Sediment cells and Shoreline Management Plans

Overview

Sediment cell and shoreline Management Plan.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a sediment cell?

   Model answer

   A length of coastline, together with its nearshore zone, within which the movement of sand and shingle is largely self-contained, so that sediment moving within the cell stays in it and little is exchanged with neighbouring cells. Its boundaries are usually major physical barriers to longshore transport, such as large headlands, deep estuaries or changes in the direction of the coast. It is treated as a closed system for practical purposes, though in reality some material does move between cells and offshore, so the closure is an approximation rather than a fact.

2. 2

   Describe a sediment cell as a system, using the language of sources, transfers and sinks.

   Model answer

   Sources are the inputs where sediment enters the cell: eroding cliffs, rivers delivering material at their mouths, the offshore zone from which constructive waves bring material shorewards, and material moved in by the wind. Transfers are the processes moving sediment about within the cell: longshore drift along the shore, swash and backwash across the beach face, tidal and wave-driven currents in the nearshore zone, and wind moving sand from beach to dune. Sinks are the stores where sediment accumulates: beaches, spits, bars, dune systems, salt marsh and offshore deposits, some temporary and some long-lasting. If inputs and outputs balance the cell is in dynamic equilibrium; if a source is cut off or a sink is enlarged, the balance shifts and shorelines within the cell adjust.

3. 3

   Why does the sediment cell concept make coastal management decisions interdependent?

   Model answer

   Because within a cell the sediment is shared, so every stretch of coast is connected to the stretches around it by the material moving between them. The eroding cliff at one place is the source of the beach a few kilometres along, and that beach is what protects the town behind it. If a sea wall is built to stop the cliff eroding, the supply of sediment into the cell is cut, and the beach downdrift starves. If a groyne field is built to widen one town's beach, it does so by intercepting material that was on its way somewhere else. In each case a defence that succeeds locally exports the problem along the coast, so a decision taken by one authority on one frontage changes the erosion risk for its neighbours. Management therefore has to be planned for the whole cell rather than frontage by frontage, which is precisely why sediment cells are used as the units for planning.

4. 4

   Explain what happens downdrift when a stretch of eroding cliff is protected by a sea wall.

   Model answer

   The wall stops the sea reaching the cliff, so the cliff no longer retreats and no longer supplies rock debris to the coastal system. That debris was, on many coasts, a major input into the sediment cell, feeding beaches further along by longshore drift. Once it is cut off, the outputs from those downdrift beaches continue unchanged while the input has stopped, so the beaches narrow and lower. A narrower beach absorbs less wave energy, so more energy reaches the cliffs or defences behind them, and erosion there accelerates. The neighbouring authority then faces a worse problem than before and is under pressure to build its own defences, and the effect is passed on again. This is why the sediment supplied by an eroding cliff is now treated as an asset rather than simply as a loss.

5. 5

   What is a Shoreline Management Plan?

   Model answer

   A large-scale, long-term plan that sets out the policy for managing the flood and erosion risk along a whole stretch of coast, drawn up for a sediment cell or a sub-cell so that the physical connections between adjacent frontages are taken into account. It divides the coast into management units, assesses the risks, the assets at stake and the natural processes operating, and assigns each unit one of a small number of policy options, looking ahead over the short, medium and long term, conventionally the next twenty years, the following thirty and the fifty years after that. It is produced in consultation with local authorities, environmental bodies, landowners and residents.

6. 6

   What are the four policy options available in a Shoreline Management Plan?

   Model answer

   Hold the existing defence line, meaning maintain, repair and if necessary replace the defences so that the shoreline stays where it is; this is chosen where the assets behind justify the cost. Advance the existing defence line, meaning build new defences seaward of the present line to reclaim land; it is rare and is used only where there is a very strong economic case. Managed realignment, meaning allowing the shoreline to move landwards in a controlled way, often by deliberately breaching an old defence and setting a new one back, and letting intertidal habitat develop in front of it. No active intervention, meaning building and maintaining nothing and allowing natural processes to operate, which is chosen where the land at risk does not justify the expense or where erosion is supplying sediment the coast needs.

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