Summary • Description • Constraints • Potential effects • Monitoring • Locations • Further information

Case Study

Wellhouse, West Mersea, Essex

Summary

The practice of using sedimentation fences was originally pioneered in Holland and Germany and was first applied in the UK in the late eighties/early nineties. It was originally applied in areas where ongoing intertidal erosion was a problem, particularly in southeast England. Following several experimental studies, the sedimentation field technique is believed only to be successful if the local sedimentary trend is towards accretion. In areas where the trend is towards erosion, the fields have proved ineffectual (Environment Agency, Anglian Region internal monitoring reports). Consequently, its use in recent years has declined and it is now used only in combination with a number of other techniques on a small scale, rather than as an exclusive, stand alone technique.

Description

Sedimentation fences are structures designed to slow the passage of water, thereby facilitating the deposition of sediment in suspension. Essentially there are two types; brushwood groynes and brushwood sediment fields or ‘polders’.

Brushwood groynes

Brushwood groynes generally consist of two parallel rows of wooden stakes, spaced approximately 300mm apart at 600mm intervals, driven deep into the mud. Different orientations of the fences have been employed but, in general, the best orientation is shore normal (i.e. at right angles to the foreshore). A variety of materials can be used as infill between the stakes, including willow brushwood, geotextile claddings and straw. Overall, however, brushwood has been found to be the most durable. The groynes minimise wave action, slow currents, promote sedimentation and, to some extent, delay the departure of the ebb tide. Tidal velocities are reduced by the ponding effect and the erosive effects of wave and tide-generated shear stress are diminished, thus allowing the fine-grained fraction of the sediment to settle out (Colenutt, 2001). As a result, the sedimentation of suspended matter is enhanced, both behind the groynes and in front of the saltmarsh edge.

Sedimentation polders

Sedimentation polders enclose a width of mature upper marsh together with a similar width of mudflat seaward of the marsh, by the construction of a perimeter fence. Ditches are dug in a regular pattern across the polder to collect deposited sediment which is cleared and piled on the banks between the ditches. The fields can be up to 400m square, although many of the experimental sites constructed in the UK have been smaller, varying between simple groynes 30-50m apart and larger, more complex fields 100-150m square. Gaps in the fencing along the seaward line of each enclosure allow the tide to flow into a series of channels within the area. These are maintained to control the flow and sediment deposition. The main ditches are dug perpendicular to the coast while other trenches (or grips) are dug parallel to it. The main ditches direct the waters of the flooding tide onto the upper areas of the marsh sufficiently rapidly for them to carry the sediment towards the shore, instead of depositing it further offshore (Colenutt, 2001). This approach also involves re-excavating the ditches and grips and placing this sediment in the intervening space thus, over time, the general level is raised (as the ditches in-fill and create a new surface) until the process is no longer required. The pattern of gripping can be seen clearly in some eroding saltmarshes and, in the erosional phase, can provide preferential erosion lines.

Constraints

Ongoing maintenance is essential, as the fences tend to loose the infill material which is swept away by the tide and deposited on adjacent areas of saltmarsh, potentially causing vegetation mortality if not removed immediately. As the infill and damaged stakes are lost, the fences become less effective and erosion of the accreted material occurs. The grips must also be constantly re-dug to maintain their effectiveness and prevent the sediment from being washed out of the ditches.

Potential effects

The construction of the fences can have a major impact on the environment through trampling and disturbance during construction and maintenance. The infill material can be washed out of the fences and deposited on the marsh, with potentially significant deleterious effects on the vegetation (and in extreme cases to navigation). The structures themselves can have a local impact by increasing scour immediately adjacent to the fences. There is also a visual intrusion into the estuarine landscape at low tide and, potentially, a hazard to boat traffic at high tide. Rapid accretion of sediment can cause swamping of benthic intertidal invertebrates and, thus, may reduce the overall food resource available to birds (at least in the short term). Rapidly deposited sediment can also be unstable and erode away again on higher tides.

Monitoring

Pre-scheme monitoring should try to assess the accretion/erosion status of the mudflat/saltmarsh system. This can be achieved by analysing historic O.S. maps and/or aerial photographs. For a more detailed assessment, topographic surveys could be carried out to assess changes in elevation. Note that the technique is believed only to be successful if the local sedimentary trend is towards accretion.

Locations

Several sedimentation fences experiments were trialled in the late eighties/early nineties at various locations in the UK, particularly in Essex (see Holder & Burd, 1990); although monitoring of these sites suggested that their effectiveness was limited. Examples include Cudmore Grove and various sites along the Dengie peninsula and a sedimentation fence technique implemented on the Strood Channel, Essex.

Further information

Beeftink, W.G. (1977). Salt-marshes. In: The Coastline, Barnes, R.S.K (ed.). John Wiley & Sons, Chichester , 93-122.

Colenutt, A. (2001). Saltmarsh Management Techniques. A review. New Forest District Council Coast Protection Group

Holder, C.L. & Burd, F. (1990). Overview of Saltmarsh Restoration Sites in Essex: an Interim Report. Nature Conservancy Council.

Wagret, P. (1968). Polderlands. Methuen and Co., London .

http://www.english-nature.org.uk/livingwiththesea