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Evidence Project Final Report

NoteIn line with the Freedom of Information Act 2000, Defra aims to place the results of its completed research projects in the public domain wherever possible. The Evidence Project Final Report is designed to capture the information on the results and outputs of Defra-funded research in a format that is easily publishable through the Defra websiteAn Evidence Project Final Report must be completed for all projects.

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Project identification

1. Defra Project code ME1101

2. Project title

Development of Approaches, Tools and Guidelines for the Assessment of the Environmental Impact of Navigational Dredging in Estuaries and Coastal Waters

3. Contractororganisation(s)

CefasLowestoft LaboratoryPakefield RoadLowestoftSuffolkNR33 0HT

54. Total Defra project costs £ (agreed fixed price)

5. Project: start date................. 01/09/2008

EVID4 Evidence Project Final Report (Rev. 06/11) of 15

mailto:[email protected]

end date.................. 31/03/2010


6. It is Defra’s intention to publish this form. Please confirm your agreement to do so...................................................................................YES NO (a) When preparing Evidence Project Final Reports contractors should bear in mind that Defra intends that

they be made public. They should be written in a clear and concise manner and represent a full account of the research project which someone not closely associated with the project can follow.Defra recognises that in a small minority of cases there may be information, such as intellectual property or commercially confidential data, used in or generated by the research project, which should not be disclosed. In these cases, such information should be detailed in a separate annex (not to be published) so that the Evidence Project Final Report can be placed in the public domain. Where it is impossible to complete the Final Report without including references to any sensitive or confidential data, the information should be included and section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No" answer.In all cases, reasons for withholding information must be fully in line with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.

(b) If you have answered NO, please explain why the Final report should not be released into public domain

Executive Summary7. The executive summary must not exceed 2 sides in total of A4 and should be understandable to the intelligent

non-scientist. It should cover the main objectives, methods and findings of the research, together with any other significant events and options for new work.

Navigational dredging is a fundamental and essential activity for most ports and harbours in the UK to maintain navigable depths for vessels. It is vital to social and economic development of a country that is heavily dependant on maritime trade. In the UK 25–50 million wet tonnes of material are dredged for disposal to licensed sites each year. Navigational dredging can impact both the seabed through disturbance and removal of sediment and the water column through sedimentary material being introduced into it. Areas subject to regular maintenance dredging are likely to have disturbed or impoverished seabed faunas but this would not be the case for areas previously un-dredged or occasionally dredged. The potential environmental impacts of navigational dredging include turbidity plume effects in the water column and subsequently effects on the seabed when the suspended sediment settles out, release of contaminants, organic matter, bacteria/viruses and nutrients from the disturbed or suspended sediment into the dissolved state and associated reductions in dissolved oxygen. In addition, dredging may induce changes in sediment transport rates by reducing the stability of seabed sediments and mobilizing sediments via turbidity plumes.

With the advent of the Marine Bill and the Marine Framework Strategy Directive as well as the implementation of the Water Framework Directive, we will need to be in a position to carry out sound assessments of the impacts of dredging and understand the available measures that can be used to mitigate those effects in order to ensure integrated management of the marine environment.

A lot of work has been done in assessing dredging impacts, particularly in the US but also in the Netherlands. Thus the first step was to carry out a literature review to assess what R&D issues we need to address before making specific proposals.

The project was intended be in 3 phases although the scheduling of these phases did not need to be strictly sequential:

Phase 1

Objective 1. Establish links with the Environment AgencyObjective 2. Literature review of significant impacts of dredging activity and it’s monitoringObjective 3. Literature review of existing mitigation measures and their efficacy in addressing these impacts


Objective 4 Stakeholder workshop Objective 5. Revised Research Plan

Phase 2

Objective 6. Additional research on dredging impactsObjective 7. Identification of potential options for improving mitigation of dredging impacts

Phase 3

Objective 8. Development of approaches and tools Objective 9. Best Practice GuidelinesObjective 10. Workshop

Note that Phases 2 and 3 of the project were not funded and thus did not take place.

