Depositional Landforms

Where sediment accumulates along the coast, a rich variety of depositional landforms develop. These features are dynamic — constantly changing in response to wave energy, sediment supply and sea level. At A-Level, you must understand the formation, characteristics and specific examples of each landform, as well as how they relate to the broader coastal system.

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Beaches

Beaches are the most common and widespread depositional landform. They are accumulations of sediment deposited between the low water mark and the highest point reached by storm waves.

Types of Beach

Beach Type	Sediment	Gradient	Permeability	Key Characteristics
Sand beach	Fine to coarse sand (0.0625-2 mm)	Gentle (1-5°)	Moderate	Wide, flat profile; well-developed runnels and ridges; strong backwash due to low permeability
Shingle beach	Pebbles and cobbles (4-256 mm)	Steep (10-20°)	High	Narrow, steep profile; prominent berms; weak backwash (water percolates through gaps)
Mixed beach	Sand and shingle	Variable	Variable	Often zonated — shingle at top (storm berm), sand on lower beach

Beach Cusps

Beach cusps are crescent-shaped features consisting of alternating ridges (horns) of coarse material and shallow bays of finer sediment. They form at regular intervals along the beach face.

• Horn spacing is typically 1-50 m, related to wave conditions
• Two theories explain their formation: edge wave theory (Guza and Inman, 1975) attributes them to interactions between incoming waves and reflected edge waves; self-organisation theory (Werner and Fink, 1993) suggests they emerge spontaneously from feedback between swash flow and sediment transport
• They are best developed on steep, coarse-sediment beaches
• Classic examples can be found at Lulworth Cove, Dorset, and Chesil Beach

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Spits

A spit is an elongated ridge of sand or shingle that extends from the coastline into open water, usually across a river mouth, estuary or bay.

Formation

1. Longshore drift transports sediment along the coast in the direction of the prevailing waves
2. Where the coastline changes direction (at an estuary mouth, bay or change in orientation), sediment continues to be transported into open water
3. The spit grows progressively longer, extending across the bay or estuary
4. Changes in wind direction or wave approach cause the tip of the spit to curve, forming a recurved tip (hook)
5. Multiple recurved tips may develop as the spit extends, recording past changes in wave direction
6. The sheltered area behind the spit experiences low-energy conditions, allowing mudflats and salt marshes to develop
7. The spit cannot grow completely across an estuary because the river current prevents deposition in the channel

graph TD
    A["Longshore drift transports sediment along coast"] --> B["Coastline changes direction"]
    B --> C["Sediment deposited into open water"]
    C --> D["Spit extends, tip curves in response to wave changes"]
    D --> E["Salt marsh develops in sheltered area behind spit"]
    E --> F["River current maintains open channel"]

Case Study: Spurn Point, East Yorkshire

Spurn Point (also called Spurn Head) is a narrow sand and shingle spit extending 5.5 km southward across the mouth of the Humber Estuary:

• Fed by sediment from the rapidly eroding Holderness coast (cliff retreat ~1.8 m/year)
• The spit is only 50 m wide in places and is extremely vulnerable to storm breaching
• Historical records suggest a 250-year cycle of spit growth, breaching and regrowth — identified by de Boer (1964). The current spit began forming around 1650
• Major breaches occurred in 1849, 1856 and most dramatically during the December 2013 storm surge, when the road was destroyed and the Spurn lifeboat station was cut off
• Following the 2013 breach, the Environment Agency decided to let the spit evolve naturally rather than repair defences — a form of managed retreat
• Supports important habitats including sand dunes, salt marsh and mudflats, and is managed by the Yorkshire Wildlife Trust

Case Study: Hurst Castle Spit, Hampshire

Hurst Castle Spit extends 2 km south-eastward from the Hampshire coast towards the Isle of Wight, narrowing the western entrance to the Solent to just 1.2 km:

• Composed primarily of flint shingle derived from erosion of the Hampshire cliffs to the west
• Longshore drift moves material eastward along the coast
• Henry VIII built Hurst Castle on the spit in 1544, recognising its strategic position guarding the Solent approach
• The spit has been reinforced with rock armour and beach nourishment to protect the castle (a Scheduled Ancient Monument) — raising questions about the sustainability of defending a naturally evolving landform

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Bars and Barrier Beaches

A bar is a ridge of sand or shingle that extends across a bay, connecting two headlands. It differs from a spit in that it completely closes off the bay from the open sea.

Formation

1. A spit grows across a bay through longshore drift
2. If conditions allow (no strong river current to prevent closure), the spit extends to reach the opposite headland
3. The enclosed body of water behind the bar becomes a lagoon, which may gradually fill with sediment and develop into freshwater or brackish wetland

Case Study: Slapton Ley, Devon

Slapton Ley is the largest natural freshwater lake in south-west England, enclosed behind a shingle bar:

• The bar is approximately 4.5 km long and composed of flint and quartzite shingle
• It was formed during the post-glacial sea level rise (approximately 3,000 years ago) as rising seas pushed beach material landward, eventually enclosing a freshwater lagoon
• The lagoon is a Site of Special Scientific Interest (SSSI) and National Nature Reserve, supporting rare aquatic plants and bird species
• The A379 road runs along the top of the bar, making it vulnerable to storm damage — major breaches occurred in 2001 and 2014
• The bar is not currently maintained by longshore drift (it is essentially a relict feature), making it increasingly vulnerable to future storm damage and sea level rise

Barrier Beaches and Islands

In some locations, bars form offshore as barrier beaches or barrier islands, separated from the mainland by a lagoon or marsh. These are common on low-gradient coastlines:

• Chesil Beach, Dorset — a 29 km tombolo/barrier beach connecting the Isle of Portland to the Dorset mainland, enclosing The Fleet lagoon
• Outer Banks, North Carolina, USA — a chain of barrier islands stretching over 300 km, subject to frequent hurricane damage and coastal retreat

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Tombolos

A tombolo is a bar of sand or shingle that connects an offshore island to the mainland.

Formation