Beach Management
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Methods for managing barrier beaches Beaches can be managed in different ways depending on individual circumstances. When selecting schemes for specific beaches, their effectiveness, cost, performance and environmental conditions must be evaluated. Typically a combination of structures is implemented to achieve the best effect. Please find a list of the most common management methods below. Seawalls Seawalls are the most common form of defence along the UK’s urban seafront (Brampton et al., 2004) and can be defined as continuous shore parallel structures that aim to provide a final line of protection to the coastline and absorb some wave energy (Simm et al., 1996). Their influence on cross-shore transport can be such that beach levels are significantly affected. They are generally constructed from concrete as it is cost effective and quick to build whilst still being strong enough, and are typically vertical or sloping revetments. However, seawalls are impermeable to wave action and not only experience high local impact pressures but may also reflect much of the waves’ energy, both of which have associated impacts. Many of the UK’s seawalls are Victorian structures which have been repaired/modified in the intervening years. New seawalls are built with more consideration for the potential long-term impacts of a vertical or near-vertical structure on the beach, and are therefore more often in the form of sloped revetments, stepped walls and/or exploit a combination of materials and methods. Seawall slope is the most critical factor determining beach response, with steep or vertical walls more likely to cause scour, and impermeable ones more detrimental than permeable where the wall itself does not receive the full impact of the waves. Wave attack may also cause problems with wave overtopping and flooding, especially in historical cases where the defence was often built on, or seaward of, the mean high water level crest for recreational purposes. Seawalls also, by providing a fixed limit to the backshore, remove the possibility of an inshore supply of material to the beach such as wind-blown sand from dune systems. Seawalls, in some form, are present at many barrier beaches. Breastworks and revetments Breastworks and revetments are placed at the foot of the beach ridge to protect the ridge and stabilise it under wave action. These structures take a number of forms with common occurrences including timber breastworks used in conjunction with groyne fields and to retain beach material, rock armouring revetments or concrete armour units at the foot of a shingle ridge to strengthen and stabilise it, and concreted slopes to dissipate wave energy. The design of these features varies, and can include stepped walls, and crests with different slopes (essentially turning the beach ridge into an artificial structure), depending on original beach stability and predominant local conditions. Revetments may be permeable, such as rock armouring, which allows water to flow through the structure and thus relieve pressure on it, or impermeable, where there is danger of building up water pressure behind the structure. Their increased roughness and, in most cases, permeability, reduces wave overtopping and can reduce toe scour (Brampton et al., 2004). A number of breastworks are used at UK barrier beaches including the timber breastworks on the eastern exposed frontage at Hayling Island and defence works to the proximal end of Hurst Castle Spit, maintaining its volume and position. This includes rock armouring and zig-zag timber breastwork. Groynes Groynes are implemented in many places around the UK. Groynes are structures, extending across part or all of the intertidal zone, designed to modify longshore drift and therefore retain sediment within the groyne bays, and are generally aligned perpendicular to the shoreline. In the UK, groynes are typically constructed from timber, rock or concrete, and can be permeable or impermeable, although the majority are impermeable. However, other materials have been used including steel tubing, asphalt and gabions. Impermeable groynes are generally concrete or sheet piling structures, but may include grouted rock and rubble mound groynes (Van Rijn, 2004). The crest of these structures is generally above mean high water level. Impermeable groynes, however, have a tendency to reflect wave energy and to create offshore-directed currents, especially on their downdrift side. Permeable groynes are used on beaches where there is sufficient sediment to allow a proportion to be transported through or over the structure without reducing its efficiency (Fleming, 1990). Their implementation is more for creating a more regular beach plan-shape on shores where erosion is less of a problem. Groynes are built to serve three main purposes (Van Rijn, 2004):
