Summary 9: ARTIFICIAL HEADLANDS
| Appropriate locations | Rapidly eroding dunes with important backshore assets at discrete intervals along the shore. |
|---|---|
| Costs | Moderate, but low maintenance (£20,000-£60,000/100m of structure, plus minor works for unprotected frontages). |
| 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
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.
Rock headland protecting a golf course tee.
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 (Summary 11).
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.
Cross-section of rock headland protecting an eroding dune face
Planview of rock headland
Methods
Small rock headland schemes can be implemented without specialist assistance, but normally the services of a competent coastal consultant and contractors are required. 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. Temporary structures can also be formed using sand bags (Summary 6) or gabions (Summary 8), although gabions can be more difficult 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, 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. Rock size may need to increase if the toe is within the zone of regular wave action.
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 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.
Concrete armour units of various types can be used instead of rock, but are normally considerably more expensive and may be considered much more visually obtrusive. The potentially greater hydraulic efficiency offered by some types of concrete units is of little importance to a dune defence structure.
Headlands should be built just seaward of the dune toe, rather than as a revetment. Increasing the distance down the beach will allow the protected dunes to retain some of their natural dynamics and appearance, and will create a more embayed shoreline, with wave energy being dissipated over a longer frontage. However if the distance is too great the structure may be outflanked, effectively forming a detached breakwater and allowing the protected dune face to be subject to erosion once more.
The approximate limits of wave run-up can be established by observing and recording the location of the strand line over Spring tide periods during both winter storms and more normal wave conditions. The toe of a freshly eroded dune face is normally just below the run-up limit of the most recent severe sea.
The length of the structure must be sufficient to protect the backshore assets at risk while adjacent dunes continue to be eroded. The structure ends must return to the eroding face, and may need extending from time to time to prevent outflanking. These end extensions can often be lower than the main structure face as they will be subject to less wave attack. By keeping the crests low there will be a greater opportunity for wind blown sand to reach the dune face.
Structures can be built to protect a single location, such as an outfall or important building, or as part of a series of headlands intended to create an embayed shoreline. Single structures may well need regular extensions to prevent outflanking if erosion of adjacent beaches is severe. Closely spaced headlands can be designed and managed to develop a stable, embayed shoreline, but each site must be considered separately by specialist consultants if a successful scheme is to be developed.
The area behind the structure can be vegetated, thatched, fenced or replenished with recycled sand (Summaries 2, 3, 4 and 5) to stabilise the dunes, and possibly encourage regeneration. This is only possible if the structure is built forward of the dune toe. If the structure is built against the dune face as a short length of revetment, the dunes will be less likely to grow forward and more erosion will occur before the adjacent shorelines begin to stabilise to a new plan shape.
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, although the benefits may be only short to medium term.
Impacts
Even though this form of defence is intended to give only partial protection to the dunes the impacts on shoreline processes and landscape will still be high, and may be unacceptable in environmentally sensitive areas. Erosion may well continue along the unprotected frontages, and, without ongoing management, the structures may be outflanked allowing erosion of the protected frontage as well.
On frontages affected by longshore transport the headlands may reduce drift rates, resulting in the erosion of downdrift stretches of coast, but helping to stabilise the updrift shore. As with all fixed dune defences, the headlands will interfere with the natural dynamic interchange of material between beach and dune. They will also influence the longshore transfer of sand, modify dune habitats, disrupt the natural landform and potentially result in localised dune face scour at their terminal ends.
Gabions forming an artificial headland and protecting outfall headworks. The gabions have been extended landward to prevent outflanking.
Best practice and environmental opportunities
Headlands can provide good protection to discrete points along the shoreline, either temporarily or over longer periods. They can also be used to transform a length of eroding dunes into a shoreline of small bays and headlands, in which part of the dune system is maintained as a dynamic system, while other lengths are artificially fixed. The width of the upper beach in the centre of the embayments may increase, providing improved recreation. The dune faces in the lee of the headlands may redevelop as blown sand can pass around the structures.
Temporary structures can be removed when no longer required, with little lasting damage to the dune system as a whole. Sandbags structures are the easiest to remove, but rock structures can also be taken away for re-use elsewhere.
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 artificial headlands:
- 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”.
- Where possible recycling, fencing and transplanting should be undertaken to establish a new line of foredunes behind the headlands and along the unprotected 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.
- Headland ends should be extended alongshore and landwards to avoid outflanking if the adjacent dunes erode rapidly. Other structure dimensions, such as crest elevation, can also be modified if monitoring indicates that the initial layout is not achieving the required objectives. Any observed storm damage, such as displaced rocks, should be rectified during maintenance operations.
- 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.