Good practice management of in-channel sediments

Design Guidance

Assessing whether sediment removal is appropriate

Three overarching founding policy related premises are stated in Policy Guidance:

1) there is a general presumption against the removal of sediment for a watercourse;

2) the justification to move or remove sediment should be based on evidence and understanding; and,

3) when removal of sediment is found to be justified, best practice must be used to carry out the necessary work to minimise adverse effects on the environment. (Environment Agency 2004).

 

The SEPA (2006) policy statement also provides a useful framework for assessing when the removal and active management of in-channel sediments is likely to be justified:

 

  • Bridge or culvert maintenance: Strong justification for removal where the function or integrity of the structure could be compromised.  A more sustainable solution should be sought if recurring works are likely to be necessary. 
  • Removal behind impounding structures: Potential justification for removal if sediment accumulation disrupts the efficient operation of the impoundment.  The potential impacts of reducing sediment supply in this manner should be considered.
  • Flood management: Potential justification for removal if there is a demonstrable link between in-channel sedimentation and flood risk.  A more sustainable solution should be sought if accumulation is a long term problem.  Potential for scour during floods should be considered before removal is undertaken. 
  • Habitat works and fisheries improvements: Potential justification if an underlying sediment issue is identified (e.g. at the reach scale) and is addressed as part of a wider catchment remediation strategy, or where wider improvements to the river and the ecology it supports can be demonstrated.  Works targeted at a single reach are unlikely to be justified, as are actions where an underlying sediment issue has not been demonstrated. 
  • Aggregate extraction: Potential justification for small scale removal where resources are abundant and habitats are insensitive to sediment removal.  Consideration should be given to the effects of sediment removal on the sediment budget of the river, and to the use of alternative sources that are not directly linked to the river channel. 
  • Land drainage: Unlikely to be justified in natural watercourses.  May be justifiable in minor watercourses that have been routinely managed for drainage purposes in the past.  Alternatives such as control of sediment sources or channel restoration to restore natural sediment conveyance should be considered on larger watercourses. 
  • Removal of gravel for use as bank protection: Unlikely to be justified, since it can cause considerable disturbance to the river channel.  Alternative options which do not disturb the substrate should be considered. 

On the basis sediment management should not be undertaken unless there is clear evidence suggesting it is needed  (for example, given one of the reasons above) – the reasoning behind the decision must be clear and justified. This requires the decision to be objective and evidence based. A standard approach to aid decision-makers to come to reasoned and sound justification has been proposed and articulated in a set of ‘Guiding Principles’. This set of Guiding Principles have been generated by the Environment Agency Project  “River Sediments and Habitats Review of Maintenance and Capital Works” Research and Development project (SC040015) (Environment Agency, 2010).  This is a systemic approach to the assessment of the needs, methods and feedback from sediment-related activities and provide a step by step guide to ensure maintenance activities are sustainable:

Six guiding principles for sediment management in watercourses:

 

1.       Identify why you are considering action

2.       Understand the sediments related issues and identify the causes

3.       Identify and prioritise the function of the river

4.       Identify and appraise possible management options based on risk

5.       Balance the multiple goals of watercourse management

6.       Inspect and appraise management options performance with respect to prioritised functions.

Further background to these principles can be found at http://learning.environment-agency.gov.uk/courses/sediment/launcher.html

The River Sediments and Habitats Review of Maintenance and Capital Works report also highlights General Guiding Principles relating to sediment and habitat issues that require consideration. This specifically includes the need to consider sediment removal alongside vegetation management due to vegetation and sediment interactions (Environment Agency, 2008, Chapter 2.8).

Direct removal of sediments

Different sediment management practices will have different impacts on habitats and the selection of the appropriate management will depend upon the type of habitats that are to be promoted (Environment Agency, 2008).  Existing guidance as to how the impact of sediment removal may be minimised is provided within Environmental Options for Flood Defence Maintenance Works (Environment Agency, 2004b) and The Drainage Channel Biodiversity Manual (Association of Drainage Authorities and Natural England, 2008).  Advice is primarily targeted at channels where fine sediment is to be removed and aims to ensure rapid recovery from sediment removal and minimisation of disturbance of channel margins. Suggested approaches include:

  • Dredging from only one side of the channel.
  • Cutting and weed raking vegetation on the other side of the channel with a retained margin of approximately 100 m in length with a minimum retained margin of 20% of the channel bed. 
  • Avoidance of silt removal from the toe of the bank.
  • Restriction of sediment removal to silt material.  Consolidated bed material, such as gravels, should not be removed.
  • Removal of only the top of established berms, whilst retaining part of these features.  

Guidance applicable to smaller watercourses and drainage channels typically maintained by Internal Drainage Boards is provided within The Drainage Channel Biodiversity Manual: Integrating Wildlife and Flood Risk Management (Association of Drainage Authorities and Natural England, 2008). Channel measures CL1-CL4 are particularly relevant.

Sediment traps

As an alternative to the direct removal of fine sediments from the channel bed, it may be possible to install sediment traps to collect fine sediments for subsequent removal.  This technique avoids the need for dredging long lengths of channel, and therefore has a considerably reduced impact on channel morphology and ecology.  There are a wide variety of different traps that can be installed, depending on the amount of sediment that needs to be controlled and the size of the watercourse. 

