cefas contract report me5403 - module 19 - defra,...

Cefas contract report ME5403 - Module 19

Project ME5403 – Applied Science to support

the licensing of dredging, disposal, renewables and general construction and associated monitoring under the future Marine Act

Module 19 - Hydrodynamic Dredging and Plough Dredging Guidance Note

Authors: Andrew Birchenough & Lara Howe

Issue date: October 2011

Hydrodynamic Dredging and Plough Dredging Guidance Note Page i

Cefas Document Control

Title: Hydrodynamic Dredging and Plough Dredging

Guidance Note

Submitted to: Defra (Cathal Linnane)

Date submitted: October 2011

Project Manager: Sonia Kirby

Report compiled by: Andrew Birchenough

Quality control by: Chris Vivian

Approved by & date: Stuart Rogers 17th

October 2011

Version: V5

Version Control History

Author Date Comment Version

A. Birchenough 03/10/11 Draft V1

A. Birchenough 12/10/11 Draft (CV comments) V2

A. Birchenough 12/10/11 Draft (SR comments) V3

A. Birchenough 24/10/11 Draft (SK comments V4

S. Kirby 26/10/11 FINAL V5

Hydrodynamic Dredging and Plough Dredging Guidance Note

Project ME5403 –

of dredging, disposal, renewables and general construction

and associated monitoring under the future Marine Act

Module 19 - Hydrodynamic Dredging

Authors:

Head office

Centre for Environment, Fisheries & Aquaculture

Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK

Tel +44 (0) 1502 56 2244 Fax +44 (0) 1502 51 3865

www.cefas.defra.gov.uk

Cefas is an executive agency of Defra


Applied Science to support the licensing


sociated monitoring under the future Marine Act

Hydrodynamic Dredging and Plough Dredging Note

Authors: Andrew Birchenough & Lara Howe

Issue date: October 2011

Centre for Environment, Fisheries & Aquaculture Science

Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK

Tel +44 (0) 1502 56 2244 Fax +44 (0) 1502 51 3865

Page ii

Applied Science to support the licensing


sociated monitoring under the future Marine Act

and Plough Dredging Guidance

Hydrodynamic Dredging and Plough Dredging Guidance Note Page iii

Hydrodynamic Dredging and Plough Dredging Guidance Note Page iv

Executive Summary

1.1. The MMO will be responsible for the licensing of hydrodynamic dredging and plough dredging

activities under the MCAA from April 2012.

1.2. This report has been compiled to provide guidance to the MMO on hydrodynamic dredging

and plough dredging operations and the issues that will need to be considered in assessing

applications for a Marine Licence.

1.3. Hydrodynamic dredging and plough dredging can have advantages over more conventional

dredging techniques such as being cost effective, operating where other dredging equipment

cannot, helping to maintain sediment budgets and having a low carbon footprint.

1.4. Hydrodynamic dredging and plough dredging activities can range from relatively small scale,

low risk activities such as small plough dredging/bed levelling operations, to larger scale WID

(Water Injection Dredging) operations that can dredge large volumes of sediment and pose a

higher potential risk to the marine environment and other uses of the sea.

1.5. In the licensing of hydrodynamic dredging and plough dredging activities it is recommended

that all applications are assessed on a case by case basis and that the level of assessment

required and management measures employed should be determined on a ‘risk-based’

approach.

1.6. It is also recommended that the MMO consider the following when licensing hydrodynamic

and plough dredging activities: sediment contamination; hydrodynamic assessments;

employing license conditions to reduce the risk of dredging operations, such as restricting the

timing of the activity; monitoring and liability.

Hydrodynamic Dredging and Plough Dredging Guidance Note Page v

Table of contents

1 Introduction ................................................................................................................................... 1

2 Overview of Dredging Techniques .............................................................................................. 2

3 Hydrodynamic Dredging .............................................................................................................. 4

3.1 What is Hydrodynamic Dredging? .......................................................................................... 4

3.2 Water Injection Dredging (WID) .............................................................................................. 5

3.3 Agitation Dredging .................................................................................................................. 7

4 Plough Dredging (or Bed Levelling)….……………………………………………………………... 8

5 Advantages and Disadvantages of Hydrodynamic Dredging and Plough Dredging ……… 9

5.1 Advantages ............................................................................................................................. 9

5.2 Disadvantages ....................................................................................................................... 10

6 Environmental Impacts ............................................................................................................... 10

7 Other Considerations ................................................................................................................. 12

7.1 Other legitimate uses of the sea ........................................................................................... 12

8 Environmental Assessment and Management ........................................................................ 12

8.1 Environmental assessment ................................................................................................... 12

8.2 Management measures ...................................................................................................... 124

