Information and Advisory Note Number 23                                                Back to menu

Rivers and their catchments: river dredging operations

1. Introduction

1.1 This Information and Advisory Note considers the physical impacts of dredging operations within and adjacent to river channels, and also the effects upon instream and riparian biota. It reviews the management practices designed to minimise the impacts of such works, as well as current and planned legislation relating to land drainage and flood alleviation schemes. This Note should be read in association with Information and Advisory Note No. 18.

1.2 Dredging operations involve the removal of bed material and associated vegetation from a river channel (Figure 1). Removal of scrub from river banks and herbicidal spraying to remove vegetation adjacent to the river are also associated with dredging. There are two reasons for undertaking dredging works within a particular reach of a river for flood alleviation purposes, or for land drainage improvement. In both cases the objective is to increase the river channel capacity and its ability to convey water.

1.3 The nature of any river channel is a result of three main variables: the material properties of the bed and banks, the flow hydraulics and the sediment transport within the river. All of these variables interact to produce the channel form, and therefore an alteration in any one of them will cause a knock-on effect upon the others. For example, an increase in sediment load will mean that more of a river's energy is used in transporting sediment, and consequently less energy is available for erosion.

1.3 Instream and riparian habitats have adapted to the natural flow regimes associated with individual rivers. Often rivers with a more variable discharge have more diverse local habitats. The flora and fauna therefore make up an important part of the river ecosystem.

2. Impacts of dredging on the river environment

2.1 In gravel bed rivers there are permanent features which remain stable over a range of flows; for example, pools, riffles, point bars, floodplains and bankside vegetation. Dredging the channel bed with a mechanical digger usually destroys, or at least disrupts, these features, creating a more uniform, less stable and less diverse environment.

2.2 The suspended sediment load and turbidity of a river are increased during the removal of bed or bank material (Figure 2) and as a consequence other water quality characteristics, such as temperature, are affected. The effects may persist for some distance downstream. Settling out of the material in suspension will alter the composition of the substrate (see Note No. 22).

2.3 Deposition of dredged material on the bankside effectively creates a barrier between the floodplain and the river. In addition, this material may be eroded and washed back into the river if left unconsolidated.

2.4 The loss and alteration of natural habitats caused by dredging operations can have serious ecological impacts, in both the short-and long-term.

3. Impacts of dredging on instream biota

3.1 Fish

3.1 1 Substrate removal will inevitably affect spawning, which takes place in gravel substrates, and juvenile fish which inhabit the substrate

3.1.2 Substrate siltation is the settling of fine sediment on to the substrate. This is known to affect the spawning, incubation and emergence of some species offish, such as salmonids. Fine sediment can reduce the suitability of gravels for spawning and as a habitat for young fish by:

3.1.3 Suspended sediment in the water affects the respiratory system of fish. Growth may also be affected since food supply and feeding success are reduced in the turbid conditions. Trout have been found to have a lower resistance to disease in high suspended sediment concentrations. The varying nature and size of the suspended sediment, along with the sensitivity of different species, mean that impacts are also varied; salmonids are more susceptible than coarse fish, for example (see Information and Advisory Note No. 22).

3.1.4 Increased turbidity reduces light penetration and therefore primary productivity, which has a knock-on effect throughout the food chain. Most fish species will migrate under increased turbidity conditions and many will have a reduced capacity to find and capture prey

3.1.5 Channel morphology alteration can have a number of impacts on local fish populations:

3.2 Macroinvertebrates

3.2.1 Substrate siltation. Species such as mayflies (Ephemeroptera), caddisflies (Trichoptera) and stoneflies (Plectoptera) are adapted to live in crevices beneath and between stones, particularly in riffle areas. The presence of silt on stones is capable of reducing invertebrate abundances for prolonged periods. The major impacts of siltation are to increase species mortality and to alter community structure by:

3.2.2 Suspended sediment and turbidity
increases will:

3.2.3 There is limited information on the physical habitat requirements of macro invertebrates, but shallower water depth will favour some species due to the associated increase in temperature.

3.3 Other wildlife

3.3.1 Reduced primary productivity and reduced invertebrate and fish populations can affect local mammal and bird populations which rely on these sources of food.

3.3.2 Material deposited on the river banks affects the hydrological continuity which exists between the river channel and the floodplain. Wetlands in the riparian zone and floodplain provide habitats for invertebrate and breeding and wintering waders and wildfowl such as lapwing(Vanellus vanellus), snipe (Gallinago gallinago) and redshank (Tringa totanus). Wintering waterfowl also exploit a range of water depths; for example, teal (Anas crecca) feed on seeds in shallow water, while diving ducks, such as tufted ducks (Aythya fuligula), feed down to depths of 2.5m. Birds such as kingfishers (Alcedo atthis) and sand martins (Piparia riparia) make their nests in river banks; others including sedge warblers (Acrocephalus schoenobaenus) nest close to the bankside, while redshank inhabit tussock grass or sedge in the floodplain. Dredging is therefore likely to disturb or destroy suitable feeding and nesting sites.