Silt traps which consist of an area of lowered bed within the channel in which sediment accumulates preferentially are recommended for use on drainage channels of all sizes (Association of Drainage Authorities and Natural England, 2008).  An existing pool or slack water area can be enlarged to reduce flow velocities and encourage fine sediments to accumulate.  Material can then be physically removed from this area, reducing the sediment load of the watercourse while minimising the size of the dredging footprint.  Sediment needs to be removed regularly to prevent the material being re-suspended, and this technique is unlikely to be suitable for application in channels with rapid flows. 

A silt trap in the bed of a drainage channel (Association of Drainage Authorities and Natural England, 2008)

An alternative design has been used on the River Eye SSSI near Melton Mowbray, Leicestershire, to reduce the supply of silt to a flood storage reservoir (Bridle et al., 2001).  These traps consist of 500m-long sections of river channel that have been widened to reduce flow velocities and encourage sedimentation.  In addition, the traps also include a floodplain “green dam” consisting of a stand of willow trees.  These are intended to reduce flow velocities during periods of overbank flow and encourage sedimentation on the floodplain. 

Wooden sediment traps can also be installed to impound flows and encourage sediment deposition.  A wide range of potential designs are available, including small weirs and bundles of alder or willow.  These traps are generally only suitable for application in small watercourses (Agate and Brooks, 2003). 

Straw bales can also be inserted into a channel as a physical barrier to sediment.  The bales are permeable, and allow water to pass through, but significantly reduce flow velocities and encourage sediment deposition.  Bales are cheap to install, can be used to trap both fine and coarse sediments, and can be removed alongside sediments once they are full (IECA Australasia, 2007).  They are generally considered as suitable for short term use only, because they break apart during higher flows and allow some sediment to pass through. 

Detention basins

A detention basin is an enlarged area of channel which is characterised by low velocity flows and sediment accumulation.  The technique uses the same principles as the sediment trap recommended by the Association of Drainage Authorities and Natural England (2008), but is typically much larger and used on a natural river channel with greater flow variability.  Detention basins generally incorporate a flow control structure such as a small weir, slot or gates, which creates a pool upstream (Saldi-Caromile et al., 2004).  The pool is frequently excavated to create a larger detention basin.  These structures can be used to control both fine and coarse sediments, and periodically emptied to maintain their effectiveness. 

It is recommended that sediment detention basins are only installed once detailed investigations into alternative sediment control measures, impacts on channel morphology and sediment regime, and potential mitigation to offset the potential effects of sediment starvation on the river channel and in-channel habitats downstream of the structure (Saldi-Caromile et al., 2004). 

Conceptual design of a sediment detention basin (Saldi-Caromile et al., 2004)

Sediment removal in gravel bed rivers

Policy documents on the removal of gravel from rivers (Environment Agency, 2004a; SEPA, 2005) indicate that government bodies are generally against the removal of gravel from rivers. Gravel removal is only allowed where there is an overriding interest (e.g. for navigation or where proven essential for flood risk management or water supply purposes). The Environment Agency’s policy states that studies to ensure that the practice is sustainable and environmentally acceptable over the long term will be developed, together with best practice guidelines or each particular case (Environment Agency, 2004a). These guidelines must include the following information:

  • A demonstrable strategic approach to the problem.
  • The consultation and liaison procedures to be followed.
  • Pre-works surveys to define the location/extent of biodiversity interests.
  • Assessment of impact on geomorphology.
  • Location, extent and timing of works.
  • Method for working and use of machinery.
  • Specific measures to minimise the mobilisation of sediments.
  • Restoration measures.
  • Disposal of material.
  • Appropriate monitoring programme to assess the impact of works and aim achievement.

Environment Agency (2004c) provides a “good practice guide” for shoal digging and gravel removal in rivers – refer to measures D1-D4. The guide recommends:

  • to carry out pre-work surveys to establish the location and extent of biodiversity interests,
  • to assess the geomorphological impact and
  • to define the location, extent and timing of works.

Works should be carried out between the 14th of August and the 30th of September to avoid disturbances to spawning fish and breeding birds. The dredging must be carried out in as small and shallow an area as practicable, not removing material below summer water levels and moving along one bank only. A proportion of the gravel shoal must be left untouched so re-colonisation can be granted. Specific measures should be taken to minimise the mobilisation of sediments and restoration measures considered when the works have finished. The extracted material must be disposed of and re-use of gravels for biodiversity projects should be considered. The good practice guide provides a set of tables where the appropriate level of Environmental Impact Assessment is defined for each of the maintenance actions (i.e. control of aquatic vegetation and gravel removal). A chart diagram that informs of all the legal steps/ legal documents to be completed before each activity is also provided.

Timing of sediment removal

Sediment does not accumulate in a channel at a steady rate.  The transport of sediment depends in a highly non-linear way on the flow velocity.  This means that significantly more sediment is transported in floods than during low flow periods. Since the need for removing sediment from a given reach may not occur at regular intervals, removal therefore must depend upon the observed condition of the channel rather than on a fixed temporal programme of maintenance activities (Environment Agency, 2008).

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