***************************Project outputs from Phase 1 of the project were:

1. Literature Review of Dredging Activities: Impacts, Monitoring and Mitigation – This combines the outputs from objectives 2 and 3 and covers:

Objective 2: Literature review of significant impacts of dredging activity and it’s monitoring:

The literature review consisted of the following sections:

1. Dredging, dredger types and associated impacts 2. Release of contaminants during dredging3. Effects of navigation dredging in water quality: nutrients, dissolved oxygen and re-suspension of bacteria4. Radionuclides in dredged material5. The impacts and monitoring of maintenance dredging - related to

turbidity6. Capital dredging impacts7. Mathematical models used to assess the impact of dredging activity

Objective 3: Literature review of existing mitigation measures and their efficacy in addressing these impacts

When an environmental assessment determines that impacts may be significant, consideration is given to means to conduct the project in such a manner that impacts are avoided, reduced or mitigated. Many alternatives exist, collectively called dredging project mitigation measures, whereby unacceptable environmental impacts of dredging (including dredged material placement and sediment transport) can be offset. The need for mitigation measures is not only limited to those dredging projects that are dealing with contaminated sediments. Even when dredging clean sediments the evaluation of potential environmental effects and identification of mitigation measures to reduce such effects is necessary.

PIANC (2009) recently developed a framework to aid the transparent selection of such mitigation measures for minimising the environmental impacts of dredging projects. As part of this undertaking, the document comprehensively reviewed the mitigation measures available to any such project and, as such, PIANC (2009) provides the most up-to-date review of mitigation measures for a dredging project. Consequently, we have used this as a firm basis for the present report, drawing upon its relevance, but also adding to it in aspects which were not as specific as those required for the present study. Although the scope of PIANC (2009) was global, very few measures indicated were deemed to have a non-UK application and, therefore, needed to be left out of the present report.

The report on this objective adopts the classification of PIANC (2009) where 6 broad types of mitigation measures were designated:

Contractual and project management Project design and planning Equipment related


Institutional Dredging-related Control-related

2. Report of stakeholder workshop

A stakeholder workshop was held on 11 March 2010 to provide feedback on both the literature reviews and guide any future work programme. The report of the workshop detailed gaps in the literature review report under the following headings:

Details of dredgers and dredging techniques Level of knowledge re source output of various dredging techniques. Monitoring Mitigation Measures Cumulative effects of combined dredge operations Modelling dredge operations Biological aspects Contaminants Case studies

Project Report to Defra8. As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with details of

the outputs of the research project for internal purposes; to meet the terms of the contract; and to allow Defra to publish details of the outputs to meet Environmental Information Regulation or Freedom of Information obligations. This short report to Defra does not preclude contractors from also seeking to publish a full, formal scientific report/paper in an appropriate scientific or other journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms. The report to Defra should include: the objectives as set out in the contract; the extent to which the objectives set out in the contract have been met; details of methods used and the results obtained, including statistical analysis (if appropriate); a discussion of the results and their reliability; the main implications of the findings; possible future work; and any action resulting from the research (e.g. IP, Knowledge Exchange).

Navigational dredging is a fundamental and essential activity for most ports and harbours in the UK to maintain navigable depths for vessels. It is vital to social and economic development of a country that is heavily dependant on maritime trade. In the UK 25–50 million wet tonnes of material are dredged for disposal to licensed sites each year. Navigational dredging can impact both the seabed through disturbance and removal of sediment and the water column through sedimentary material being introduced into it. Areas subject to regular maintenance dredging are likely to have disturbed or impoverished seabed faunas but this would not be the case for areas previously un-dredged or occasionally dredged. The potential environmental impacts of navigational dredging include turbidity plume effects in the water column and subsequently effects on the seabed when the suspended sediment settles out, release of contaminants, organic matter, bacteria/viruses and nutrients from the disturbed or suspended sediment into the dissolved state and associated reductions in dissolved oxygen. In addition, dredging may induce changes in sediment transport rates by reducing the stability of seabed sediments and mobilizing sediments via turbidity plumes.

With the advent of the Marine Bill and the Marine Framework Strategy Directive as well as the implementation of the Water Framework Directive, we will need to be in a position to carry out sound assessments of the impacts of dredging and understand the available measures that can be used to mitigate those effects in order to ensure integrated management of the marine environment.

A lot of work has been done in assessing dredging impacts, particularly in the US but also in the Netherlands. Thus the first step was to carry out a literature review to assess what R&D issues we need to address before making specific proposals. We also need to develop approaches, tools and associated guidance so that both Cefas and the Environment Agency (EA) can effectively assess the environmental impact of proposals for navigational dredging to contribute to the integrated management of the marine environment. Given that the EA has well-developed water quality models for quite a number of major estuaries in England and Wales, it would make sense to try to develop tools that utilise or link to those


models where practical.

