Sediment is a very important input in coastal systems. It can come from many different sources:
- Offshore sand banks
- Estuaries
- Streams/rivers flowing into the sea
- Material from a biological origin including cells, coral fragments and skeletons of marine organisms
- Cliff erosion
There are various types of sediment:
- Clastic sediment: comes from weathering of rock and varies from very small clay particles to sand/pebbles/boulders
- Biogenic sediment: skeletons and sediments of marine organisms
- Non-cohesive sediment: larger particles (like sand) moved grain by grain
- Cohesive sediment: very small clay and mud particles that bond together
Sediment cells
There is dispute about the topic, but it is generally believed that sediment around the UK coast is moved in distinct areas called sediment cells – within a cell inputs and outputs are balanced, but generally, they are considered closed systems with no inputs or outputs, but sediment can move past boundaries and therefore between cells.
Boundaries between cells are usually in the form of a headland or stretch of deep water. On the east coast of Britain, Flamborough Head is a headland boundary and The Wash is a deep-water boundary. St David’s Head in SW England is another headland boundary.
There is variation in size of sediment cells. Larger cells are often split into smaller sub-cells which allow closer management and study. A sub-cell is in operation between Flamborough Head and the Humber Estuary.
If a cell draws or loses material to offshore, it is referred to as a sediment sink.
Coastal sediment budget
Essentially, this is the balance between sediment being added to and removed from the coastal system (generally lying between individual sediment cells).
Sediment budget for deposition:
More material added to cell than is removed – net accretion of material
Positive budget or surplus of sediment
Shoreline builds towards to sea
Sediment budget for erosion:
More material is removed from the cell than is added
Negative budget or a deficit in sediment supply
Shoreline retreats landward
Problems with calculating sediment budget:
- Seasonal variation/changes in sediment patterns with the weather
- Very difficult to accurately the changes in sediment
- Identifying sediment sinks
- There are lots of sources of sediment