Based on kindly provided information by the Scottish Natural Heritage
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.
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.
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.
Methods
Small rock headland schemes can be implemented without specialist assistance, but normally the services of a competent coastal consultant and contractors are required. As with all rock structures on the shoreline the rock size, face slopes, crest elevation and crest width must be designed with care. 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.
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 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.
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.
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.