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 13: BEACH DRAINAGE

Appropriate locations	Low tidal range sand beach sites with a high amenity value, low to moderate wave energy
Costs	Low to moderate, with high maintenance (£5,000 - £20,000/100m length, plus running costs).
Effectiveness	Increases upper beach width and therefore dune stability, variable life expectancy.
Benefits	Non-intrusive technique resulting in wider, drier beach
Problems	Storm erosion of beach is likely to damage the system

General description

Beach drains comprise perforated land drain pipes buried below the upper beach surface, and connected to a pump and discharge. The concept is based on the principle that sand will tend to accrete if the beach surface is permeable due to an artificially lowered water table. The system is largely buried and therefore has no visual impact.

Function

Mild upper beach and dune erosion can be controlled by beach drains. The system actively lowers the water table in the swash zone, thereby enhancing the wave absorption capacity of the beach, reducing sand fluidisation and encouraging sand deposition. The deposited sand forms an upper beach berm that protects the dune face during storm events that might otherwise cause erosion.

Benefits are greatest on micro-tidal (<2m range), high value amenity beaches where landscape issues preclude the use of other management approaches. Important backshore assets should not rely on drainage systems for erosion protection during storms, even as a temporary measure.

Active drainage systems have been found to enhance sand deposition at sites in the USA and Europe. Trial sites in the UK have had a mixed reception, partly as a result of poor site selection, inadequate design and lack of management. Installation costs can be relatively low, but maintenance and management commitments are high. Although the drains should increase upper beach volume during low to moderate wave conditions they will not be significantly effective during storms, with the result that beach draw down may lead to exposure of the pipework and system failure.

Methods

Beach drainage systems are relatively simple to install but should always be designed by a competent hydraulic consultant. Little information is available to guide designs, with the exception of that provided by organisations providing a patented installation service. In simple terms, perforated plastic drain pipes with a geotextile sleeve are laid into excavated trenches within the high tide swash zone. The pipes are connected to a pump and discharge system. Pipework must be laid at least 1m below the expected minimum beach level, but not so deep as to be ineffective at draining the surface layer of the beach. The geotextile sleeve serves to filter sand from the seawater collected in the drains. A surround of gravel has sometimes been used to improve flows. Pumping facilities must be appropriately housed and represent the major system installation and maintenance cost. Discharge of collected water should be designed to minimise any interference with natural beach processes.

Consideration must be given to the real time control of the pumping facilities. The system will only operate efficiently while the pipework is within the swash zone: at lower tide levels no wave driven sand will reach the system’s area of influence, and once the still water level has risen over the drains the pumps will be overwhelmed by the total saturation of the beach. A water level sensitive pump control is therefore required, although it would also be possible to produce a time switch system based on predicted tide levels. Regular beach monitoring and system management are required to obtain optimum results from a drainage scheme.

The system will be most effective in areas with a low tidal range (less than 2m) in which the pump would be active for a large part of each tidal cycle. Most of the Scottish coast has a tide range of at least 3m, with a 4 - 5m range being common, so drains installed on or above high water line of the beach will be redundant for at least 75% of each year. Drainage systems will also be most effective over relatively short lengths of shoreline. A single system should be capable of achieving some benefits over lengths of 100m to 400m. Small embayments or discrete lengths of longer frontages could be appropriate sites.

Associated vegetation transplanting and fencing/thatching (Summaries 2, 3 and 4) will enhance dune growth. Beach recycling (Summary 5) may also be beneficial to the initial installation or to restoring the system if pipework is exposed by storm erosion.

Costs for drainage schemes are dependent on the number of lines laid, the problems of providing appropriate pumping and outfall facilities and the amount of maintenance required to keep the system active.

To date no information is available on which to assess life expectancy. With regular maintenance and repairs to storm damage, a system could remain active for at least 10 years before requiring a major rebuild. Sand clogging within the pipework is likely to be a limiting factor on life expectancy, assuming storm damage can be avoided.

Impacts

Construction of a drainage system will require trenching of the upper foreshore to a depth of 1m – 2m. Once installed the system should be largely invisible, although storm erosion of the upper beach may expose the pipework. Permanently visible elements will be the pump housing and discharge pipes. These may be incorporated into existing shoreline features in some instances, but will otherwise be intrusive on the landscape.

Best practice and environmental opportunities

A successful beach drainage system will increase the elevation and width of the upper beach, providing an improvement to amenity value and encouraging the formation of foredunes. The raised upper beach will provide storm protection to the dunes. Recycling or nourishment can initiate beach level increases, and fencing, thatching and transplanting can encourage dune growth.

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 beach drainage:

• Trench excavation must be sufficiently deep to avoid pipework becoming exposed during storms.
• Pumping and outfall works should not be visually intrusive or impact on natural beach processes.
• Systems must be regularly monitored to ensure that pumping is optimised for greatest effect.
• If erosion continues despite the installation of a drainage system, pipework should be removed before it is exposed and damaged by storm waves.
• Where possible recycling, fencing and transplanting should be undertaken to establish a new line of foredunes to landward of the drains. 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.
• If the system is effective in accreting new foredunes, further lines of drain pipes should be installed and redundant lines disconnected from pumps.

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