Siltation in Rivers A Review of Monitoring Techniques
Conserving Natura 2000 Rivers Conservation Techniques Series No. 6
Siltation in Rivers A Review of Monitoring Techniques Conserving Natura 2000 Rivers Conservation Techniques Series No. 6 Pamela Naden1, Barnaby Smith1, Helen Jarvie1, Neville Llewellyn1, Peter Matthiessen2, Hugh Dawson3, Pete Scarlett3 & Duncan Hornby3 1CEH Wallingford, 2CEH Windermere
and 3CEH Dorset
For more information on this document, contact: English Nature Northminster House Peterborough PE1 1UA Email:
[email protected] Tel: +44 (0) 1733 455100 Fax: +44 (0) 1733 455103 This document was produced with the support of the European Commission’s LIFE Nature Programme. It was published by Life in UK Rivers, a joint venture involving English Nature (EN), the Countryside Council for Wales (CCW), the Environment Agency (EA), the Scottish Environment Protection Agency (SEPA), Scottish Natural Heritage (SNH), and the Scotland and Northern Ireland Forum for Environmental Research (SNIFFER). © (Text only) EN, CCW, EA, SEPA, SNH & SNIFFER 2003 ISBN 1 85716 780 5 A full range of Life in UK Rivers publications can be ordered from: The Enquiry Service English Nature Northminster House Peterborough PE1 1UA Email:
[email protected] Tel: +44 (0) 1733 455100 Fax: +44 (0) 1733 455103 This document should be cited as: Naden P, Smith B, Jarvie H, Llewellyn N, Matthiessen P, Dawson H, Scarlett S & Hornby D (2003). Siltation in Rivers. A Review of Monitoring Techniques. Conserving Natura 2000 Rivers Conservation Techniques Series No. 6. English Nature, Peterborough. Technical Editor: Lynn Parr Series Ecological Coordinator: Ann Skinner Cover design: Coral Design Management, Peterborough. Printed by Astron Document Services, Norwich, on Revive, 75% recycled post-consumer waste paper, Elemental Chlorine Free. 1M. Cover photo: Sue Scott
Siltation in Rivers
Conserving Natura 2000 Rivers This report on methods of monitoring siltation in rivers has been produced as part of Life in UK Rivers, a project to develop methods for conserving the wildlife and habitats of rivers within the Natura 2000 network of protected European sites.The project's focus has been the conservation of rivers identified as Special Areas of Conservation (SACs) and of relevant habitats and species listed in annexes I and II of the European Union Directive on the Conservation of Natural Habitats and of Wild Fauna and Flora (92/43/EEC) (the Habitats Directive). One of the main products is a set of methods for monitoring species and habitats, while a complementary series contains the best available information on their ecological requirements. Each report has been compiled by ecologists who are studying these species and habitats in the UK, and has been subject to peer review, including scrutiny by a Technical Advisory Group established by the project partners. In the case of the monitoring techniques, further refinement has been accomplished by fieldtesting and by workshops involving experts and conservation practitioners. Conservation strategies have also been produced for seven different SAC rivers in the UK. In these, you can see how the statutory conservation and environment agencies have developed objectives for the conservation of the habitats and species, and drawn up action plans with their local partners for achieving favourable conservation status. This report is part of the project’s series of publications on conservation techniques.The series also deals with reintroduction methods for the white-clawed crayfish (Austropotamobius pallipes), geomorphological surveying for river conservation, and captive breeding of the freshwater pearl mussel (Margaritifera margaritifera). Life in UK Rivers is very much a demonstration project, and although the reports have no official status in the implementation of the directive, they are intended as a helpful source of information for organisations trying to set conservation objectives and to monitor for 'favourable conservation status' for these habitats and species.They can also be used to help assess plans and projects affecting Natura 2000 sites, as required by Article 6.3 of the directive. Titles in the monitoring and ecology series are listed inside the back cover of this report, and copies of these, together with other project publications, are available on the project website: www.riverlife.org.uk.
