1. Erosion of the cliffs can provide direct sediment input- However, there are lots of other sources of sediment such as currents bring in material from the sea bed
2. In all areas the sediment is either stored as a depositional landform or as a nearshore feature such a bank or offshore bar. Alternatively it is transported as a throughput and become outputs from the system being deposited either in deeper water or away from the coastal area in question.
3. Sediment can be divided into 2 types
a) Clastic Sediment and
b) Biogenic sediment
Clastic sediments are from rock weathering and erosion- these can vary in size from really small clay and mud particles to sand, pebbles and boulder size Biogenic sediments are the shells and skeletons of marine organisms.
Sediment Transport and Deposition
1. Waves, currents, tides and wind provide the energy inputs for the erosion and transport of sediment from the source areas to create coastal landforms which exist in a state of dynamic equilibrium with the local conditions
2. These depositional landforms such as beaches, sand dunes, salt marshes and mudflats, act as a dynamic sediment store and sediments are transported onshore, offshore, and alongshore to create them. These stores in turn provide sediment for stores further down the coast.
3. The
sand and larger particles are transported grain by grain as non-cohesive
4. You would expect smaller clays and muds to require very little energy but they are sticky or cohesive. These particles tend to cling together by electromagnetic bonding. These sediments require a much larger velocity to become entrained ( get moving) than their individual grain size would suggest. One they have been dislodged and set in motion they are moved very easily with little velocity needed
5. As particles are transported they become rounded by attrition.
6- Large sediments are deposited in high energy environments to form beaches. Smaller particles such as silt and clays are carried in suspension and settle in areas of reduced wave energy but high tidal energy to form salt-marshes, mudflats etc
7. The landforms which reflect these local conditions are in a state of dynamic equilibrium. In areas of mobile clastic sediment these adjustments take place continuously In response to short term wave and tidal conditions as well as eventually reaching a long term equilibrium. The erosion, transport and deposition processes sustain the equilibrium both offshore and onshore. At low tide and on sandy beaches, the wind can act as a transport agent by entraining and transporting small sand and silt particles. These particles are quickly deposited again and may form sand dunes.
8. What are Sediment cells? - the movement of sand and shingle in the nearshore zone by longshore drift (littoral drift) has been found to occur in discrete, functionally separate sediment cells. There are 11 such cells around England and Wales. Smaller sub cells have been identified within these. The main 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. Interruptions to movement of sand and shingle within one cell should not affect beaches in an adjacent sediment Cell
9. These sediment cells are now forming the basis for coastal management schemes,
10. Sub-cell boundaries identify smaller cells associated within the major cells. There Is some movement of sediment between cells. These cells are called open systems.
11. There are 2 sorts of sediment cell boundaries Littoral drift divides and Sediment sinks.
Littoral drift divides( longshore drift divides) =where the coastline abruptly changes direction such as at major headlands. They also occur where wave conditions cause a change in longshore drift direction as at Sheringham. Since material is moved outwards from a drift divide there is a net output of sediment from the area. This results in a dominance of erosional processes and landforms e.g. eroding beaches and cliffs.
We can explore the movement of sediment around the coast by analysing sediment budgets. These show the ins and cuts of sediment at various locations
Sediment sinks: These are where sediment transport paths meet so that sediment builds up in depositional environments. Sediment sinks occur in deeply indented bays and estuaries, although spits and cuspate forelands may form sub-cell sinks