Based on kindly provided information by the Scottish Natural Heritage
General description
Thatching of exposed dunes faces or blowouts using waste cuttings from forestry management, or other low cost materials, is a traditional way of stabilising sand, reducing trampling and protecting vegetation. Materials are low cost if locally available and no machinery or skilled labour is required to achieve success, but continual maintenance is important. The approach is normally carried out with dune grass planting to encourage dune stability. Thatching materials are often removed for bonfires by beach users.
Function
Well laid thatch will encourage dune recovery and will resist some erosion, but cannot prevent erosion where wave attack is frequent and damaging. The thatch reduces surface wind speeds, encouraging deposition of blown sand. Success depends on the amount of blown sand, the frequency of wave attack and the availability of vegetation. Transplanting dune grasses after thatching will enhance dune recovery and longer term stability. Continual maintenance and replenishment of cuttings is required.
Methods
Thatching has been practised for centuries and well established methods are presented in various publications. Materials can include any form of timber or brushwood cuttings, although conifer brashings (lower branches) from spruce or fir are preferred for their flat, fan shapes. Thorny species, such as sea buckthorn (Hippophae rhamnoides), have the advantage of discouraging trampling and they are hard to burn, but can be highly invasive if cuttings or seeds become established as new growth. Thatch should be laid to cover 20% - 30% of the exposed sand surface. Dune grasses should be transplanted through the thatch to promote sand retention and restoration of natural habitats. Where public pressure is significant, access routes should be left between the beach and backshore, and fencing may be required to further protect slopes.
A similar effect can be achieved by mulching the dune face with coarse vegetative matter such as straw, seaweed or reeds. Transplants can be introduced through the mulch. This approach may not be appropriate for areas with very strong winds as the material may blow away. Mulching also brings concerns over introducing foreign plant seeds to the dune ecology, but has the advantage of increasing the organic content and water retention of the dune sand.
Biodegradable mats (jute or similar natural fibres) can be laid over the surface and pinned down. Transplants can be introduced through holes. However, mats are not usually recommended for marine erosion sites as they are likely to be disturbed by waves and left partly attached to the dune face where they will be unattractive and ineffective.
Thatching should not be undertaken on steep, freshly eroded slopes. The dune face should be regraded or built out with recycled sand prior to further works. A maximum slope of 1:2 is recommended.
Thatching will be quickly damaged by wave action and should not extend seaward of the line of normal wave run-up. The approximate limit 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 at the run-up limit of the most recent severe sea.
Thatching requires regular maintenance to repair damage caused by erosion or the public, and to extend coverage as the dune system evolves. Work can be undertaken at any time of year, unless combined with transplanting when the spring is preferred. On exposed shorelines thatching may be completely lost during storms.
Costs for thatching are dependent on labour, material sources, extent of works, the need for ongoing management and the cost of ancillary works to help stabilise the dune face. Small schemes using volunteer labour and free brushwood will be very low cost. Larger schemes undertaken by contractors with large volumes of brushwood transported to site may cost up to £20,000/km, plus ongoing management costs. Cost assessments should allow for the possibility that dune thatching may be substantially damaged by the first storm event following works.
Impacts
Thatching limits public access to the dunes and beach, and alters the appearance of the dune face. Blown litter and strand line debris can become trapped. The thatch material may introduce unwanted seeds or live cuttings to the dune ecology. Regular maintenance and provision of controlled access routes will minimise adverse public reaction to the visual impact and loss of recreational areas.
Public access through the dunes and trampling of vegetation can be controlled at low cost by thatching. Wastage from forestry operations, roadside tree clearing and discarded Christmas trees can be used beneficially. Associated dune grass planting will help to stabilise the foredunes and will extend the dune habitat. Thatching is low cost and materials are degradable so failure will not jeopardise long term environmental interests.