The project was intended be in 3 phases although the scheduling of these phases did not need to be strictly sequential:

Phase 1

Objective 1. Establish links with the Environment Agency

Prior to the project starting, we will explore opportunities for the Environment Agency to participate in the project. Early in this phase of the project links with be established with appropriate Environment Agency staff to explore ways to work together on the assessment of the impact of navigational dredging in estuaries and coastal waters. This objective was completed in December 2008.

Objective 2. Literature review of significant impacts of dredging activity and it’s monitoring

To underpin the development of best practice guidance for dredging activities, it is necessary to review current knowledge of dredging impacts and it’s monitoring to provide a firm basis for determining best practice mitigation and monitoring requirements for different types of dredging activity later in the project. The literature review will conclude with a prioritised list of additional research needed to ensure appropriate development of best practice approaches, tools and guidelines for assessing the impacts of navigational dredging. This objective was completed in February 2010.

Objective 3. Literature review of existing mitigation measures and their efficacy in addressing these impacts

Information on existing mitigation measures and their efficacy will be obtained from a variety of sources including a literature review, environmental statements for developments involving capital dredging, monitoring data and discussions with relevant bodies. The literature review will conclude with a prioritised list of gaps in existing mitigation measures that need to be addressed to ensure appropriate mitigation of the impacts of navigational dredging. This objective was completed in March 2009.

Objective 4 Stakeholder workshop

After the reviews under objectives 2 and 3 have been completed, a workshop will be held to provide feedback on both the literature reviews and guide the proposed future work programme. This workshop was held in March 2010.

Objective 5. Revised Research Plan

Following the workshop, a revised research plan will be submitted to Defra for its approval. Once that is agreed, the SID3 will be revised and submitted to Defra. Note that this objective was not done since it was scheduled to occur in the early part of 2010/11 and there was no funding available for the project in that financial year.

Phase 2

Objective 6. Additional research on dredging impacts

Based on the outcome of Objectives 2 and 4 above, key research issues will be addressed. It is not possible to say what this will consist of or the costs of the work until the outcome of the first phase is known.

Objective 7. Identification of potential options for improving mitigation of dredging impacts

Based on the outcome of Objectives 3 and 4 above, potential options for improving mitigation of dredging impacts, where required, will be developed through consultation and discussion (possibly a small workshop) with port and harbour authorities, regulators, dredging contractors and others taking into account the cost/benefit of potential measures. It is not possible to say what this will consist of or the costs of the work until the outcome of the first phase is known.

Phase 3

Objective 8. Development of approaches and tools


Based on the outcome of Objectives 2, 4 and 5 above, we will develop approaches and tools as deemed appropriate and necessary for the assessment and monitoring of the impact of navigational dredging. This could include for example a decision-support system or a risk assessment approach that would, where appropriate, draw upon the Marine Aggregate Risk Assessment Framework project funded by the MEPF fund of the ALSF (Project MEPF 04/03). We will also liaise with the project co-ordinator of the Defra Project ‘ME 1104 - Contaminated Dredged Marine Sediments: Developing a Management Framework’ to incorporate any relevant experience from that project.

Objective 9. Best Practice Guidelines

Based on the outcome of all the above objectives, we will develop best practice guidelines for the assessment of the impact of navigational dredging, for monitoring the impacts and for options to mitigate dredging impacts. The best practice guidance will be illustrated with a number of case studies.

Objective 10. Workshop

We will hold a workshop to communicate the outcome of the project to stakeholders at the end of the project.

Note that Phases 2 and 3 of the project were not funded and thus did not take place.

***************************************

Project outputs from Phase 1 of the project were:

1. Literature Review of Dredging Activities: Impacts, Monitoring and Mitigation – This combines the outputs from objectives 2 and 3 and covers:

Objective 2: Literature review of significant impacts of dredging activity and it’s monitoring:

A lot of work has been done in assessing dredging impacts, particularly in the US but also in the Netherlands. Thus, the first step was to carry out a literature review to assess the current state of knowledge and thus determine what R&D issues may need to be addressed. The degree of contamination of sediments clearly plays a very important role in determining the significance of any mobilisation of contaminants from those sediments. In this context it should be noted that the Environment Agency’s ‘Clearing the Waters’ guidance for assessing compliance with the Water Framework Directive utilises in its assessment Cefas Action Levels for metals, TBT, halogenated organics and PAHs in sediments.

