Summary 7: BEACH NOURISHMENT

Appropriate locations High value amenity beaches. Shorelines suffering erosion due to updrift construction works or channel dredging. Mixed sand/gravel beaches with moderate to high value backshore assets.
Costs Moderate to high, and requires ongoing maintenance (£5,000-£200,000/100m frontage, plus control structures, ongoing management and minor works).
Effectiveness Short to medium term reduction in erosion. Enhancement to natural recovery. 1 to 10 year life before first major recharge.
Benefits Erosion protection without hard structures. Natural beach processes retained. Recreational value of beach enhanced.
Problems Appropriate materials may be unavailable or expensive. Dissimilar imported material may alter geomorphology or ecology. Sand may be blown inland causing a nuisance.

General description

Beach nourishment (also known as beach recharging) involves the importing of sand or gravel to make good losses due to erosion. If the source of material is local and related by coastal processes to the eroding area then this approach is known as recycling (Summary 5). Nourishment schemes can vary from a few truckloads to repair a blow out or other small eroded area up to multi-million pound schemes requiring sea delivery of sand dredged from the seabed.

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The beach at Portobello has been renourished with sand dredged from offshore.

Function

The imported material may be placed on the intertidal foreshore where it will help to protect the dunes by increasing wave energy dissipation across the beach. Alternatively the material may be placed directly at the dune face to form an artificial foredune.

Nourishment with sand is normally only appropriate to high value amenity beaches or small pocket beaches. The sand is quickly redistributed alongshore and offshore by waves, currents and winds, so large volumes and continuing top ups are required to achieve a lasting benefit. In areas with existing mixed sand/gravel beaches a gravel nourishment can be cost effective. The gravel tends to form a narrow storm ridge along the toe of the dunes from where it is redistributed alongshore by wave action at high tide only.

The success of a nourishment scheme will be enhanced by vegetation transplanting and fencing or thatching (Summaries 2, 3 and 4). In some instances the nourishment may also benefit from the construction of groynes, reefs, breakwaters or sills (Summaries 10, 11 and 12), that will reduce longshore transport losses. Where high value assets are to be protected, the nourishment scheme may be backed by a fixed line of defence such as a gabion or rock revetment (Summaries 8 and 14), possibly buried by the nourishment.

Method

Before giving serious consideration to this approach the dune manager should investigate potential sources of nourishment material. Appropriate materials in the required volumes may not be available at a cost that is acceptable. There are few licensed dredge areas around Scotland, and there is strong competition for both the material and for the dredge vessels. Alternatively materials may come from navigation maintenance dredging or onshore sites. Dredge spoil may be available from ports, but must be carefully sampled prior to use to ensure that materials do not contain too much mud, and that they are not contaminated in any way. Land based sources may be most suitable for small recharge schemes and for top ups, as start up costs are likely to be lower and materials are likely to be more readily available in small quantities. If material is available it may not conform to the ideal size or quality standards desired for the dune management site. Compromises are likely to be required.

Nourishment should only be undertaken where the shoreline manager has a good knowledge of local processes and beach movements, developed from measurements and observations over at least several years. Nourishment methods are dependent on the scale of operations. The Guide does not consider appropriate methods for nourishment schemes involving transfer of sediment from the sea; these larger scale operations are covered by the CIRIA Beach Management Manual and should be designed by competent consultants.

Delivery from land (whether port dredge spoil or quarried) will involve large numbers of vehicles using local roads, accessing the beach and placing the material. Each phase of this operation will have impacts on people and the natural environment. Work schedules will be controlled by the tide, and cost efficiency may demand night work or a schedule that is damaging to habitat. The significance of these issues will be site specific, but must be carefully considered. Delivery from the sea is less damaging, but cost effectiveness depends on the economies of scale and the source of the material.

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Major renourishment operations typically require sand to be pumped onshore from dredgers

The nourishment material should be as similar as possible to the indigenous sediment. Sediment size, grading, shell content and material should match the upper beach and dune face. The material should also be clean and free of seeds. If these conditions are not met then the nourishment may cause unwanted changes to the beach and dune profiles, to the dune ecology and the dune appearance. Coarser sediments can be used, and may create a more stable upper beach, but they are likely to cause the beach gradient to increase and will be less likely to form new foredunes. Environmental and hydraulic assessments should be carried out to determine the acceptability of available sediments if they do meet the ideal criteria.

There is some debate as to the most effective position to place the nourishment to achieve optimum protection. Possible locations include the upper beach and dune face, the mid to lower beach and the shallow nearshore zone. The former is most obvious as the benefits of nourishment are immediate, but, if the sediment is primarily sand, it will be rapidly redistributed alongshore or across the beach face by waves and currents to form a new equilibrium profile (shingle tends to remain on the upper beach, but may be redistributed alongshore). Placement of sand on the mid-beach anticipates this redistribution, and provides shoreline protection by helping to dissipate wave energy before it reaches the dunes. Placement below the water line also anticipates the redistribution and allows sand to be fed into the beach system gradually. This latter approach is only appropriate to very large recharge schemes and is unlikely to be considered for dune management.