Groynes perform best at sites with a significant longshore drift component and an oblique angle of wave approach. Groyne spacing is variable and depends on the nature of beach material, with spacing on shingle beaches less than that on sand. This is due to shingle beaches recovering from storm conditions more quickly than sandy beaches. Spacing is also related to groyne length, with spacing being 2-4 times groyne length to prevent the generation of rip currents and excessive erosion in the groyne bays (Van Rijn, 2004). As the sediment within the groyne bay aligns itself towards the dominant wave direction, a saw-tooth pattern evolves, and once the bays become “full”, sediment passes the down-drift end. Recharge operations are often required in conjunction with groynes to ensure beach levels are healthy at the time of construction, and to decrease unwanted downdrift effects. One of the main problems with groynes is the transfer of the erosion problems along the coast, as has been the case at Selsey Bill where the groyne fields have increased erosion at Medmerry. Groynes are used on many barrier beaches, in various states of repair and extent, including Pevensey, Reculver and Sandbanks. Fishtail groynes Fishtail and T-head groynes are a recent development from the standard linear structures, and as such their performance is not well-documented. These so-called head extensions may improve groyne efficiency. Their purpose is to increase the distribution of wave energy by diffraction and they are designed to affect the incoming waves as well as the longshore currents that the waves and tides produce. Groyne head extensions may also allow groyne spacing to be increased and thus the number of structures required to decrease by extending the alongshore influence. Fishtail and T-head groynes have been implemented at a number of barrier beaches including Dinas Dinlle, Jaywick and Sand Bay. Breakwaters Detached breakwaters are structures built in the nearshore zone aligned parallel to the shoreline and designed to dissipate wave energy and decrease wave activity at the beach, promoting sediment deposition in their lee. Breakwaters are generally constructed as a series and are built from rock with a rubble core, although geo-textiles, concrete armour units and other materials are sometimes used. They may be submerged or emergent. Emergent breakwaters have crests which are typically ~2 m above MHWS and the structure is designed to prevent wave overtopping. All of the breakwater schemes applied at UK barrier beaches have been emergent. As with groynes, beach recharge is generally necessary in conjunction with detached breakwaters to initially increase beach width and thus reduce downdrift problems. Waves break against the structures and are diffracted through the gaps between them before they reach the beach. This causes frontages in the lee of the breakwaters to accrete, resulting in the formation of salients behind individual structures, which may form tombolos linking the structure to the beach if there is sufficient sand supply. However, detached breakwaters may induce significant changes to the coastal process regime and therefore it is crucial that prediction and understanding of their impact is undertaken before their construction. Beaches may develop a pattern of alternate wide and narrow stretches, rather than an overall wider frontage. Again, spacing of the structures is important in determining how the beach is affected. Pope and Dean (1986) developed a simple series of rules for predicting beach effects, which were dependent on breakwater length, distance between adjacent breakwaters and their distance offshore. To ensure the formation of a tombolo, breakwater length should be greater than gap lengths. If breakwaters are positioned so that adjacent structures have a gap wider than their length, there is likely to be a problem of erosion between them. This erosion may be a significant problem, to the extent that in some cases groynes have been constructed in the gaps to retain some of the sediment. With regards to barrier beaches, breakwaters are currently in place at Elmer along the Bognor Regis frontage, and at Sea Palling in Norfolk. Crest stabilisation Crest stabilisation can help prevent roll-back of barrier beaches by fixing them in place, and strengthening ridges that are weakening and beginning to fail. Stabilisation also ensures that the beach crest stays at an appropriate height to prevent overtopping or breaching. Stabilisation may be implemented through ‘soft’ engineering by the planting of vegetation to trap the beach material or through ‘hard’ engineering such as the placement of gabion or concrete mattresses on the beach crest. Gabions are rock-filled mesh baskets which are assembled on site, and can be placed as vertical baskets, or as sloping mattresses. They are flexible and permeable. The sloping mattresses are more appropriate for crest stabilisation as they reflect less wave energy than vertical cages. At the eastern end of Chesil Beach, in the vicinity of Chiswell, there is a history of overtopping and flooding, including two severe attempts in the winter of 1978/9. This led to crest stabilisation works in the form of flexible gabion mattresses to maintain the crest height and position. The scheme requires an ongoing programme