The literature review consisted of the following sections:

1. Dredging, dredger types and associated impacts

1.1 Introduction Dredging can be described as the process of removing part of the seabed or its overlying sediments with the aim of deepening the area commonly for the purposes of navigation or associated with construction projects.

There are two main types of navigational dredging activities; capital dredging and maintenance dredging.

Capital dredging is the removal of material to create a greater depth than had previously existed. The Marine Management Organisation (MMO) definition of capital dredging is:

‘Material arising from the excavation of the seabed, generally for construction or navigational purposes, in an area or down to a level (relative to Ordnance Datum) not previously dredged during the preceding 10 years.’

Maintenance dredging is required to maintain water depths in areas where sedimentation occurs. It mainly involves the removal of recently deposited unconsolidated sediments, such as mud, sand and gravel. It is undertaken by many operators (ports, berth operators and marinas) to maintain navigable channels and berths. It is generally an ongoing activity that can consist of cycle or a series of repeat dredges. The MMO definition of maintenance dredging is:

‘Material (generally of an unconsolidated nature) arising:


From an area where the level of the seabed to be achieved by the dredging proposed is not lower (relative to Ordnance Datum), than it has been at any time during the preceding 10 years; or

From an area for which there is evidence that dredging has previously been undertaken to that level (or lower) during that period.’

1.2 Dredging methodsDredging used in the marine industry for a variety of navigational purposes ranging from routine maintenance of small marinas and harbours to the creation and deepening of navigation channels and berths at major ports. The choice of dredging plant is largely dependant on environmental conditions such as the hardness and quantity of material to be dredged, site exposure, the method of disposal etc.

Different types of dredging equipment and techniques are employed to achieve the required project outcomes in the most efficient way. There are 3 main dredging methods which are based on the physical processes involved in the excavation and transport of the dredged material, these are described below:

Hydraulic dredgers: use a centrifugal pump and pipe system to raise loosened material in suspension to the surface. There are three main types of hydraulic dredger, suction dredgers (SD), cutter suction dredgers (CSD) and trailer suction hopper dredgers (TSHD). Mechanical dredgers: use of mechanical excavation equipment to loosen the seabed sediment and raise it to the surface. There are three main sub-groups of mechanical dredger, grab dredger (GD), backhoe dredger (BD) and bucket ladder dredger (BLD).

Hydrodynamic dredgers: use the re-suspension of sediments and their transport away from the dredge area by means of natural forces. The term ‘Hydrodynamic dredging’ is often used to group the following dredging techniques, water injection dredgers (WID), forms of agitation dredgers, that use mixing to make a density current and underwater plough dredgers (UPD), which stir or rake sediments into suspension.

The main types of dredgers used for capital and maintenance dredging are briefly described below:

1.3 Dredgers used in capital dredging projectsCapital dredging projects are often large-scale projects which involve the creation or deepening of access channels and/or berths. With capital dredging the full range of materials may be encountered and soft materials, such as clays, sands and silts, but generally the material dredged has been undisturbed for several years and therefore tends to be consolidated such as stiffer clays, boulders and rocks.

Cutter suction dredgers (CSD) are static dredgers usually associated with capital dredging or areas of harder soils, which have to be removed in thick layers (Bray 2008). The cutter suction dredge dislodges material with a rotating cutter equipped with cutting teeth. The loosened material is sucked into the suction mouth located in the cutter head by means of centrifugal pump installed on the pontoon or ladder of the dredger.

Backhoe dredgers (BD) are basically pontoon mounted or shore-based excavators or diggers that are used to dredge marine sediments. Sediment is excavated by the crane bucket which is then raised above water by the crane arm. This method of dredging can be highly accurate which may be of benefit when dredging contaminated sediment or in environmentally sensitive areas.

Bucket Ladder dredger (BLD) consists of a large pontoon with a central well in which a ladder equipped with an endless chain of buckets, is mounted. During dredging, the endless chain rotates along the ladder. The lowest bucket digs into the bed material and the cut material falls into the bucket. It is then carried upwards as the bucket chain rotates. This type of dredger is used infrequently in dredging projects today.

