Good practice management of in-channel sediments

Description of measure

Historical modifications and activities have resulted in the removal of natural substrate from the channel bed along many UK rivers, for example through dredging activities or channelisation.  Although there has been a reduction in dredging or de-silting activities, sediment removal is still undertaken in some rivers to satisfy particular functions, such as navigation, flood risk management and water supply.  Sediment removal, particularly of gravel substrate, can have significant adverse impacts on the hydromorphology and biology of rivers and should be avoided where possible.  Removal is not a sustainable practice as deposition of sediment is a natural response of the river to prevailing flow and sediment conditions.

In most cases it is recommended that sediment is retained in the river channel because it is an intrinsic part of the functioning of the river system, contributes to diverse channel morphology, and provides vital habitats for aquatic organisms.  SEPA and the Environment Agency both have policy statements stating that in general there is have a presumption against sediment management. However, individual situations and site specific conditions may necessitate sediment manipulation. In cases where sediment removal is necessary to maintain river function or would be of demonstrable benefit to ecology and geomorphology, and where sediment supply cannot be controlled, it may be possible to actively manage in-channel sediments.  This measure is concerned with ensuring that appropriate approaches are taken to limit the impact of further sediment removal on hydromorphology and biology, in situations where sediment removal is deemed necessary. 

Application

This measure is relevant to all rivers, including both gravel bedded rivers and those with mixed bed substrates.  The approaches described in relation to sediment removal are relevant to rivers where it has already been identified that sediment removal is necessary.  Since sediment removal is not a sustainable practice in the long-term, it is recommended that the justification for sediment removal is reviewed and examined in detail before proceeding.  Before removing sediment, consideration should be given as to whether the sediment that is present in the channel is having a significant impact on the conveyance.  To minimise or remove the need for dredging, it is recommended that investigations are made into upstream sediment sources that may be the source or contributing to the problem.

The decision that sediment removal is necessary should be dependant on establishing whether the benefits of removal (e.g. for flood risk, water supply) outweigh the potential environmental impacts. It may be decided that there are overriding reasons of public interest that justify the environmental impacts of sediment removal. However, sediment removal is untaken by a variety of parties, including government bodies, Internal Drainage Boards and local landowners. It may be difficult to influence decision-making with regard to sediment removal on a “non-main” river over which government bodies do not have permissive powers. The consequences of sediment removal in upstream reaches may subsequently have an impact on the river further downstream.

Benefits for Water Framework Directive

Good practice management of in-channel sediments can help deliver objectives of the Water Framework Directive by:

  • Allowing the development of sediment-related features which occur naturally in alluvial channels.  The presence of such features is important in providing a range of flow velocities, flow depths and substrates and hence a diversity of habitats within a channel. 
  • Limiting sediment removal and reinstating sediments where they have previously been removed, which can be used to assist recovery of more natural channel morphology and provide more diverse bed conditions and habitats upon which macrophytes, invertebrates and fish depend.
  • Ensuring that where sediment removal is deemed necessary, use of approaches that are more sensitive to the need to work with river systems can be used to reduce the degree to which natural processes are disturbed and the time that the system takes to recover.
  • Improving water quality status, for example, through the oxygenation of water over riffles and reducing the re-suspension of fine sediment that can cause turbidity and spread of sediment-bound contaminants downstream.

Good practice management of in-channel sediments can also bring about a number of other benefits. For example, limitation of sediment removal may result in significant cost savings in both undertaking channel maintenance and disposal of sediments removed.

To read more about the effectiveness of the measure within academic literature please click here: Effectiveness for Biological Quality Elements

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