5.2 Engineering considerations

There are a number of engineering issues that need to be considered prior to selection of an appropriate approach and option. These include:

5.2.1 Materials

Dune management may require anything from marram glass transplants and brushwood up to 6 tonne armour stone and reinforced concrete. Availability, applications and costs will vary depending on location, and may dictate the viability of a scheme. Other considerations governing choice of materials will be appearance, environmental impact, life expectancy and public safety.

Vegetation transplanting, thatching and fencing may well be considered as a part of any scheme, no matter how small or large. Transplants for a small scheme over tens of metres may well be available at no cost from the local dune system, but schemes covering longer lengths of shoreline will probably need a commercial supply or the development of a managed nursery. Either way, supplies will be limited and work will have to be planned well in advance. Fencing or thatching supplies should be readily available at any location. Chestnut palings or synthetic fabrics can be purchased and delivered to any site in the required quantities, though transport costs to islands may be high. Thatching materials, such as brush wood or forestry waste, are often available for the cost of transport, but supplies may be limited on exposed west and north coast sites. These materials have a life expectancy of less than 5 years and are likely to need regular maintenance to retain effectiveness, an acceptable appearance and minimal hazard to beach users.

Sediment for re-cycling or nourishment can be a problem. Re-cycling involves moving material from a point of accretion to a point of erosion within a local process area. The accretion point may be further along the shoreline, for example next to a breakwater or at an estuary mouth, or it may be down the beach face at a low tide bar. Prior to any detailed consideration of hydraulic or ecological issues, the ownership and licensing arrangements for re-cycling must be established. Much of the Scottish foreshore up to HWMOST belongs to the Crown Estate, but other owners include Unitary Councils, private estates and the National Trust. The beach above HWMOST normally belongs with the adjoining backshore property. Movement of sediment across any ownership boundaries may lead to conflicts unless early consultation has established a consensus. Before implementation appropriate legal consents (Chapter 3) will have to be sought.

Wherever the source of the re-cycled sediment it is critical to establish whether removal will create a new erosion problem or whether the impact will be negligible. It is also important to establish that the source material is compatible with the erosion area in terms of hydraulic response, impact on the ecosystem and impact on recreation. Annual, or even post-storm, re-cycling may be required to provide on going protection.

Beach nourishment uses material imported from outside the local coastal process unit. Sources may be quarries, licensed dredging areas or spoil taken from navigation dredging (Plate 22). Quarried sand or gravel is often appropriate for smaller schemes that are close to the quarry or have very easy road access. Larger schemes normally rely on dredged material transported by sea and pumped ashore via floating pipelines or directly from the vessel to the beach. Licensed dredging areas are limited to a few estuary sites around Scotland and schemes will be competing with the aggregate industry for both sediment and delivery vessels. In some locations material may be available from dredged harbours and navigation channels, but this source is less reliable in terms of available quantities, material grading and quality. High proportions of potentially contaminated muds may render this source inappropriate, especially if dredged from estuarine locations. As with re-cycling, nourishment is likely to need regular top ups to maintain effectiveness as a dune protection measure, making long term management costs higher than might be expected.

Plate 22

Plate 22 Sand for a recent beach renourishment scheme at Montrose was obtained by re-using uncontaminated maintenance dredgings from the adjacent South Esk channel instead of dumping these at sea.

Sand bag structures normally make use of sturdy geotextile bags, filled on site using local sand. In most cases both the bags and sand are easily available. As the bags are liable to damage, both from natural causes and vandalism, structures are best buried with re-cycled sand, only to be exposed during extreme storms when they will act as a final line of defence. Damaged bags will release the enclosed sand harmlessly back to the beach, leaving only the geotextile to be retrieved. Structures are likely to have a life of only about 10 years.

Rock filled gabion baskets have frequently been used for coastal defence, and have earned a poor reputation. Gabions have been known to fail when used in unsuitable locations, placed and filled with insufficient attention to recommended construction practices, or when managers have not attended to proper maintenance. Damaged baskets release cobbles or quarried rock onto the beach, and the baskets themselves become a hazardous eye-sore of rusting wire. However, when designed, built and maintained correctly gabions can provide good service with minimal ecological or visual impact. Life expectancy before a major re-build can be as much as 20 years, though 10 years is more likely. Gabions will last longer, have less environmental impact and be less intrusive on the coastal landscape if they are buried into the dune face, only reappearing during severe erosion events. Under accreting conditions gabions can be buried by wind blown sand, and will allow dune vegetation to be re-established. On open coasts, gabions may only be appropriate as sloping revetments. In estuaries, where wave attack will be less severe, they can also be used to form other structures such as groynes. Baskets and appropriate rock are easily obtained for most sites.

