OCS Report 

MMS 2004-076 

 

REVIEW OF EXISTING AND EMERGING ENVIRONMENTALLY 

FRIENDLY OFFSHORE DREDGING TECHNOLOGIES 

PREPARED FOR: 

US DEPARTMENT OF THE INTERIOR 

MINERALS MANAGEMENT SERVICE 

Under Contract Number No. 0103CT71516 

 November 2004
OCS Report 

MMS 2004-076 

Top Image courtesy of Jan de Nul 

Bottom Image from Newell and Seiderer (2003)
OCS Report 

MMS 2004-076 

REVIEW OF EXISTING AND EMERGING 

ENVIRONMENTALLY FRIENDLY OFFSHORE DREDGING TECHNOLOGIES 

Prepared for: 

Leasing Division, Sand and Gravel Unit 

Minerals Management Service 

US Department of Interior 

Herndon, Virginia 

Prepared by: 

W.F. Baird & Associates Ltd. 

Madison, Wisconsin 

Research Planning, Inc. 

Columbia, South Carolina 

November 2004 

DISCLAIMER 

This report has been reviewed by the Minerals Management Service and approved for 

publication. Approval does not signify that the contents necessarily reflect the views and policies 

of the Service, nor does mention of trade names or commercial products constitute endorsement 

or recommendation for use. 

i 
TABLE OF CONTENTS 

1.0 INTRODUCTION.......................................................................................................................................1

1.1 Background............................................................................................................................................1

1.2 Project Goals..........................................................................................................................................1

1.3 Study Approach ....................................................................................................................................1

1.4 Team Organization ................................................................................................................................2

1.5 Report Structure.....................................................................................................................................2

2.0 IDENTIFICATION OF DREDGING IMPACTS.......................................................................................3

2.1 Introduction ...........................................................................................................................................3

2.2 Approach ...............................................................................................................................................3

2.3 Review and Description of the List of Key Impacts..............................................................................7

2.3.1 Short-term and Cumulative Impacts F rom Dredging That Lead to Loss or Reduced Stability of

Benthic Habitats, Including Re-colonization by an Altered Biological Community........................7

2.3.2 Injury and Death of Special Species of Concern (e.g., sea turtles) From Being Sucked Into the 

Draghead During Dredging Operations Using Hopper Dredges. .....................................................9

2.3.3 Changes in the Substrate Characteristics (Grain Size, Dissolved Oxygen, Compaction and Organic 

Content) That Lead to a Reduction in Benthic Communities and Suitability of the Area for Future 

Dredging. ........................................................................................................................................11

2.3.4 Changes in Bathymetry that can Alter the Wave Climate Reaching the Shore, Resulting in 

Shoreline Changes. .........................................................................................................................12

2.3.5 Sedimentation (Burial) Impacts to Adjacent Hard/Live Bottom or Other Sensitive Habitats ......12

2.3.6 Creation of Depressions and Furrows From Removal of Substrate................................................14

2.3.7 Short-term Increased Turbidity from Cutterhead or Draghead and Overflow (from hopper 

dredges) that Affects Benthic Communities. ..................................................................................15

2.3.8 Spatial and Seasonal Conflicts Between Dredging and Commercial and Recreational Fisheries. .21

2.3.9 Potential to Break an Active or Abandoned Pipeline, Resulting in a Release of Petroleum

Hydrocarbons..................................................................................................................................22

ii 
2.3.10 Collisions with Marine Mammals and Sea Turtles During Vessel Operations..........................22

2.3.11 Damage to Archaeological Resources........................................................................................23

2.3.12 Potential Harmful Alteration or Destruction of Essential Fish Habitat......................................23

2.4 Summary of Potential Impacts and Focus for this Investigation of Environmentally Friendly

Approaches...........................................................................................................................................25

3.0 LITERATURE REVIEW OF ENVIRONMENTALLY-FRIENDLY APPROACHES ...........................28

3.1 Methodology........................................................................................................................................28

3.2 Trailing Suction Hopper Dredges........................................................................................................28

3.3 Cutter Suction Dredges........................................................................................................................31

3.4 Other Dredging Devices and Equipment.............................................................................................32

3.5 Monitoring Instruments and Approaches ............................................................................................33

3.6 Summary of the Literature Review......................................................................................................39

4.0 DREDGING INDUSTRY REVIEW .......................................................................................................42

4.1 Introduction .........................................................................................................................................42

4.2 Selection of a European Partner ..........................................................................................................42

4.3 Development of a Questionnaire .........................................................................................................43

