EQ2 2B.5 Flashcards
(24 cards)
How is material transported ?
Traction – large heavy sediment rolled along sea bed
Saltation – smaller sediment bounces along the sea bed
Suspension – small sediment is carried within the water column
Solution – dissolved material is carried within the water
What is the angle of wave attack
Main determinant of the direction of sediment transport
how does LSD work
1) Swash carries sediment onto beach at an angle determined by the direction of the prevailing wind
2) Waves push sediment in this direction
and up the beach in the swash
3) Backwash carries sediment backward 90’ from coast
4) This moves sediment along the beach over
time
What is a current and how does it transport sediment
Flow of water in a particular direction which can be driven by winds or initiated by differences in water density, temperature, salinity
-Global thermohaline circulation connects four oceans (500 years for one complete circuit)
-Rip currents on the beach transport sediment a few metres out to sea for a few hours when the wind is blowing directly onshore with the right strength
What is a tide and how does it transport sediment
Changes in sea level produced by the gravitational pull of the moon and the Sun
-Incoming and ebbing tide can create tidal currents in nearshore and offshore zones
What is deposition
Occurs when waves no longer have sufficient energy to continue transporting material
Caused by:
-wind dropping
-resistance by obstruction
-dissipation of energy through refraction
-friction from extended transport across shallow angled nearshore and foreshore zone
Two ways in which sediment is deposited
Gravity settling - water become too low to move sediment so deposits large sediment first
Flocculation - small particles suspended in water, clump together through electrical or chemical attraction, become large enough to sink
Formation of beaches
Accumulation of sand/shingle
Formed by stronger swash, weaker backwash, deposits material on beach
Formation of bayhead beach
Curved beaches found at the back of a bay
Common on swash-aligned coastlines where wave refraction disperses wave energy around the bay perimeter
Formation of spits
Linear ridges of sand or shingle beach stretching into the sea beyond a turn in the coastline
Form on drift-aligned coastlines, LSD continues past turn, energy dispersed as wave refracts, leading to deposition
Formation of recurved spits
Spit whose end is curved landwards, into a bay
Hook or a recurve may form at the end of the spit
-wave refraction round the distal end transports and deposits sediment for a short distance in the landward direction
-wind and wave front are frequently at an opposing angle to the prevailing wind, generating short periods of longshore drift in the landward direction
-strong incoming tidal current can also create a recurved spit.
Formation of double spits
Two spits extend out in opposite directions from both sides of the bay, towards the middle
Form where longshore drift is operating in different directions on opposite sides of the bay
Formation of offshore/breakpoint bars
Ridges of sand or shingle running parallel to the coast in an offshore zone
Form from sediment eroded by destructive waves and carried out by backwash
Sediment deposited at boundary of nearshore and offshore
Formation of bars/barrier beaches
Linear ridges of sand/shingle extending across a bay and are connected to land on both sides, traps body of water behind (lagoon)
Drift-aligned coasts, LSD extends a spit across the entire width of the bay
Rising sea levels cause constructive waves to drive a ridge of sediment onshore to coastlines with a gently sloping shallow sea bed (barrier beach)
Formation of tombolos
Linear ridges of sand and shingle connecting an offshore island to the coastline of the mainland
Drift aligned coasts- LSD builds a spit out from land until it contacts with an offshore island
Swash aligned coasts- Wave refraction around both sides of the island, causes collision of wave fronts on the landward side, cancelling each other out and producing a zone of still, calm water where deposition occurs between the island and the coast
Formation of cuspate forelands
Low lying triangular shaped headlands, extending out from a shoreline, formed from deposited sediment
1) LSD from opposing directions converge at boundary of sediment cell
2) Sediment deposited out into the sea by both currents creating a triangular shaped area of deposited material
What is a swash and drift aligned coastline
Swash-aligned
-wave crests approach parallel to coast so there is limited longshore drift so sediment doesn’t travel up the beach far (berms)
Drift-aligned
-waves approach at a significant angle, so longshore drift causes the sediment to
travel far up the beach
What is a sediment cell
Linked system of sources, transfers and sinks of sediment along a section of coastline
What are the sources of a sediment cell
Places where sediment is generated
Cliff erosion,
Onshore currents
River transport
Wind blown (aeolian) sediment from land
Subaerial processes
Marine organisms
What are transfers of a sediment cell
Places where sediment is moving alongshore through longshore drift and offshore currents
Longshore drift
Swash
Backwash
Tidal currents
Sea/ocean currents
Wind
What are sinks of a sediment cell
Locations where the dominant process is deposition and depositional landforms are created
Backshore depositional landforms - sand dunes
Foreshore depositional landforms - beaches
Nearshore depositional landforms - bars
Offshore depositional landforms - barrier islands
Why are sediment cells considered a dynamic system
Sediment is constantly generated in the source region, transported through the transfer region and deposited in the sink region
Dynamic equilibrium - inputs of sediment from the source region = amount being deposited in sinks
-In balance, but constant movement of sediment through the system
Negative feedback cycle
Change creates effects that reduce/work against the original change
-erosion leads to block fall mass movement, debris acts as a barrier protecting the cliff base, slowing erosion for a period of time
-major erosion of sand dunes leads to excessive deposition offshore, creating an offshore bar that reduces energy, allowing the dunes time to recover.
Positive feedback cycle
When change operates to increase the original change
-wind erosion of a dune section during high velocity storms removes stabilising vegetation allowing further wind erosion at low velocity
