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.

General description

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.

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Major rock armour revetment in front of dune system. 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 (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.

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Revetment cross section

Methods

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.

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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.

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Abrupt ending to rock revetment provides focus for future marine erosion, exacerbated by trampling and potential blow-out formation.

Impacts

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:

In addition to these general guidelines, the following are of specific importance to rock revetments.