8.3 Consideration of other legislation ...................................................................................... 126

8.3.1 Water Framework Directive (WFD) ...................................................................................... 16

8.3.2 Waste Framework Directive ................................................................................................. 16

9 Monitoring .................................................................................................................................... 17

10 Conclusions and Recommendations for Licensing ................................................................ 18

11 References ................................................................................................................................. 212

12 Annex 1 ........................................................................................................................................ 22

1


1 Introduction

Prior to the Marine and Coastal Access Act 2009 (MCAA) the statutory control of dredging

operations was complex, being the responsibility of a number of regulatory authorities including

port and harbour authorities who operate under local acts which empower them to undertake

dredging works within the limits of their jurisdiction. In most cases a licence was also required under

the Food and Environment Protection Act 1985 (FEPA) to dispose, or place for beneficial use,

dredged material anywhere below the mean high water spring tide mark (MHWS). An operator may

have also required consent under the Coastal Protection Act (CPA) if the dredging affected

navigation.

Under the previous licensing regime the MMO (as the licensing authority for FEPA and CPA) and

Cefas (as scientific advisor to the MMO) had experience and knowledge of the types of dredging

methods that required the need for the disposal of material at sea. However there are certain

hydrodynamic dredging methods that did not require a disposal to sea licence as the dredging

operations did not involve the excavation and removal of sediment from the river/sea bed.

Therefore the knowledge of these dredging methods, within the MMO and Cefas, and the

implications for licensing is currently limited.

Under the MCAA dredging is now a licensable activity, unless Section 75 of the Act applies which

exempts dredging or disposal from requiring a Marine Licence if it is carried out by or on behalf of a

harbour authority in accordance with a Harbour Order or Local Act. Therefore, dredging activities

outside of harbour authority jurisdiction will require a Marine Licence from the MMO and this will

include hydrodynamic dredging and plough dredging activities that were not previously licenced

under FEPA. The MCAA introduced definitions for dredging, describing the activity as ‘including use

of any device to move any material (whether or not suspended in water) from one part of the sea or

sea bed to another’.

The new marine licensing system was introduced in April 2011 however there is a transitionary

period of a year for dredging activities that did not previously require a FEPA licence or consent

under CPA. Therefore newly regulated dredging, including hydrodynamic dredging and plough

dredging operations, will not require a Marine License until April 2012.

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In anticipation of the MMO licensing of dredging this report has been compiled to provide guidance

to the MMO on hydrodynamic and plough dredging activities and the issues that will need to be

considered in assessing applications for a Marine Licence.

2 Overview of Dredging Techniques

There are several methods of dredging used in the marine industry for a variety of purposes ranging

from routine maintenance of small marinas and harbours to the 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 categories of dredging equipment which are

based on the physical processes involved in the excavation and transport of the dredged material:

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 whilst

being acknowledged that it is not 100% accurate for all of them (PIANC, in press):

• Water injection dredging (WID)

• Plough dredging/Bed levelling

• Agitation dredging

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However in the normal use of a plough dredger the vast majority of the seabed material is moved by

a mechanical push of the equipment and so differs from rakes and beams which generally aim to re-

suspend the bed material to move it (forms of agitation dredging). Therefore plough dredging is not

strictly a hydrodynamic technique so is described in a separate section (Section 4) in this guidance

note.

The techniques of hopper overflow1 and sidecasting

2 are also sometimes regarded as types of

hydrodynamic dredging (PIANC, in press) as they place material back into the water column into

suspension to be transported by natural forces. However these methods are considered by Cefas to

be forms of dredged material disposal as the material is lifted from the water column and can be

returned at the dredge site or alternative locations away from the dredge area. These techniques are

not considered here as hydrodynamic dredging and are not considered further in this guidance note.

The following are considered to be hydrodynamic dredging techniques in this guidance note and

described in Section 3:

• Water injection dredging (WID)

• Agitation dredging (including raking, jetting and plough dredging where material is

deliberately raised into the water column)

After hydrodynamic dredging and plough dredging are described in Section 3 and 4, respectively,

they are considered together in the subsequent sections (Sections 5 -10).

In general, hydraulic and mechanical methods are considered as conventional dredging techniques

and are the main types of dredging employed for navigation dredging in the UK and Europe. For

example TSHD is estimated to be responsible for in excess of 90% of maintenance dredging by

volume in Europe (Bates, 2005). Small ports, harbours and marinas tend to use mechanical dredgers

such as GD and BD working with barges because they can work in more confined areas, such as

alongside wharfs. However, hydrodynamic dredgers are being used increasingly to dredge ports and

harbours around the UK (Sullivan, 2000).

