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Coastal Erosion Management: Techniques and Approaches

River and Sea Erosion
In order to provide a comprehensive guide to the options available for the management of coastal erosion all principle coast protection and erosion management techniques are covered. It must be recognised, however, that finely all of these can be damaging to the natural environment, to a greater or lesser degree, in inappropriate situations. The inclusion of any particular approach herein does not, therefore, indicate that it is, necessarily, environmentally sensitive, nor are universally appropriate as a means of managing erosion. Rather, the summaries highlight and encourage the pursuit of good practice, from an environmental perspective, which ever approach is deemed necessary by the circumstances concerned.

The various approaches to management of coastal erosion in beach and dunes are as follows:
ADAPTIVE MANAGEMENT
Appropriate locations Locations with low value, life expired or moveable backshore assets
Effectiveness Short term loss of assets, but highly sustainable over medium to long term
Benefits Allows natural processes to continue with possible strategic benefits spread over adjacent areas. No ongoing management costs.
Problems Backshore assets are lost or moved, often causing conflict due to differing perceptions of values

General description
River and Sea Erosion
Many cases of dune erosion may be best managed by not interfering with the natural processes, but instead accepting that erosion will occur and adapting backshore management accordingly. This approach will involve relocation and monitoring costs, but these may be much lower than the cost of protection. Adaptive management should be considered at all sites before considering any of the other options set out in this guide.
Function
Adapting to erosion, or the potential for erosion, by moving, replacing or demolishing structures or other assets that are at risk will avoid the need for interference with coastal processes. Better still is the initial control of any form of development along the shoreline dunes, though in many instances it will be too late to adopt this preventative approach.


DUNE GRASS PLANTING
Appropriate locations Above normal limit of wave run-up at any location with available blown sand. Unlikely to succeed where erosion is severe.
Effectiveness Enhancement to natural dune recovery. Reservoir of sand held in planted foredunes will provide a buffer to resist storm erosion.
Benefits Compliments natural system. Can be used to improve other management options. Potentially self sustaining.
Problems Normally requires dune fencing or thatching to achieve success. May be completely lost to storm erosion.

General description
River and Sea Erosion
Vegetation encourages dune growth by trapping and stabilizing blown sand. Transplanting marram grass (Ammophila arenaria) to the face of eroded dunes will enhance the natural development of yellow dunes above the limit of direct wave attack. Sand couchgrass (Elymus farctus) or lyme grass (Leymus arenarius) can be transplanted to encourage the growth of new foredunes along the toe of existing dunes, as these species are tolerant to occasional inundation by seawater. These natural dune grasses act to reduce wind speeds across the surface, thereby trapping and holding sand. They grow both vertically and horizontally as the sand accumulates. Marram grass is particularly effective as it positively thrives on growing dunes, and is perhaps the easiest to transplant.
Function
Transplanting vegetation will not prevent erosion, but it will accelerate natural recovery after storm damage creating a reservoir of sand within the foredunes that will make the dunes better able to withstand the next period of erosion. Additional works are often necessary to increase the potential for success. Thatching and beach recycling will assist in the accretion of sand, will provide minor protection from waves and will reduce damage due to trampling. Once grasses are well established they may well become self-sustaining, although any storm erosion damage will need to be rapidly made good.


DUNE THATCHING
Appropriate locations Above normal limit of wave run-up at any location with available blown sand. Unlikely to succeed where erosion is severe.
Effectiveness Enhancement to natural dune recovery. Modest resistance to storm erosion. Enhanced by vegetation transplanting.
Benefits Minimal impact on natural system. Materials are all natural degradable and low cost.
Problems Without maintenance thatching will last no more than 1 year. Materials are often used to build bonfires.

General description
River and Sea Erosion
Thatching of exposed dunes faces or blowouts using waste cuttings from forestry management, or other low cost materials, is a traditional way of stabilising sand, reducing trampling and protecting vegetation. Materials are low cost if locally available and no machinery or skilled labour is required to achieve success, but continual maintenance is important. The approach is normally carried out with dune grass planting to encourage dune stability. Thatching materials are often removed for bonfires by beach users.
Function
Well laid thatch will encourage dune recovery and will resist some erosion, but cannot prevent erosion where wave attack is frequent and damaging. The thatch reduces surface wind speeds, encouraging deposition of blown sand. Success depends on the amount of blown sand, the frequency of wave attack and the availability of vegetation. Transplanting dune grasses after thatching will enhance dune recovery and longer term stability. Continual maintenance and replenishment of cuttings is required.


