Problem setting
Pevensey Bay sea defences consist of a 9 km long shingle (gravel) barrier beach between Eastbourne and Bexhill in East Sussex, on the English Channel coast of southern England (See map). Many properties have been constructed on the crest of the shingle bank. Immediately inland there is the Pevensey Levels, an area of about 50 km2, which would be flooded at high spring tide if the sea defence becomes breached. The Pevensey Levels themselves are an area of low-lying marshland prized for its high conservation and agricultural value. This area also contains over 10,000 properties, plus caravan parks, roads and a rail link.

Barrier beaches can be overtopped by large waves, may leak or roll-back landward, and ultimately may breach (Technical Report Understanding barrier beaches, Defra). All of these events give rise to a risk of flooding, and temporary flooding events did occur at Pevensey in 1926, 1935, 1965 and 1999. The volume of mobile beach material leaving the barrier beach at its eastern end is greater than volume of mobile beach material naturally reaching the frontage at the south-west. The natural response of the barrier beach would be roll-back of shingle ridge and re-alignment to form a swash-aligned bay beach. This natural response is prevented by:

	Blockage of the coarse sediment supply by the construction of groynes and breakwaters, including the construction of Sovereign Harbour marina in 1992;
	Construction of approximately 150 groynes between Eastbourne and Bexhill in three main phases from 1870;
	Nourishment of the barrier beach by adding fresh gravel from offshore;
	Recycling gravel from parts of the beach with more than enough gravel to areas that are lacking; and
	Reprofiling the barrier beach using diggers to push shingle back up to the barrier’s crest;

The problem at Pevensey is how to manage the barrier beach within the context of the UK developing strategy of flood and coastal erosion risk management (Defra, Making Space for Water Homepage)

Description of the coastline
The western limit of the shoreline considered here is Beachy Head, the moderately resistant chalk cliff that forms the western end of the sediment sub-cell 4c for beach sand and the western limit of the local South Foreland to Beachy Head Shoreline Management Plan (SMP). This is not a closed boundary for sediment and the beaches of Eastbourne have traditionally relied on a supply of gravel from the west, passing round Beachy Head. However, the construction of man-made structures has reduced the supply of beach-grade sediment from the west. Sediment is also provided by the erosion of the shore platform and cliffs.

In 1992 Sovereign Harbour marina, was dredged out of the accumulation of shingle that formed a cupate foreland known as the Crumbles and as Langley Point on the eastern side of Eastbourne. The Crumbles is believed to have developed through the onshore migration of a gravel bar between 1100 AD and 1600 AD. Longshore drift then moved gravel north-eastwards along the coastline. This loss of sediment caused the shoreline to retreat by around 1m/year, although erosion has been largely halted by the building of groynes from 1884. Today, gravel accumulates on the western side of the harbour and erosion occurs on the eastern side, caused by the harbour. This area is believed not to receive a supply of gravel from offshore sources any more.

Pevensey and Norman’s Bay are areas of low-lying land (Hooe Level and Pevensey Level) fronted by a continuous gravel barrier beach. Pevensey Levels used to be a tidal inlet, where in 1066 William the [Norman] Conqueror landed at a site now over 1km inland (marked Castle at Pevensey on map). Wave-driven nett sediment transport on the beach is from west to east and this eventually closed the inlet. This shoreline becomes increasingly exposed to wave attack towards the east as the shelter from Beachy Head and Sovereign Harbour diminishes. Groynes were first constructed at the western end of this frontage in1907, with groynes added at Pevensey Bay between 1952 and 1962 and at Norman’s Bay between 1962 and 1967. Many of these groynes are in a poor state of repair.

The eastern limit of the shoreline considered here is Cooden Cliffs at Bexhill, which lie to the East of Hooe levels and consist of cretaceous thin shale, clay and sandstone. The foreshore is a mix of shingle and sand, which overlies an occasionally exposed shore platform of Tunbridge Wells silts and sandstones. Analysis of historic Ordnance Survey maps shows the cliffs experienced erosion between 1982 and 1950. The cliffs are now largely defended, but the foreshore is undergoing erosion, which will, in time, undermine the cliff defences.