1 When dredging materials, overflowing may occur to increase the solid loads in the hopper and improve the

efficiency of dredging operations. 2 Sidecasting dredgers discharge dredged material via a pipeline supported by a boom structure to one side of

the ship rather than the dredged material passing through the ship’s hopper.

4


Hydrodynamic dredging definition:

Hydrodynamic dredging is a method where sediment is placed into suspension

in the water column, without the physical removal from the water body, and

then the horizontal transport of the dredged material takes place by natural

forces without the need for an additional energy input.

3 Hydrodynamic Dredging

3.1 What is hydrodynamic dredging?

For hydrodynamic dredging the water column is used as the primary transport medium for the

dredged material (instead of pipes, barges or hoppers, as with the more conventional dredging

techniques). The dredged material is lifted into the water column and remains there to be

transported away by natural forces, either under the influence of natural gravitation forces (WID) or

by currents (agitation dredging) (CEDA/IADC, 2008).

The following definition of what constitutes hydrodynamic dredging is proposed:

In WID the vertical movement of the sediment is purposely limited to about a metre above the

sea/river bed. During agitation dredging the sediment is put into suspension, which can be over the

whole of the water column (Figure 1).

Process I II

Method Injection Agitation

(sediment into suspension in water

column)

Technique Water Injection Raking

Jetting

Ploughing (where sediment deliberately

stirred in water column)

Figure 1. Hydrodynamic dredging processes (adapted from CEDA/IADC, 2008)

5


3.2 Water Injection Dredging (WID)

This technique involves the injection of large volumes water into the seabed to fluidise and mobilise

sediments. The water is injected at low pressure, through a fixed array of water jet nossels, into the

near surface layers of the seabed sediment to fluidise the bed material (Figure 2 and 3). This is

known as a fluidised soil layer3 (PIANC, in press) and it is formed when the density of the material

decreases to a point where it becomes liquid. This layer can then flow by gravity or current from the

dredge site horizontally or down very gentle slopes to deeper water. When the fluidised soil layer is

on the move it is called a density or turbidity current, it can also be referred to as a density flow

(PIANC, in press).

The fluidised soil layer created by WID remains close to the bed and therefore the overlying water is

not affected by this method, unlike agitation dredging. A small difference in density between the

fluidised soil layer and the surrounding water is sufficient for it to flow underneath the surrounding

water to a deeper area. The distance of flow can vary from a few meters to a few kilometres

depending on the soil properties, seabed gradient and the hydrodynamic conditions. Currents

moving in the same direction will drag the fluidised soil layer and increase its speed; conversely

currents in the opposite direction will reduce it.

In areas of low currents (low energy areas) the combined conditions of current and slope will

determine the speed and direction of turbidity current (PIANC, in press). In turbulent waters (high

energy environments) the dredged material will not settle but may be redistributed into suspension

by the natural currents, which would then transport the material to where conditions are suitable

for settlement, often areas with greater water depth.

3 During the water injection process the distance between the sediment/soil particles increases until the

sediment/soil layer becomes fluid, creating a fluidised soil layer. In the literature this is also referred to as fluid

mud, fluid bed or density cloud (the term density cloud can be misleading as it implies that the material is

suspended in the water column).

6


Figure 2. Basic principles of WID (PIANC, in press)

Figure 3. WID with an artists view of the underwater operation. Courtesy of CEDA / IADC from

Environmental Aspects of Dredging (CEDA/IADC, 2008)

WID is mostly used for maintenance dredging of recently deposited sediments in ports, harbours and

marinas, and navigation channels to maintain sufficient depth for vessel movements. However it

can also be used to: (1) clean locks, jetties and pipelines (or other submerged structures) that have

issues with siltation; (2) dredge sand dunes (shallows) that arise on river beds that can impede or

7


endanger navigation; and (3) level off high spots created by hopper tracks from a Trailer Suction

Hopper Dredger (TSHD), improving the performance of the TSHD.

3.3 Agitation Dredging

Hydrodynamic dredging by means of agitation is a method of dredging which aims to lift the

excavated material into the overlying waters to be transported by currents and tides. It includes the

use of specifically designed trailing equipment such as rakes or beams (sometimes referred to as

agitators). These tools are towed by vessels dragging them along the sea/riverbed to stir up the

material into the water column to be moved by natural forces. It is difficult to stir up the material

throughout the whole water column using this type of equipment; however, the propeller of the

ship towing the equipment can create enough turbulence to spread the material (Bray et al, 1997).

As discussed in Section 3.1 a plough dredger can also be employed in a similar way. Agitation

dredging also includes jetting, whereby air is blown into the sediment at high pressure through a

pipe to disturb the sediment.

There have also been a number of other types of equipment designed to perform agitation dredging

including:

• Wing excavator - a type of platform which is lowered near to the seabed to scour the bed by

means of two powerful thrusters/downward facing impellers.

• Hydrodigger – a hydrodynamic excavation system which uses high mass jetting to direct a

column of water vertically downward at the seabed to excavate the bed material.

• Rotech Aquaflow T8000 – similarly, this piece of equipment generates a high velocity, high

volume water column to excavate up to 1.5 tonnes of bed material per second.

In addition the propellers of vessels can also be used to stir up sediment into the water column

therefore this activity can also be considered a form of agitation dredging.

Generally these techniques are used for maintenance dredging projects in areas with strong tidal

currents and high background turbidity but have also been used in capital projects. When currents

are variable agitation dredging can be restricted to periods when currents are high (CEDA/IADC,

2008).

8


4 Plough Dredging (or Bed Levelling)

A plough can be described as a frame that is pulled over the seabed by a tug boat to relocate bed

material. The plough is lowered to the seabed to the required depth and then pulled over the area

to be dredged. The plough frame consists of an open steel box with a cutting blade on the front that

scrapes over the bed cutting the bottom layers. As the plough is pulled along the cut material is

caught in the box, as the box has an open bottom the material will fall out as soon as the water

depth increases beyond the depth that the plough is set, creating a level bottom (hence the

alternative name ‘Bed Leveller’). If material continues to gather in the box or in front of the cutting

blades the transport process is either stopped or the material falls off the blade and can go partially

into suspension. As with WID, ploughing should not lead to significant re-suspension of sediment

but if the sediment ploughed is soft it may be sufficiently disturbed to rise in to suspension.

Figure 4. Plough or Bed leveller

Ploughs can be used for maintenance dredging in tidal basins, small marinas and harbours as a stand

alone technique. They are used to move material over relatively short distances from shallow to

deeper areas to re-establish safe navigation depths, without the need to transport material to a

disposal site. The material may also be dragged into the main tidal stream to reintroduce it back into

the channel’s natural transport process. Because the vessels required for ploughs are small and

9


manoeuvrable they are also used to relocate bed material from areas inaccessible to other dredging

equipment, such as a THSD, to where it can be dredged.

A plough dredger can also be used to deliberately stir material into the water column for it to be

removed by natural forces (hydrodynamic dredging), this could be achieved by increasing the vessel

speed.

5 Advantages and Disadvantages of

Hydrodynamic Dredging and Plough

Dredging

5.1 Advantages

Hydrodynamic and plough dredging can have a number of advantages over more conventional

dredging techniques. The following advantages have been identified:

• Cost effective – both hydrodynamic and plough dredging are a low cost dredging method

compared with conventional dredging methods, with relatively simple equipment used, with

low investment and low operating costs. For example WID can be a third to a tenth cheaper

than conventional mechanical methods (N. Clay, pers. comm.)

• Accurate, manoeuvrable and flexible - hydrodynamic and plough dredgers can operate

where other equipment cannot (i.e. close to jetties, berths, quay walls). They can be used to

complement conventional dredging methods or as a standalone technique.

• Help to maintain sediment budget – hydrodynamic and plough dredging ensures sediment is

retained within the system, which may be beneficial in maintaining mud flats and salt

marshes and contribute to achieving conservation objectives.

• Low carbon footprint – as hydrodynamic dredging uses natural forces to transport the

dredged material no additional energy is required unlike conventional dredging methods

which may use barges or the actual dredger to transport the material to a disposal site, this

aspect of a dredging project can entail significant energy consumption.

10


5.2 Disadvantages

Hydrodynamic and plough dredging should not be used for environmentally sensitive projects

because the material is not physically removed from the environment; it is merely shifted to another

location in a rather uncontrolled and less predictive manner (IADC/CEDA, 1998).

• More dependent on site specific conditions – certain areas will be more suited to

hydrodynamic dredging. For example areas with strong tidal currents to disperse/move the

material and, in relation to WID areas, in which the bathymetry encourages the transport of

the material in the desired direction, often this is seaward. Also, hydrodynamic dredging

techniques cannot easily move coarse sediments.

• Accuracy - the accuracy of WID can be lower than other dredging techniques as it is difficult

to control the effective penetration depth of the injected water or the cutting blade in the

bed. This depends largely on the characteristics of the material being dredged. Some control

is possible with UWP by careful handling of the frame’s suspension wires or by using rigid

arms (IADC/CEDA, 1998).

• Restricted control of material – as hydrodynamic dredging uses natural forces to transport

dredged material it has restricted control of the fate of the dredged material compared to

disposal methods.