DUNE FENCING
Appropriate locations Above normal limit of wave run-up at any location with available blown sand. Unlikely to succeed where erosion is severe.
Effectiveness Enhancement to natural dune recovery. Limited resistance to storm erosion. Enhanced by vegetation transplanting.
Benefits Minimal impact on natural system. Can be used to control public access and to improve other systems.
Problems Damaged fences and accumulated debris can be unsightly. Fences need regular maintenance and have a maximum life of about 5 years depending on material, frequency of storm wave damage and vandalism.

General description
River and Sea Erosion
Construction of semi-permeable fences along the seaward face of dunes will encourage the deposition of wind blown sand, reduce trampling and protect existing or transplanted vegetation. A variety of fencing materials can be used successfully to enhance natural recovery. Fencing can also be used in conjunction with other management schemes to encourage dune stabilisation and reduce environmental impacts.
Function
Sand fences cannot prevent erosion where wave attack is both frequent and damaging, but they will encourage foredune growth and resist some erosion. Fences reduce wind speed across the sand surface and encourage foredune deposition. They also act as a modest barrier to wave attack, reducing the erosion potential of waves near the limit of uprush.

Success depends on the void to solid ratio of the fence, the availability of blown sand, the frequency of wave attack at the fence and the amount of vegetation available to stabilise the accumulated sand. Success will be enhanced by a programme of dune grass transplanting, thatching and beach recycling/regrading to establish new foredunes. Fencing and associated works can be used to enhance the appearance and effectiveness of other erosion defences. Rock, timber or gabion structures can provide a fixed line of defence, but are incongruous along a natural dune coast: partial burial by recycled or sand accreted by fencing and grasses will create a more natural dune environment.



BEACH RECYCLING AND REPROFILING
Appropriate locations All locations, including those with limited blown sand for natural recovery.
Effectiveness Short term defence against erosion, and enhancement of natural recovery. Moderate resistance to single storms. Enhanced by fencing and vegetation transplanting, and can be successfully used to bury hard defences.
Benefits Accelerates natural recovery of fore dunes and provides short term defence against single severe storms.
Problems Removes material from other sites, possibly transferring erosion or environmental damage to a different frontage. May introduce beach debris, non-indigenous sediment and/or vegetation, potentially damaging local ecology.

General description
River and Sea Erosion
Recycling is the mechanical shifting of sand, shingle or even boulders from an area of accretion to an area of erosion. Normally recycling would be undertaken at a local level, with sediment being taken from an accreting ridge, the lower beach or an estuary bar, and transported a short distance to an eroding dune face or a blow out. Alternatively the donor area may be to landward if sand is blown onto roads or other areas where it is not wanted and from where it can be recovered.

Reprofiling is an alternative term, usually referring to the direct transfer of material from the lower to the upper beach or, occasionally, the transfer of sand down the dune face from crest to toe.

Recycling sand or shingle can be undertaken to repair minor erosion problems such as blowouts, or it can be used to rebuild long lengths of upper beach. Use of boulders is usually restricted to relocating small numbers up the beach face to provide temporary armouring of a short length of dune face suffering minor erosion.
Function
Recycling provides an artificial buffer between the dune face and the erosive forces of the sea. Where erosion is active this buffer provides a short-term defence of the dunes, possibly only lasting through a single storm. Where the beach is stable or recovering, recycling accelerates the development of new foredunes. The success of this approach will be enhanced if combined with vegetation transplanting and dune thatching or fencing .

Recycling can also be used to enhance the coastal landscape by burying hard defences (sand bags, rock, gabions or timber), with the understanding that those defences may become re-exposed and active during storms. This will only be successful if the defences are high on the beach face - structures within the normally active foreshore zone will not be successfully buried as waves will rapidly remove the recycled material.



SAND BAG STRUCTURES
Appropriate locations Low to moderate energy sandy shorelines requiring lower cost, temporary defence.
Effectiveness Provides short term fixed line of defence. Less than 5 year life. Burial may extend life.
Benefits Low cost, low skill approach using local materials that are returned to the beach when the defences no longer required.
Problems Sand bags subject to vandalism and rapid deterioration due to wave action, sunlight and public pressure. Bags are effectively impermeable and do not absorb wave energy, so beach scour may accelerate.

