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.

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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.

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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:

In addition to these general guidelines, the following are of specific importance to sand bag structures: