1.c - sources of sediment Flashcards
what are the 3 main sources of sediment?
TERRESTRIAL - including fluvial deposition, weathering and mass movement, marine erosion, aeolian deposition and longshore drift.
OFFSHORE - including marine deposition
HUMAN - including beach nourishment
examples of sources of sediment
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
how do waves input into a coastal system?
wave erosion is also the source of large amounts of sediment and makes a major contribution to coastal sediment budgets.
what could affect the relative significance of different sources of sediment?
GEOLOGY- more soft rocks = more input from cliffs = bigger beaches.
WEATHER - nature of waves = more sediment
GEOGRAPHICAL LOCATION - more exposed/destructive waves = removing sediment = decreased input
SIZE OF RIVER = big beaches come with rivers.
how do rivers input into a coastal system?
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
how do cliffs input into a coastal system?
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 compromise 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.
how do offshore sources input sediment?
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 energu than water, and so cannot transport very large particles.
what is equilibrium?
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.
how do humans input sediment?
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 ont he 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
what happens when equilibrium is disturbed?
the system undergoes self regulation to restore equilibrium
this is called dynamic equilibrium and is an example of negative feedback.
what is a positive feedback loop?
an automatice 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
what is a negative feedback loop?
an automatic response to change in a system that restores equilibrium (a good thing!)
e.g. storm removes sediment = not equilibrium,
constructive waves buld up beach, restores equilibrium.