3.3.3 Amphibians may lose habitat diversity and spawning areas, though drying out may also eliminate predatory fish such as pike (Esox lucius)

3.4 Vegetation

3.4.1 River wetland plants are able to survive in a range of habitats. Some aquatic plants, such as bladderworts (Utricularia), prefer slow-flowing or stagnant deep water; others, such as narrow-leaved water parsnip (Berula erecta), favour fast-flowing shallow water. On the river banks, species like pendulous sedge (Carex pendula) inhabit the area above the water-level, while others, including small-sweet grass (Glycerine declinata), prefer submerged conditions. Dredging operations can physically disturb, or remove entirely, any vegetation situated instream or upon the banks. Spraying of herbicides, such as Glyphosate, will drastically reduce plant species diversity and, while reducing the number of certain problem species, it can have a knock-on effect upon fauna and other plant life.

4. Alternative approaches and recommendations

4.1 The impacts of dredging work can be minimised by a number of techniques. Bank stabilisation and cover can be improved at water-level by the placement of temporary log or board overhangs, artificial metal or fibreglass overhangs, tree or brush retards and riprap. These may be used in conjunction with the revegetation of deposited material on the bankside. The structures can then be removed when the banks become stable and cover is re-established.

4.2 Deposition of dredged material should be away from the channel edge to limit damage to streamside habitats This also allows a degree of flooding to occur on the floodplain, thereby creating opportunities for wet grassland, scrub/wet woodland, wetlands and seasonally grazed rough grass. Where possible biotechnical engineering, for example geotextiles, may be used to help stabilise the material and aid re-colonisation. Other possibilities include: drying and spreading the spoil over adjacent land, which can improve soil fertility in some cases, but may also smother important flora and habitats; excavating a trench and infilling it with spoil, thus minimising disturbance to agriculture and the local environment; dumping off-site is possible but expensive, using spoil to create artificial wetlands.

4.3 If riprap is used for bank stabilisation, it must extend below the toe of the underwater slope of the bank to prevent undercutting. The structure can also be covered by soil and re-planted to improve habitat diversity and aesthetic value.

4.4 Re-creation of morphological diversity can be achieved by re-instating the pool-riffle sequence. This is possible with careful use of instream deflectors and boulders to encourage areas of scour and deposition. Morphological characteristics similar to those prior to excavation should be created, particularly with regard to the substrate

4.5 The use of settling ponds and bunds (Figure 3) creates a pool of slow-flowing water, allowing sift and sand to settle out. Although this reduces the downstream impacts during dredging works, the dredged reach does not benefit, and there may be further downstream impacts once the bunds are removed.

4.6 Disturbance can be minimised if mechanical excavators work from one bank. If the channel is too wide, the digger must work within the channel. Disruption can be minimised by diverting the river down one side of the channel and dredging the other side while it is 'dry*. Smaller plant equipment generally limits the level of impact on bankside and instream habitats.

4.7 In the past, angling associations or District Salmon Fishery Boards have electro-fished reaches of a river prior to dredging and relocated as many fish as possible in a safe area. However, this may create problems of over-population in the recipient stream.

4.8 Selective scrub and vegetation removal, is preferable to total clearance. Similarly, sensitive spraying of herbicide can be achieved. The removed vegetation should be stored and replaced on unconsolidated banks, or native species re-planted. This should be timed to allow the vegetation to take root If the channel margins are left untouched, a small amount of marginal vegetation and undredged bed will allow more rapid recolonisation.

4.9 Ultimately, implementation of mitigation techniques involves a trade-off between their lesser impacts and those of a more serious nature arising from dredging within the active channel. All work should be timed to avoid sensitive periods in the life-cycles of instream and riparian flora and fauna. For example, salmonids use gravels for spawning in the autumn, egg incubation in the winter and emergence of young fish in the spring.

4.10 There is a dearth of empirical information relating to the effects of dredging operations on Scottish rivers. The monitoring of future works would assist in drawing up further management guidelines aimed at reducing environmental impacts. There remains the need for a more strategic, catchment-based and environmentally sensitive approach to river maintenance in Scotland.