• Output rates limited - ploughs, using large blades can reach outputs of 2000 m3/hour

however output rates are limited when compared to more conventional dredging

techniques (IADC/CEDA, 1998).

6 Environmental Impacts

All 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

11


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 re-

suspension of sediments and changes to hydrodynamics and sediment 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.

Generally hydrodynamic dredging and plough dredging will have the same potential impacts as other

dredging techniques. However due to: (1) the restricted control of the material, once it is dredged,

and: (2) all of the material dredged remaining in the water column, the following impacts warrant

particular consideration.

Dispersion of contaminants – If contaminated sediments are dredged using hydrodynamic or plough

dredging techniques then there is a high potential for the remobilisation and uncontrolled spread of

contaminants, which can result in ecotoxicological effects to both water and sediment dwelling biota

(OSPAR Commission, 2004). This of particular concern with hydrodynamic dredging as transport

distance, quantity and placement of relocated sediment are difficult to both estimate and control

(Sullivan, 2000; OSPAR Commission, 2004). Therefore it is generally agreed that hydrodynamic

dredging is not to be employed to dredge contaminated sediment (PIANC, in press, CEDA/IADC,

2008).

Increased turbidity – Dredging operations are likely to cause local increases in turbidity. This can

result in a decrease in the depth that light is able to penetrate the water column which can result in

a reduction in plant photosynthesis which reduces productivity. Increases in turbidity are more likely

to be an issue for agitation dredging as this technique deliberately puts sediment into suspension in

the water column, whereas with WID it is purposely limited to a few meters above the sea/river bed.

Smothering - Increases in suspended sediment may result in the smothering of filter feeders

(Sullivan, 2000) and other sensitive species. This can be an issue for both agitation dredging but also

for WID if the fluidised soil layer travels to areas containing sensitive ecological receptors (e.g.

seagrass beds and shellfisheries).

Reduction in dissolved oxygen - Oxygen and nutrient consuming substances and other harmful

materials bonded to the sediments can be released through dredging and thus reduce oxygen or

cause increases in concentrations of nutrients or harmful material (van Raalte and Bray, 1999).

12


Deoxygenation can lead to the suffocation of benthic communities and fish and may also deter

migratory fish from passing through an affected area. This effect may only be temporary as tidal

exchange can quickly replenish oxygen levels however the impact on marine flora and fauna can be

severe.

All forms of dredging can affect water quality through increases in suspended solids and turbidity

and a reduction of dissolved oxygen however they generally occur over short timeframes and need

to be considered with regard to natural background levels.

7 Other Considerations

7.1 Interference with legitimate uses of the sea

When making licensing decisions the MMO must have regard to the need to prevent interference

with legitimate uses of the sea, in addition to protecting the environment and protection of human

health. Hydrodynamic dredging has the potential to cause interference with other activities, such as

fishing activity (e.g. smothering of pots and nets) and other port/marina operators (e.g. affecting

navigation channels or harbours and berths) through the deposition of the dredged material. The

potential for financial liabilities has been highlighted as a particular concern should sediment moved

by WID be deposited in the berth or channel areas of other operators.

8 Environmental Assessment and

Management

8.1 Environmental assessment

As with all dredging operations, hydrodynamic dredging and plough dredging should be subject to an

assessment of the likely significant environmental effects. This assessment should be undertaken on

a case by case basis. The assessment should consider the additional potential effects from the

greater volume of sediment that is introduced into the receiving environment from hydrodynamic

dredging, with particular attention given to the environmental impacts described in Section 4.

Often hydrodynamic and plough dredging operations are small scale and repetitive maintenance

operations, for example some plough dredging activities in marinas and berths may be very localised

13


with only small volumes of material being dredged. In such cases a detailed environmental risk

assessment may not be proportionate to the environmental risk of the project. In such cases where

a large scale environmental risk assessment may be costly and disproportionate, PIANC (in press)

recommend that at least an ‘expert study’ should be made before the start of the project.

An expert study would essentially be an assessment of the environmental implications of the

dredging activity by the MMO and their scientific advisors, Cefas. To undertake this there would be a

requirement for certain information to be supplied with the application. The minimum requirement

would be information on:

• Location – areas to be dredged (e.g. within the marina, port, access channel) and the wider

environment.

• Amount and type of material to be dredged.

• Dredging method.

• Previous dredging activities – e.g. is this an ongoing activity or has there been other dredging

activities in the area.

In addition it is recommended that contaminant analysis is carried out on initial applications for a

Marine Licence. This information can be supplied with the licence application or undertaken through

the formal pre-application and application process:

• Contaminant analysis of sediment samples – The number of samples and determinands to

test for would be identified on a case by case basis based on knowledge of the area and

historic information, where available. However it is recommended that it would include

analysis for metals and Tri-Butyl Tin as a minimum.