4.4 Review of Responses...........................................................................................................................44

4.4.1 Discussion of Responses to Plume Related Impacts.......................................................................45

4.4.2 Discussion of Responses to Impacts to Benthic Habitats ...............................................................48

4.4.3 Discussion of Impacts to Marine Mammals....................................................................................50

4.4.4 Discussion of Miscellaneous Issues................................................................................................51

4.5 Follow-Up Conversations with Dredge Contractors ...........................................................................52

4.6 Discussion of the Anti-Turbidity Valve and the Green Pipe ...............................................................52

4.6.1 Anti-Turbidity Valve ......................................................................................................................52

4.6.2 Green Pipe System..........................................................................................................................54

5.0 ANALYSIS OF ENVIRONMENTALLY FRIENDLY TECHNOLOGY ..............................................55

iii 
5.1 Introduction .........................................................................................................................................55

5.2 Workshop Approach............................................................................................................................55

5.3 Evaluation of the Key Equipment and Approaches Discussed at the Workshop ................................56

5.3.1 Removal of Benthic Communities..................................................................................................57

5.3.1.1 Creation of Temporal or Spatial Refuge Areas (effectively “environmental windows at a 

borrow deposit scale”) ...........................................................................................................57

5.3.2 Entrainment of species of concern..................................................................................................59

5.3.3 Changes in Substrate Characteristics..............................................................................................60

5.3.3.1 Limitation of Pit Depths to a Fixed Value.............................................................................61

5.3.3.2 Site Specific Evaluation of Local Sedimentation Potential and Dissolved Oxygen Conditions 

Through the Application of Numerical Models or Analytical Methods................................62

5.3.3.3 Use of Monitoring to Avoid Development of a Mud Layer and/or Anoxic Conditions .......63

5.3.4 Wave Climate Alterations by Changes in Bathymetry ...................................................................64

5.3.5 Damage to Hard/Live Bottom Habitats..........................................................................................64

5.3.5.1 Implementation of a Blanket Buffer Zone for All Situations................................................64

5.3.5.2 Implementation of a Region/Habitat Specific Minimum Buffer Zone Width Together with

Monitoring at the Sensitive Habitat .......................................................................................65

5.3.5.3 Use of Analysis to Determine a Site-Specific Buffer Zone...................................................66

5.3.5.4 Monitoring Turbidity to Meet a General Stipulation.............................................................67

5.3.5.5 Development of Site-Specific Limits Based on Background Levels ....................................69

5.3.5.6 Use of “Green Pipe” (re-circulation of overflow to draghead) ............................................70

5.3.5.7 Use of Anti-Turbidity Valve ................................................................................................71

5.3.6 Creation of Depressions and Furrows.............................................................................................73

5.3.7 Increased Short-Term Turbidity (Turbidity Limits)........................................................................73

5.3.7.1 Implementation of a General Stipulation..............................................................................73

5.3.7.2 Development of Site Specific Limits Based on Background Levels.....................................74

5.3.7.3 Use of “Green Pipe” (re-circulation of overflow to draghead) ............................................76

5.3.7.4 Use of Anti-Turbidity Valve ................................................................................................77

iv 
5.3.8 Commercial and Recreational Fisheries..........................................................................................79

5.3.9 Seafloor Pipeline Breakage and Leakage........................................................................................79

5.3.10 Collisions With Marine Mammals and Sea Turtles ...................................................................79

5.3.11 Archaeological Resources..........................................................................................................79

5.3.12 Alteration or Destruction of Essential Fish Habitat ...................................................................79

5.3.12.1 Dredge Depositional Areas that are Undergoing Constant Natural Recovery (and Burial) in a 

Physical Sense........................................................................................................................80

5.3.12.2 Post-Project Monitoring of Changes.....................................................................................80

5.3.12.3 Complete a Sediment Dynamics Study of the Shoal to Determine the Most Appropriate

Areas to Dredge to Avoid or Minimize Impacts....................................................................81

5.3.12.4 Limit Removal of Sediment to Some Threshold Level (to Avoid Deflation or Irreversible 

Damage to the Morphologic Integrity of the Shoal) ..............................................................82

6.0 CONCLUSIONS AND RECOMMENDATIONS...................................................................................84

6.1 Conclusions .........................................................................................................................................84

6.2 Recommendations................................................................................................................................86

6.2.1 Recommendation on Spatial/Temporal Refuge Areas to Promote Re-Colonization of Benthic 

Communities...................................................................................................................................86