1.4 Dredgers used in maintenance dredging projectsAs outlined above maintenance dredging mainly involves the removal of recently deposited unconsolidated sediments, such as mud, sand and gravel. Most ports that have to carry out substantial maintenance dredging each year use TSHD, they are mainly used for dredging of unconsolidated sediments of lower strength. Small ports, harbours and marinas tend to use mechanical dredgers such as GD and BD working with barges. These dredgers are used in more confined areas, such as alongside wharfs, because they are smaller than suction dredges. Until recently WID had limited use in the UK but they are being used increasingly to dredge ports and harbours around the UK (Sullivan, 2000).

The main types of dredgers used for maintenance dredging are briefly described below:


Trailing Suction Hopper Dredgers (TSHD) They are normal sea going ships with a large hopper and equipped with single or double trailing suction pipes that end in a draghead. The function of the draghead is to allow the material to flow to the suction inlet as efficiently as possible as the ship moves slowly forward. The material is lifted through the trailing pipes by one or more pumps and discharged into a hopper contained within the hull of the dredger.

Grab dredgers (GD) are relatively simple and involve the collection of sediments in a crane-mounted bucket, the jaws of which are opened and closed to trap the sediment. Depending on the type of material to be dredged different grab bucket designs can be employed, such as mud grabs, sand grabs and heavy digging grabs.

Backhoe dredgers (BD) are described above and can be used for both capital and maintenance dredging projects. Spillage can occur throughout the complete height of the water column.

Water injection dredgers (WID) inject high quantities of water under low pressure into bed sediments to reduce their density to the point that they act as a fluid and flow over the bed naturally through the action of gravity. The distance the water-sediment mixture travels is dependent on a number of factors including the sediment composition, density, morphology and the gradient of the bottom slope.

Underwater plough dredgers (UPD) also know as bed levellers have a basic design of a large frame or steel bar that is pulled over the bed. The frame has a cutting blade which scrapes the bed cutting the bottom sediment layers, these build up in front of the frame and are moved away from the dredge area.

1.5 Associated impacts Navigation dredging operations have the potential to effect the environment in a number of ways ranging from impacts to habitats and species and effects on physical processes to disturbance to humans and birds from noise. However not all these effects will necessarily cause an impact on the marine ecosystem. Whether an effect actually causes an impact depends on a number of factors such as frequency, duration and magnitude.

The effects of the dredging process can be both direct/indirect and short/long term. Direct impacts of dredging can include direct removal of habitat and species and smothering of benthic habitats, indirect impacts include contaminant release through resuspension of sediments and changes to hydrodynamics and sediments regimes. Impacts can be short term, for example an increase in turbidity due to excavation of sediment or the direct removal of a habitat, or long-term such as changes in flow rate.

The following sections focus on the impacts associated with maintenance dredging however capital dredging can result in the same impacts but can also have additional impacts, therefore these are discussed in section 6.

The report goes into further details of the different types of dredgers.

2. Release of contaminants during dredging

Sediment disturbance events, such as dredging and other natural anthropogenic activities can result in alteration in the chemical properties of the sediment. These properties, together with the nature of the pollutant concerned, govern the mobilisation mechanisms for the release of such contaminants.

Contaminant release from dredging operations needs to be quantified to estimate short-term exposures and risk of dredging and potential impacts on long-term risk following completion of dredging. The degree of risk is multi factorial dependent upon methodology and duration of the dredging operation, physical nature of the dredge material, characteristics of contaminants associated with the sediment, current velocities and a range of other physical and chemical factors. Contaminants adsorbed on and to resuspended particles may partition to the water column and be transported great distances downstream in dissolved form along with dissolved contaminants in the released pore water. The resuspended sediment particles will settle and contaminants may be released to the water column by densification, diffusion and bioturbation. This release may have significant implications for the long-term flux of sediment associated contaminants into the water column. Understanding of both contaminant release mechanisms and magnitude of release is limited. Bioavailability and bioaccumulation of contaminants in the aquatic environment are largely governed by partitioning characteristics of the contaminant to sediment, although bioavailability is also influenced by the ability of an organism to metabolise contaminants. The development of new methodology and techniques for mitigation of potentially adverse environmental impacts caused by the dredging of contaminated sediments are ongoing.

The degree of contamination of sediments clearly plays a very important role in determining the


significance of any mobilisation of contaminants from those sediments. This applies to all types of contaminants including those mentioned in this and subsequent sections of the review. In this context it should be noted that the Environment Agency’s ‘Clearing the Waters’ guidance for assessing compliance with the Water Framework Directive utilises in its assessment Cefas Action Levels for metals, TBT, halogenated organics and PAHs in sediments.