Rock, from small boulders to large armour stone can be used effectively in erosion protection schemes. Established design guidance is available to ensure that rock structures are built to give optimum performance and cost (CIRIA/CUR, 1991). Structures may include headlands, revetments, breakwaters, reefs or groynes. The required rock sizes will vary according to structure function, wave conditions, water levels, structure slopes, permeability and acceptable damage criteria. Rock has the advantage of durability and flexibility, meaning that structures can suffer some damage without failing and can be rebuilt or even removed if necessary. As with gabions, rock revetments can be buried to reduce their visual impact under normal conditions, but may be re-exposed during storms. In Scotland, good quality rock is generally available and will have an effectively unlimited life.

Timber structures are less popular today than in the past. Softwood only lasts a few years in the harsh and abrasive marine environment. Hardwood lasts longer, but is expensive and should only be sourced from acceptably managed forests. However, timber is a flexible building material, and can be used to form solid or permeable revetments, vertical breastwork and groynes. A 20-30 year life would be considered successful. Impermeable structures built in areas subject to regular wave activity may suffer from localised toe scour that may cause structural failure.

Solid revetments and seawalls are not normally considered appropriate for dune protection, but can be built from asphalt, blockwork, pre-cast concrete units or in situ concrete. These materials are readily available in urban areas but are costly to transport to remote locations. Structures can be designed to enhance public access to the foreshore and can accommodate promenades and seafront roads, but they normally disrupt the natural dynamism of dune systems and replace it with an artificial shoreline. Construction costs are very high and the services of civil engineering consultants are required. Assuming no major hydraulic failure these structures should have a design life of up to 50 years.

Other materials seen along the shoreline include building rubble, broken concrete slabs, re-cycled tyres, patented mattresses of linked concrete blocks, re-cycled construction materials such as concrete pipes, slag, etc. Apart from the block mattresses, the costs of these materials are very low, as are the aesthetic qualities. If the dumped materials are sufficiently large and robust (such as broken concrete slabs) then they may provide a long lasting defence. More often the materials are unable to withstand wave attack and break down rapidly to be spread across the beach. Patented concrete block mattresses are not low cost, but suffer equally from rapid damage as they are easily undermined or lifted by hydraulic forces.

5.2.2 Transport and placement

A significant proportion of the costs for most coast protection materials is in the transport and placement. Work on dune systems can impose additional costs due to concerns over destruction of landforms and habitats, and the problems of working in remote locations. Delivery from the sea of bulk materials (rock or beach sediment) is often preferred as backshore damage is minimised, although land access will still have to be provided for plant, labour and additional materials.

Haul roads will have to be built across the dunes unless access can be provided from an existing route along the shoreline. Delivery from the sea avoids these problems, but will suffer from nearshore navigation hazards, weather downtime and tidal restrictions.

5.2.3 Costs

The costs of dune erosion management can vary from almost nothing to several million pounds per kilometre. Major cost elements are labour and materials for initial works, with secondary elements being preliminary investigations, consultant’s fees, pre- and post-project monitoring, site equipment, site access, permissions and ongoing maintenance.

Small dune grass transplanting or thatching schemes can be undertaken by unskilled volunteer labour using locally available materials. As the consequences of inappropriate implementation are minimal, there would be no need to undertake significant preliminary investigations, involve consultants or establish any rigorous monitoring programmes. Costs would be very low, but the life of the scheme may be short and the benefits very localised.

Plate 23

Plate 23 Where major industrial facilities are at risk of erosion, the economic benefits of protection will usually outweigh the costs.

Larger schemes involving the transport and placement of rock and / or imported beach material, possibly combined with transplanting, fencing and recreational management will need the services of specialists. Preliminary investigations, benefit cost studies, site monitoring, legal procedures, tender preparation and long term management planning will all need the attention of a competent coastal consultant. Contractors will need to source and transport materials, provide acceptable site access routes for heavy plant, ensure site safety for staff and the public, and make good any damage to the environment.

5.2.4 Environmental impacts

Erosion management operations may have unwanted impacts on the ecology, geomorphology and appearance of a dune system. Decisions on the most appropriate management approach at a given site should be driven in part by the desire to minimise these impacts so as to preserve the natural characteristics of the dunes. It is important to bear in mind that erosion of dunes is a natural process and should not normally be regarded as a problem. Problems only arise when erosion threatens human activities or assets, or when the erosion is the result of human interference with coastal processes along an adjacent frontage.

All human activity within a dune system will cause some damage. Simply walking over the dunes to reach the beach will damage the vegetation, and repeated use of an uncontrolled track may well result in a blow out through the yellow dunes. Moving machinery through the dunes will cause much greater damage, through direct destruction of vegetation, disturbance of previously stable sand and compaction of soils. Dune slacks are equally susceptible to damage as the dunes themselves. Heavy machinery working in the intertidal area, particularly in estuaries, will also damage benthic communities, although these are generally quick to recover.