of maintenance but has proved successful. At Weybourne, unusually, a rock-filled gabion basket was placed beneath the shingle ridge during the 1980s. As it has yet to be exposed, any erosion or landward retreat of the feature must be slow. Recharge and reprofiling Beach nourishment is becoming an ever-more necessary option due to natural sediment losses from open coast beaches and with increasing anthropogenic pressure. It involves the placement of beach sediment – from either inland sources, or, more commonly, offshore sources such as channel dredge material or licensed dredge areas. Sediment is generally pumped ashore either through floating pipes, or directly discharged from the dredge vessel through a nozzle and then distributed across the shoreline to create the required profile. The recharge sediment is typically equally coarse, or coarser, than the natural beach sediment to minimise losses through longshore drift, although this is more difficult when recharging shingle beaches (Brampton et al., 2004). Other criteria often include colour matches for aesthetic reasons, and minimal fines proportion to prevent turbidity. Nourishment is frequently used in conjunction with other forms of defence to stabilise the beach and reduce the effects of defences on downdrift areas. However, recharge is also an option to be used singly to create and maintain a higher and wider beach profile that is more stable and provides more protection to the backshore assets. A beach is often recognised as the best form of coastal defence as unless it is constrained, it naturally has the ability to respond to changing conditions and dissipate wave energy, and therefore the addition of sediment to the beach is a direct way of overcoming beach sediment losses. Nourishment, although it can be costly, is a low-impact solution, inducing little long-term adverse environmental or aesthetic impacts and potentially increasing the recreational value of the beach, and thus local tourism. However, nourishment is often only a short-term solution unless used with other defences, or if the longshore drift can be controlled – otherwise the added sediment is removed from the system just as the naturally occurring sediment was. Therefore re-nourishment is often required, and periodic recharge/ recycling schemes established. Nourishment material may be difficult to acquire, particularly in the volumes which are often required. Nourishment is undertaken at many of the UK’s barrier beaches, including major schemes at Hunstanton/Heacham and at Hurst Castle Spit where sediment is added to increase the height of the beach at both the proximal and distal ends. Beach nourishment is by default, associated with reprofiling, as the additional material placed on the beach must be re-distributed across the shore. However, beaches may be re-profiled without the addition of extra material, simply to create a more stable cross-section. It involves the artificial re-shaping of the profile, for example moving sediment from the inter-tidal area and depositing it above high water. Re-profiling is a low-cost method which can be undertaken immediately, but in most cases the beach is steepened, allowing larger waves to attack upper beach areas, potentially leading to rapid erosion and thus re-distribution. Recycle Beach recycling is a low-cost and short-term solution, where sediment is removed from an accreting area and re-distributed over an eroding area, often on the same beach. The mechanics are the same as nourishment, but the sediment is related to the source sediment rather than an unconnected source. In many cases recycling is undertaken periodically (often annually) to re-distribute the sand over the beach – as at Hayling Island where sand is taken from the accreting Eastoke Point to the eroding beach further west as an additional activity to the renourishment activities. Do nothing All of the previous management options have involved the construction of beach defences or the addition of sediment into the beach system. However, to do nothing is another option. This is when the decision is taken not to undertake defence works and no expenses are incurred. The do nothing option should always be considered, even if simply to predict the future evolution of the area without intervention. Beaches are evaluated for their importance, and the land-use behind them. In areas where the land-use is deemed of little value, or where the creation/modification of habitats due to allowing current processes to continue is considered important, the do nothing option may be adopted. However, generally it is advised that monitoring is continued to determine the need for possible future intervention. Porlock Bay is an example of a beach where a do nothing approach was adopted following the formation of a breach in the 1996 storm surge. In fact, previous management strategies of reprofiling and renourishing the crest are thought to have been detrimental to the crest’s stability and strength and thus partially responsible for causing the breach. References
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