3. Effects of navigation dredging in water quality: nutrients, dissolved oxygen and re-suspension of bacteria

While the potential for biostimulatory nutrient release during dredging has been suggested, there have been very few studies of this and associated processes. Dredging activities in estuarine and coastal regions result in the relocation of large volumes of anaerobic porewater and particles, potentially leading to release of nutrients and stimulation of heterotrophic microplankton. Investigations of the extent and the geochemical mechanisms of nutrient release have mainly focused on organic matter mineralization and mineral dissolution during periodic resuspension of surface sediments. While sediment particles may also bind some nutrients, re-suspension of sediments following disturbance also causes rapid release of nutrients to the water column. However, some studies suggest that the effects of the dredging activities on water chemistry in coastal waters appear to be localized, temporally short-lived, and well-within the range of natural variability in the system at short and long time scales. Results of modelling studies also suggest that no significant changes in the levels of water column dissolved oxygen take place at disposal sites after placement of significant volumes of dredged material.

There is limited information on the resuspension of faecal bacteria in contaminated sediment caused by dredging, but some studies suggest that neither turbidity nor bacteriological effects of dredging extended more than 2km from the dredging area.

4. Radionuclides in dredged material

This section of the report reviewed the information on studies concerned with the assessment of dredging activities involving the potential impact of radionuclides contained within the dredged material. However, little information was found implying that such specific assessments are not frequent or widespread. With regard to disposal of dredged material, the report references the London Convention process for assessing whether the material has de minimis levels of radioactivity.

5. The impacts and monitoring of maintenance dredging - related to turbidity

Turbidity is an index of the scattering of light by suspended particles as it passes through water. The principal cause of dredging-related turbidity is the presence of sediment suspended in the water column. The effect of suspended sediment on turbidity is dependent on the sediment particle size and the sediment concentration. Increasing sediment concentration (without affecting sediment size) increases turbidity. Fine sediment such as clays and silts generate much higher turbidity than a similar concentration of coarse sediment. The environmental effects of dredge-related plumes may result from elevated turbidity (e.g. reduced light penetration) or elevated suspended sediment concentration (e.g. physical abrasion), or a combination of both factors. The nature of the sediment and the hydrodynamic conditions will determine the area over which material in a turbidity plume may be distributed (Bolam and Rees 2003; Erftemeijer and Lewis 2006).

The dredging process typically involves four distinct phases:

1. Dislodging of the in-situ material2. Raising the material3. Horizontal transport of the material4. Placement of the material (Bray 2008).

Elevated turbidity can result from the direct disturbance of the seabed during all of the phases. The type of dredging method and type of equipment employed has a bearing on turbidity.

This section of the report considers the generation, impacts, mitigation and monitoring of turbidity with the use of the 3 main types of dredging methods referred to above.

6. Capital dredging impacts

Capital dredging generally involves the removal of ‘virgin’ material that is relatively stable and has become consolidated under the existing hydraulic regime. However it also includes the removal of material from previously dredged areas where sedimentation has since occurred and has not been disturbed by further


dredging over a period of time (10 years). In such cases consolidation of the deposited material occurs and the physical properties of the bed will revert to similar characteristics to the virgin material and is therefore treated as capital dredged material.

Unlike maintenance dredging which is removal recently deposited material chemical contamination associated with capital dredged material is most likely to be from historic rather than current anthropogenic inputs as it is undisturbed, consolidated material. The type and extent of any contamination will vary depending on the location due to human activities and geology.

Capital dredging can also involve the uses of explosives in the dredging of hard rock. The explosives are placed in boreholes drilled into the rock, the work is usually carried out from a floating jack-up pontoon. After blasting the rock is removed by bucket, grab or cutter suction dredgers. Considerable resuspension can be associated with hard rock dredging (Dearnaley et al 1996). Noise effects on fish and mammals from the blasting are another concern.

The previous sections outlined impacts associated with maintenance dredging and these impacts can also occur with capital dredging due to the range of material involved. However as capital dredging is usually associated with removal from areas that have previously been undisturbed it can have additional impacts, that are outlined in this section under the headings:

Physical effects Chemical effects Biological effects Noise effects Socio-economic effects Cumulative/in –combination effects

Gaps in knowledge are also addressed.