General description
River and Sea Erosion
Sand bags of various sizes and lengths can be used to form temporary reefs, breakwaters, groynes, headlands or revetments on sand beaches. Sturdy geotextile bags are filled in-situ with local beach sand and therefore have a relatively low cost.
Function
Sand bag structures can be placed without the need for costly equipment or skilled labour. They can be used to form any form of shoreline structure but will have a short life expectancy due to lack of resistance to physical damage (wave borne debris impacts or vandalism) and the effects of UV sunlight. They are potentially most useful as a buried revetment under the dune face, where they will form a final line of protection after the overlaying sand has been eroded by storm waves. An alternative use is to form temporary headlands to protect backshore assets while other, longer term, options are planned and implemented.


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.
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
River and Sea Erosion
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 .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.
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 some instances the nourishment may also benefit from the construction of groynes, reefs, breakwaters or sills 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, possibly buried by the nourishment.


GABION REVETMENTS
Appropriate locations Sandy beach sites suffering periodic moderate to severe erosion where backshore assets are at risk. Useful for estuary bank protection.
Effectiveness Well placed gabions provide reasonable fixed defences, but have a limited life of 5-10 years due to deterioration of the baskets.
Benefits Useful solution where armour rock is considered inappropriate or too costly. Various forms available. Can be buried by sand and vegetation. Permeable face absorbs wave energy and encourages upper beach stability.
Problems Limited life, leading to unsightly and hazardous wire baskets along beach and the release of non-indigenous cobbles to the beach system. Wire affected by saltwater, vandalism and abrasion by trampling or gravel beach impacts.

General description
River and Sea Erosion
Gabions are wire mesh baskets filled with cobbles or crushed rock. They are filled insitu, often with locally available material and therefore have a relatively low capital cost. Because they are flexible and porous they can absorb some wave and wind energy, thereby reducing the scour problems associated with impermeable sea defences such as concrete seawalls. Gabions can be placed as sloping "mattresses" or as near vertical cubic baskets. The latter are intended for bank or cliff stabilisation and are not normally suitable for use in shoreline situations.
Gabion revetments (foregound) are generally preferred to gabion walls (background) in coastal environments being less reflective of wave energy and more stable. Blown sand is also better able to accumulate on revetments, potentially softening their appearance.
Function
The purpose of a gabion revetment is to provide short term (5-10 years) protection from backshore erosion by absorbing wave energy along the dune face. Their application is restricted to the upper part of sandy beaches, since they are not sufficiently durable to withstand regular direct wave action. They should not be installed on shingle beaches because wear and tear will rapidly cause damage to the baskets. As they are porous structures they will tend to trap wind blown sand and allow the growth of vegetation under favourable conditions. This only applies to sloping structures: steep walls of cubic baskets will not attract sand or allow dune vegetation regrowth.
Gabions provide a short term alternative to rock armour structures in areas where large rocks are not available at an acceptable cost, or where long term protection is not appropriate.


ARTIFICIAL HEADLANDS
Appropriate locations Rapidly eroding dunes with important backshore assets at discrete intervals along the shore.
Effectiveness Good temporary or long term protection for protected length. Allows natural processes to continue elsewhere. Can be used with other low cost methods. Unlimited structure life for rock headlands.
Benefits Provides local protection with minimum disturbance to dune system as a whole. Can be modified or removed at later date.
Problems Visually intrusive. Do not control erosion along the whole frontage. Structures may interfere with longshore transport, particularly on sand-gravel beaches, and may require periodic extension or relocation landward to avoid outflanking.

