Sediment Cells and Coastal Systems
Understanding coastal geomorphology at A-Level requires a thorough grasp of sediment cells — the fundamental organising units of the coastline. AQA specification section 3.1.3 frames the coast as a system, and sediment cells provide the spatial framework within which that system operates.
What Is a Sediment Cell?
A sediment cell (also called a littoral cell) is a stretch of coastline that forms a largely self-contained system for the movement of sediment. Within each cell, sediment is sourced, transported, deposited and lost with minimal transfer across the cell boundaries.
Key Definition: A sediment cell is a section of coast within which the movement of sediment is largely self-contained, bounded by features that prevent or minimise the transfer of material to adjacent cells.
England and Wales are divided into 11 major sediment cells, as identified by the Department for Environment, Food and Rural Affairs (DEFRA). These range from St Abb's Head to the Solway Firth and encompass the entire coastline.
The 11 Major Sediment Cells of England and Wales
| Cell Number | Boundaries | Key Features |
|---|
| 1 | St Abb's Head to Flamborough Head | Holderness coast, Humber Estuary |
| 2 | Flamborough Head to The Wash | Lincolnshire coast, Spurn Head spit |
| 3 | The Wash to Thames Estuary | East Anglian coast, Dunwich erosion |
| 4 | Thames Estuary to Selsey Bill | North Kent, Sussex coast |
| 5 | Selsey Bill to Portland Bill | Solent, Isle of Wight |
| 6 | Portland Bill to Land's End | Dorset, Devon, Cornwall |
| 7 | Land's End to Hartland Point | North Cornwall |
| 8 | Hartland Point to St Abb's Head | Severn Estuary, Welsh coast |
| 9 | Great Orme to Solway Firth | North Wales, Cumbrian coast |
| 10 | Solway Firth to Mull of Galloway | Scottish borders |
| 11 | Mull of Galloway to St Abb's Head | Southwest Scotland |
Each major cell is subdivided into sub-cells, which allow more detailed analysis of local sediment movement patterns.
Sediment Sources, Stores and Sinks
Within each sediment cell, material follows a pathway from source to sink, with temporary residence in stores.
Sources of Sediment
Sediment enters the coastal system from several origins:
- Cliff erosion — the most significant source on many UK coastlines. At Holderness, cliff retreat averages 1.8 m per year, contributing approximately 3.4 million m³ of material annually
- River input — fluvial sediment delivered to estuaries and the coast. The River Humber delivers around 1.26 million tonnes per year
- Offshore sources — material moved onshore from the continental shelf, much of it deposited during the last glaciation (Devensian, ending c. 11,700 years ago)
- Biological material — shell fragments, coral debris, and organic matter
- Wind-blown material — aeolian transport from inland or alongshore sources
- Longshore input — sediment arriving from an adjacent cell or sub-cell
Stores of Sediment
Stores are locations where sediment resides temporarily or semi-permanently:
- Beaches — the primary store, holding sand, shingle and pebbles between low and high tide marks
- Sand dunes — aeolian accumulations stabilised by marram grass and other vegetation
- Salt marshes and mudflats — fine sediment deposited in sheltered, low-energy intertidal environments
- Offshore bars — submerged ridges of sediment running parallel to the coast
- Spits, bars and tombolos — depositional landforms that act as semi-permanent stores
Sinks
Sinks are locations where sediment is effectively removed from the active system:
- Deep water — material transported beyond the wave base by rip currents or storm activity
- Estuarine infilling — fine sediment trapped within estuaries
- Human extraction — dredging and aggregate removal (the UK extracts approximately 20 million tonnes of marine aggregate annually)
- Permanent deposition — buried sediment that becomes lithified over geological time
The Sediment Budget
The sediment budget is the balance between inputs and outputs within a sediment cell. It is expressed as:
Sediment Budget = Inputs − Outputs
- If inputs > outputs → positive budget → net deposition → coastline advances seaward (progradation)
- If inputs < outputs → negative budget → net erosion → coastline retreats landward (retrogradation)
- If inputs = outputs → balanced budget → dynamic equilibrium
Exam Tip: AQA frequently asks students to evaluate how human intervention disrupts the sediment budget. Groynes, for example, trap sediment updrift but starve beaches downdrift, creating a negative budget in adjacent sub-cells.
