Coastal Systems: How a Sediment Cell and Sediment Budget Work

This study note explains sediment cells and sediment budgets.

Sediment cells

Sediment cells are areas along the coastline and in the nearshore area where the movement of material is largely self-contained. They can be considered as a closed coastal sub-system as far as sediment is concerned. They are often determined by the topography and shape of the coastline which directs the movement of the sediment within the cell. It is for this reason that they are therefore thought to be closed systems: sediment is largely recycled within them rather than having significant new inputs or outputs. The boundaries of the sediment cells tend to be headlands and peninsulas which act as natural barriers to stop the further movement of the sediment.

Despite the fact that the majority of sediment stays in the cell, changes in wind direction and movements of ocean currents can affect some of the sediment under high-energy conditions and cause some sediment to move offshore into long-term ocean floor stores of sediment. Within each sediment cell, there can be smaller sub cells.

For example, there are 11 large sediment cells in England and Wales but these can then be further subdivided into smaller cells.

The defining of sediment cells helps consideration of coastal process as a system and assists coastal management plans by illustrating the links between inputs, components, stores, transfers and outputs. Human intervention in a coastal system, in the form of coastal defences for example, is likely to have repercussions elsewhere in the system

Sediment Budgets

A sediment budget is the balance between changes in the volume of sediment held within the system and the volume of sediment entering or leaving the system.

A positive budget is when there are more inputs than outputs to the system and a negative budget is when outputs are higher than inputs. The budget can alter according to the following factors:

Input changes: volume of fluvial material being deposited into the coastal system and the impact that human intervention can have on that, e.g. damming a river. Coastal defences can impact upon the inputs too with reduced cliff face erosion taking place. Sea level rise may add more sediment with increased coastal erosion.

Output changes: human intervention, such as removing large amounts of sand from an area for industrial or coastal protection use. Also, sea level rise can increase the likelihood of changing ocean currents and material being removed from sediment cells.