Coastal Deposition and Management

Deposition occurs when the energy available to transport sediment falls below the threshold needed to keep it moving. At A-Level, AQA requires detailed knowledge of depositional landforms, ecological succession on coastal deposits, and the principles and practice of coastal management.

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Longshore Drift

Longshore drift (littoral drift) is the movement of sediment along the coast by wave action. It is the dominant process of sediment transport in the nearshore zone.

Process

1. Waves approach the coast at an angle determined by the prevailing wind direction
2. The swash carries sediment up the beach at this oblique angle
3. The backwash drags sediment directly down the beach under gravity, perpendicular to the shoreline
4. The result is a net movement of sediment along the coast in a zigzag pattern

Quantification

• The rate of longshore drift can be measured by monitoring sediment accumulation against groynes or natural obstacles
• Typical rates on the English coast: 100,000 to 500,000 m³ per year
• At Holderness, longshore drift moves approximately 500,000 m³ of sediment southward annually
• Tracer studies using fluorescent pebbles or radioactive tracers can track sediment movement over short periods

Evidence of Longshore Drift Direction

• Sediment accumulation on the updrift side of groynes
• Progressive reduction in pebble size in the direction of transport (attrition)
• Growth direction of spits (extending in the drift direction)
• Orientation of beach cusps

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Depositional Landforms

Spits

A spit is an elongated ridge of sand or shingle that extends from the coastline into the sea or across an estuary mouth.

Formation:

1. Longshore drift transports sediment along the coast
2. Where the coastline changes direction (e.g., at an estuary mouth), sediment continues to be deposited in the original direction of transport
3. The spit grows outward, extending into open water
4. The distal end often curves landward due to wave refraction around the tip (recurved end)
5. Continued growth creates multiple recurved ridges, indicating former positions of the spit tip
6. A sheltered area of low-energy water forms behind the spit, where fine sediment accumulates to create salt marshes

Example: Spurn Head, East Yorkshire

• Extends 5.5 km into the Humber Estuary
• Formed by southward longshore drift of material eroded from the Holderness cliffs
• Has been breached and reformed several times over recorded history (approximately every 250 years)
• Currently managed by the Environment Agency as a dynamic natural feature

Bars

A bar is a ridge of sand or shingle that extends across a bay, completely cutting off the water behind it from the open sea.

Formation:

1. A spit grows across a bay
2. If the bay is not fed by a significant river, the spit may eventually reach the opposite headland
3. The enclosed water body behind the bar becomes a lagoon, which may eventually infill with sediment

Example: Slapton Ley, Devon — a freshwater lagoon trapped behind a shingle bar

Tombolos

A tombolo is a bar that connects an island to the mainland.

Formation:

1. Wave refraction around an offshore island creates areas of reduced wave energy in the lee of the island
2. Sediment transported by longshore drift and wave action is deposited in this sheltered zone
3. Over time, the deposit builds up to connect the island to the coast

Example: Chesil Beach connects the Isle of Portland to the Dorset mainland (although Chesil Beach is also classified as a barrier beach)

Barrier Islands

Barrier islands are elongated ridges of sand running parallel to the coast but separated from it by a lagoon.

• Form through a combination of longshore drift, wave action, and sea-level change
• Common on low-gradient coasts with plentiful sediment supply
• Examples: The Outer Banks, North Carolina (USA); the Frisian Islands (Netherlands/Germany)

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Salt Marshes

Salt marshes are coastal wetlands that form in sheltered, low-energy environments where fine sediment (silt and clay) accumulates in the intertidal zone.

Formation and Ecological Succession

Salt marshes develop through a process of ecological succession (a prisere beginning in salt water — a halosere):

Stage 1: Mudflat

• Fine sediment (silt, clay) is deposited in sheltered water behind spits, in estuaries, or in lagoons
• The surface is submerged at every tide
• No rooted vegetation; some algae and diatoms

Stage 2: Pioneer Zone (Lower Marsh)

• Pioneer species colonise the mudflat: Salicornia (glasswort), Spartina (cord grass)
• These plants are adapted to salt water, regular submersion and unstable substrate
• They trap more sediment, raising the surface level and reducing submersion time
• Spartina anglica (common cord grass) is particularly effective at trapping sediment

Stage 3: Lower-Middle Marsh

• Surface level rises; flooding frequency decreases
• More species can colonise: Aster tripolium (sea aster), Limonium (sea lavender), Puccinellia (sea meadow grass)
• Creeks develop to drain tidal water, creating a dendritic drainage pattern

Stage 4: Upper Marsh

• Only flooded during spring tides
• Greater species diversity: Juncus (rushes), Festuca (fescue grass), Armeria maritima (thrift)
• Soil begins to develop with increasing organic content
• Salinity decreases as rainwater accumulates

Stage 5: Climatic Climax

• Rarely or never flooded by the sea
• Terrestrial vegetation (scrub, trees) may colonise
• True soil forms — a transition from salt marsh to terrestrial habitat

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Sand Dune Succession

Sand dunes form on coastlines with a plentiful supply of sand, onshore winds, and an obstacle to trap windblown sand. They develop through a psammosere (a succession beginning on sand).

Succession Stages