General description
River and Sea Erosion
Artificial headlands are rock structures built along the toe of eroding dunes to protect strategic points, allowing natural processes to continue along the remaining frontage. This is significantly cheaper than protecting a whole frontage and can provide temporary or long term protection to specific assets at risk. Temporary headlands can be formed of gabions or sand bags, but life expectancy will normally be between 1 and 5 years.
Function
Artificial headlands stabilise discrete lengths of the dune face while allowing the intervening stretches to erode naturally, forming an increasingly embayed shoreline. As the shoreline becomes more indented so the wave energy will be dissipated over a longer frontage and ultimately a more stable plan shape can develop. Stability will depend on the length and spacing of the headlands. Short structures with long gaps will provide local protection but may not allow a stable planshape to develop. If ongoing erosion is severe the headlands may need to be extended or relocated to prevent outflanking or structural failure, although they will continue to provide some protection as nearshore breakwaters The embayments between headlands will not become independent units as sand will be transported by wind, waves and currents along the lower foreshore to seaward of the structures. If gravel is present the headlands may restrict longshore movement along the upper beach; this can be useful to control losses if renourishment or recycling is undertaken.


ARTIFICIAL REEFS
Appropriate locations Exposed dunes of high ecological and landscape value.
Effectiveness Causes lee side accretion, but least effective during storm surge conditions. Unlimited structure life.
Benefits Natural processes are only partly disrupted, allowing dunes to stabilise. Rocks create new intertidal habitat.
Problems May cause navigation hazard. Visually intrusive at low tide. Disrupt amenity use of beach.

General description
River and Sea Erosion
Artificial reefs are shore parallel rock mound structures set part way down the beach face. They may be long single structures or form a series of reefs extending for some distance alongshore. They are distinguished from Nearshore Breakwaters by being submerged for at least part of the tidal cycle, and are therefore less intrusive on the coastal landscape, have less impact on upper beach longshore processes and add a new intertidal habitat to sandy foreshores.
Function
Reefs dissipate part of the incident wave energy before it reaches the dune face, protecting the upper beach from erosion and encouraging deposition. Long structures (sills) reduce wave energy over an extended frontage, resulting in a more stable upper beach and dune face. Shorter, segmented reefs protect short lengths of the shore, allowing erosion to continue elsewhere. The result is an embayed shoreline with upper beach deposits (salients) forming behind the reefs.

Salients will allow new foredunes to develop, but this accretion may be at the expense of continued erosion elsewhere. Recycling or nourishment, followed by fencing, thatching and transplanting may address this problem, and will enhance the rate of dune-beach recovery. Reefs have less impact on upper beach transport processes than nearshore breakwaters, and can be used on open beaches. In particular, tombolos will not form behind low level reefs, but can form with higher breakwaters; tombolos would significantly disrupt longshore drift, potentially causing downdrift erosion.

Reefs are of little use within estuaries where currents, rather than waves, are the main erosive force


NEARSHORE BREAKWATERS
Appropriate locations High value frontages with low rates of longshore transport, and weak near shore tidal currents.
Effectiveness Cause lee side accretion and erosion behind gaps. Offers good protection within enclosed bays, but potentially damaging to open coasts.
Benefits Dunes not directly disturbed, increases area of dry upper beach, may allow new fore dunes to stabilise. Unlimited structure life.
Problems Visually intrusive, alter upper beach morphology, may cause fine sediment, seaweed or debris to accumulate along upper beach. Can cause locally strong currents and may be a hazard to beach users.

General description
River and Sea Erosion
Nearshore breakwaters are segmented, shore parallel structures built along the upper beach at approximately high water mark. They are normally built of rock, but can be formed of concrete armour units. At maximum tide levels their crests are still visible, but they may be separated from the shoreline. The gaps allow some wave energy to reach the upper beach and even the dune face. These structures are distinguished from Artificial Reefs that are built further down the foreshore and are submerged at high tide.
Function
River and Sea Erosion
Breakwaters reduce the energy of waves reaching the shoreline, but do not completely isolate dunes from the natural beach processes. The structures act as a direct barrier to waves, but at very high water levels they allow some overtopping. The gaps between segmented structures allow some wave energy to reach the upper beach and dune face, but this is dissipated by refraction and diffraction. Erosion may continue in the lee of the gaps leading to formation of an embayed shoreline as sand moves into the shelter of the structures.

Sand build up in the lee of the structures (salients) may grow seawards sufficiently to connect with the structure, forming a �tombolo�. If the salient is stable, new foredunes may develop. Recycling or nourishment followed by fencing, thatching and transplanting may be used to accelerate formation of stable salients and dunes.

Breakwaters can have a strong influence on longshore drift and should not normally be used on long expanses of open coast or within estuaries if strong wave or tidally induced currents are present. Breakwaters can cause downdrift erosion or result in dangerous conditions for beach users.


