Sediment sources, cells and budgets

In this section, you will learn about:

• the main sources of sediment in coastal systems
• how sediment cells operate
• how sediment budgets influence coastal change
• the relationship between inputs, transfers, stores and outputs within coastal systems

Sediment is one of the most important components of coastal systems. Without a supply of sediment, many coastal landforms — including beaches, spits, dunes and mudflats — could not exist. The movement and storage of sediment help shape coastlines and influence whether a coastline experiences erosion, deposition or relative stability.

Coastal systems are therefore closely linked to the availability and movement of sediment through time.

Sources of sediment

Sediment enters coastal systems from a variety of sources. These sources vary depending on geology, climate, wave energy and human activity.

Rivers

Rivers are one of the largest sources of coastal sediment globally. Weathered material is transported downstream and deposited at river mouths and estuaries, where it may then be redistributed by waves, tides and currents.

River sediment supply is often greatest:

• in regions with high rainfall
• where rates of weathering and erosion are high
• during floods and storm events

Large river systems can therefore play a major role in maintaining beaches and coastal depositional landforms.

Cliff erosion

Eroding cliffs provide large quantities of sediment directly to the coastal system. Soft, unconsolidated coastlines can supply especially high volumes of material.

Along rapidly eroding coastlines, cliff retreat becomes a major sediment input into nearby beaches and offshore bars.

However, where cliffs are protected by sea walls or revetments, sediment supply may decline significantly, creating sediment shortages further along the coast.

Offshore sediment

Sediment can also be transported onshore from offshore deposits by:

• waves
• tidal currents
• storm activity

Some offshore sediment originates from material deposited during previous geological periods, including glacial deposits left after the last Ice Age.

Storm waves and surges may temporarily move large amounts of sediment into coastal systems.

Longshore drift

Longshore drift transfers sediment between different parts of the coastline. Sediment leaving one stretch of coast may become an input elsewhere within the same sediment system.

This demonstrates how coastlines are interconnected rather than isolated environments.

Wind

Wind can transport fine sediment inland from beaches, particularly sand-sized particles. This is important in the development of:

• sand dunes
• dune slacks
• coastal dune systems

Aeolian transport is most effective where beaches are dry, wide and exposed.

Glacial and biological sources

In high-latitude environments, glaciers and ice sheets may deliver sediment directly to the coast through melting and calving.

Biological material can also contribute sediment, including:

• shell fragments
• coral material
• skeletal remains of marine organisms

These inputs are especially important in tropical coastal environments.

Sediment cells

Coastlines can be divided into distinct areas known as sediment cells.

A sediment cell is a section of coastline within which sediment movement is largely self-contained. Sediment is transferred between different parts of the cell, with relatively limited movement beyond its boundaries.

Boundaries of sediment cells

Sediment cells are usually separated by major physical features such as:

• headlands
• deep water zones
• changes in coastline orientation

These boundaries reduce the movement of sediment between neighbouring cells.

Although sediment cells are often described as closed systems, in reality, small amounts of sediment may still move between them, particularly fine material transported offshore.

The UK sediment cell system

The coastline of England and Wales is divided into 11 major sediment cells, many of which are further subdivided into smaller sub-cells for management purposes.

Studying sediment cells helps geographers understand:

• patterns of erosion and deposition
• sediment transfers
• the impacts of coastal management

This systems approach is particularly important for shoreline management planning.

Inputs, transfers, stores and outputs

Sediment cells can be understood using systems terminology.

Inputs
These are sources adding sediment into the system, such as:

• rivers
• cliff erosion
• offshore deposits

Transfers (flows)
These are processes moving sediment through the system:

• longshore drift
• tidal currents
• offshore-onshore movement
• wind transport

Transfers connect different parts of the coastal system.

Stores (sinks)
Stores are locations where sediment accumulates temporarily or over long periods.

Examples include:

• beaches
• dunes
• spits
• bars
• offshore banks
• estuarine mudflats

Some stores are relatively stable, while others may change rapidly during storms.

Outputs
Sediment may leave the system through:

• offshore transport
• submarine canyons
• movement into neighbouring sediment cells

Extreme storm events can remove very large quantities of sediment from coastal systems in short periods of time.

Sediment budgets

A sediment budget measures the balance between sediment entering and leaving a coastal system.

Sediment budgets help explain whether coastlines are:

• building outward
• remaining stable
• retreating inland

Positive sediment budget

A positive sediment budget occurs when:

sediment inputs exceed outputs
This creates a surplus of sediment, encouraging deposition and coastal accretion.

Associated features may include:

• beach growth
• spit extension
• dune development

Negative sediment budget

A negative sediment budget occurs when:

• sediment outputs exceed inputs

This creates a sediment deficit, increasing the likelihood of erosion and coastal retreat.

Negative budgets may develop where:

• sediment supply is reduced
• storm erosion increases
• coastal management interrupts sediment transfer

Dynamic equilibrium

Many coastal systems operate in a state of dynamic equilibrium, where inputs and outputs fluctuate but remain broadly balanced over time.

However, this balance can be disrupted by:

• storms
• sea-level rise
• dredging
• coastal engineering
• changes in river sediment supply

Human activity can therefore significantly alter sediment budgets and coastal behaviour.

The impact of coastal management

Coastal protection schemes often affect sediment movement within a sediment cell.

For example:

• groynes trap sediment locally
• sea walls reduce cliff erosion inputs
• dredging removes offshore sediment stores

While management may protect one location, it can unintentionally increase erosion elsewhere by starving down-drift coastlines of sediment.

This highlights the interconnected nature of sediment systems.

Why sediment cells matter

Sediment cells are important because they provide a framework for understanding:

• coastal change
• shoreline management
• erosion risk
• depositional processes

Modern coastal management increasingly adopts a sediment-cell approach because actions in one location often have consequences elsewhere within the same system.