Selection of dredging methods

Dredging practice and equipment has evolved considerably in recent years to increase dredging efficiency and to minimise the potential adverse effects on the environment (Bray, Bates & Land 1997; Bates 1998). To some extent the environmental effects due to the resuspension and settlement of sediments during the excavation process can be minimised by selecting the most appropriate method of dredging. A summary of the main dredging methods used in the UK, their potential to cause the resuspension of sediments and how dredging equipment can be modified to improve environmental performance is shown in Appendix L. The characteristics of the dredging sites have a significant bearing on the type of dredger which can be used and on the extent that precautions to minimise sediment resuspension are needed (Bray, Bates & Land 1997). Subject to appropriate modification, most types of dredger can be operated in a manner that does not cause excessive loss of sediment to the surrounding environment.

The type of dredger used may not be an important consideration for all dredging operations. For example when dredging in enclosed areas, such as docks or within locks, where there is little potential for any adverse effects on the wider marine environment or in highly turbid environments where any adverse effects due to sediment resuspension are unlikely. Consideration should be given to the type of dredger used where adverse effects on marine animals and plants due to suspended solids have been predicted which cannot be avoided by careful programming of the timing of the works. Assessments on the most suitable dredger to use must be made on a case by case basis, giving consideration to both practical and economic considerations. The type of dredger employed is often determined by the depth of water, scale of the maintenance operations, the type of material to be dredged, and can be a question of meeting supply and demand.

Protective silt curtains or screens can be used with certain dredging equipment (grab and backhoe dredgers) in order to decrease the amount suspended sediment being transported outside the dredging area or can be placed around sensitive marine features. The use of silt curtains is reported to considerably reduce the loss of suspended sediments from the dredge area, by up to 75% where current velocities are very low. However they are generally ineffective in areas with high wave action and current velocities which exceed 0.5 m/s.

Over recent years, certain dredging methods have been used in ports and harbours that are not presently regulated under the FEPA licensing process, such as water injection dredging or sea bed levelling (Appendix L). These methods operate by moving material from one place to another along the seabed and as sediments are not raised from the surface of the water, then strictly speaking no disposal takes place. Although the aim of these methods is to keep sediments in the vicinity of the seabed, there is potential for increased suspended sediments to occur possibly causing disturbance to marine animals and plants, especially where sediments are contaminated. Agitation dredging, which encompasses a number of different techniques, is an example of a type of operation that is outside the remit of FEPA. Unlike other types of dredging, as its name implies, agitation dredging aims to disturb sediments and raise them into suspension in order to move material through the water column. It is therefore inevitable that there will be greater increases in suspended solids and siltation levels, and subsequently the magnitude and extent of impacts on the nature conservation interests of the site may possibly be greater, although they may remain within the range of natural variation, depending on the local conditions at the site.

As with other types of dredging, where these dredging methods occur in systems with high background levels of suspended sediments there is likely to be little problem, however in other areas more caution may need to be applied particularly with regard to agitation dredging. Although, it should be noted that the amounts of material redistributed during agitation dredging may be no more than occurs during natural phenomena, such as storm events.

When these dredging methods are proposed within the harbour area, either by the port themselves or a third party, consideration by the port authority should be given to the potential affects of such an activity on safe navigation and the potential for effects on designated marine features. This should be based on information provided by those proposing to undertake the dredging, including answers to questions such as, where, when, over what area, how much material, and how often? When considering whether there are likely to be any effects on the communities of the designated features of the site, ports and harbours may consult with the country conservation agencies for advice. Any identified effects of the proposed activities on designated features should be addressed and minimised by careful operation and by planning the dredge to avoid particularly sensitive times, as described in these guidelines.

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