6.2.2 Changes to Substrate Characteristics and Recommendation on a Pit Depth Rule .....................87

6.2.3 Recommendation on Protecting Sensitive Habitat from Sedimentation .........................................87

6.2.4 Recommendation on Addressing the Impact of Turbidity .........................................................88

6.2.5 Recommendation on Protecting Essential Fish Habitat, and Specifically, Shoal Integrity........88

7.0 REFERENCES..........................................................................................................................................89

v 
LIST OF FIGURES

FIGURE 2.1 Schematic diagram showing the likely re-colonization rates............................................ 8 

FIGURE 2.2 Dredging scars from a CSD operation ............................................................................ 15 

FIGURE 2.3 Pre- and post-dredge surveys of a borrow pit off Miami Beach..................................... 15 

FIGURE 2.4 Hopper dredge sedimentation processes......................................................................... 16 

FIGURE 2.5 Acoustic backscatter images........................................................................................... 19 

FIGURE 2.6 Longitudinal section of the sedimentation plume ........................................................... 20 

FIGURE 2.7 Map of the essential fish habitat for adult summer flounder .......................................... 24 

FIGURE 3.1 Control center window................................................................................................... 35 

FIGURE 3.2 On board dredge display ................................................................................................. 35 

FIGURE 3.3 Load summary display.................................................................................................... 36 

FIGURE 3.4 Dredge disposal history .................................................................................................. 36 

FIGURE 3.5 Plotting options display menu......................................................................................... 37 

FIGURE 3.6 Plot example ................................................................................................................... 37 

FIGURE 3.7 Dredge track plot ............................................................................................................ 38 

FIGURE 4.1 Beach nourishment scenarios.......................................................................................... 46 

FIGURE 4.2 Hopper dredge in operation ............................................................................................ 46 

FIGURE 4.3 Dredge operators display from the Liberty Island TSHD............................................... 49 

FIGURE 4.4 New MMS stipulation on tracking dredge position........................................................ 50 

FIGURE 4.5 Sketch showing the anti-turbidity or environmental valve ............................................. 53 

FIGURE 4.6 Sketch showing the “Green Pipe” approach ................................................................... 54 

vi 
LIST OF TABLES 

TABLE 2.1 Environmental studies on OCS sand resource issues.......................................................... 5 

TABLE 2.2 Federal and State agency staff and researchers who were contacted.................................. 7 

TABLE 2.3 Summary of perceived impacts and focus for this assessment ........................................ 26 

TABLE 3.1 Summary of turbidity generated by different dredge types.............................................. 41 

vii 
GLOSSARY OF ACRONYMS 

ADCP Acoustic Doppler Current Profiler 

ALSF Aggregate Levy Sustainability Fund 

BMAPA British Marine Aggregates Producers Association 

CEFAS Centre for Environment, Fisheries & Aquaculture Science 

COE US Army Corps of Engineers 

CSD Cutter Suction Dredge 

DERM Miami-Dade County Department of Environmental Resources Management 

DOER Dredging Operations and Environmental Research of the USACE 

DRL Dredging Research Ltd. 

DSS Dredge Specific System

EFH Essential Fish Habitat 

ERDC Engineering Research and Development Center of the USACE 

JTU Jackson Turbidity Unit 

LISST Laser In-Situ Scattering and Transmissometry 

MIRO UK Mineral Industry Research Organization 

MMS Minerals Management Service 

NMFS National Marine Fisheries Service 

NOAA National Oceanographic and Atmospheric Administration 

NTU Nephelometric Turbidity Units 

OCS Outer Continental Shelf (here referring to Federal waters) 

REMOTS Remote Ecological Monitoring of the Seafloor 

ROV Remotely Operated Vehicle 

SI Silent Inspector 

SPOT An earth observation satellite 

TSHD Trailing Suction Hopper Dredge 

TSS Total Suspended Solids 

USACE US Army Corps of Engineers 

USGS/BRD US Geological Survey / Biological Research Division 

USFWS US Fish and Wildlife Service 

WID Water Injection Dredging 

WES Waterways Experiment Station, USACE 

viii 
ACKNOWLEDGEMENTS 

This report was prepared under contract to the Marine Minerals Branch, Leasing Division, 

Minerals Management Service, Contract No. 01-02-CT-85139. Barry Drucker was the MMS 

Project Manager and contributed significantly to the project objectives and the overall success of 

the project. Dr. Rob Nairn of Baird and Associates was the Project Manager. The senior authors 

were Dr. Rob Nairn, Mr. Tim Kenny, and Dr. Fernando Marván of Baird & Associates, Dr. 