It is recommended that a detailed environmental assessment and possibly appropriate studies (e.g.

assessment of the fate of the material) are carried out for hydrodynamic dredging activities where:

• There are new areas to be dredged;

• There has been a change in the dredging technique;

• There is an increase in the scale of the dredging activities;

• There is a change in contamination levels or;

• There has been environmental damage reported.

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Repeated assessments for ongoing maintenance dredging activities may not be considered

necessary unless there is a significant change to the dredging operation, or the sensitivity of the

receiving environment or the material is considered to have behaved differently to that assessed.

8.2 Management measures

The outcome of the expert study/environmental risk assessment will determine the level of risk of

the dredging project. If the risks of the project are moderate or high then management measures

could be required to further reduce those risks, these can include:

Tidal constraints - Restricting dredging operations to certain tidal windows can limit the potential for

suspended sediments to move towards sensitive areas but can also aid the direction of travel and

dispersion of the dredged material, for example restricting operations to the ebb tide can facilitate

the seaward movement of the dredged material.

Seasonal or timing constraints - Dredging can be restricted to avoid the seasonal

spawning/migratory periods of specific species, to avoid certain tidal states or night/day times.

Reduction of the operating time - The operational time of the dredging activities can be reduced to

limit the amount of suspended sediment that is put into the water column over a certain period.

Modelling of the plume dispersion - Modelling can be undertaken to assess the potential fate of the

sediment to be moved by the dredging operation. The results of the modelling may then lead to the

requirement for other management measures described in this section.

Monitoring - Appropriate monitoring can confirm the predictions made in an environmental impact

assessment, providing information and assurances on the operation to the regulator. Monitoring can

vary from routine monitoring, such as pre- and post-bathymetric surveys, to a more detailed

monitoring programme that includes water quality monitoring with trigger levels (see Section 7).

With specific regard to WID the following measures could also be employed to reduce the risk of the

dredging operation:

15


Trial dredge - A small scale trial dredge can be undertaken to confirm the predictions of the

sediment transport pathways from modelling studies. The feedback from the trial can then be used

to determine if the full dredging operation can be licensed/proceed.

Dredging in layers - WID can be carried out by dredging in layers, this limits the amount of sediment

is transported at one time.

Dredging areas in sequence - In areas where there are a number of berths or areas that require WID

it is important to consider the sequence in which they are dredged. If WID is to take place on an ebb

tide to facilitate the seaward movement of the material then the furthest dredge location upstream

should be dredged first to ensure that dredged material is not deposited in berths after they have

been dredged.

Creating flow/access channels - Channels can be dredged to facilitate the flow of the sediment

fluidised by WID thus providing more certainty that it will travel in the required direction. For

example this may be required to convey the material into the outgoing harbour tidal flow, or to

avoid sensitive areas.

Despite the fact that hydrodynamic and plough dredging operations have not been licensed under

the previous licensing regime they have often been undertaken in a responsible manner. Particularly

with regard to WID, harbour authorities and the contractors involved in the operations have

undertaken modelling and pre- and post-dredging surveys to determine the fate of the material and

to improve the effectiveness of the technique. Therefore in some cases information (e.g.

hydrodynamic assessment and monitoring data) may already exist and be included with the Marine

Licence applications for maintenance dredging. Where this is available further hydrodynamic

assessment may not be required and the level of monitoring reduced.

For example, WID has been used to maintain depths for marina operators in Portsmouth Harbour for

a number of years. Consent was required from the harbour authority (under its local powers) for the

operations. The Harbour Authority has required sediment quality sampling followed by pre- and

post-dredge monitoring to understand the likely fate of WID material, and to ensure the mobilised

sediment was not contaminated. Monitoring campaigns have focused on the fluidised soil layer

created by WID, successive years of monitoring have confirmed that the layer retains most of the

16


mobilised material, which stays relatively close to the seabed, creating virtually no turbidity higher in

the water column (ABPmer, 2011).

8.3 Consideration of other legislation

8.3.1 Water Framework Directive (WFD)

The WFD aims to protect and enhance water bodies within Europe and covers all estuarine and

coastal waters out to one nautical mile. All dredging activities need to be compliant with the WFD

and all applicants for a Marine Licence will be required to undertake a WFD compliance assessment

in accordance with the Environment Agency’s ‘Clearing the Waters’ guidance4, to ensure that

ongoing maintenance dredging activities are not having a detrimental impact on water body

ecological objectives.

8.3.2 Waste Framework Directive

The EU Waste Framework Directive (2008/98/EC) provides the overarching legislative framework for

the collection, transport, recovery and disposal of waste and includes a common definition of waste.