Dune grass planting and thatching are generally seen as beneficial, but care should be taken not to introduce non-indigenous seeds or shoots that may change the composition of the dune flora; sea buckthorn (Hippophae rhamnoides) is particularly invasive when used for thatching. Fencing is also seen as generally beneficial, but lack of planning for suitable public access may encourage people to make their own routes through the dunes, while lack of maintenance may leave derelict fences as an eyesore on the coastal landscape.

Construction of continuous defences along an eroding dune face will dramatically change the coastal landscape and will disrupt the natural interchange of sand between beach and dune (Plate 24). This should be avoided if at all possible as the impacts are effectively irreversible. Seawalls and large revetments generally allow some wind blown sand to reach the dunes, but prevent the return of sand to the beach during storm erosion events. The reduced amounts of blown sand prevent grasses like marram from thriving, and the natural succession of dune habitats from unstable foredunes, through marram dominated yellow dunes to stable grey dunes and machair tends to break down to a simple jump from upper beach to stable backshore vegetation. Interspersing lengths of protection with open dune faces retains at least some of the natural succession and allows natural cycles of erosion or accretion to continue. Wherever possible any form of continuous hard structure should be placed as a final line of defence along the backshore rather than within the active foreshore. With the exception of artificial reef breakwaters all forms of discontinuous hard structures should also be built high on the beach and certainly above HWMOST.

Plate 24

Plate 24 ‘Hard’ structures of rock or concrete may offer greater medium term security than most other defences but have a strong landscape impact and obscure or destroy backshore habitats and landforms. Impacts are effectively irreversible.

Increasing the width and elevation of the upper beach and dune face by recycling or nourishment is an attractive option for erosion management, but has environmental drawbacks. The physical and chemical properties of the new material should be consistent with the existing beach and dunes (Plate 22). Coarser material (shingle) may be more stable than sand, but will increase the slope of the upper beach and change the appearance of the shoreline. Finer material (silt) mixed in with imported sand will tend to be taken into suspension by wave action and removed from the beach; the increased suspended load can be damaging to nearshore fisheries and benthic communities. Sediment with a different chemical composition (e.g. acidity) or from a heavily polluted source (e.g. dredged from a port channel) may have an adverse effect on dune vegetation. Dumping and spreading of the new material may well damage the existing intertidal and dune face communities by smothering or by compaction under the wheels of heavy earth moving plant. Any large schemes involving recycling or nourishment should be preceded by an environmental impact assessment to establish the significance of these impacts on the local site.

Finally, consideration should be given to the full life environmental impacts of proposed management operations. The manager must consider not only the local short term impact of a scheme, but also:

Management plans should allow for these environmental impacts during the decision process, particularly where costs are being passed on to future generations. Mitigation measures and good working practices to minimise impacts should be built in to designs, agreed with contractors and monitored rigorously during initial and ongoing operations.

5.2.5 Work windows

Coastal works are affected by limited work windows. Seasonal conditions often restrict work to the spring and autumn: in winter the weather may be too harsh and the days too short, while in summer work may disrupt recreational use of the beach. Daily tidal cycles will restrict work below the high tide line and access along the beach, although dune face work may be largely unaffected. Other restrictions may be imposed by natural events such as bird nesting or migration, or by optimum seasons for vegetation transplanting or dune recovery. Each site will have its own set of restrictions, and these may well influence decisions on the initial scheme or the long term management options.

5.2.6 Maintenance

Short term works such as transplanting, fencing, re-cycling, sand bagging and even gabion placement will need a defined maintenance and replacement programme as part of the preliminary scheme appraisal. Post-storm, seasonal or annual inspections will be required, followed by appropriate maintenance and repair work. This commitment must be costed and programmed from the outset if erosion is to be managed effectively.

Other more substantial schemes may require a much lower level of long term commitment, but will have to be monitored to ensure ongoing effectiveness. At many sites foreshore erosion will be an ongoing process and may cause local scour or general beach level reduction, resulting in structural instability. Solid defences may need to have toe aprons added to prevent undermining, while gabions may deform to the point of basket failure. Rock structures are unlikely to collapse, but may need some re-shaping or extension to maintain the expected standard of defence. Monitoring may also reveal outflanking problems as the dune face at the end of a structure suffers ongoing erosion, requiring lateral extensions to retain effectiveness. The economic and environmental costs of these future works must be allowed for during the selection and planning stage of scheme development. Failure to do so may place an unacceptable burden on future generations and result in the loss of important environmental assets.