Jacqueline Michel of Research Planning, Inc., Dr. Richard Newell of Marine Ecological 

Surveys, and Mr. Nick Bray of Dredging Research Ltd. The cooperation of the UK Mineral 

Industry Research Operation in the survey of the dredging industry in Europe is also gratefully 

acknowledged. 

ix 
1.0 INTRODUCTION 

1.1 Background 

The US Minerals Management Service (MMS) Leasing Division has the 

responsibility for administering the Department of the Interior's role in mineral resource 

development other than oil, gas, and sulfur on the US outer continental shelf (OCS). 

MMS does not develop and maintain a schedule of lease offerings for OCS sand 

resources. Rather, the leasing process for OCS sand must begin by a request from

potential users of the sand. Only recently have OCS sand resources been considered as 

feasible sources of sand for beach nourishment. Between 1995 and 2001, MMS 

conveyed 14,600,000 cubic yards of OCS sand for ten projects. 

MMS expects that the OCS sand resources will be long-term sources of sand borrow 

material for coastal erosion management because of: 

• The general diminishing supply of onshore and nearshore sand; 

• Impact of sea level rise and other natural and human-induced factors leading to 

increased erosion; 

• The re-nourishment cycles for beaches or coastal areas requiring quantities of 

sand not currently available from State sources; and 

• Immediate/emergency repair of beaches and coastal damage from severe coastal 

storms. 

MMS has responsibility for providing environmental analysis and assessment 

information enabling the responsible management of the OSC sand resources. There is a 

range of environmental concerns, including both direct and indirect impacts, with the 

dredging operations necessary for sand borrow extraction. This project was initiated to 

evaluate the extent to which recent developments in offshore dredging equipment and 

practices may lead to more environmentally friendly results. 

1.2 Project Goals 

The goal of the project is to evaluate dredging equipment and techniques on a 

worldwide basis to identify existing and emerging dredging technologies that aim to 

reduce or avoid potential adverse effects on the offshore biological and physical 

environment. Based on the results, recommendations are developed for an 

implementation strategy for any promising technologies. 

1.3 Study Approach 

The project approach was comprised of four main areas of activity as described as 

follows: 

1 
• A literature review was completed for two main areas. The first was to update the 

understanding of the impacts of dredging in order to provide the backbone for the 

overall study. The second main thrust of the literature review was to assess the 

existing and emerging environmentally friendly dredging technologies. 

• Representatives of the various regulatory agencies at the Federal and State level 

that are responsible for the offshore environment of concern were interviewed to 

determine the priority of key dredging impacts. 

• The second main area of investigation consisted of obtaining information through 

direct contact with the dredging industry. This task took the form of 

questionnaires, follow-up calls, and meetings. 

• Finally, the various approaches and techniques were evaluated for their 

appropriateness, practicality, and effectiveness as they relate to the key impacts. 

A central activity in this assessment was a workshop attended by regulating 

agency representatives, consultants, dredging industry representatives, and MMS 

staff from the Offshore Minerals Management Sand and Gravel Program. 

1.4 Team Organization 

The project team was comprised of the following firms and staff members: 

Baird & Associates 

Robert Nairn, Ph.D., M.Sc. P.E., Project Manager 

Tim Kenny, B.Sc. Baird, Dredging Specialist – Industry Review, USA 

Fernando Marván Ph.D., Literature Review 

Research Planning, Inc. 

Jacqueline Michel, Ph.D., Impact Evaluation (USA) 

Marine Ecological Surveys 

Dr. R.C. Newell B.Sc., Ph.D., D.Sc. (Lond), Impact Evaluation 

(UK/Europe/Worldwide) 

Dredging Research Ltd. 

Mr. Nick Bray, B.A. MICE, Dredging Specialist – Industry Review, Overseas 

1.5 Report Structure 

The remainder the report is divided into the following sections: 

2. Identification of Dredging Impacts 

3. Literature Review of Environmentally Friendly Approaches 

4. Dredging Industry Review 

5. Analysis of Environmentally Friendly Technology 

6. Conclusions and Recommendations 

2 
2.0 IDENTIFICATION OF DREDGING IMPACTS 

2.1 Introduction 

This section describes the development of a prioritized list of key ecologic impacts 

of offshore dredging activities from the perspective of the regulatory agencies in the USA 

in 2004. It draws on the literature review of dredging impacts and related previous MMS 

studies to define the current understanding of the impacts together with direct input from

representatives of the appropriate regulatory agencies. 

