Based on kindly provided information by the Scottish Natural Heritage
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.
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, 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.
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. 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.
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.
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.
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.
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.