Present day management approach
The shoreline management policies considered within the SMP (www.se-coastalgroup.org.uk) were those defined by Defra:

• Hold the line: maintain or upgrade the level of protection provided by defences.
• Advance the line: build new defences seaward of the existing defence line.
• Managed realignment: allowing retreat of the shoreline, with management to control or limit movement.
• No active intervention: a decision not to invest in providing or maintaining defences.

The present day policy is to hold the line, which is implemented by Pevensey Coastal Defence Limited (www.pevensey-bay.co.uk) in the following way:

• removal of about 140 relect groynes, leaving only 10 groynes at locations with a significant change in beach orientation;
• periodic renourishment of the beach using shingle imported from an offshore dredge site (Owers Bank);
• bypassing shingle round Sovereign Harbour to the main beach;
• recycling shingle to depleted areas (generally involving movement of shingle in lorries along the beach from east to west as net transport is from west to east);
• re-profiling of beach to push shingle back towards the ridge crest.
  

Application of the Frame of Reference approach
The Frame of reference approach provides a systematic framework for the development and implementation of a policy for coastal management. The application of the Frame of Reference to Pevensey is illustrated in the figure below and contains the following elements:

Strategic Objective
The UK has adopted a Flood and Coastal Erosion Risk Management Approach, whereby the coasts and benefits of each proposed scheme are assessed using a standard appraisal methodology. The Environment agency then collates information on the costs and benefits and looks at how the benefits would contribute to its outcome targets. Funding is then prioritised. There is therefore no guaranteed level of protection against flooding or coastal erosion in the UK. The strategic objective has been summarised as “affordable risk management”.

Operational or Tactical objective
A tactic must be chosen to implement the operational objective. In the UK, four tactical (formerly operational) objectives are considered at the level of a Shoreline Management Plan :

• Hold the line;
• Advance the line;
• Managed realignment;
• No active intervention.

The SMP identifies policies to manage risk, assesses the coasts and benefits and identifies a preferred tactical objective for each management unit (lengths of coastline). A more detailed and more local strategy study is then carried out to identify appropriate schemes that will put the policy into practice. This process identifies a preferred approach. At Pevensey the preferred tactical objective at SMP and Strategy Study level is to hold the line. This tactical objective has two explicit characteristics:

• position; and
• standard of protection.

The remaining 4 elements of the Frame of Reference constitute a decision making process.

Quantitative State Concept
The first element of the decision making process is to identify a ‘Quantitative State Concept’ which is the outcome from an analysis of the current state of the coast, including its behaviour, land use and functions. This requires the identification of coastal state indicators, which are a reduced set of parameters that can simply, adequately and quantitatively describe the dynamic-state and evolutionary trends of a coastal system (Jiménez and van Koningsveld 2002). Ideally they should relay a complex message in a simple and useful manner.

At Pevensey we are working with Bradbury’s (2000) concept of barrier inertia as a means of identifying the threshold of breaching of gravel barrier beaches. This is a non-dimensional measure of a barrier’s ability to withstand breaching given by:

Bi =Rc.Ba/Hs³

where:
Bi = barrier inertia;
Rc = barrier freeboard above mean water level (including tide and storm surge);
Ba = cross-sectional area of barrier above mean water level; and
Hs = significant wave height at the toe of the barrier.

The threshold of breaching is a function of barrier inertia and wave steepness. Assumptions about characteristic barrier shape are made to link barrier inertia to a measurable cross-shore profile. A novel method of determining the minimum barrier-cross-section to withstand breaching for conditions with different return periods is being developed and tested. This will be compared to the existing method of determining the required barrier cross-section.

Benchmarking procedures
The benchmarking procedure consists of regularly measuring the gravel barrier, dividing it into sections and calculating the barrier profile in each. This is then compared to the threshold value of the cross-shore profile.

Intervention
If the measured profile is smaller than the target profile then the manager must choose an intervention procedure. In this case the available choices are nourishment, recycling and / or re-profiling.

Evaluation
Periodically the success of the intervention should be evaluated against the tactical objective and, less often, the strategic objective. Most simply this can be achieved by comparing the measure profile (after intervention) to the threshold profile. If the intervention was planned and executed successfully the new measured profile will be above the threshold profile.

However, other measures of success are available. As the management policy is designed to prevent flooding from the breaching of the gravel barrier beach. The evaluation could be expressed in terms of having a minimal annual damage cost from flooding.