The prioritized list and description of key ecological impacts will be used to evaluate 

the appropriateness and effectiveness of the various environmentally friendly equipment 

and approaches. In other words, this information will help answer such questions as: 

• Does a specific development in dredging equipment or practice address an 

important ecologic impact?

• Given our understanding of the impact, to what degree does the specific 

development mitigate the impact? 

In addition, it was necessary to determine a focus for this investigation, specifically 

addressing areas that were not being considered in other ongoing MMS studies or by 

other agencies. 

2.2 Approach 

MMS has been conducting studies of both generic and site-specific impacts of 

dredging OCS sand borrow sites (Table 2.1). These studies were reviewed and potential 

physical and biological impacts were summarized in Research Planning, Inc. et al. 

(2001). For the current study, this initial list was updated with new information from the 

stipulations required for the 2003 dredging test off Louisiana, discussions during the 

Louisiana Sand Management Working Group meetings in 2003 and 2004, issues raised 

during consultations with the US Fish and Wildlife Service (USFWS), the National 

Oceanic and Atmospheric Administration (NOAA), and the National Marine Fisheries 

Service (NMFS) as part of the negotiated lease process for recent and pending lease 

agreements, and the results of new and on-going MMS studies. Also, Newell and 

Seiderer (2003) prepared a summary of the ecological impacts of dredging for marine 

aggregates, with emphasis on sand and gravel dredging in the UK, specifically as input to 

this study (this report is included in its entirety as Appendix A). The UK Mineral 

Industry Research Organization (MIRO) draft report on best practices to assessing the 

impacts of aggregates dredging, particularly Sections 5 (Mitigation) and 6 (Monitoring), 

was also reviewed and found to be very useful (Royal Haskoning, 2004). Potential sand 

borrow sites off the Atlantic and Gulf of Mexico coasts were the focus of our study. 

3 
Discussions were held with researchers and staff from Federal and State resource 

agencies that are actively dealing with OCS dredging issues to refine and prioritize the 

list of potential impacts (Table 2.2). Based on these discussions, the prioritized list of 

concerns from OCS dredging operations on marine biological and physical resources 

included: 

1. Short-term and cumulative impacts from dredging that lead to loss or reduced 

stability of benthic habitats, including re-colonization by an altered biological 

community. 

2. Injury and death of special species of concern (e.g., sea turtles) from being 

sucked into the draghead during dredging operations using hopper dredges. 

3. Changes in the substrate characteristics (grain size, dissolved oxygen, 

compaction and organic content) that lead to a reduction or alteration in benthic 

communities and suitability of the area for future dredging. 

4. Changes in bathymetry that can alter the wave climate reaching the shore, 

resulting in shoreline changes. 

5. Sedimentation (burial) impacts to adjacent hard/live bottom or other sensitive 

habitats. 

6. Creation of depressions and furrows from removal of substrate. 

7. Impacts from short-term increased turbidity from cutterhead or draghead and 

overflow from hopper dredges on benthic communities. 

8. Spatial and seasonal conflicts between dredging and commercial and 

recreational fisheries. 

9. Potential to cause a break in an active or abandoned pipeline, resulting in a 

release of petroleum hydrocarbons. 

10. Collisions with marine mammals and sea turtles during vessel operations. 

11. Damage to archaeological resources. 

12. Potential harmful alteration or destruction of Essential Fish Habitat. 

4 
TABLE 2.1 Environmental studies on OCS sand resource issues funded 

or supported by MMS. Copies of completed reports and status reports for 

ongoing studies are available at: www.mms.gov/sandandgravel.

Site-Specific Environmental Baseline Studies 

Environmental Investigation of the Use of Shoals Offshore Delaware and Maryland by 

Mobile Benthos and Finfish Species. Final Report January 2005 

Field Testing of a Physical/ Biological Monitoring Methodology for Offshore Dredging and 

Mining Operations (being conducted at Sandbridge Shoal, offshore Virginia via 

Cooperative Agreement with VIMS). Final Report 2005 

Environmental Surveys of Potential Borrow Areas Offshore Northern New Jersey and 

Southern New York and the Environmental Implications of Sand Removal for Coastal and 

Beach Restoration. Draft Report Spring 2003 

Environmental Surveys of Potential Borrow Areas on the East Florida Shelf and the 

Environmental Implications of Sand Removal for Coastal and Beach Restoration. OCS 

Study MMS 2004-037 

Collection of Environmental Data within Sand Resource Areas Offshore North Carolina and 

the Environmental Implications of Sand Removal for Coastal and Beach Restoration. OCS 