GROYNES
Appropriate locations High value frontages influenced by strong long shore processes (wave induced or tidal currents) where nourishment or recycling are undertaken. Best on shingle beaches or within estuaries.
Effectiveness Good on exposed shorelines with a natural shingle upper beach. Can also be useful in estuaries to deflect flows. Unlimited structure life for rock groynes.
Benefits Encourages upper beach stability and reduces maintenance commitment for recycling or nourishment.
Problems Disrupts natural processes and public access along upper beach. Likely to cause downdrift erosion if beach is not managed.

General description
River and Sea Erosion
Groynes are cross-shore structures designed to reduce longshore transport on open beaches or to deflect nearshore currents within an estuary. On an open beach they are normally built as a series to influence a long section of shoreline that has been nourished or is managed by recycling. In an estuary they may be single structures.

Rock is often favoured as the construction material, but timber or gabions can be used for temporary structures of varying life expectancies (timber: 10-25 years, gabions: 1-5 years). Groynes are often used in combination with revetments to provide a high level of erosion protection.

Function
Groynes reduce longshore transport by trapping beach material and causing the beach orientation to change relative to the dominant wave directions. They mainly influence bedload transport and are most effective on shingle or gravel beaches. Sand is carried in temporary suspension during higher energy wave or current conditions and will therefore tend to be carried over or around any cross-shore structures. Groynes can also be used successfully in estuaries to alter nearshore tidal flow patterns.

Rock groynes have the advantages of simple construction, long-term durability and ability to absorb some wave energy due to their semi-permeable nature. Wooden groynes are less durable and tend to reflect, rather than absorb energy. Gabions can be useful as temporary groynes but have a short life expectancy. Groynes along a duned beach must have at least a short "T" section of revetment at their landward end to prevent outflanking during storm events. The revetment will be less obtrusive if it is normally buried by the foredunes.

Beach recycling or nourishment is normally required to maximise the effectiveness of groynes. On their own, they will cause downdrift erosion as beach material is held within the groyne bays.


BEACH DRAINAGE
Appropriate locations Low tidal range sand beach sites with a high amenity value, low to moderate wave energy
Effectiveness Increases upper beach width and therefore dune stability, variable life expectancy.
Benefits Non-intrusive technique resulting in wider, drier beach
Problems Storm erosion of beach is likely to damage the system

General description
River and Sea Erosion
Beach drains comprise perforated land drain pipes buried below the upper beach surface, and connected to a pump and discharge. The concept is based on the principle that sand will tend to accrete if the beach surface is permeable due to an artificially lowered water table. The system is largely buried and therefore has no visual impact.
Function
Mild upper beach and dune erosion can be controlled by beach drains. The system actively lowers the water table in the swash zone, thereby enhancing the wave absorption capacity of the beach, reducing sand fluidisation and encouraging sand deposition. The deposited sand forms an upper beach berm that protects the dune face during storm events that might otherwise cause erosion.

Benefits are greatest on micro-tidal (<2m range), high value amenity beaches where landscape issues preclude the use of other management approaches. Important backshore assets should not rely on drainage systems for erosion protection during storms, even as a temporary measure.


ROCK REVETMENTS
Appropriate locations Sites suffering severe and ongoing erosion where important and extensive backshore assets are at risk.
Effectiveness Good long-term protection. Can be extended or modified to allow for future shoreline change. Unlimited structure life.
Benefits Low risk option for important backshore assets. Permeable face absorbs wave energy and encourages upper beach stability.
Problems Strong landscape impact. Can alter dune system permanently as sand tends not to build up over the rocks if beach erosion continues.

General description
River and Sea Erosion
Rock revetments may be used to control erosion by armouring the dune face. They dissipate the energy of storm waves and prevent further recession of the backshore if well designed and maintained. Revetments may be carefully engineered structures protecting long lengths of shoreline, or roughly placed rip-rap protecting short sections of severely eroded dunes.

Though offering long-term security, the landscape impact and damage to habitat are considerable.
Function
Rock revetments are widely used in areas with important backshore assets subject to severe and ongoing erosion where it is not cost effective or environmentally acceptable to provide full protection using seawalls he function of permeable revetments is to reduce the erosive power of the waves by means of wave energy dissipation in the interstices of the revetment.

