Dredging is the general term used for the excavation of material below water level either as a maintenance activity or as part of channel enlargement works. The main purpose of dredging is either to maintain the navigation depth or the flood capacity, or sometimes both.
Based on "A T Pepper, C E Rickard (2009): 8 Works in the river channel. 36p. In: UK Environmental Agency (2009): Fluvial Design Guide (Contains public sector information licensed under the Open Government Licence v3.0.)"
Dredging is an expensive operation and can have severe environmental drawbacks. The expense is not only in the dredging operation itself but also the disposal of the dredged material, which may be contaminated and require disposal to a licensed landfill, with associated transportation costs. Dredging should therefore be avoided wherever possible, especially since it is almost never a ‘one off’ operation.
Environmental damage can be reduced by carrying out the dredging in the appropriate season, for example to avoid polluting fish spawning grounds with fine sediment disturbed by the dredging process. Avoidance of the summer season when boating and other river-based activities are at their peak is also advisable, but this may leave a narrow window for the dredging operations when other restrictions are taken into account.
The environmental impacts can also be minimised by choosing the right plant and equipment for the dredging. Mechanical removal of material from the bed (for example, using a dragline or a grab) is likely to create the most disturbance and hence sediment pollution, although measures can be taken to reduce this impact.
The sediment derived from the dredging can be re-used for DRR measures. How this can be achieved, can be read here.