Summary 12: 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.|
|Costs||Moderate, but must include for recycling or nourishment (£10,000-£100,000 per structure, plus recycling).|
|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.|
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.
Recently built rock groyne at estuary mouth, constructed in association with beach renourishment of adjacent foreshore.
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 (Summaries 5 and 7) 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.
Groynes can have a significant impact on the shoreline, and schemes should always be undertaken under the supervision of a competent coastal consultant. Information on the design of rock structures is available from the CIRIA/CUR “Manual on the use of rock in coastal and shoreline engineering” with further detailed guidance on the use of groynes found in the CIRIA “Beach Management Manual”. The accompanying figures provide initial guidance but this should be confirmed for each site. Temporary structures can be formed using sand bags (Summary 6) or gabions (Summary 8), although gabions can be more to remove or relocate than rock.
As with all rock structures on the shoreline the rock size, face slopes, crest elevation and crest width must be designed with care. Rock size is dependent on incident wave height, period and direction, structure slope, acceptance of risk, cross-sectional design, and the availability/cost of armour rock from quarries. In general 1-3 tonne rock will suffice for the landward parts of the groynes, provided that it is placed as at least a double layer, with a 1:1.5 to 1:2.5 face slope, and there is an acceptance of some risk of failure. Larger rock, probably 3-6 tonne, may be needed for the more exposed body and seaward head of each structure.
Randomly dumped rock with a high void to solid ratio is hydraulically more efficient than placed and packed rock. However, rock structures on recreational beaches should be built with a view to minimising the potential for accidents involving beach users slipping between rocks.
The groynes should be built prior to nourishment, with the rocks being laid into a shallow trench. On gravel beaches a geotextile is not normally required, as upward sediment migration is less important than on a sand beach. The groyne berm should be built to the anticipated crest level of the beach. The groyne berm length should equal the intended crest width of the updrift beach. The groyne should extend down the beach at a level of about 1m above the anticipated updrift shingle beach, normally at a slope of about 1:5 to 1:10. The groyne head should extend down into the sand beach, allowing for some future erosion. On a shingle beach there is not significant benefit to creating any novel head extensions
The groyne-dune interface may need additional protection to reduce the possibility of outflanking. Short lengths of revetment, longer on the downdrift side, will ensure greater resistance to storm erosion. Where a high degree of erosion protection is required it may be necessary to construct a full rock revetment (Summary 14) to provide a fixed line of defence along the shoreline.
As a general rule, groynes should not be built on an open beach unless construction is accompanied by a commitment to regular recycling or nourishment. Without this commitment the groynes are likely to cause downdrift erosion as the upper beach becomes starved of sediment. Where there is a plentiful sediment supply, or where downdrift erosion is not considered to be a significant issue, then recycling may not be required.
Groynes should normally only be considered for beaches with a significant proportion of gravel. Structure length should extend across the full width of the steeper upper beach, allowing for beach reorientation after construction and recycling/nourishment. Further extension across the sandy lower beach is generally not effective as the sand will be transported over and around the groynes as suspended load. Groyne lengths should be reduced at the downdrift end of a series to reduce the tendency for local erosion.
Groyne spacing will depend on the nearshore direction of the dominant waves and the expected orientation of the upper beach after construction. The design of larger schemes should make use of numerical models to assess the optimum lengths and spacings.
Within estuaries groynes are used primarily to deflect tidal flows away from an eroding shoreline. To be effective structures must be large, both in elevation and lengths. Impacts can be significant on other areas of the estuary, and are difficult to predict with certainty. The services of specialist estuary consultants should be commissioned at preliminary appraisal stage.
Construction costs are mainly dependent on structure dimensions, but can be heavily influenced by the availability of suitable rock (or other material), transport and the associated costs of recycling or nourishment. Rock structures can be assumed to have an unlimited life with respect to economic assessments.
Groynes have a significant impact on the landscape and can create barriers to the recreational use of the upper beach. They often cause downdrift erosion unless there is a long term management commitment to beach recycling or nourishment. Downdrift erosion may well lead to pressure for further defence works.
Timber groynes must be built from hardwood to endure the harsh shoreline environment. Much hardwood comes from tropical sources, making it both costly and potentially environmentally unacceptable. Timber groynes tend to reflect, rather than absorb, wave energy making them significantly less effective than rock on exposed coasts. They are also more likely to structural failure due to formation of scour channels around their seaward ends.
