A guide to managing coastal erosion in beach/dune systems

• Index
• Tables
• Figures
• Appendices

Contents

• Preface
• Introduction
• Dunes, beaches and the sea
• Long-term evolution of dunes
  • Sources of sand for dunes
  • Losses of sand from dunes
  • Consequences for dune evolution
• Short-term dune evolution processes
  • Accretion of dunes
  • Dune erosion
• Attributes of coastal dunes
• Classification of Scottish dunes
• The legal framework
• Coast Protection Legislation in Scotland
  • Introduction
  • Summary of Legislation
• Nature Conservation Issues and Legislation
• Strategic approach to dune management
• Processes
• Risks
• Funding
• Consultation
• Coastal Cells and Shoreline Management Plans
• Management options
• Possible approaches
• Engineering considerations
  • Materials
  • Transport and placement
  • Costs
  • Environmental impacts
  • Work windows
  • Maintenance
• Recommended approaches
• References and Bibliography
• Scottish regional information
• Further guidance for dune management
• Coastal processes and shoreline management
• Glossary
• Abbreviations
• Appendix 1 Erosion management options
• Adaptive management
• Dune grass planting
• Dune thatching
• Dune fencing
• Beach recycling and reprofiling
• Sandbag structures
• Beach nourishment
• Gabion revetments
• Artificial headlands
• Artificial reefs
• Nearshore breakwater
• Groynes
• Beach drainage
• Rock revetments
• Timber revetments
• Impermeable revetments and seawalls
• Novel coast protection methods
• Appendix 2 Monitoring erosion and change in dune systems by simple and specialist techniques
  • Introduction
  • Aims and objectives for monitoring
  • Priorities for measurement
  • Frequency, timing and location of measurements
  • Monitoring techniques – low cost approaches
  • Monitoring techniques – aerial photography
  • Measurements of the physical environment
  • Data Storage and Analysis
  • Data Sheets for Download
• Credits

Summary 3: DUNE THATCHING

Appropriate locations	Above normal limit of wave run-up at any location with available blown sand. Unlikely to succeed where erosion is severe.
Costs	Low, but labour intensive and requires ongoing maintenance. (£200 - £2000/100m length plus cost of transplanting and annual maintenance)
Effectiveness	Enhancement to natural dune recovery. Modest resistance to storm erosion. Enhanced by vegetation transplanting.
Benefits	Minimal impact on natural system. Materials are all natural, degradable and low cost.
Problems	Without maintenance thatching will last no more than 1 year. Materials are often used to build bonfires.

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 (Summary 2) 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, including the British Trust for Conservation Volunteers “Sand Dunes Handbook”. 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 (Summary 2). Where public pressure is significant, access routes should be left between the beach and backshore, and fencing may be required to further protect slopes (Summary 4).

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 (Summary 5) 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.

Best practice and environmental opportunities

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.

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 dune thatching:

• Further detailed guidance on dune thatching is available from several publications including the BTCV “Sand Dunes Handbook”.
• Thatch must be regularly maintained to maximise effectiveness and to minimise impact on public use and visual amenity.
• Materials must be degradable and should not introduce foreign seeds, live cuttings or pollutants that may damage the dune ecology. Sea buckthorn is a particularly invasive species and should be avoided as a thatch material if there are specific concerns about maintaining the local dune species distribution.
• Attention should be paid to areas suffering wind erosion as well as wave attack in order to prevent the formation of large blowouts.
• Vegetation transplanting will encourage dune growth and enhance the shoreline environment. Thatching without transplanting will have a short term impact only as any accumulated sand will remain unstable.

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