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Imported sand or gravel placed along an eroding dune face

Although there is some merit in the alternative locations it is generally considered appropriate that deposition of the nourishment material should be along the upper beach, above the high water line and along the eroding dune face. Natural redistribution of the placed material alongshore and cross-shore will occur, particularly for sand. Nourishment operations should anticipate this redistribution by ensuring that the volume deposited to the upper beach is in excess of the amount required for immediate protection to the dune. The above figure provides some rough guidance as to appropriate beach profiles that will allow for continued erosion, but each site should be assessed separately. Consideration should be given to the past rates of erosion, the likely rate of redistribution and the intended life of the nourishment before the next top-up operation will be required. Fencing or thatching of the new dune line, followed by vegetation transplanting should enhance the success of the recharge operation (Summaries 2, 3 and 4). Ongoing management will allow the future nourishment to be fine tuned for each site.

Nourishing with gravel can be very effective. A ridge of shingle along the dune face can provide good protection from wave erosion. The gravel may well be partially covered by sand during natural recovery, and may only be exposed by storms. A gravel ridge can be successfully controlled by groynes (Summary 10), although great care must be taken with design of these.

The approximate limits of wave run-up can be established by observing and recording the location of the strand line over Spring tide periods during both winter storms and more normal wave conditions. The toe of a freshly eroded dune face is normally at the run-up limit of the most recent severe sea.

Prior to placing material along the dune face it may be prudent to remove some of the existing vegetation for later transplanting. Vegetation buried by imported shingle or wet sand to a depth of more than a few centimetres may not recover.

Nourishment is most appropriate to pocket beaches where little material is lost from the system. On long, exposed beaches where longshore transport is likely to redistribute the nourishment material rapidly, there may be a need to provide beach control structures such as headlands, reefs, breakwaters or groynes. Attempting to control sand beaches in areas with high tidal ranges is unlikely to be successful unless massive cross-shore structures are used. Gravel ridges along the upper beach are much easier to control and require relatively small structures.

The dimensions suggested by the accompanying sketch are basic minimums to serve as a guide for small nourishment schemes. Greater and longer term success can be achieved by increasing volumes and by undertaking regular top-ups to maintain effective erosion protection. Large schemes should be designed by competent coastal consultants, following preliminary studies of the physical, natural and human environment.

Schemes are best implemented in the spring and early summer when work windows are least restricted and the nourishment has the greatest chance to stabilise before winter storms start to erode and redistribute the beach.

Costs for a nourishment scheme depend on the source of material, transport methods, volumes required, the need for beach control structures, (groynes, breakwaters etc.), the need for secondary defences, expected scheme life before topping-up and the amount of minor works undertaken to enhance the dune system. Small recharges using locally quarried gravel that is transported easily to site by road may cost as little as £5000/100m frontage. Schemes requiring large volumes of offshore dredged sand, pumped ashore from barges and requiring substantial top ups at, say, 5 yearly intervals may have an initial cost of £2,000,000/1km with ongoing management costs of £400,000 each year. If secondary defences or control structures are also included, the initial costs may rise to as much as £4,000,000/km but future management costs could drop to, say, £100,000/km each year.

Impacts

Sand deposited on the upper beach may be subject to wind erosion, causing an unwanted increase in blown sand across the back shore; fencing and vegetation transplanting will reduce this problem. The deposits may also bury existing vegetation and intertidal invertebrate communities, reducing the natural stability of the foredunes and destroying habitats. As a general rule impacts will be less for regular, small nourishment operations than for occasional large operations.

If the source material contains sediments dissimilar to the indigenous dune beach sand (e.g. gravels or estuary silt), or if it contains vegetation or seeds that are foreign to the dune system then the dune geomorphology and ecology may be adversely affected. Any fine materials will be washed out of the nourished beach, potentially impacting on nearshore fisheries and invertebrate communities.

Best practice and environmental opportunities

Nourishment will increase the volume of beach and/or dune material, and will enhance the natural recovery of dune face erosion. Where large volumes of material are imported, nourishment can provide a wide upper beach for recreational use, and will gradually feed sediment to adjacent beaches due to longshore drift. The initial artificial appearance of the upper beach and dune face will be quickly transformed by wind, waves and vegetation to a more natural form.

Nourishment can be combined with fencing, thatching and transplanting to encourage development of new foredunes and an extended dune habitat. Hard defences built high on the beach face can be buried by the nourishment, followed by fencing and transplanting, to create a much more natural shore.

All dune management schemes should observe the following guidelines to maximise the probability of success and minimise impacts on the natural and human environment:

In addition to these general guidelines, the following are of specific importance to beach nourishment: