Coastal defence concerns

Barrier beaches can be problematic to manage due to their natural ability to ‘roll-back’ and capacity for breaching or flooding of the landward side. A number of the problems that may be encountered are described here, with case-studies. The effects of these processes will depend upon the land-use behind the beaches, and the timescale involved. One of the consequences of many of these problems, particularly associated with flooding, is the landward movement of sediment, and therefore the barrier beach itself.

Breaching
A barrier beach may breach as a result of a one-off event or through the shingle ridge becoming weaker over time through constant attack, eventually failing. Breaching is when the ridge collapses, with beach crest levels effectively falling to below sea level leading to the ‘weiring’ of flows through the beach. Storm surges, high tides, fetch lengths, management methods and sediment availability are mechanisms which all affect the likelihood of breaching. However the most common is a storm event coupled with extreme water levels causing ridge failure.

Porlock Bay in Somerset, is a shingle barrier beach which has experienced ‘roll-back’, overwashing and flooding for many years. Previous management has attempted to stabilise the crest through renourishment and artificial profiling of the seaward face. However, in October 1996, a combination of sediment depletion, a storm surge and associated high wave heights, a high spring tide and an overly-steepened artificial profile caused a breach, which then widened and became a permanent overwash channel.

Flooding
Along with breaching, there are two other flooding mechanisms, namely through-flow or seepage, and overtopping.

• Through-flow is where water flows through the sediment below the beach crest. It is more problematic in coarser sediments, where permeability improves. The formation of through-flow channels may cause the beach ridge to weaken and become unstable, potentially leading to a breach. In cases where the barrier protects a freshwater lagoon, seepage may have significant consequences to the freshwater habitat and the species inhabiting it. However, it may be important for maintaining a beach’s equilibrium and, in the case of Chesil, is partially responsible for maintaining the Fleet’s brackish habitat.
  
  
• Overtopping occurs when waves and tides force water over the beach crest, often transporting sediment with it. This can cause severe flooding of the landward zone, as well as problems, in developed areas, with the deposition of sediment – which during severe storm events can damage structures immediately behind. The formation of overwash fans, fan-shaped deposits of beach sediment found behind the crest, is indicative of overwashing events. Wave overtopping is usually a result of high water levels coupled with large waves, and the likelihood is also affected by the mean still water level relative to the beach crest. Overtopping is a flooding issue which is generally related to severe weather conditions, but affects a large number of beaches, including Hayling Island. The eastern end has a low backshore and the houses behind the beach are built at a much lower level than the beach crest, with sediments overtopping causing structural damage.

Erosion
The erosion of barrier beaches, and indeed any change in their morphology and location, may affect not only the stability and structure of the barrier itself, but also the assets behind. Erosion may occur for a plethora of reasons including high-energy storm events transporting sediment away, a change in the dominant wind and wave conditions, removal of sediment from the source area and updrift coastal defences. In many cases, the construction of beach defences updrift, although protecting the immediate shoreline, will cause sediment deficiencies elsewhere.

Medmerry is a typical example of an eroding barrier which is retreating landwards, mainly as a result of reduced sediment supply. It has breached frequently, including a severe episode in 1910 which resulted in the Selsey Bill temporarily becoming an island. Beach recharge and recycling has been in operation since the 1970s and there are breastworks and a timber groyne field in place.