7. Mathematical models used to assess the impact of dredging activity

The report reviewed the different types of mathematical models used in this area and described several that have been specifically applied to dredging applications. Computer modelling systems predicting hydrodynamic and morphological processes are powerful tools that can assist in the design, planning and efficient execution of dredging operations, and the assessment of their likely impacts. In the case of maintenance, dredging their primary use is likely to be in the assessment of the likely fate of disposed material.

Objective 3: Literature review of existing mitigation measures and their efficacy in addressing these impacts

The overall management goal of any dredging project should be to achieve a sustainable solution, subject to sound environmental, social and financial impact evaluations, weighing and balancing all the associated risks. Sediments are a basic component of habitats that support aquatic life. Because dredging inherently involves disturbance of existing substrates, either by the dredging itself or subsequent deposition of sediments, impacts on the environment may result. To avoid or limit any negative impacts many national and international conventions have been developed concerning the placement of dredged material in the aquatic environment.

When an environmental assessment determines that impacts may be significant, consideration is given to means to conduct the project in such a manner that impacts are avoided, reduced or mitigated. Many alternatives exist, collectively called dredging project mitigation measures, whereby unacceptable environmental impacts of dredging (including dredged material placement and sediment transport) can be offset. These practices encompass a diverse array, from relatively minor operational changes (e.g., to alter the rate of sediment removal) to application of innovative technologies, or to restrictions that constrain dredging to certain times of the year. The need for mitigation measures is not only limited to those dredging projects that are dealing with contaminated sediments. Even when dredging clean sediments the evaluation of potential environmental effects and identification of mitigation measures to reduce such effects is necessary.

PIANC (2009) recently developed a framework to aid the transparent selection of such mitigation measures for minimising the environmental impacts of dredging projects. As part of this undertaking, the


document comprehensively reviewed the mitigation measures available to any such project. Professionals from all over the world encompassing a variety of disciplines (e.g., regulators, contractors, consultants, port authorities) were involved in developing this review and, as such, PIANC (2009) provides the most up-to-date review of mitigation measures for a dredging project. Consequently, we have used this as a firm basis for the present report, drawing upon its relevance, but also adding to it in aspects which were not as specific as those required for the present study. Although the scope of PIANC (2009) was global, very few measures indicated were deemed to have a non-UK application and, therefore, needed to be left out of the present report.

Mitigation measures for dredging projects

There is a wide range of mitigation measures that may be applicable to a dredging project. These vary greatly in type, environmental effectiveness, economic cost and in the actual stage when they can be implemented during a dredging project. For example, while some measures can be applied or adopted at the planning stage by the ‘owner’ of a project (e.g. the contractor-selection basis) others pertain to restrictions regarding when the dredge can and cannot be undertaken as required by a regulator. Clearly, therefore, any appraisal of the efficacy of mitigation measures available for dredging projects requires a carefully-designed basis within which to categorise the entire list of measures. The report on this objective adopts the classification of PIANC (2009) where 6 broad types of mitigation measures were designated:

Contractual and project management Project design and planning Equipment related Institutional Dredging-related Control-related

A summary of the measures within each category is presented in the report on this objective; more detail is provided regarding each mitigation measure, its limitations and advantages and disadvantages in the succeeding tables. Note that PIANC (2009) provides further information for each, indicating to what extent the effectiveness of each measure can be either predicted or monitored, together with providing examples where each measure has been used and references for further reading.

2. Report of stakeholder workshop

A stakeholder workshop was held on 11 March 2010 to provide feedback on both the literature reviews and guide any future work programme. The report of the workshop was as follows:

Gaps in the Literature review report

Details of dredgers and dredging techniques

Lots of information was provided on dredging method and technologies but some further amendment to update this with descriptions of new developments to increase efficiency was felt to be required. Further detail could also have been provided with respect to smaller scale operations e.g. JCB on a barge , as although the nature of the impacts resultant from the dredging process is consistent, its impact in relative terms could be greater. There appeared to be a gap in the knowledge on the specifics of dredging practice i.e. the timing and duration of the process. .Specific knowledge regarding agitation and plough dredging is very limited.

Noted that trailer dredging is omitted in error from the list provided.

It was felt that inclusion of images of the equipment (available from PIANC or other) would have been helpful.

Would also have been useful to include images of the dredging plumes generated by differing methodology and subsequent differences in behaviour in the water column together with relative quantification of overflow predictions which have significant impact on monitoring.

Level of knowledge re source output of various dredging techniques.

