2B.5C: The Sediment Cell Model

• Inputs

  Sources are places where sediment is generated, such as cliffs or eroding sand dunes. Some sources are offshore bars and river systems and these are an important source of sediment for the coast.  Some examples of sediment inputs are:

  • Cliff erosion,
  • Onshore currents
  • River transport
  • Wind blown (aeolian) sediment from land
  • Subaerial processes
  • Marine organisms
• Transfers

  Places where sediment is moving alongshore through longshore drift and offshore currents. (Drift-aligned) beaches and parts of dunes and salt marshes perform this function. Some examples of sediment transfers are:

  • Longshore drift
  • Swash
  • Backwash
  • Tidal currents
  • Sea/ocean currents
  • Wind (onshore, offshore or along shore)
• Outputs

  Sinks are locations where the dominant process is deposition and depositional landforms are created, including spits and offshore bars. 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 
• Dynamic system

  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. 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. With a dynamic equilibrium, the size of the landforms in the transfer zone will remain the same. (But not the ones in the source and sink regions)
• Dynamic system: Dynamic Equilibrium

  • E.g. climate change creating more frequent storms or erosion of the cliff line to a more resistant rock type. 
  • The system's equilibrium may be interrupted (e.g. during a storm event) but they tend to return to balance on average over time due to negative feedback. 
  • Seasonal change (e.g. storms and strong winds during winter) will change the dynamic equilibrium. 
• Negative feedback

  • When the change produced creates effects that operate to reduce or work against the original change. 
  • 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.  
• Positive feedback

  • When the changed produces an effect that operates to increase the original change.
  • 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.