Best practice and environmental opportunities
Provided that groynes are used in appropriate locations, they reduce dependency on regular recycling or nourishment, and therefore reduce future disturbance of the shoreline environment. Localised accumulations of beach material will encourage new dune growth. Recycling, fencing and transplanting will help to keep the revetment sections buried, thereby enhancing habitat regeneration.
All dune management schemes should observe the following guidelines to maximise the probability of success and minimise impacts on the natural and human environment:
- Each dune erosion site must be considered independently, with management approaches tailored to the specific site.
- A policy of “Adaptive management” (Summary 1) should be considered for all sites before other options are assessed.
- Work should not be undertaken unless the beach-dune system and nearshore coastal processes have been monitored over several years and a reasonable understanding of the physical and natural environment has been established. Hasty responses to erosion may prove to be either unnecessary or damaging.
- No work of a permanent nature should be undertaken unless important immovable or irreplaceable backshore assets are at risk.
- Local interest groups, such as landowners, nature trusts, fishing associations and recreational users, should be consulted early to ensure that a broad view of the shoreline and nearshore zone is considered prior to implementing any particular management approach.
- Consideration must always be given to both long term “average” and short term extreme weather and sea conditions to determine the life expectancy of any operations.
- Consideration must be given to the consequences of failure, such as construction debris spread along the beach, public safety hazards, loss of amenity access, deterioration of the landscape, etc.
- Work should be planned and scheduled to limit damage to fragile ecosystems and to recreation. Consideration should be given to vegetation, bird nesting and migration, intertidal invertebrates, fisheries, public access, noise levels and public safety.
- All site staff must be made aware of the need for careful working practises to avoid environmental damage, and to avoid hazards associated with steep and unstable dune faces.
- Temporary or permanent management access routes to the dune face for materials, equipment and labour must be planned and constructed to minimise trampling damage to the dunes and to limit the formation of blowouts. Boardwalks or other temporary surfaces should be laid and should follow the natural contours of the dunes rather than cutting straight lines susceptible to wind erosion. Fencing should be used to stabilise sand adjacent to the track.
- Public access routes to the beach should be clearly laid out and fenced where necessary to prevent trampling that may lead to blowouts.
- Educational displays at backshore car parking areas or along footpaths should be used to explain management schemes and encourage public interest and support for the management objectives.
- Warning signs should be set up highlighting the dangers of unstable dune faces, any construction work in progress or any other hazards associated with the management schemes (gaps in rock structures, slippery algal growth, buried defences, submerged structures, mud deposits, etc)
- Post project monitoring should be undertaken at least bi-annually to assess the beach-dune evolution and the success of the scheme relative to the objectives. Appendix 2 of this guide provides monitoring guidelines.
In addition to these general guidelines, the following are of specific importance to groynes:
- Further guidance on the design of rock structures is available from the CIRIA/CUR “Manual on the use of rock in coastal and shoreline engineering” and from the CIRIA “Beach management manual”.
- Groyne construction should normally be accompanied by an ongoing programme of beach recycling or nourishment. Regular monitoring and management is required to establish a successful scheme. Monitoring must include adjacent shorelines as well as those immediately within the groyne scheme.
- Groyne heights, lengths and profiles can be modified if monitoring indicates that the initial layout is not achieving the required objectives. Modification is easier to achieve with rock structures than with timber. Any observed storm damage, such as displaced rocks, should be rectified during maintenance operations.
- Timber used for groyne construction should be derived from sustainably managed forests.
- Groynes in estuaries may need navigation marks to ensure public safety.
- Where possible fencing and transplanting should be undertaken to establish a new line of foredunes along the stabilised upper beach. These dunes will enhance the coastal landscape, provide additional erosion protection and re-establish a natural succession of dune habitats from the shoreline to the backshore.
- The use of local rock should not be a requirement of design unless there are genuine landscape considerations, such as adjacent rocky outcrops; even in this instance local rock should only be used if it is readily available in the size range required and is a sound material for coastal construction.
- The use of builder’s rubble is unlikely to ever be appropriate for dune management. Most material is too small to be effective and will be drawn down the beach during any significant storm. The rubble may contain material that is either hazardous to beach users, toxic or simply unattractive. Large concrete slabs may be acceptable from an engineering perspective but are unlikely to meet approval with respect to their landscape impact or their safety for use in a public area.