How does the sea transport and deposit sediment to build the constructive landforms of a coastline?
Explain longshore drift and other transport processes and how deposition produces beaches, spits, bars, tombolos and barrier features
A focused answer to the H2 Geography outcome on coastal deposition. Longshore drift and sediment transport, the conditions favouring deposition, and the formation of beaches, spits, bars, tombolos and barrier islands.
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What this dot point is asking
SEAB wants you to explain how sediment is transported along a coast (chiefly by longshore drift) and how deposition under low-energy, well-supplied conditions builds beaches, spits, bars, tombolos and barrier features. The central insight is that depositional landforms appear wherever the sea loses the energy to carry its sediment load, so it drops material faster than it removes it.
The answer
Sediment transport
The sea moves sediment by four marine processes (the same names as transport in rivers): traction (rolling large material), saltation (bouncing), suspension (carrying fine material in the water) and solution (dissolved load). Along the shore, the dominant transport is longshore drift.
Longshore drift
When waves approach the beach at an angle set by the prevailing wind, the swash carries sediment up the beach at that angle, but the backwash returns it straight down the steepest slope under gravity. Repeated, this moves sediment along the coast in a zigzag path; the net movement is longshore (littoral) drift. It is the conveyor belt that supplies sediment to depositional landforms downdrift.
Conditions favouring deposition
Deposition dominates where:
- Wave energy is low, in sheltered bays (where refraction reduces energy), behind spits, reefs or islands;
- Sediment supply is high, from rivers, eroding cliffs or offshore banks;
- Constructive waves prevail, so swash exceeds backwash and material is left on the shore.
Depositional landforms
- Beaches: accumulations of sand or shingle where swash deposits sediment; the profile reflects the wave regime.
- Spits: ridges of sediment extending from a change in coastline direction (a headland or river mouth), built as longshore drift continues across open water; a change in wind can recurve the distal end, and salt marsh accumulates in the sheltered water behind.
- Bars: where a spit grows across a bay, or an offshore bar is driven onshore, it can seal off a lagoon.
- Tombolos: a spit or bar that links an island to the mainland.
- Barrier islands: long offshore sediment ridges on low-energy, sediment-rich coasts, separated from land by a lagoon.
Examples in context
Example 1. Singapore's reclaimed and nourished shorelines. Much of Singapore's modern coastline is engineered, with reclaimed land at Changi and the East Coast supplied by imported sand and protected by groynes and breakwaters that trap longshore drift. East Coast Park beaches are maintained by nourishment, illustrating how depositional processes are deliberately managed where natural sediment supply is limited in a low-energy setting.
Example 2. Spurn Head, England. Spurn Head is a long recurved spit at the mouth of the Humber Estuary, built by longshore drift carrying sediment southward along the rapidly eroding Holderness coast. Its hooked end and the salt marsh in its sheltered lee are classic spit features, and its survival depends on continued sediment supply from updrift cliff erosion.
Try this
Q1. Explain why sediment moves along a beach in a zigzag path. [3 marks]
- Cue. Waves push swash obliquely up the beach at the angle of approach, while backwash returns sediment straight down the slope under gravity; repeated, this produces a net zigzag movement, longshore drift.
Q2. State two conditions that favour coastal deposition. [2 marks]
- Cue. Low wave energy (sheltered, refracted or behind a barrier) and a high sediment supply (from rivers, eroding cliffs or offshore sources), with constructive waves dominating.
Q3. Explain how a tombolo forms. [2 marks]
- Cue. Longshore drift or wave refraction deposits sediment in the sheltered water between an island and the mainland, building a bar or spit that eventually links the island to the shore.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksExplain how longshore drift transports sediment along a coast and how it leads to the formation of a spit.Show worked answer →
Argument: longshore drift moves sediment along the shore in a zigzag path driven by waves approaching at an angle, and where the coast changes direction this transport builds a spit out into open water.
Process to explain: when waves approach the beach at an angle (set by the prevailing wind), the swash carries sediment up the beach at that angle, but the backwash, under gravity, returns it straight down the steepest slope. Repeated, this moves sediment along the coast in a zigzag, the net direction being longshore drift.
Spit formation: where the coastline changes direction, for example at a river mouth or bay, longshore drift continues in its original direction across the open water, depositing sediment as a long ridge attached to the land at one end. The sheltered, lower-energy water behind it allows finer sediment and salt marsh to accumulate. A change in wind direction can curve, or recurve, the distal end, giving a hooked spit. The spit cannot grow across a river mouth indefinitely because the river current keeps a channel open.
Markers reward the swash-at-an-angle, backwash-straight-down mechanism, the role of a change in coastline direction, deposition in lower-energy water, and recurving and the marsh behind.
Original8 marksExplain the conditions and processes that favour coastal deposition, with reference to depositional landforms.Show worked answer →
Argument: deposition occurs where wave energy is low and sediment supply is high, so the sea drops more material than it removes, building constructive landforms.
Conditions: sheltered locations such as bays where wave refraction reduces energy; areas behind spits, reefs or islands; coasts with a low gradient and a large sediment supply from rivers, cliff erosion or offshore sources; and constructive wave conditions where swash exceeds backwash.
Landforms to reference: beaches accumulate where swash deposits sediment; spits extend from a change in coastline direction; a bar can form where a spit grows across a bay or where an offshore bar is driven onshore, sometimes enclosing a lagoon; a tombolo links an island to the mainland; barrier islands form offshore on low-energy, sediment-rich coasts.
Markers reward linking low energy and high sediment supply to deposition, the role of constructive waves and shelter, and accurate reference to named depositional landforms.
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