Study MMS 2000-056 

Surveys of Sand Resource Areas Offshore Maryland/Delaware and the Environmental 

Implications of Sand Removal for Beach Restoration Projects. OCS Study MMS 2000-055 

Environmental Surveys of OCS Sand Resources Offshore New Jersey. OCS Study MMS 

2000-052 

Environmental Survey of Identified Sand Resource Areas Offshore Alabama. OCS Study 

MMS 99-0051 

Use of Federal Sand Resources for Beach and Coastal Restoration in New Jersey, Maryland, 

Delaware and Virginia. OCS Study MMS 99-0036 

Environmental Studies Relative to Potential Sand Mining in the Vicinity of the City of

Virginia Beach, Virginia. OCS Study MMS 97-0025 

West Florida Shelf Benthic Repopulation Study. OCS Report MMS 95-0005 

Wave Modeling/Shoreline Erosion 

A Numerical Modeling Examination of the Cumulative Physical Effects of Offshore Sand 

Dredging for Beach Nourishment – New Jersey, Virginia, North Carolina, Florida. OCS 

Study MMS 2001-098 

Wave Climate and Bottom Boundary Layer Dynamics with Implications for Offshore Sand 

Mining and Barrier Island Replenishment, South-Central Louisiana. OCS Study MMS 

2000-053 

Wave Climate Modeling and Evaluation Relative to Sand Mining on Ship Shoal, Offshore 

LA, for Coastal and Barrier Islands Restoration. OCS Study MMS 96-0059 

A Methodology and Criteria to Assess the Impact of Sand Volume Removed in Federal 

Waters on the Offshore Wave Climate. OCS Study MMS 99-0046 

Development of Criteria to Evaluate Wave Refraction Models. OCS Study MMS 99-0096 

5 
TABLE 2.1 Cont. 

Generic Studies Applicable to all Offshore Marine Mineral Efforts 

Analysis of Potential Biological and Physical Dredging Impacts on Offshore Ridge and Shoal 

Features/Engineering Alternatives and Options to Avoid Adverse Environmental Impacts. 

On-going 

Worldwide Analysis of Shipwreck Damage Caused by Offshore Dredging: Recommendations 

for Pre- Operational Surveys and Mitigation to Avoid Adverse Impacts. OCS Study MMS 

2004-0005 

Model Development or Modification for Analysis of Benthic and Surface Plume Generation 

and Extent During Offshore Dredging Operations. Final model delivered December 2003. 

Development and Design of Biological and Physical Monitoring Protocols to Evaluate the 

Long-Term Impacts of Offshore Dredging Operations on the Marine Environment. OCS 

Report MMS 2001-089 

Integrated Study of the Biological and Physical Effects of Marine Aggregate Dredging. OCS 

Study MMS 2000-054 

Study of the Cumulative Effects of Marine Aggregate Dredging. OCS Study MMS 99-0030 

Marine Aggregate Mining Benthic and Surface Plume Study. OCS Study MMS 99-0029 

Impacts and Direct Effects of Sand Dredging for Beach Re-nourishment on the Benthic Organisms and 

Geology of the West Florida Shelf. OCS Report MMS 95-0005 

Marine Mining Technologies and Mitigation Techniques. A Detailed Analysis with Respect to 

the Mining of Specific Offshore Mineral Commodities. OCS Report MMS 95-0003 

Synthesis and Analysis of Existing Information Regarding Environmental Effects of Marine 

Mining. OCS Study MMS 93-0006 

Marine Mining Literature Search Study. OCS Study MMS 93-0006 

6 
TABLE 2.2 Federal and State agency staff and researchers who were contacted to 

prioritize the list of physical and biological concerns associated with OCS 

sand dredging.

Ken Duffy Louisiana Department of Natural Resources 

Syed Khalil Louisiana Department of Natural Resources 

David Burkholder Louisiana Department of Natural Resources 

Heather Finley Louisiana Department of Wildlife and Fish 

Bob Van Dolah Director, Marine Resources Research Institute 

South Carolina Department of Natural Resources 

Jeff Normant New Jersey Division of Fish and Wildlife 

Mark N. Mauriello New Jersey Department of Environmental Protection 

Ron Williams Florida Department of Environmental Protection, 

Bureau of Beaches & Coastal Systems 

Russ Watson US Fish and Wildlife Service, Lafayette, La. 

Carlos Mendoza US Fish and Wildlife Service, Houston, Tx. 

Richard Hartman National Marine Fisheries Service, Habitat 

Conservation Division, Baton Rouge, La. 