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Contents Executive summary
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1 Introduction
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2 Essential background
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2.1 Species requirements
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2.2 Definitions
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2.3 UK context
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2.4 SAC context
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2.5 Summary
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3 Measuring suspended solids
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3.1 Sampling suspended solids
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3.2 Estimating suspended solid characteristics from proxy measurements
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3.3 Laboratory techniques
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3.4 Summary and recommendations
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4 Measuring silt in substrates 4.1 Monitoring scour and fill on riverbeds
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4.2 Collecting samples for analysis
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4.3 Proxy assessment of riverbed siltation
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4.4 Laboratory particle size analysis
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4.5 Summary and recommendations
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5 River habitat and geomorphological survey methods
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5.1 River Habitat Survey
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5.2 Geomorphological audit
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5.3 River geomorphology surveys
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5.4 Substrate data from General Quality Assessments
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5.5 Conclusions and recommendations
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6 Measuring the chemical quality of suspended and bed sediments
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6.1 Sediment sampling methods for chemical analysis
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6.2 Analysing phosphorus in sediment
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6.3 Determining sediment-associated metals
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6.4 Determining organic compounds
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6.5 Recommendations
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7 Measuring bioaccumulation of contaminants from sediments
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7.1 Bioavailability of contaminants in sediments
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7.2 Sediment bioassays
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7.3 Selection criteria for sediment bioaccumulation tests
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7.4 Measurement of bioaccumulation in wild organisms
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7.5 In-situ tests for bioaccumulation from sediments
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7.6 Laboratory-based sediment bioaccumulation tests
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7.7 Conclusions and recommendations
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8 Summary of conclusions and recommendations
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References
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Appendix A: Consultees
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Appendix B: Standard methods
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Executive summary This report reviews the techniques available for monitoring particulates in water columns and substrates, with particular reference to the conservation objectives and characteristics of rivers designated as Special Areas of Conservation (SACs). Before reviewing the techniques, essential background information is provided on sediment transport processes and the key controls on suspended solids concentrations and siltation.
Key findings and recommendations 1. Mean concentrations of suspended solids in the UK vary between approximately 1 and 100 mg l-1. A limited study of Environment Agency data from Yorkshire rivers suggests that mean values of 5 mg l-1 may be representative of semi-natural sites, 27 mg l-1 of cropped land and 62 mg l-1 of urban areas. Suspended sediment loads are highly variable. It is common for over 90% of sediment transport to occur in less than 10% of the time. 2. No routine monitoring of siltation, in terms of percentage of fine sediment in substrates, has been carried out, although recent publications suggest that levels of 3–8% silt and 11–74% sand are not uncommon.There has been little or no analysis of existing data in terms of catchment or river type. With regard to SAC rivers, siltation data only exist for the Hampshire Avon and these data only provide a single snapshot in time. 3. Given the generally low levels of suspended solids in SAC rivers and the low threshold concentration required in the draft Favourable Condition Tables (FCTs), direct measurement (either manual or automatic bottle sampling and subsequent laboratory analysis) to determine suspended solids concentrations is recommended, rather than the surrogate measure of turbidity. 4.The number of samples required to characterise the mean and to test compliance depends on the suspended solids concentration and its temporal variability. Suspended sediment concentrations are highly skewed and can range over three orders of magnitude at a single site during a flow event. It is therefore anticipated that, in most cases, a combination of manual and triggered automatic sampling will be most efficient.This will need to be carried out in conjunction with either continuous depth or flow measurements.The calculation of suspended solids loads also requires the availability of flow measurements. 5. Direct measurement of siltation is also recommended.The most popular methods are either freezecoring or baskets. both of which provide different information. Freeze-coring provides a spot sample in time, while baskets allow measurement of accumulation over a set period of time. Freeze-coring is a more reliable method, but the measured fine sediment content has to be interpreted in relation to the sediment transport events experienced between samples.This can be done qualitatively by reference to nearby flow gauging, or quantitatively using scour chains at some sites. Freeze-coring is very destructive, requiring the removal of 30–50 kg sediment at each site.This method is therefore inappropriate for some species and habitats. 6.While the use of dissolved oxygen as a proxy method for monitoring siltation is not recommended, it may be a more relevant species requirement. 7. River habitat and geomorphological survey methods are reviewed. However, although they can provide useful anciliary information, since they are all based on visual assessment, data are limited to what can be seen on the surface of the bed. Furthermore, only gross measures of surface siltation are included, and the identification of sources of silt needs to be used with caution. 8.With respect to understanding the sediment transfer processes within a catchment on a reach-byreach basis, and within the historical context of channel change and river management, it is recommended that fluvial audit methods are carried out on each of the SAC rivers to provide the geomorphological context for the conservation strategy.