5. Case studies

5.1 The River Knaik, Perthshire and Kinross

5.1.1 During June 1995 an 870m reach of the River Knaik was dredged as part of the statutory drainage scheme under the Land Drainage (Scotland) Act 1941 (see section 6). Bunds and a settling pond were provided at the downstream end of the works and flow was diverted to create dry sections of river bed to be dredged. Despite this, the morphology of the reach was destroyed and the previously observed pool-riffle sequence was replaced by a shallow, flat, uniform, silt-covered bed. A 23,000% increase in fine sediment concentrations in the substrate was recorded in the dredged area.

5.1.2 Dumping of unconsolidated gravel on the bank has left this material prone to erosion back into the channel. Drainage pipes from adjacent fields were covered by spoil, thus increasing the likelihood of bank failure when water begins to percolate through them.

5.1.3 Approximately 38,000 salmonids died during the operations, though stone loach (Neomacheilus barbatulus) populations were Iittle affected (McKenzie, 1995). Invertebrate were also affected; mayflies suffered in particular, with a 50% reduction in their total number.

5.2 The River Spey, Highland

5.2.1 A scheme to alleviate flooding by rechannelling and regrading the River Spey from Kincraig Bridge to Spey Bank was proposed. The hydrological impacts of this scheme were predicted using a computational river channel model and conclusions were then drawn as to likely impacts on flora and fauna (Maitland, 1991).

5.2.2 The associated lowering of the level of Loch Insh, upstream of the dredged reach, would potentially increase nutrient concentrations and increase light penetration, thus intensifying algal blooms. Algae shade out macrophyte populations, but the reduced flooding frequency downstream of channel works could benefit macrophytes. Vegetation encroachment would also be encouraged downstream due to low flows.

5.2.3 Dredging of the main channel would seriously affect the irivertebrate community of the reach since it is unlike that elsewhere in the main river. The reach could therefore not be recolonised after dredging. An area of river bed would have to be left undisturbed to allow subsequent recolonisation.

5.2.4 Fish stocks would be disrupted but would be expected to return to normal as the new channel matured. Recovery of the river bed would depend upon the morphology and type of substrate left in the new channel.

5.2.5 The Insh marshes might have a greater tendency to dry out in the summer, affecting the diversity of the aquatic fauna and flora. Dewatering, for example, would lead to acidification of large areas of the site and to peat accumulation. If amphibian populations declined, this would affect otters adversely. Drying out of areas of fen would reduce nesting sites for waterfowl.

5.2.6 Scottish Natural Heritage objected to the proposed scheme and in March 1993 a public inquiry resulted in a Nature Conservation Order being placed under Section 29 of the Wildlife and Countryside Act 1981, to prevent the scheme being undertaken. However, a smaller flood relief channel was excavated through an instream island just below the confluence of the Rivers Spey and Feshie. This is designed to alleviate risks posed by peak flood events and consent was given, based on results from the model, prior to the public inquiry.

5.3 The River Devon, Clackmannanshire

5.3.1 The River Devon is part of the Land Drainage (Scotland) Act 1941 statutory drainage scheme (see section 6). The scheme involves dredging of the river bed and river bank maintenance along 9.3 km of channel. A survey of breeding birds on the river noted limited bird territories on a straightened and dredged reach near Alva. The removal of alluvial banks and willow scrub, characteristic of adjacent reaches, has led to a reduction in sedge warbler and reed bunting numbers at the site. Monitoring of fish populations will also be undertaken in future years.

6. Legislation

6.1 The following information draws attention to some relevant legislation regarding drainage works, but it is not comprehensive.

6.2 Under the Flood Prevention (Scotland) Act 1961 the Regional and Islands Councils (to become the new local authorities as of 1 April 1996) have discretionary powers to undertake works to prevent flooding of non-agricultural land, whereas flood prevention on agricultural land is generally the responsibility of the landowner. After 1 April 1996 any person proposing to undertake drainage works must consult the Scottish Environment Protection Agency (SEPA) as to the precautions to be taken to prevent pollution to controlled waters under Section 24(1) of the Environment Act 1995. The Secretary of State may make Regulations listing types of drainage to which this provision does not apply.