Permeable revetments can also be built from gabions timber) or concrete armour units. Concrete units are normally too costly for use as dune protection, but may be appropriate where high value back shore assets must be protected and armour rock is difficult to obtain. They are often considered to be more unattractive than rock.

Revetments may not prevent on going shoreline recession unless they are maintained, and, if necessary, extended. If the foreshore continues to erode, the rock revetment may slump down, becoming less effective as a defence structure, but will not fail completely. Repairs and extensions may be necessary to provide continued backshore protection at the design standard.


TIMBER REVETMENTS
Appropriate locations High value sites suffering modest and periodic erosion.
Effectiveness Provide good protection if only occasionally exposed to waves. 5-30 year life.
Benefits Normally acceptable to the public. Less expensive than seawalls or rock revetments
Problems Limited life, particularly where exposed to wave action. Visually intrusive. Alters beach-dune processes as sand interchange is disrupted.

General description
River and Sea Erosion
Timber revetments can range from substantial, impermeable breastwork to temporary permeable upper beach wave barriers. The former is a final line of dune erosion protection, while the latter serves to partially dissipate wave energy before it reaches the dune face.
Function
Timber revetments have been widely used in the UK for coast protection where the costs or impacts of a seawall may have been unacceptable. Construction flexibility allows timber revetments to serve various purposes. They can provide a partial barrier to wave energy when built as a permeable "fence" along the upper beach. Alternatively they can form a final wave protection wall when built as an impermeable vertical breastwork along the dune face.

Temporary structures can be built relatively cheaply of pressure treated softwood but more substantial structures are usually built of imported hardwood. Concerns over the sustainability of hardwood sources have increased material costs considerably, making it unlikely that large scale timber defences will be used in the future. Timber is now only likely to be viable for smaller schemes in relatively low energy areas. On an open beach exposed to large storm waves, hardwood structures will be abraded, giving a life expectancy of only 15-20 years. Within estuaries or on low energy beaches the timber may last 25-30 years before abrasion and wood boring invertebrates cause significant damage. Softwood structures are likely to have a life of only 5-10 years.


IMPERMEABLE REVETMENTS AND SEAWALLS
Appropriate locations Exposed frontages with extensive and high value backshore assets.
Effectiveness Provides good medium term protection, but continued erosion will cause long term failure (30-50 year life expectancy).
Benefits Fixed line of defences allowing development up to shoreline. Allows amenity facilities along backshore and easy access to beach.
Problems Continued erosion may cause undermining and structural failure. Complete disruption of natural beach-dune processes.

General description
River and Sea Erosion
Impermeable revetments are continuous sloping defence structures of concrete or stone blockwork, asphalt or mass concrete. Revetments are built along the dune face, preferably above the run-up limit of waves under normal conditions. Where frequent wave attack is anticipated, the revetment may be topped by a vertical or recurved wall to reduce overtopping.

Seawalls are near vertical structures of concrete, masonry or sheet piles, designed to withstand severe wave attack. Their use was popular in the past but they are now normally considered to be costly, detrimental to the stability of beaches and unsuitable for erosion management along a dune shoreline.
Function
The rock armour was placed after beach lowering exposed the toe of the revetment. The good medium term protection of such structures has to be balanced against considerable landscape impact and habitat damage.

Impermeable revetments provide a fixed line of defence for frontages with high value backshore assets. They are intended to withstand storm wave attack over a life expectancy of 30 to 50 years. Amenity facilities such as promenades, slipways and beach access steps can be built into the revetment.

Revetments will severely disrupt natural beach-dune interactions, and should not be used on frontages valued for natural heritage. Ongoing beach erosion may result in undermining of the revetment toe, leading eventually to structural failure or the need for repairs and extensions. In common with many other fixed structures the natural interchange of sand between beach and dunes is prevented with the consequent loss of transitional habitats.


NOVEL COAST PROTECTION METHODS
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 or breastwork
  • River and Sea Erosion
  • Artificial seaweed
  • Seaweed planting
  • Bubble barriers
  • Alternative breakwaters
  • Sunken vessels
  • Tyre revetments
  • Interlinked concrete block mattresses
  • Bitumen spraying

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 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.


Bubble barriers
River and Sea Erosion
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:
  • Layered plate frameworks
  • Floating breakwaters
  • Perforated caissons
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.

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