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9. Phosphorus plays a key role in eutrophication of surface waters, and it is important to understand the process of uptake and release of phosphorus by bed sediments.The most relevant measurement is that describing the sorption characteristics of the bed sediment expressed in terms of the Equilibrium Phosphorus Concentration (EPCo). Measurements of EPCo have been developed in a research context but could be applied within routine monitoring. As they are relatively expensive to perform, they should be focused on areas where high phosphorus concentrations are of concern and ecological monitoring indicates that eutrophication is a problem. 10. Methods for the analysis of toxic contaminants (heavy metals and organic) are reviewed, along with methods for measuring bioaccumulation of contaminants from sediments. Given the expense of such methods, it is recommended that a tiered approach be taken to minimise costs, initially using biological monitoring to identify sites that may be subject to contamination before going on to detailed chemical and biological measurements.
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1 Introduction Both suspended solids and condition of the substrate, particularly with regard to siltation, are important for many of the species found in UK rivers. For example, high suspended solids concentrations can affect the feeding and health of individual species either indirectly through increased turbidity of the water, or directly through clogging of gills. Siltation is a potential problem both with regard to access to suitable substrate – for example, for the establishment of the freshwater pearl mussel (Margaritifera margaritifera) and Ranunculus spp. plant communities – and with regard to egg and fry survival in Atlantic salmon (Salmon salar), lamprey (Lampetra and Petromyzon spp.) and bullhead (Cottus gobio) populations. The UK approach to managing and monitoring species and habitats entails setting conservation objectives or minimum thresholds at which these features can be assumed to be in favourable condition. Minimum thresholds relating to sediment are currently defined in terms of annual average concentrations of suspended solids and maximum levels of substrate silt content at different depths. These thresholds are currently being refined, and monitoring recommendations will need to address both the current and, potentially more stringent, future criteria. An additional question is sediment quality.This is important in relation to both phosphorus and contaminants (heavy metals and organic compounds). Phosphorus plays a key role in eutrophication of surface waters. Elevated phosphorus concentrations in rivers have been linked to increasing rates of plant growth, changes in species composition and proliferation of planktonic, epiphytic and epibenthic algae, resulting in shading of higher plants. A key element governing phosphorus levels is the uptake and release by riverine sediments.The presence of contaminants in the fine fraction of aquatic sediments can also lead to acute or chronic toxicity to both sediment-dwelling and feeding organisms, and, through re-suspension of the sediment or release of the contaminants, organisms inhabiting the water column. Key factors are both the presence of the contaminant and its bioavailability. This report summarises the available techniques for monitoring and assessing suspended solids concentrations, siltation, river geomorphology, sediment quality and bioaccumulation.These are considered in relation to the conservation requirements, costs and ease of use, and applicability to SAC rivers.The work has been carried out largely through consultation (see Appendix A) and literature review, but also draws upon direct experience of some of the techniques and analysis of existing data from the SAC rivers.
2 Background 2.1 Species requirements Draft FCTs have been developed by the Joint Nature Conservation Committee (JNCC) for the species and habitats listed in annexes to the Habitat Directive.Table1 summarises the requirements relating to suspended solids concentrations and siltation.This has been done according to species, river and season to provide a guide for discussing appropriate techniques and monitoring strategies. At present, the FCTs quote a mean suspended sediment concentration (generally 10 or 25 mg l-1) and a maximum percentage of fine material within the top 20 or 30 cm of substrate. Fine material is defined in terms of
