Summary 17: NOVEL COAST PROTECTION METHODS

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Dumping of rubble is unsightly, poses a health hazard and does little to impede natural shoreline recession.

There are a number of coast protection techniques that are of marginal use for dune protection. They are either unproven or inappropriate. These include the following:

Open revetments, sills or breastwork

Large rocks, concrete tank traps or timber “soldiers” (vertical piles) placed at discrete intervals in a line or as an open array along the mid to upper beach will have a limited influence on cross-shore wave energy. During mild conditions they may have a positive impact on upper beach and dune stability, but their impact during storms may be negligible. As the units are brought closer together to form a tight array they begin to act as a standard permeable headland, breakwater, reef or revetment (Summaries 9, 11, 12 and 14) and become increasingly effective at damping wave energy. To achieve storm protection a void to solid ratio of 0.3 – 0.5 would be required, preferably with a cross-shore width of at least two units (e.g. 2 rows of touching rocks). A properly designed structure would also have the advantage of greater stability of individual elements (i.e. single rocks or concrete units on the beach are less stable than rocks forming a structure).

Artificial seaweed

There have been several attempts at placing artificial seaweed mats in the nearshore zone in an effort to decrease wave energy by the process of frictional drag. The field trials have generally been inconclusive as regards wave energy attenuation. The most successful trials have been in areas of very low wave conditions, low tide range and relatively constant tidal current flows, when some sedimentation was found to take place.

On open coast sites there have been major problems with the installation of the systems and the synthetic seaweed fronds have shown very little durability even under modest wave attack. The synthetic seaweed has tended to flatten under wave action, thereby having minimal impact upon waves approaching the coast.

Field trials in the United Kingdom have been unsuccessful and the experiments were abandoned in all cases, due to the material being ripped away from the anchorage points.

In the Netherlands experiments were more successful with synthetic seaweed being placed in relatively deep water, where sedimentation up to 0.35m took place soon after installation, although this would result in only a very minor decrease in shoreline wave conditions.

The cost of the artificial seaweed is low but the costs and frequency of maintenance works make this option not worth pursuing in an exposed coastal environment, where it would be subject to severe wave conditions and would become damaged rapidly.

Seaweed planting

Seaweed planting can be considered as an alternative to the installation of synthetic seaweed, and works on the same principle of dissipating wave energy by friction. Artificial planting of Posidonia in relatively protected water is now well understood. However there are problems with applying this technique in open coastlines including growth time and creating sufficient plant area. The technique is unproven as far as the significant damping of open coast waves and is unlikely to be of use in dune management.

Bubble barriers

The principle behind the bubble barrier technique is the creation of a continuous curtain of bubbles rising from the seabed to dampen wave energy. The concept was developed with the aim of stilling wave energy at the mouths of harbours, where it would be possible to create suitable conditions over a short distance. The installation costs of such techniques are high, and the maintenance problems are likely to be difficult.

The bubble barrier technique is inappropriate for an open coast location where the costs of installation over hundreds of metres or greater would be considerable. The technique is still very much in an experimental stage with respect to shore protection.

Alternative breakwaters

A considerable amount of research has been carried out on the potential performance of various types of breakwater including:

These techniques involve the attenuation of wave energy by means other than providing a direct barrier. The numerous designs that have been tested or built are usually specific to a particular wave environment, and are usually aimed at vessel protection over relatively short distance. Design, construction and management costs are high. None have been shown to be practical as far as dune protection is concerned.

Sunken vessels

Another method of shoreline protection involves the placement of vessel hulks parallel to the shoreline to dissipate incoming wave energy. Parts of the east coast of the UK are fringed by marshes and these are particularly sensitive to changes in the wave climate, changed sediment supply, etc. On the borders of the Dengie peninsula at the mouth of the Thames Estuary there are extensive salt marshes affected by erosion. Attempts have been made to create conditions conducive to regeneration of salt marsh growth by grounding a line of barge hulks several hundred metres off the shore on shallow mud flats.

The cost of protection is relatively low and few maintenance costs are attached to this form of protection. However, this method does not easily lend itself to the protection of dune systems, if only because of the various adverse impacts that may be encountered. The hulks act like a series of breakwaters, having a strong influence on coastal processes. Not only is wave energy dissipated, but littoral transport is strongly affected. The inability to fine tune such structures means that the adverse impacts down coast may be as severe as any local beneficial impacts, in terms of protection, that are achievable. The hulks are also very intrusive on the coastal landscape and may be unstable under the high wave energy conditions found off many Scottish dune systems.

Tyre revetments

Because of the availability of scrap tyres a number of trials have been undertaken in the U.S.A. to use them as low cost shoreline protection. Much of the shoreline in the U.S.A. is in private ownership, hence there is a need for the development of solutions utilising cheap materials and simple to construct installation methods.

The intended function is to dissipate wave energy by means of the porosity characteristics of the tyre structures. This is by no means easily achievable due to problems with holding tyres together adequately.

This system is not recommended for any but very sheltered conditions. Even then, scrap tyres cannot be recommended for general use. Tests made in the U.S.A. categorised the wave height range under which tyres could be used, as being below 2 feet (less than 0.6m). The main problem is that individual tyres are much stronger than the interconnections between them.

A revetment constructed of scrap tyres will be visually intrusive. The tyres are almost indestructible hence there is a potential for adding a highly unattractive element to the shoreline if they were to come loose.

Inter-linked concrete block revetments

Patented concrete block mattress systems are widely used for protecting estuary and river banks, and for protecting the face of earth embankments. The individual blocks are linked to form flexible mattress, often using flexible cables, allowing large surface area to be covered rapidly.

None of these mattresses are suitable for use as dune face protection in situations where wave attack is experienced. They require a well laid and compacted base to prevent hydraulic uplift forces from buckling the mattress, and they require solid fixing at the crest, toe and ends to prevent slumping or outflanking. Neither of these criteria can be met in most dune situations. At best they can be used as a costly temporary structure.

Bitumen spraying

Various surface stabilising sprays, including bitumen, have been trialed in the UK and elsewhere with the aim of reducing wind erosion of dunes,. The sprays temporarily fix the dune surface and allow newly transplanted grasses to become rooted if they are placed in very mobile sand. Any surface distribution (i.e. trampling or wave attack) will quickly break through the surface layer leaving the sand in its original form.

This approach may have some benefit to backshore blowout management, but will have little, if any, benefit to an eroding seaward face.