sediment transport & deposition

• What is a sediment cell?

  a linked system of sources, transfers and sinks of sediment along a section of coastline
• What are some examples of sediment inputs?

  cliff erosion

  onshore currents

  river transport

  wind blown (aeolian) sediment from land

  subaerial processes

  marine ecosystems
• What are the inputs in a sediment cell?

  sources are places where sediment is generated e.g cliffs and eroding sand dunes

  some sources are offshore bars and river systems, these are an important source of sediment for the coast
• What are transfers in a sediment cell?

  Places where sediment is moving alongshore through longshore drift and offshore currents.

  Examples:

  • longshore drift
  • swash
  • backswash
  • tidal currents
  • sea/ocean currents
  • wind
• What are outputs in a sediment cell?

  sinks are locations where the dominant process is deposition and depositional landforms are created, including spits and offshore bars.
• Outputs in a sediment cell?

  Some examples of sinks are:

  • Backshore depositional landforms
  • E.g. sand dunes
  • Foreshore depositional landforms
  • E.g. beaches
  • Nearshore depositional landforms
  • E.g. bars
  • Offshore depositional landforms
  • E.g. barrier islands 
• Why are sediment cells dynamic?

  Sediment cells are dynamic because the sediment is constantly generated in the source region, transported through the transfer region and deposited in the sink region. 
• How is dynamic equilibrium reached in a sediment cell?

  Dynamic equilibrium (in this instance) is reached when inputs of sediment from the source region are balanced by the amount being deposited in sinks.

  It's dynamic because although it's in balance, there's a constant movement of sediment through the system. 
• What is negative feedback?

  when the change produced creates effects that operate to reduce or work against the original change.  
• What are 2 examples of negative feedback?

  • E.g. when erosion leads to blockfall mass movement. The collapsed debris acts as a barrier protecting the cliff base, slowing or preventing erosion for a period of time.
  • E.g. major erosion of sand dunes could lead to excessive deposition offshore, creating an offshore bar that reduces energy, allowing the dunes time to recover.  
• What is positive feedback?

  when the changed produces an effect that operates to increase the original change.
• What is an examples of positive feedback?

  E.g. When wind erosion of a dune section during high velocity storms may removing stabilising vegetation. 

  • Further wind erosion now occurs in later low velocity wind conditions, increasing the depletion of dune sand. 
• What is solution?

  substances that can dissolve are carried along in the water

  for example, limestone is dissolved into water that's slightly acidic
• What is suspension?

  very fine material, such as silt and clay particles are whipped up by turbulence (erratic swirling of water) and carried along in the water column.

  most eroded material is transported this way
• What is saltation?

  larger particles, such as pebbles or gravel, are too heavy to be carried in suspension

  instead, the force of the water causes them to bounce alobng the seabed
• What is traction?

  very large particles such as boulders are pushed and rolled along the sea bed by the force of the water
• How does longshore drift transport sediment along the shore?

  swash carries sediment such as shingle up the beach, parallel to the prevailing wind

  backwash carries sediment back down the beach, at right angles to the shoreline

  when there's an angle between the prevailing wind and the shoreline, a few rounds of swash and backwash move the sediment along the shoreline in a zig-zag pattern
• How do beaches form?


  when constructive waves deposit sediment on the shore

  from the top of the beach to the shoreline, there is 'natural sorting' of beach sediment: larger and more angular sediment is at the top of the beach and smaller, more rounded sediment is closer to the shore

  this is because, while the swash deposits material of all sizes, the backwash only has enough energy to move the smallest material, leaving the larger material at the top.
• What are berms?

  ridges of sand and pebbles standing 1-2m high

  they occur at the high tide mark
• What run below berms?

  runnels (grooves) which run in the sand parallel to the shore

  they form when backwash drains back to the sea
• Define current?

  • Current is the flow of water in a specific direction which transports sediment
  • Surface currents are formed mainly by wind
  • Deepwater currents are also called thermohaline circulation and are caused by density differences 
• Define tide?

  The gravitational pull of the sun or moon causes tides
• What is tidal range?

  • The difference between high and low tide
  • Larger tidal ranges produce stronger tidal currents and this can transport more sediment, particularly in the areas around estuaries
• Difference between high energy and low energy coastlines?

  • High energy coastlines deposit large rocks and shingle but generally maintain enough energy to carry smaller sediment particles 
  • This is why these types of coastlines tend to have rocky beaches, such as Brighton on the south coast of England
  • Low energy coastlines deposit smaller sediment due to low wave velocity, creating sandy beaches
• What is a spit?

  an extended stretch of sand or shingle that extends out to sea from the shore 
• How are spits formed?

  spits tend to form where the coast suddenly changes direction, e.g across river mouths

  longshore drift continues to deposit materials across the river mouth, leaving a bank of sand and shingle sticking out into the sea

  the area behind the spit is sheltered from is sheltered from the wave and often develops into mudflats and saltmarshes
• What is a simple spit?

  a straight spit that grows out roughly parallel to the coast
• how are recurved spits (hooks) formed?

  occasional changes to the dominant wind and wave direction (refraction) may lead to the spit having a curved end
• What is a double spit?

  these occur on opposite sides of a bay, both extending towards its middle

  the outflow of river discharge stops the two spits from joining
• What are cuspate forelands?

  these occur when 2 spits form in opposite directions and merge into a low-lying triangular shape

  in some cases they can stretch several kilometres out to sea
• How are bars formed?

  when a spit joins two headlands together

  this can occur across a bay or a river mouth
• what forms behind a bar?

  a lagoon as seawater is trapped

  over time, if there is a stream flowing into this water the water can become less saline
• What is a tombolo?

  a bar that connects the shore to an island

  a tombolo can also be formed by wave refraction occurring around an island
• What are barrier beaches?

  long, narrow islands of sand or gravel that run parallel to the shore and are detached from it

  they tend to form in areas with a good supply of sediment, a gentle slope offshore fairly powerful waves and a small tidal range