Sediment in the coastal system.

Sediment sources.

Sediment inputs within coastal system is from erosion processes.

These sediments are stored as depositional landforms or as nearshore features.

They are tend transported (throughputs) and become outputs from the coastal system as they moved into deep waters of the ocean and away from immediate coastal zone.

The sediment can be divided into two types:

(i)Clastic sediments – from rock weathering and erosion. (vary in sizes; clay, mud, sand and

larger cobbles and boulders)

(ii) Biogenic sediments – shells and skeletons of marine organisms.

Sediment transport and deposition.

1. Waves, currents, tides and wind provide energy inputs for erosion and transport of sediment from the source to create

coastal depositional landforms.

(Depositional landforms: beaches, dunes, salt marshes and mud flats) –sediment stores

2. These sediments are transported onshore, offshore and alongshore to create them.

3. Small particles such as silt and clay are carried in suspension, and settle as

depositional landforms in areas of reduced wave energy but high tidal energy to form salt marshes and mud flats.

Sediment cells:1. The movement of sand and shingle-sized sediment in the nearshore zone by littoral (longshore) drift found to occur in discrete, functionally separate sediment cells.

2. Around the coastline of England and Wales, 11 main sediment cells with smaller subcells have been identified.

3. These major cells are defined as a length of

coastline and its associated nearshore area within which the movement of coarse sediment (sand and shingle) is largely self-contained.4. Interruptions to the movement of sand and shingle within one cell should not affects beaches in an adjacent sediment cell.

5. These sediment cells, as functional systems, form

basis for coastal management schemes.

6. Sub cell boundaries identify smaller cells associated within the major cells.

7. There is some movement of sediment between the cells, i.e. they are open not closed systems.

8. There are two main types of boundary:(i) Littoral drift divides

These occur when the coastline abruptly changes direction such as at major headlands, e.g. Portland Bill in Dorset.

These also occur without dramatic change in the shape of the coastline but where wave conditions cause a change in drift direction e.g. near Sheringham in Norfolk.

Material is moved outwards from a drift divide, there is a

net output of sediment from the area.

This result in a dominance of erosional processes and landforms e.g. eroding beaches and cliffs.

(ii) Sediment sinksThese are where sediment transport paths meet so that sediment builds up in major depositional environments.

Sediment sinks occur in deeply indented bays and estuaries although spits and cuspate forelands may form subcell sinks.

Sediment cells are an important concept in coastal geomorphology, but there are problems with this approach.

The cells are based upon the movement of coarser particles, rather than the movement of suspended

fine material that can be moved long distances.

Cell boundaries in such a dynamic system cannot be fixed and static.