Quality measurement data required of quantities of output and physical state, required in order to facilitate representative modelling of dredging event. Ongoing USACE project which will provide useful information when complete but largely related to backhoe and grab dredger campaigns. Some measurement


information available for trailer suction dredging, conducted by Dutch companies. Measurement data also needs to be available for other techniques widely used in the UK e.g. cutter suction also agitation and hydrodynamic, in order to allow validation and calibration of modelling of output and subsequent application in licence conditions. Possible that current estimation of quantity of source output could be up to order of magnitude of 10x out. Importance of validation increases with potential magnitude for output of operation. Outputs of water injection and agitation dredging methods need to be addressed in the light of any site specific sensitivities e.g. contaminated material, conservation interests.

Monitoring

Discussion of approach adopted in the absence of real output data for licensing consent and its limitations. Needs to be site specific particularly in terms of local ambient suspended sediments data and variations observed. Knowledge is required of temporal baseline data in order to set reliable and realistic threshold values, including impact of storm events.

The importance of having a clear understanding of what it is that might potentially be affected by the process was agreed i.e. the biota present and its recovery success/ rate. It would appear that there is a scarcity of data with respect to the sub-tidal zone. The ALSF sensitivity project may provide the basis for consideration but more offshore species would be required for inclusion.It is possible that there may be a large quantity of grey literature available on recovery but this needs to be sourced and assimilated.

Work undertaken by the EA on the reliance of estuaries to climate change may also provide another source of information.

Literature report- Para 5.3 optical monitoring and 5.4 acoustic monitoring - Some caution necessary in the adoption of such techniques, list provided is not a panacea and not possible to employ without calibration.

Mitigation Measures

Agreed thorough project planning is ultimately most effective mitigation available- dredging equipment, timing restriction, dredging schedule etc. The application of adequate back ground data and baseline information is crucial in the selection of appropriate methodology.

Physical barriers to prevent dredge material migration e.g. silt curtains, bubble screens only of limited value in very narrow range of ambient conditions- estimated current speeds of <0.5m/s. Additionally any accumulated sediments potentially released ultimately once barrier removed. Possible to successfully employ specialist dredging equipment in sensitive dredging locations or in campaigns involving contaminated sediments e.g. Estimated that closed buckets could reduce load losses by around 0.5 in fairly mobile sediments, specialised enclosed grabs for use in removal of contaminated sediments.

? ongoing development of any supplementary techniques.

Considered PIANC report to provide good coverage, cost and timescale implications and comparative risks.

Cumulative effects of combined dredge operations

Water quality considered probably not to be the issue but possible effects on seabed morphology. The necessity to have regard to the potential for cumulative effects from smaller operations was agreed. Discussion of potential approach including adoption of code of practice with respect to dredging operations within estuaries coordinated by Harbour Authorities, River Hamble was considered to be a good example where such a collaborative exercise could be implemented.

Modelling dredge operations

Considered important to model the processes that contribute to sediment movement i.e. flow modelling of advection, dispersion etc. and consider the use of particle tracking. The significance of non-tidal events such as surges and freshwater inputs should also be investigated further.

In particular, it was considered beneficial to move from modelling on a small scale towards investigations over a longer temporal period, particularly for larger, long term dredge projects therefore removing variations due to seasonality. Particularly with respect to nutrient modelling where more experience is


required to supplement that already conducted by the EA. Apparent that variation in toxicity is very dependent upon site specific conditions.

The provision of more feedback from the assessor on the models utilised was considered to be helpful.The successful application of flood risk and coastal erosion models appeared to require more development.

Biological aspects

There appeared to be a lack of information on some biological aspects of assessment such as impacts of dredging on migratory fish and protected species in particular. The effect of raised turbidity levels and oxygen depletion on salmon was considered particularly important.

Contaminants

It was agreed that further information on the WFD determinants in the sediments is required.It was considered that interpretation of contaminant levels was confusing due to differences between Cefas action levels and EA limits for water quality. However, the reason for this is unclear since they relate to different phases?

Case studies

Suggested that case studies could be introduced into the table of the mitigation measures at the back of the report. Information from completed EIA’s could be utilised to populate. Possible that the MMO in their new role could contribute a knowledge base to promote best practice through presenting case studies of ‘successful’ projects. Useful to address dredging and disposal as a holistic project


References to published material9. This section should be used to record links (hypertext links where possible) or references to other

published material generated by, or relating to this project.

None published but used internally in advisory work.


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