The Directive requires all Member States to ensure waste is recovered or disposed of without

endangering human health or causing harm to the environment.

Section 75 of MCAA included an exemption from licensing for dredging and disposal of dredged

material if carried out by or on behalf of Harbour Authorities. However to comply with the Waste

Framework Directive the MCAA was amended (The Marine and Coastal Access Act 2009

(Amendment) Regulations 2011). The purpose of this Statutory Instrument was to ensure that the

Government can continue to issue permits for the disposal of dredged material by Harbour

Authorities. The instrument achieved this by amending Section 75 and the effect of this was that:

(1) all disposal at sea of dredged material by Harbour Authorities or anyone else would likely

need a Marine Licence;

(2) Harbour Authorities will not need to deposit dredged material for the purposes of land

reclamation, managing waters and waterways, preventing floods and droughts within

surface waters provided the activity is authorised by Local Act or Harbour Order and they

have demonstrated to the MMO’s satisfaction that the sediments are non-hazardous; and

(3) Forms of dredging which do not involve deposits (e.g. plough dredging, water injection

dredging and agitation dredging) will not need a licence if they are carried out by a Harbour

Authority in accordance with a Harbour Order or Local Act.

4 http://www.environment-agency.gov.uk/business/sectors/116352.aspx

17


9 Monitoring

During hydrodynamic and plough dredging operations, none of the material dredged is brought

above the surface therefore methods to establish the results of the dredging activity are limited

(PIANC, in press). However there are a number of techniques used to monitor sediment pathways

and concentrations, the volume of material dredged and fate and deposition of the sediment. These

include:

• Bathymetric survey techniques: to establish the dredged volume by pre- and post- survey.

These can be employed at the dredge location and to identify areas of sediment deposition.

• Particle size analysis (PSA): determined from grab samples, has been used to monitor

sensitive areas to determine if any sediment deposited was attributable to the dredging

operation (assuming that the characteristics of the dredged sediment were different to the

area of concern).

• Fixed and mobile turbidity or suspended sediment meters, echo sounders and Acoustic

Doppler Current Profilers (ADCP): to monitor suspended sediment levels and sediment

plumes. They have also been used to attempt to detect the fluidised soil layer which is

created by WID.

The most effective method for establishing the dredged volumes is by pre- and post-bathymetric

surveys (PIANC, in press).

It is important that any environmental monitoring programme should be proportionate to the size of

the dredging operation and potential impacts caused by the project. Not all monitoring methods will

be applicable or necessary for smaller projects. As stated above, where dredging projects have been

subject to monitoring and the results of that monitoring is available, it may be reasonable to require

reduced monitoring effort if the dredging operations remain within similar parameters.

18


10 Conclusions and Recommendations for

Licensing

Hydrodynamic dredging and plough dredging have been widely employed for maintenance dredging

activities. These can range from relatively small scale, low risk activities such as small plough

dredging/bed levelling operations in marinas and berths, to larger scale WID operations that can

dredge large volumes of sediment and pose a higher potential risk to the marine environment and

other uses of the sea.

MMO as an ‘enabling regulator’ needs to be consistent and proportionate in the licensing of marine

activities ensuring that licensing decisions are risk-based and, where appropriate, in line with the

precautionary principle.

In the licensing of hydrodynamic dredging and plough dredging activities it is recommended that all

applications are assessed on a case by case basis and that the level of assessment required and

management measures employed should be determined on a ‘risk-based’ approach. For dredging

activities where there is sufficient knowledge and evidence of operation and for those identified as

low risk it is recommended that they would only require an initial assessment. However in order to

undertake that assessment certain information would need to be supplied with the application, at a

minimum this would include:

• Location

• Amount and type of material to be dredged

• Dredging method

• Previous dredging activities

• Assessment of sediment quality (this could be undertaken during the Marine Licence

application process)

Repeated assessments for ongoing maintenance dredging activities may not be considered

necessary unless there is a significant change to the dredging operation, or the sensitivity of the

receiving environment or the material is considered to have behaved differently to that assessed.

This would simplify the licensing process for low risk dredging activities.

Outlined below are further recommendations that the MMO should consider in the licensing of

hydrodynamic and plough dredging activities.

19


Sediment contamination - It is recommended that an assessment of the quality of the material to be

dredged is undertaken for all Marine Licence applications for hydrodynamic dredging.

Given the restricted control over the fate of the material it is recommended that the assessment of

sediment suitable for hydrodynamic and plough dredging should conform to contaminant levels that

would be considered acceptable for beneficial use projects in near shore environments. This is

determined on a case by case basis and would need to take into consideration background levels,

however it is generally only acceptable for those sediments that have contaminants at or around

Cefas Action Level 1 (see Annex 1).

