1.C - Sources of Sediment

Question 1

what are the 3 main sources of sediment?

Answer 1

1. TERRESTRIAL - including fluvial deposition, weathering and mass movement, marine erosion, aeolian deposition and longshore drift.
2. OFFSHORE - including marine deposition
3. HUMAN - including beach nourishment

Question 2

examples of sources of sediment

Answer 2

• offshore bars/sand banks
• wave/ocean sediment (marine)
• nearshore bank
• longshore drift (beaches mostly drift aligned in UK)
• rivers carrying sediment (alluvium)
• cliffs eroding
• aeolian processes

Question 3

what could affect the relative significance of different sources of sediment?

Answer 3

1. GEOLOGY- more soft rocks = more input from cliffs = bigger beaches.
2. WEATHER - nature of waves = more sediment
3. GEOGRAPHICAL LOCATION - more exposed/destructive waves = removing sediment = decreased input
4. SIZE OF RIVER = big beaches come with rivers.

Question 4

how do rivers input into a coastal system?

Answer 4

• rivers are main sources of sediment input to the coastal sediment budget and this is particularly true of coasts with a steep gradient, where rivers directly deposit their sediments at the coast.
• sediment delivery to the shoreline can be intermittent, mostly occuring during floods. in some locations, as much as 80% of coastal sediment comes from rivers.
• the origin of the sediment is the erosion of inland areas by water, wind and ice as well as sub-aerial processes of weathering and mass movement

Question 5

how do waves input into a coastal system?

Answer 5

wave erosion is also the source of large amounts of sediment and makes a major contribution to coastal sediment budgets.

Question 6

how do cliffs input into a coastal system?

Answer 6

• cliff erosion can be increased by rising sea levels and is amplified by storm surge events.
• the erosion of weak cliffs in high energy wave environments contributes as much as 70% of the overall material supplied to beaches, although typically it contributes much smaller amounts.
• some of this sediment may comprise large rocks and boulders, esp. if derived directly from the collapse of undercut cliffs.
• longshore drift can also supply sediment from 1 coastal area by moving it along the coast to adjacent areas.

Question 7

how do offshore sources input sediment?

Answer 7

• constructive waves bring sediment to the shore from offshore locations and deposit it (marine deposition), adding to the sediment budget
• tides and currents do the same
• wind also blows sediment from other locations, including exposed sand bars, dunes and other beaches elsewhere along the coast
• this aeolian material is generally fine sand, as wind has less energy than water, and so cannot transport very large particles.

Question 8

how do humans input sediment?

Answer 8

• when a coastal sediment budget is in deficit, beach nourishment is one way in which a sediment equilibrium can be maintained.
• this type of management has been adopted all over the world to preserve and protect the coastal environment
• sediment can be brought in by lorry and dumped on the beach before being spread out by bulldozers.
• alternatively, sand and water can be pumped onshore by pipe line from offshore sources. low bunds hold the mixture in place while the water drains away and leaves the sediment behind.
• in addition, hard engineering coastal management systems can affect the coastal sediment budget e.g. input from longshore drift is affected by building groynes

Question 9

what is equilibrium?

Answer 9

• when a system’s inputs and outputs are equal
• e.g. rate of sediment added to beach = the rate its being removed, ∴ beach remains same size.

Question 10

what happens when equilibrium is disturbed?

Answer 10

• the system undergoes self regulation to restore equilibrium
• this is called dynamic equilibrium and is an example of negative feedback.

Question 11

what is a positive feedback loop?

Answer 11

• an automatic response to a change in a system which GENERATES FURTHER CHANGE (an issue)
• causes rate of change to increase
• often caused by humans
• one small change leads to more change away from equilibrium

Question 12

what is a negative feedback loop?

Answer 12

• an automatic response to change in a system that restores equilibrium (a good thing!)
• e.g. storm removes sediment = not equilibrium,
  constructive waves build up beach, restores equilibrium.