Summary 14: ROCK REVETMENTS
|Appropriate locations||Sites suffering severe and ongoing erosion where important and extensive backshore assets are at risk.|
|Costs||High, but with relatively low maintenance (£100,000-£300,000/100m length).|
|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.|
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.
Major rock armour revetment in front of dune system. Though offering long-term security, the landscape impact and damage to habitat are considerable.
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 (Summary 16). The 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 (Summary 8), timber (Summary 15) 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.
Rock revetment schemes can have a significant impact on the shoreline and should not be implemented without specialist assistance from a competent coastal consultant and contractors. Information on the design of rock structures is available from the CIRIA/CUR “Manual on the use of rock in coastal and shoreline engineering”. The accompanying figures provide initial guidance but this should be confirmed for each site.
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, provided that it is placed as at least a double layer, with a 1:1.5 to 1:3 face slope, and there is an acceptance of some risk of failure. Rock size may need to increase if the beach fronting the structure is expected to drop below HWMOST.
Structure face slopes are a compromise between flatter faces that absorb more wave energy, and therefore suffer less toe scour and allow use of smaller rock, and steeper faces that give the structure a smaller footprint and require less rock volume. A slope of 1:2 is a reasonable compromise and is in keeping with natural dune slopes.
The structure should be constructed within a shallow trench and a geotextile filter should be laid under the rocks to prevent the migration of sand upwards and the settlement of the rocks into the beach. The geotextile should be wrapped around the base layer of rocks, and the rock toe should be set below the lowest expected beach level.
The length of the structure must be sufficient to protect the backshore assets at risk. To avoid localised scour the structure ends must return into the eroding dune face over the final 20m-40m and should be buried by as much as 5m-10m, depending on the expected rate of future erosion. The face slope over this final section can be flattened to 1:3 or 1:4 to increase wave absorption. The revetment length may need extending from time to time as erosion of the adjacent dune frontages may continue.
The structure crest elevation must be above the wave run-up limit during storms to prevent further dune erosion. During very extreme storms some overtopping damage will be inevitable, and the designers must determine the acceptable risk depending on potential damage and the probability of extreme events.
If the beach-revetment interface is well above normal spring tide levels there may be opportunities to use beach recycling (Summary 5), fencing and vegetation transplanting (Summaries 2 to 4) to encourage dune growth. However, as revetments are only likely to be used where erosion problems are severe, then it is unlikely that this opportunity will arise. In the latter case fencing and transplanting should be used along the structure crest to soften the landscape impact and encourage dune recovery.
Large rock revetments in areas open to the public will be a safety hazard. To increase hydraulic efficiency the rocks should be placed randomly to form a rough surface with large voids. The rocks may move when walked on and the voids may be large enough to fall or climb into, and may result in children becoming trapped or injured. Where structures extend down the shoreline below the normal spring tide levels the lower rocks will be covered by algae and other marine growth, and may be extremely slippery.
Natural recovery has allowed dunes to reform over the rock revetment. The revetment crest forms a shoreline path.
Safe access routes, usually concrete steps with hand rails, should be built at intervals across large revetments and should lead to controlled paths through the dunes. The crest of large rock revetments should be blinded, with the interstitial voids infilled with smaller rock to form a reasonable surface for safe walking. During severe storms this surface may be damaged and need maintenance to replace dislodged rocks and refill voids. In all cases the structures should be well signed to warn the public of hazards and discourage access except at controlled points.
The above discussions relate to large, engineered revetments. Less substantial defences may be formed as rip-rap slopes, but only in low energy situations. Widely graded rock from small boulders up to armour rock can be placed along estuary shores or well protected coastal sites. This approach may well be subject to regular storm damage, requiring maintenance to reform the slopes. Necessary maintenance work may well be harmful to the environment as heavy equipment will be active on the beach and may need to gain access through the dunes. Burial of the rip-rap slopes will reduce the visual impact, while fencing, thatching and transplanting may encourage covering dune growth.
Construction costs for revetments are mainly dependent on structure dimensions, but can be heavily influenced by the availability of suitable rock and transport methods. Rock structures can be assumed to have an unlimited life with respect to economic assessments, while smaller rip-rap slopes will require regular maintenance costs to be included in the budget.
Abrupt ending to rock revetment provides focus for future marine erosion, exacerbated by trampling and potential blow-out formation.
The construction of any substantial defence along a dune face will have a significant impact on the landscape and on the natural interchange of sand between beach and dune. The natural succession of dune habitats from foredunes back to grey dunes or machair will be disrupted. Sand can be blown from the beach and over the structure to reach the dunes, but cannot be returned to counter erosion of the upper beach during storms. The consequences can be increasing shoreline recession, with the need to extend the revetment to cope with increasing wave attack. Where the revetment is built high on the beach face the erosion pressures are much reduced.
Public access will be disrupted and may be hazardous unless steps are provided.
Erosion may well continue along adjacent frontages leaving the revetment seaward of the general line of the shore and exposed to ever larger waves. Again this may necessitate on-going extension and upgrade of the defences.
Best practice and environmental opportunities
Rock revetments provide robust, long term protection for important backshore assets. The revetment crest can form a public walkway, reducing trampling of the dunes. Inclusion of safe access routes down to the beach will improve amenity value.
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 rock revetments.
- Further detailed guidance on the design of rock structures is available from the CIRIA/CUR “Manual on the use of rock in coastal and shoreline engineering”.
- Revetment design must anticipate ongoing erosion that may result in toe scour, overtopping or outflanking and may cause partial structural failure.
- Where possible the revetment toe should be at HWMOST or higher, and recycling, fencing and transplanting should be undertaken to establish a new line of foredunes in front of and over the revetment. These dunes will reduce visual impact, providing additional erosion protection and re-establish a natural succession of dune habitats from the shoreline to the backshore.
- Voids between armour rocks can be filled along the crest to provide a public walkway.
- Steps with handrails should be built into the revetment at intervals to provide safe public access to and from the beach.
- 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.