Stan Gorski National Marine Fisheries Service, Sandy Hook, NJ 

Tim Goodger National Marine Fisheries Service 

Eric Hawk National Marine Fisheries Service, Protected Species 

Division, St. Petersburg, FL 

Stan Riggs East Carolina University 

Richard Condrey Louisiana State University 

Woody Hobbs Virginia Institute of Marine Sciences 

Bob Diaz Virginia Institute of Marine Sciences 

Chris Slay Coastwise Consulting, Inc. 

2.3 Review and Description of the List of Key Impacts 

In the following sections, each of the impacts identified in Section 2.2 is described 

and discussed with respect to whether it is appropriate to address the impact under this 

study. Where there are existing stipulations in use by MMS to address the impact of 

concern, these are also indicated. 

2.3.1 Short-term and Cumulative Impacts F rom Dredging That Lead to 

Loss or Reduced Stability of Benthic Habitats, Including Recolonization by an Altered Biological Community 

This concern is based on the direct removal of benthic habitat along with infaunal 

and epifaunal organisms that are incapable of avoiding the dredge, resulting in significant 

reductions in the number of individuals, number of species, and biomass. Benthic 

7 
resources are important in the food web for commercially and recreationally important 

fishes and invertebrates, and they contribute to the biodiversity of the pelagic 

environment. Although short-term losses and changes in benthic community structure 

have been documented to occur following sand dredging (Blake et. al, 1996; Van Dolah 

et al., 1994), the ecological significance to the benthic community is uncertain. Studies 

investigating the recovery of benthic communities following dredging (Blake et. al., 

1996; Newell et al, 1998; Van Dolah et al., 1994) have indicated that communities of 

comparable total abundance and diversity can be expected to re-colonize dredge sites 

within several years. Newell and Seiderer (2003) summarized recovery rates of benthic 

communities post-dredging for different substrate types (Figure 2.1). Sandy substrates 

typically recover within 2 to 4 years. However, even though these re-colonized 

communities may be similar in terms of total abundance and species diversity, their 

taxonomic composition, in terms of dominant species and species abundance, is often 

very different from pre- to post-dredging. 

FIGURE 2.1 Schematic diagram showing the likely re-colonization rates for the 

benthic communities of estuarine muds, sand, gravels, and rocky reefs (Newell and 

Seiderer, 2003 – see Appendix A). 

There are distinct patterns of re-colonization, with initial colonization by mobile 

“opportunistic” species that have planktonic larvae within days, or even during the 

dredging process. These species are capable of rapid colonization within months of space 

being made available for colonization and growth. This phase is followed by an 

increasing variety of colonizing species, an increase in the population densities of the 

component species, and finally by growth of the individuals which leads to restoration of 

the biomass. The rate at which recovery of the species diversity occurs is dependent on 

8 
the complexity of the fauna and the inter-relationships that control larval recruitment and 

settlement. Many species do not re-colonize regularly, and most require specific 

physicochemical and biological cues to induce settlement, implying that even if the 

deposits in a dredge site post-dredging remain similar to those pre-dredging, there may be 

a significant interval before all the species components are present in the community 

(Newell and Seiderer, 2003). Longest recovery rates would be for slow-growing species 

that do not have planktonic larvae. Also, recovery rates are long for sites that are 

intensely (repeatedly) dredged. Newell and Seiderer (2003) reported on several studies 

where there were significant differences in microfaunal assemblages subjected to 

different dredging intensities. Therefore, impacts to benthic communities are of even 

greater concern for sand borrow sites that are repeatedly dredged. 

The key ecological concern with a change in benthic community is whether the new 

benthic communities fill the same trophic function and provide the same energy transfer 

to higher trophic levels, as did the original communities. If they do not, then the potential 

long-term and cumulative ecological impacts of sand dredging may be far greater than 

predicted to date, a condition that may be unacceptable as more sites along the coast are 

dredged and others are dredged on a regular basis. The MMS monitoring protocols 

(Research Planning, Inc. et al., 2001) were designed specifically to determine the effects 

of dredging activities on benthic communities and the transfer of energy from benthic 

communities to fishes. 

All resource managers and researchers raised concerns about direct impacts to 

benthic communities. The greatest concern is in known benthic-associated fishery areas, 

such as the surf clam fishery off New Jersey and the shrimp fishery in the Gulf of 

Mexico. There is less concern in areas of general biological productivity or dynamic 

processes, such as in South Carolina. 

