Landforms of coastal deposition

In this section, you will learn about:

• how depositional coastal landforms develop
• the formation of beaches, spits, tombolos and bars
• the development of dunes and barrier systems
• the role of sediment supply, wave energy and longshore drift in shaping depositional landscapes
• examples of depositional coastal landscapes from the UK and beyond

Depositional coastal landscapes

Depositional landscapes develop where coastal systems experience a surplus of sediment, and wave energy is relatively low. In these environments, waves and currents lose energy and can no longer transport their sediment load effectively.

Deposition commonly occurs:

• in sheltered bays
• along low-energy coastlines
• in estuaries and lagoons
• where constructive waves dominate
• where sediment supply is abundant

Depositional landscapes are dynamic environments that constantly adjust to changes in:

• wave energy
• sea level
• sediment supply
• wind direction
• human activity

Beaches

Beaches are accumulations of sediment deposited between the low-water and high-water marks.

Beach material may include:

• sand
• shingle
• pebbles
• cobbles

The size and shape of beach sediment depend on:

• geology
• wave energy
• transportation processes
• sediment supply

Beach formation

Beaches form when:

• wave energy decreases
• sediment accumulates faster than it is removed
• constructive waves dominate

Constructive waves:

• have strong swash and weak backwash
• transport material up the beach
• encourage deposition

Sediment may originate from:

• cliff erosion
• rivers
• offshore deposits
• longshore drift

Beach profiles

Beach profiles vary according to wave energy and sediment size.

Constructive beach profiles

Constructive waves create:

• wide beaches
• gentle gradients
• berms formed by swash deposition

Sand beaches are common in low-energy environments where constructive waves dominate.

Destructive beach profiles

Destructive waves create:

• steeper beach gradients
• narrower beaches
• offshore bars formed by strong backwash

Shingle beaches are often steeper because larger particles allow water to infiltrate more easily, reducing backwash.

Beach features

Berms

Berms are ridges of deposited sediment formed by constructive waves near the back of the beach.

Storm beaches may contain several berm ridges representing different wave conditions.

Cusps

Beach cusps are crescent-shaped depositional features formed by interactions between swash and backwash.

They often develop on shingle beaches.

Spits

Spits are elongated ridges of sand or shingle extending from the coastline into the sea or across estuaries.

They form primarily through longshore drift.

Formation of simple spits

Simple spits develop when:

• Longshore drift transports sediment along the coastline
• The coastline changes direction or crosses an estuary
• Wave energy decreases
• Sediment is deposited beyond the headland
• The end of the spit may become recurved if wave direction changes due to prevailing winds or wave refraction.

Compound spits

Compound spits contain several recurved ridges extending from the main spit.

These recurves form because:

• wind and wave directions change over time
• periods of deposition alternate with storm activity

Each recurved ridge represents a former position of the spit as it gradually extended seawards.

Salt marsh development

The sheltered water behind spits often becomes a low-energy environment where:

• fine sediment accumulates
• mudflats develop
• salt-tolerant vegetation colonises the area

Over time, this may lead to the development of salt marshes.

Tombolos

A tombolo is a ridge of sand or shingle connecting an island to the mainland.

Tombolos form when:

• wave refraction causes sediment deposition between the island and mainland
• longshore drift supplies sediment to the sheltered zone

As deposition continues, the sediment ridge eventually links the island to the coast.

Bars

Bars develop when a spit extends completely across a bay.

This traps water behind the bar, forming a:

• lagoon
• salt marsh
• wetland environment

Bars usually form in areas where:

• wave energy is relatively low
• sediment supply is high
• the coastline is gently curved

Barrier beaches and barrier islands

Barrier systems are long ridges of sediment running parallel to the coastline.

Barrier beaches

Barrier beaches remain attached to the mainland but are separated from it by lagoons or marshes in places.

They often form through:

• offshore sediment deposition
• longshore drift
• sea-level change

Barrier beaches absorb wave energy and help protect coastlines from erosion.

Barrier islands

Barrier islands are detached ridges of sand running parallel to the coast.

They are particularly common along low-gradient coastlines with large sediment supplies.

Barrier islands:

• migrate landward over time
• are highly dynamic
• change shape during storms and sea-level rise

They are especially common along the Atlantic and Gulf coasts of the USA.

Sand dunes

Sand dunes form where wind transports dry beach sediment inland.

Dune systems are important coastal sediment stores and often develop behind beaches in sheltered environments.

Conditions needed for dune formation

Dunes require:

• a large supply of dry sand
• strong onshore winds
• obstacles to trap sand
• vegetation to stabilise sediment
• a wide beach exposed at low tide

Stages of dune development

Embryo dunes

Embryo dunes form when sand accumulates around obstacles such as:

• driftwood
• pebbles
• vegetation

Pioneer species such as marram grass help trap additional sediment.

Foredunes

As dunes grow larger and become more stable, they develop into foredunes with increasing vegetation cover.

Yellow dunes

Yellow dunes contain more vegetation and organic matter.

Roots help stabilise the dunes further.

Grey dunes

Grey dunes are older and more stable, with richer soils and greater biodiversity.

Dune slacks

Dune slacks are low-lying areas between dunes where the water table reaches the surface.

These areas often support distinctive wetland vegetation.

Factors influencing depositional landscapes

The development of depositional landforms depends on several interacting factors.

Sediment supply

Large sediment supplies encourage deposition and landform growth.

Sediment may originate from:

• rivers
• cliff erosion
• offshore deposits
• longshore drift

Wave energy

Low-energy environments favour deposition because waves lose the ability to transport sediment.

Constructive waves are particularly important.

Wind direction

Wind influences:

• wave approach
• longshore drift direction
• dune development

Tidal range

Tides affect:

• sediment movement
• exposure of beaches
• dune formation opportunities

Large tidal ranges may expose extensive areas of sediment for aeolian transport.

Depositional landscapes in the UK

Spurn Head, England

Spurn Head on the Holderness Coast is a recurved spit formed through longshore drift transporting sediment southwards along the coastline.

Changing wave directions have created recurved ends.

Chesil Beach, England

Chesil Beach in Dorset is a barrier beach connecting the Isle of Portland to the mainland.

Longshore drift and offshore sediment movement contributed to its formation.

Sediment size increases from west to east because of longshore sorting.

Depositional landscapes beyond the UK

Cape Cod, USA

Cape Cod in Massachusetts is a depositional landscape formed largely from glacial sediments reworked by waves and longshore drift.

It contains:

• spits
• dunes
• barrier beaches

Outer Banks, USA

The Outer Banks of North Carolina are a series of barrier islands shaped by:

• wave action
• storms
• longshore drift
• sea-level change

The islands migrate over time and are highly dynamic environments.

Ninety Mile Beach, New Zealand

This extensive sandy coastline contains large dune systems formed by:

• abundant sediment supply
• strong onshore winds
• active aeolian transport

Landscape evolution

Depositional landscapes evolve continuously because sediment movement and wave conditions constantly change.

For example:

• spits may extend further through longshore drift
• storms may breach barrier beaches
• dunes may migrate inland
• sea-level rise may alter depositional environments

This demonstrates that depositional coastlines are highly dynamic systems.