the beach in the direction of the prevailing wind.
Over time, this zigzag process moves enormous quantities of material along a coastline. The direction of longshore drift along any particular stretch of coast is determined by the dominant wind direction. On the east coast of England, for example, longshore drift generally moves sediment southward; on much of the south coast it moves sediment broadly eastward.
Understanding longshore drift is essential because it explains where depositional landforms build up — at places where energy drops and sediment accumulates — and why coastal managers sometimes build groynes (wooden or rock barriers running perpendicular to the shore) to interrupt drift and trap sediment to maintain beach width.
What is a beach and how does it form?
A beach is the most common coastal depositional landform — the accumulation of sediment between the low-tide and high-tide marks, and sometimes beyond. Beaches form wherever wave energy is low enough for deposition to exceed erosion, such as in sheltered bays between headlands.
Beach sediment varies by energy level:
- High-energy coasts (exposed to strong waves): coarse pebbles and shingle, as fine material is washed away
- Low-energy coasts (sheltered): fine sand and even silt, as gentle waves deposit finer particles
Beach profiles vary too. Steep beaches (shingle) are shaped by constructive waves — low-frequency waves with a strong swash that builds material upward. Gently sloping sandy beaches result from destructive waves that flatten material. A berm is a ridge of material deposited at the upper limit of the swash.
What is a spit and how does it form?
A spit is an elongated ridge of sand or shingle that extends out from the coast, usually from a headland or where a coastline changes direction. It projects into open water rather than crossing to the other side of a bay.
Formation process:
- Longshore drift carries sediment along the coast to a point where the coastline bends or ends — for example, at the mouth of an estuary.
- As the wave energy decreases when the coastline is no longer providing shelter, sediment is deposited.
- Material continues to accumulate, extending the ridge in the direction of longshore drift.
- The end of the spit often curves due to secondary wind directions or wave refraction — creating a "hooked" or recurved tip.
- A sheltered area of water (often a lagoon or marsh) forms behind the spit.
UK example: Spurn Head (Humber Estuary, East Yorkshire) is a classic spit extending about 5.5 km southward into the Humber. It is narrow, dynamic, and gradually migrating westward as the sea erodes its western side and deposits material to the east.
What are bars and tombolos?
| Landform | Definition | UK example |
|---|---|---|
| Spit | Ridge of sediment extending into water from a headland or bend | Spurn Head, East Yorkshire |
| Bar (bay bar) | Spit that has grown across the mouth of a bay, enclosing it | Slapton Sands, Devon — bar encloses Slapton Ley lagoon |
| Tombolo | Bar connecting an offshore island to the mainland | Chesil Beach connecting the Isle of Portland to Dorset |
| Lagoon | Enclosed water body behind a bar or spit | Slapton Ley (Devon), The Fleet (behind Chesil Beach) |
A bar forms when a spit continues to grow across a bay until it connects to the other headland. The water enclosed behind the bar becomes a lagoon or freshwater lake.
A tombolo forms when a bar grows out from the mainland and connects to an offshore island, anchoring it. Chesil Beach in Dorset is a spectacular tombolo: an 18 km shingle bank that connects the Isle of Portland to the Dorset mainland and encloses a lagoon (The Fleet).
How do sand dunes form?
Sand dunes are depositional landforms found just inland of sandy beaches. They form through the action of wind rather than waves:
- Embryo dunes: Small mounds form around obstacles (driftwood, seaweed) where wind-blown sand is trapped.
- Yellow (foredunes): Pioneer plants such as marram grass colonise embryo dunes. Marram grass is highly adapted to sand — it has rolled leaves to reduce water loss and deep roots to access groundwater. Its roots bind the sand and allow the dune to grow.
- Grey (mature) dunes: Further from the sea, more stable dunes develop richer soil as organic matter accumulates; more diverse plant communities establish.
- Dune slack: Low-lying areas between dune ridges where the water table reaches the surface; wetland habitats.
Dune systems provide habitat, coastal protection (acting as a buffer against storm waves), and recreational space, but are vulnerable to trampling and erosion if visitor pressure is high.
Frequently asked questions
What is the difference between coastal erosion and coastal deposition?
Erosion removes material from the coast; deposition adds material. The two processes are closely linked — material eroded from one location is transported by waves and currents and eventually deposited somewhere else, often further along the coast in the direction of longshore drift. A headland being eroded may be the source of sediment building a spit 20 km away. Managing one process without considering the other can have unintended consequences: protecting a cliffed coast with a sea wall can starve beaches further down the coast of their sediment supply.
Why do some beaches have sand while others have shingle?
The type of sediment reflects both the energy of the waves and the type of rock being eroded nearby. High-energy coasts grind material down to coarser particles; lower-energy or sheltered coasts retain finer material. The local geology also matters — softer rocks like sandstone produce sand-grade material; harder rocks like flint produce shingle. Beaches in Devon and Cornwall often have sand because the local sediment is fine-grained; beaches in Sussex are often shingle because flint from chalk cliffs is the dominant local material.
How do groynes prevent longshore drift?
Groynes are barriers built at roughly right angles to the shoreline. They interrupt the longshore drift process by trapping sand and shingle on their updrift side. This keeps the beach wider at the point where groynes are installed, which in turn absorbs wave energy and protects the coast. The problem is that groynes starve the coast immediately down-drift of sediment, causing beach loss there. This is the "terminal groyne effect" — a classic example of how engineering interventions in coastal systems can solve one problem while creating another.
What is the difference between a spit and a bar?
Both are ridges of deposited sediment extending across water, but a spit leaves the bay mouth at least partly open while a bar closes it entirely. A spit is attached to the land at one end and extends into open water at the other, with its end typically curved or hooked. A bar extends all the way across a bay entrance, enclosing the water behind it as a lagoon. The distinction matters geographically because a bar fundamentally changes the hydrology of the area behind it.
Map out coastal depositional landforms with Professor Mercator at aitutors.me.