6.3 The Farm and Conservation Grant Scheme (National), administered by The Scottish Office Agriculture and Fisheries Department, ended in February 1996. This scheme provided grant aid for a range of measures including flood protection, arterial drainage and anti-erosion measures. The Scottish Countryside Premium Scheme (SCPS), to be introduced during 1997, is a new conservation scheme which will extend
the availability of incentives for the management of the natural heritage to all farmers and crofters in Scotland. There will be no payments for drainage under SCPS, although there will be incentives for the sensitive management of flood plains. Under the Agricultural Business Improvement Scheme, part of the Highlands and Islands Agricultural Programme launched in 1994, grant aid is available for the drainage of inbye land. Under the Crofting Counties Agricultural Grants Scheme (CCAGS), grants are available to crofters and other eligible occupiers living in the former crofting counties of Argyll, Inverness, Ross and Cromarty, Sutherland, Caithness, Orkney and Shetland towards the cost of certain works on their holdings, including ditching, under drainage, hill drainage, arterial drainage and river works. Written authority from The Scottish Office Agriculture and Fisheries Department is required before work can commence on arterial drainage and river works.

6.4 SOAEFD provide civil engineering advice on land drainage and flood defence schemes. Flood prevention schemes are made by a local authority under the 1961 Act and confirmed by the Secretary of State.

6.5 The Land Drainage (Scotland) Act 1958 allows any owners of agricultural land to apply to the Secretary of State for an improvement order to execute drainage works which will improve the drainage of their land or will prevent or mitigate flooding or erosion to which that land is subject. In practice, the use of improvement orders is usually made by a group of landowners who share a common riparian system. A provision in the Act specifies that the cost of such works must be "reasonable having regard to the likely benefit to agricultural production they accrue therefrom". SEPA is not consulted until the Draft Order stage.

6.6 The Secretary of State also retains the responsibility for maintaining schemes initiated by the Land Drainage (Scotland) Acts of 1930,1935 and 1941. They are designed to allow, or maintain, the cultivation of agricultural land. In Scotland there are 13 schemes still in force (Table 1). Maintenance of these schemes to their original specifications still rests with SOAEFD. Research sponsored by SNH is in progress to provide an assessment of the impact of these schemes on the natural heritage, with a view to strengthening the case for more sympathetic river management.


6.7 The Salmon Act 1986 allows District Salmon Fishery Boards to execute any works which appear necessary to them to protect and improve the salmon stock within their district; this includes channel and bank works. Whether planning permission is required for such works is currently a matter of debate. Under the Town and Country Planning (General Permitted Development) (Scotland) Order 1992 (PDO), forestry and agricultural concerns can undertake work without planning permission. Fishery management is not classed as agriculture under the PDO, but planning permission is rarely sought unless the site is designated.

6.8 Section 66 of the Countryside (Scotland) Act 1967, as amended by schedule 10 of the Natural Heritage (Scotland) Act 1991, states that every Government department shall have regard to the desirability of conserving the natural heritage of Scotland within the meaning of the this Act, including the flora and fauna of Scotland, its geological and physiographical features, its natural beauty and amenity. The Forestry Commission have a similar obligation under the Forestry Acts 1967-79, as amended by the Wildlife and Countryside (Amendment) Act 1985.

6.9 Under Section 32 of the Environment Act 1995, the Secretary of State and SEPA have a duty to have regard to the desirability of
conserving and enhancing the natural heritage of Scotland. Under Section 34 of the Act, SEPA also has a duty generally to promote the conservation and enhancement of the natural beauty and amenity of inland and coastal waters and the land associated with such waters, and the conservation of flora and fauna which are dependent upon aquatic environments. Section 35 of the Act obliges SEPA to liaise with Scottish Natural Heritage where SEPA's activities may affect a Natural Heritage Area or Site of Special Scientific Interest (SSSI).

6.10 Under Section 1 of the Water (Scotland) Act 1980, as amended by Section 65 of the Local Government etc. (Scotland) Act, as of 1 April 1996, the Secretary of State and the new water and sewerage authorities are obliged to further the conservation and enhancement of natural beauty and the conservation of flora, fauna and geological and physiographical features of special interest.

6.11 From 1 April 1996, the Scottish Environment Protection Agency will combine the existing pollution control functions of the River Purification Authorities, Her Majesty's Industrial Pollution Inspectorate and local authorities. It will have a statutory conservation duty but its freshwater work is unlikely to extend beyond that already carried out by the RPAs.

6.12 The Conservation (Natural Habitats &c.) Regulations 1994 are relevant as regards proposed dredging works in, or affecting, European sites (Special Areas of Conservation and Special Protection Areas). Article 6(2) of the Habitats Directive states that Member States are required to take steps to avoid significant deterioration and disturbance of natural habitats and species at these sites.

6.13 European Union Directives may indirectly affect dredging works. Their requirements are transposed into UK law via Regulations:

7. Further reading

Brookes A., 1988. Channelized Rivers: Perspectives for Environmental Management. Chichester, John Wiley.

Crisp D.T., 1989. Some impacts of human activities on Trout, Salmo trutta, populations. Freshwater Biology, 21, 21-33.

