the sediment cell concept (2B.5C)

Question 1

what is the sediment cell concept?

Answer 1

a sediment cell (or littoral cell) is a linked system of sources, transfers and sinks of sediment along a section of coastline.

sediment moves along the coast in sediment cells. within each cell the sediment moves between the beach, cliffs and sea through processes of erosion, transportation and deposition.

the coastline of England and Wales is split up into 11 major sediment cells and operates as a closed system.

Question 2

the sediment cell as a system

Answer 2

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.

• within each cell, there is an amount of sediment available. 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.
  !!!dynamic equilibrium is itself dynamic because its constantly changing as energy and sediment inputs constantly alter: the amount of stuff moving through is changing!!!
• 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.

positive feedback: when the changed produces an effect that operates to increase the original change. EG when wind erosion of a dune section during high velocity storms may remove stabilizing vegetation

negative feedback: when the change produced creates effects that operate to reduce or work against the original change. EG when erosion leads to block fall mass movement. The collapsed debris acts as a barrier protecting the cliff base, slowing or preventing erosion for a period of time.

Question 3

inputs

Answer 3

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

Question 4

transfers

Answer 4

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)

Question 5

outputs

Answer 5

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 (sand dunes)
• foreshore depositional landforms (beaches)
• nearshore depositional landforms (bars)
• offshore depositional landforms (barrier islands)