Hydrodynamic assessment - For all WID operations a hydrodynamic assessment should be

undertaken to assess the potential fate of the dredged material. This information may be already

available where WID is already in use for maintenance dredging. For new applications an assessment

will be necessary but may only be required for the initial proposal unless there is significant change

to the dredging operation or the sensitivity of the receiving environment, or the material is

considered to have behaved differently to that assessed. However for lower risk activities, such as

small plough dredging operations, the practicalities undertaking such surveys may prove difficult and

their cost may be disproportionate to the cost of the dredging operation itself.

Management measures – There are a number of measures that can be included as conditions on

Marine Licences to reduce the risk of hydrodynamic and plough dredging operations. These

measures include: restricting the timing of the dredging activity (seasonal and tidal cycle

constraints); reducing the operating time; ensuring there is a hydrodynamic assessment to predict

the fate of the material. For WID operations measures can also include: undertaking a trial dredge;

dredging in layers; dredging areas or berths in sequence and; creating flow/access channels to

facilitate the movement of the material.

Post consent validation – The most effective way to establish what volumes of material have been

dredged and where the material has been deposited is through pre and post bathymetric surveys.

However the MMO will need to consider if they are prohibitively expensive for the smaller

operations to undertake such surveys.

20


Liability - The MMO will need to consider the issue of liability in relation to the licensing of

hydrodynamic and plough dredging. The potential for financial liabilities is a particular concern

should sediment moved by WID be deposited in the berth or channel areas of other operators. This

concern provides an important driver for the requirement of a hydrodynamic assessment to

determine the fate of the material.

Dredge Management Plans - It is also recommended that where possible dredging operations are

considered in the context of a Dredge Management Plan, for example, through a Baseline Document

produced under the Maintenance Dredging Protocol5. This can put a particular dredging activity in

context of a geographical area (often estuary wide) and provide a more holistic approach to

dredging management. It can ensure more efficient use of resources, reduce interference/liability

issues and enable cumulative and in-combination impacts to be assessed.

One project one licence - Under Section 66 of the MCAA where a project comprises more than one

licensable activity, then a Marine Licence for each activity will be required. Where it is appropriate

these licenses will be issued together by the MMO as one project license. Therefore projects could

include two dredging methods such as hydrodynamic and TSHD and also include disposal of the

dredged material. The assessment of such an application will need to determine the environmental

impacts at both the dredging and disposal site and therefore the MMO should considered this under

fees and charging.

5 Defra (2007). Maintenance Dredging & The Habitats Regulations 1994: A Conservation Assessment Protocol

for England.

http://archive.defra.gov.uk/wildlife-pets/wildlife/protect/documents/mdp-cap.pdf

21


11 References

ABPmer. 2011. Water injection dredging – monitoring leads to effective planning. Port Technology

International. 48. 34-35.

Bates, A. (2005). Methods of mitigating environmental impact. 2nd International Conference on

Maintenance Dredging: Proceedings of the International Conference on Maintenance Dredging.

Institute of Civil Engineers.

CDEA/IADC. 2008. Environmental Aspects of Dredging, Edited by R. N. Bray. Taylor and Francis.

IADC/CEDA. 1998. Environmental Aspects of Dredging. 4. Machines, Methods and Mitigation.

Sponsored by IADC/CEDA. Published by International Association of Dredging Companies, The

Netherlands.

PIANC.(In press). Water Injection Dredging. Report of Working Group 51.

Sullivan, N. 2000. The use of agitation dredging, water injection dredging and sidecasting: Results of

a survey of ports in England and Wales. Terra et Aqua. 78. 11-20.

van Raalte, G.H. and Bray, R.N. 1999. Hydrodynamic dredging: principles, effects and methods. CEDA

Dredging Days 1999: November 18th

– 19th

1999. Amsterdam, Netherlands.

OSPAR Commission. 2004. Environmental Impacts to marine species and habitats of dredging for

navigational purposes. Biological Diversity and Ecosystems Series. OSPAR publication no. 208/2004

22


12 Annex 1

Cefas Action Levels

ACTION LEVELS

Contaminant / Compound Action Level 1 Action Level 2

mg/kg Dry Weight (ppm)

As 20 100

Hg 0.3 3

Cd 0.4 5

Cr 40 400

Cu 40 400

Ni 20 200

Pb 50 500

Zn 130 800

Organotins; TBT DBT MBT 0.1 1

PCB's, sum of ICES 7 0.01 none

PCB's, sum of 25 congeners 0.02 0.2

Oil 100

*DDT *0.001

*Dieldrin *0.005

*these levels were set in 1994

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