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

Summary

As for ‘sedimentation fences’, the use of breakwaters as an exclusive technique to prevent saltmarsh erosion has declined in recent years. Difficulties in obtaining planning permission and the move towards soft engineering approaches has led to the decline of this technique. It is now typically used only in combination with a number of other techniques on a small scale, rather than as an exclusive stand alone technique.

Description

Offshore and nearshore breakwaters can encourage the development of a stable saltmarsh/mudflat profile through a reduction in wave energy. Several types of breakwater have been used for general shoreline protection, which largely take the form of submerged rocky reefs offshore, using a shore-parallel line of individual islands. An alternative, cheaper option to rock breakwaters is geotextile tubes. They are long tubes made from strong geotextiles, which are filled in-situ to form breakwaters, groynes or levees. They can be used in water up to a metre deep and do not present the same difficulties in equipment access that rock or concrete structures face, with costs approximately a third cheaper (Living with the Sea, 2003).

In some cases a breakwater may be combined with brushwood fences, which connect the breakwater to the shore to act as a large-scale polder. The use of this technique in high energy wave climates, however, means that fences, if used, can be subject to severe damage. Breakwaters may also be combined with foreshore recharge and vegetation planting to enhance their overall effect.

They are usually positioned at or near low water to encompass as much of the intertidal profile as possible and provide protection for most of the tidal cycle. This has the advantage of allowing the circulation of sediment between the marsh and mudflat and the intertidal profile to respond to short term changes in wave energy. Where breakwaters are used without shore-connecting structures, there is also less disruption of longshore sediment transport processes.

Breakwaters, once installed, require less maintenance than brushwood fences and polders, although repeated assessment should be made of their stability or requirement for re-orientation.

Constraints

Given their potential influence on coastal process, the biological resource (which could be beneficial in certain circumstances) and the landscape, schemes such as the placement of submerged Thames lighters as breakwaters at Horsey Island require significant scientific justification in order to achieve consent (where this included detailed modelling the down-drift effects of the lighters). With the introduction of legislation such as the Conservation (Habitats &c.) Regulations 1994, it is likely that it would be difficult to obtain consent for such an approach today (Mark Dixon, pers. comm.).

In general terms, hard engineering may not provide cost-effective and environmentally acceptable solutions to prevent saltmarsh erosion. Such techniques often conflict with “natural” processes and can require expensive repairs and regular maintenance in order to provide an adequate level of coast and flood protection from the effects associated with climate change.

Potential effects

The potential impacts of offshore wave breaks are similar to those associated with sedimentation fields, including visual intrusion, smothering of the invertebrate communities if accretion rates are high, local scour around the structures and potential hazard to navigation. Approaches such as the sinking of barges, in particular, can have an impact on the foreshore, as the fill material may be excavated from the landward side of each barge, removing this material from the mudflat.

Monitoring

It is important to determine the most suitable distance offshore, orientation and spacing of breakwater structures by modelling prior to installation. It may also be necessary to re-orientate breakwater structures as offshore conditions change, although this can be expensive. The minimum data requirements to assess site suitability are, therefore:

• Historical analysis of maps and aerial photographs to determine the rate of saltmarsh retreat and changes in the high and low water marks.
• Topographic contour mapping of the foreshore (mudflat and saltmarsh), corrected to Ordnance Datum, to give an elevation that may be related to tidal inundation.
• Measurement of the wave environment and tidal regime.
• Modelling of the wave climate, tidal currents and sediment transport to determine orientation and spacing, requirement for shore-connecting groynes or suitability of foreshore recharge.

Post scheme monitoring should be similar to that recommended for brushwood fences and polders as the aim of enhanced accretion on the foreshore is the same. In addition, however, the wave climate within and around the breakwater should be measured at intervals to check the design functions and ensure that it is still functioning as required.

Locations

Breakwaters have been placed offshore at various locations in Essex, including Horsey Island and on the Dengie Peninsula. For more information regarding specific schemes at Dengie, the reader is referred to Hodder and Burd (1990).  

Further information

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.

Simm, J.D., Brampton , A.H., Beech, N.W. & Brooke, J.S. (1996). Beach Management Manual. CIRIA. Report No 153.