Darby S.E. and Thorne CR., 1995. Fluvial maintenance operations in managed alluvial rivers. Aquatic Conservation: Marine and Freshwater Ecosystems, 5, 37-54.

Henty C.J., 1994. A Survey of the Breeding Birds of the Lower Devon, Clackmannanshire. Report submitted to Scottish Natural Heritage, Stirling.

Hoey T.B., Smart D.W J., Pender G. and Metcalfe N., 1995. Alternative Methods of River Management for Scottish Rivers. Scottish Natural Heritage Review, No. 47.

Maitland P.S., 1991. Flood Alleviation in Upper Strathspey. Modelling and Environment Study: Volume II: Supporting Arguments. Report prepared for the Nature Conservancy Council for Scotland.

Maitland P.S., Boon P.J. and McLusky D.S., 1994. The Freshwaters of Scotland: a National Resource of International Significance. Chichester, John Wiley.

McKenzie T., 1995. The Impacts of Dredging Operations on Fish Populations in the River Knaik, June 1995. lasgair Fishery Advisory Services. Unpublished.

RSPB, NRA and RSNC, 1994 The New Rivers and Wildlife Handbook. Sandy, Royal Society for the Protection of Birds.

Scottish Office, 1995. Nature Conservation: Implementation in Scotland of EC Directives on the Conservation of Natural Habitats and of Wild Flora and Fauna, and the Conservation of Wild Birds: The Conservation (Natural Habitats &c.) Regulations 1994. The Habitats and Birds Directives. Circular No. 6/1995 The Scottish Office Environment Department, Rural Affairs and Natural Heritage Division. Edinburgh, HMSO.

Werritty A., 1995. Integrated Catchment Management: a Review and Evaluation. Scottish Natural Heritage Review, No. 58, Battleby.

8. Glossary of technical terms

Benthos - organisms Irving within a stream's substrate

Biotechnical engineering - the combination of engineering materials and vegetation to provide defence against erosion, e g. geotextiles, reeds.

Bunds - barriers erected across a river to create a settling pond

Detectors - artificial, erosion resistant pant bar designed to divert the current in a river.

Dredging- operations involving the removal of sediment from a river bed and the removal of bankside vegetation and scrub.

Floodplain - flat valley-bottom area of sediments deposited in floods, and typically being reworked as the course of a river repeatedly shifts across the valley floor

Geotextile-natural or synthetic permeable fabric, used with soil, to protect against erosion.

Hydrological continuity- gradation of wetness, generally decreasing away from the river channel

Interception-rainfall caught on leaf/branch surfaces of vegetation, subsequently lost back to the atmosphere by evaporation

Intergravel flow-the flow of water through the gravel

Interstitial spaces - spaces between substrate particles

Invertebrate drift- a macroinvertebrate is an animal without a backbone, large enough to see without magnification Drift is their downstream migration in the water current

Meander- bend in a river, formed by natural channel processes

Morphology- (science of analysing) the structural form of a river or other landform

Point bar- area of fine sediment, accumulating by fluvial deposition, on the inside of a bend, and typically balanced by erosion on the opposite bank.

Pool- portion of a stream with reduced velocities, where water is deeper than surrounding areas. Used by fish for resting and cover

Pool-riffle sequence - the spacing of pool and riffle areas in a given reach

Reach - a relatively homogeneous length of stream having a similar sequence of characteristics

Redd - area of stream bed dug out by the female salmonid, before spawning, where she lays her eggs.

Riffle - shallow rapids where water flows swiftly and food supply offish (i.e. invertebrates) is concentrated Female salmonids dig their redds in riffle areas

Riparian - situated on the banks of a watercourse.

Riprap- (Revetment) layer(s) of large durable materials, usually rock, used to protect a river bank from erosion

Setting pond - area of slow flowing water, created by bunds, in order to allow fine sediment to settle out

Station - deposition of fine sediment by a watercourse.

Substrate - mineral and/or organic material fret forms the bed of a stream. Includes clays, silts, sands, gravels, cobbles, boulders and bedrock.

Suspended Sediment- portion of the sediment load of a river that moves in suspension.

Turbidity - amount of light scattered or absorbed by a fluid, as a result of its suspended particles and dissolved load.

9. Author

Jonathan Clark, Earth Sciences Branch, Scottish Natural Heritage, Edinburgh

10. Further information

Research and Advisory Services Directorate
Scottish Natural Heritage
2 Anderson Place
Tel: 0131-447 4784

Earth Sciences Branch (provide information on physical processes in the aquatic environment); Aquatic Environments Branch (provide information on biota in the aquatic environment.


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