Summary 6: SAND BAG STRUCTURES
| Appropriate locations | Low to moderate energy sandy shorelines requiring lower cost, temporary defence. |
|---|---|
| Costs | Low to moderate (£2,000-£10,000/100m frontage) |
| Effectiveness | Provides short term fixed line of defence. Less than 5 year life. Burial may extend life. |
| Benefits | Low cost, low skill approach using local materials that are returned to the beach when the defences no longer required. |
| Problems | Sand bags subject to vandalism and rapid deterioration due to wave action, sunlight and public pressure. Bags are effectively impermeable and do not absorb wave energy, so beach scour may accelerate. |
General description
Sand bags of various sizes and lengths can be used to form temporary reefs, breakwaters, groynes, headlands or revetments on sand beaches. Sturdy geotextile bags are filled in-situ with local beach sand and therefore have a relatively low cost.
Function
Sand bag structures can be placed without the need for costly equipment or skilled labour. They can be used to form any form of shoreline structure but will have a short life expectancy due to lack of resistance to physical damage (wave borne debris impacts or vandalism) and the effects of UV sunlight. They are potentially most useful as a buried revetment under the dune face, where they will form a final line of protection after the overlaying sand has been eroded by storm waves. An alternative use is to form temporary headlands (Summary 9) to protect backshore assets while other, longer term, options are planned and implemented.
Methods
Bags should be filled and closed according to manufacturer’s recommendations. Care is required throughout to avoid selection should be governed by the anticipated methods of filling and placing on site. Labour intensive operations will limit bags to about 50kg and will only be appropriate for small schemes in low wave energy conditions. Large schemes will require filling and lifting equipment. A practical bag size limit is about 3m x 1.5m x 0.5m, containing about 3 tonnes of sand. Long tubes have been used in the past, but these are more likely to fail, as a single tear will affect the whole tube.
Buried sand bag revetment
Prior to construction the dune face will need to be dressed to form a plane slope on which bags can be laid evenly. This slope should not be steeper than 1:1.5. Sandbags should be stacked against the dune face, as shown on the enclosed sketch. The bags should be placed with their long axes parallel to the beach line. A minimum thickness of 2 bag widths is recommended, with a thickness of 3 bag widths for the lowest course to reduce scour. The seaward line of bags should be treated as sacrificial. The bags should be filled in-situ, by hand or by pumping in a slurry of sand and water. Fill material can be recycled or imported sand or fine gravel. The toe of the completed revetment should be landward of the limit of normal wave run-up to avoid scour problems. The crest should be about 1m above the limit of run-up during storms to avoid overtopping damage to the dune face.
The approximate limits 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 just below the run-up limit of the most recent severe sea.
Bags intended to provide a buried revetment as a final line of defence should be covered by recycled (Summary 5) or imported sand, stabilised by transplanted vegetation and fencing/thatching (Summaries 2, 3 and 4). If a good quality geotextile is used to make the bags, and if a regular recycling programme is maintained to make good seasonal erosion, then a life expectancy of 10 years might be assumed. More realistically sandbags will have a much more limited life expectancy, say 5 years at best.
Damage to sand bag structures will be most intense at the alongshore extremes due to displacement by wave action, local scouring of beach levels and vandalism/public pressure. The ends of exposed structures should be feathered smoothly back into the dune face with the last 5m-10m of bags buried by several metres to allow for some future erosion. Maintenance attention should be focused on these potential problem areas.
Costs depend mainly on labour, structure dimensions and the need to bury the sandbags. Small schemes can be undertaken with volunteer labour with a minimum of lifting equipment, while larger schemes will require contractors with heavy lifting, pumping and earth moving plant. Costs of up to £100,000/km can be expected, with an anticipated scheme life of no more than 5 years.
Impacts
Exposed sand bags are unsightly and easily damaged. As structures are effectively impermeable they will not absorb wave energy, and may cause local beach scour to accelerate. Damaged bags will release the fill material back onto the beach, but the bags will remain as unsightly debris along the shoreline. Assuming that the fill material is taken locally or is similar to the beach material then losses will be harmless.
As with all fixed defences the sand bags 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
Buried sand bags can form a useful and low cost final line of defence in areas subject to mild, seasonal erosion. They are also useful for temporary (less than 5 years) headland protection while other options are considered, planned and implemented, but they should be superseded by a longer term solution. They have no environmental benefits except that they are temporary structures, easily removed with no significant long term impacts on the physical or natural environment.
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 sand bag structures:
- Exposed sand bag structures are only appropriate for temporary protection while other management solutions are planned and implemented; buried structures have a somewhat longer life expectancy, and have less impact on habitat, landform and landscape.
- Damaged sandbags should be repaired rapidly to avoid failure of the structure, or removed to avoid any detriment to the landscape.
- Once the structures have served their intended short term function they should be removed to prevent landscape impacts and to allow natural processes to resume.
- Where possible recycling, fencing and transplanting should be undertaken to establish a new line of foredunes over the sandbags. Burial will increase the sand bag life, while the new 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.