Calculating Sediment Budgets
A simplified sediment budget equation for a coastal sub-cell:
ΔV = Qsource + Qriver + Qonshore + Qlongshore(in) − Qlongshore(out) − Qoffshore − Qextraction
Where:
- ΔV = change in sediment volume
- Q = sediment flux (volume per unit time, typically m³/year)
Dynamic Equilibrium
The coast tends towards a state of dynamic equilibrium — a condition in which the system adjusts to maintain a balance despite continuous change. This is not a static state but rather a fluctuating balance.
Key characteristics of dynamic equilibrium on the coast:
- Short-term fluctuations — storm events may temporarily erode a beach, which is then rebuilt by constructive waves during calmer conditions
- Seasonal variation — winter storm beaches are steep and depleted; summer beaches are wider with a gentle gradient
- Negative feedback — self-regulating mechanisms that restore equilibrium. For example, if a cliff retreats, the additional sediment widens the beach, which absorbs more wave energy, reducing further erosion
- Positive feedback — self-reinforcing mechanisms that drive the system further from equilibrium. For example, removal of beach material by dredging exposes the cliff to greater wave energy, accelerating erosion, which destabilises the cliff further
Concordant and Discordant Coastlines
The geological structure of the coast fundamentally influences its morphology and the pattern of sediment cells.
Concordant (Pacific-type) Coastlines
Rock strata run parallel to the coast. This produces a relatively uniform, straight coastline because the same rock type faces the sea along its length.
Example: The coast of Dalmatia (Croatia) and parts of the Dorset coast near Lulworth.
- The resistant rock facing the sea acts as a barrier, protecting softer rocks behind it
- Where the resistant rock is breached (e.g., at Lulworth Cove), the sea erodes the softer rock behind rapidly, creating a circular cove
- Concordant coastlines tend to have fewer headlands and bays
Discordant (Atlantic-type) Coastlines
Rock strata run perpendicular to the coast. Different rock types are exposed alternately, leading to differential erosion.
Example: The Dorset coast between Swanage and Studland.
- Resistant rocks (e.g., Portland limestone, Purbeck limestone) form headlands
- Weaker rocks (e.g., Wealden clays, London Clay) are eroded to form bays
- This pattern creates the classic headland-and-bay coastline
Geological Structure and Dip
The dip of rock strata also influences cliff morphology:
| Dip Direction | Cliff Profile | Stability |
|---|
| Horizontal | Vertical or steep cliffs | Moderately stable |
| Seaward dip | Gentle, sloping cliffs | Prone to landslides |
| Landward dip | Very steep or overhanging cliffs | Prone to rockfall |
| Steeply inclined | Variable | Depends on jointing and bedding |
Joints, faults and bedding planes create lines of weakness that waves exploit through hydraulic action and weathering, accelerating erosion along these planes.
Case Study: Sediment Cell 1 — St Abb's Head to Flamborough Head
This cell includes the Holderness coast, one of the most rapidly eroding coastlines in Europe.
Key Features
- Source: Glacial till cliffs (boulder clay deposited during the Devensian glaciation) provide the primary sediment source
- Transport: Longshore drift moves material southward at an estimated rate of 500,000 m³ per year
- Store: Spurn Head spit at the mouth of the Humber acts as a major sediment store
- Sink: Material is lost to the Humber Estuary and to offshore areas
Disruption to the Sediment Budget
- Hard engineering at settlements such as Hornsea, Withernsea and Mappleton (rock armour, groynes) traps sediment locally but starves downdrift areas
- The construction of groynes at Mappleton in 1991 (costing £2 million) protected the village but accelerated erosion to the south at Great Cowden, where farms have been lost to the sea
- This demonstrates the interconnected nature of sediment cells and the consequences of disrupting the sediment budget
Summary
- Sediment cells are self-contained systems that provide the framework for understanding coastal processes
- The sediment budget determines whether a coastline is advancing, retreating, or in equilibrium
- Dynamic equilibrium is maintained through feedback mechanisms but can be disrupted by human intervention
- Geological structure (concordant vs discordant, dip, jointing) fundamentally shapes the coastline
- Understanding sediment cells is essential for effective Shoreline Management Planning
Revision Checklist:
- Can you name and locate the 11 major sediment cells of England and Wales?
- Can you explain the difference between sources, stores and sinks?
- Can you calculate a simple sediment budget?
- Can you explain how concordant and discordant coastlines form?
- Can you evaluate how human intervention disrupts sediment budgets using a case study?