In summary, there is a high priority to identify dredging methods that would speed 

the rate of recovery of benthic communities and reduce the potential for permanent 

changes in species abundance and dominant species. This potential impact warrants 

specific focus in the review of potentially environmentally friendly approaches and 

equipment. 

2.3.2 Injury and Death of Special Species of Concern (e.g., sea turtles) 

From Being Sucked Into the Draghead During Dredging Operations 

Using Hopper Dredges. 

Dredging of navigation channels has been identified as a source of sea turtle 

mortality since sea turtle deaths were first documented during hopper dredging 

operations in Canaveral Channel, Florida, in 1980 when 71 sea turtles were killed by 

hopper dredging over the period of July 11 through November 13, 1980 (NMFS, 1991). 

Hopper dredges move relatively rapidly and can entrain and kill sea turtles, presumably 

as the drag arm of the moving dredge overtakes the slower moving turtle. In contrast, 

9 
there have been no reports of injury or death of sea turtles during cutterhead suction 

dredging (NMFS, 2004). 

Gulf sturgeon is another listed species (Federal, threatened) with documentation of 

impacts from channel dredging (NMFS, 2004). NMFS and USFWS jointly designated 

critical habitat for Gulf sturgeon in 2003 (68 FR 13370). All designated critical habitats 

are in riverine, estuarine, or State marine waters; there are none in Federal waters at this 

time. 

Every Federal agency with management responsibility for species listed under the 

Endangered Species Act expressed concern about potential impacts to listed species, and 

to sea turtles in particular. There are existing stipulations for sea turtles that have 

significantly reduced impacts, but even a single “take” is considered significant. The 

USACE has an active research program working with all stakeholders to develop new 

methods to reduce impacts to sea turtles during hopper dredging, primarily from dredging 

operations in channels. In summary, it was concluded that MMS would review the results 

of this research as results become available and adopt appropriate methods to reduce 

potential impacts to sea turtles during OCS dredging activities. Therefore, this current 

study has not explored in any detail the latest innovative developments related to 

avoidance of impacts to listed species. 

Existing stipulations being used in MMS leases to protect sea turtles include: 

• Presence of trained observer(s) for a specified percent of the time who 

follows specific protocols. 

• Use of a rigid sea turtle deflector, such as the one designed by the USACE 

or similar. 

• Operation of the dredge in a manner that will reduce the risk of interaction 

with any sea turtles that might be present in the dredge area. Keep the 

draghead on the bottom except: 1) when the dredge is not in a pumping 

operation and the suction pumps are turned completely off; 2) the dredge 

is being re-oriented to the next dredge line during borrow activities; and 3) 

the vessel’s safety is at risk. 

• Dredge equipped with inflow screening baskets (4-inch mesh) to better 

monitor the intake and overflow of the dredged materials for sea turtles 

and their remains. The percent of inflow to be screened varies by region 

from 50-100 percent. 

• Assessment/relocation trawling to further assess/reduce the potential for 

incidental take during dredging. Trawling is conducted repeatedly in front 

of the dredge as it moves along the track lines. Any turtles collected are to 

be relocated. There are specifications for trawl tow time and speed. There 

may be requirements for flipper tagging and genetic analysis of tissue 

samples from turtles caught during relocation trawling. 

10 
• Filing of detailed reports with the appropriate NOAA office within 30 

days of project completion. 

No revision to these stipulations is required at this time, however, following the 

conclusion of the latest USACE research efforts it may be necessary to refine the 

stipulations. 

2.3.3 Changes in the Substrate Characteristics (Grain Size, Dissolved 

Oxygen, Compaction and Organic Content) That Lead to a Reduction 

in Benthic Communities and Suitability of the Area for Future 

Dredging. 

There are several conditions where OCS dredging can lead to changes in substrate 

characteristics. Deep dredging (greater than 3 m) can create pits that may infill with 

finer-grained sediments. Van Dolah et al. (1998) studied six dredged sites in South 

Carolina and found that, at three of the sites, the borrow area had filled with muddy 

sediments forming a cap over clean sand. Infilling with muddy sediments will change the 

benthic communities, as well as rendering the site unsuitable as a future borrow area (or 

less suitable due to the potential requirement for stripping and disposal of overlying fine 

sediment). Newell and Seiderer (2003) looked at recovery rates of benthic communities 

at a wide range of dredge sites and found that recruitment success was controlled mainly 

by whether the sediments remain suitable for settlement after cessation of dredging. 

Deep pits can also take a long time to infill. Van Dolah et al. (1998) found that 

infilling at the six sites they studied took from 1.75 years to greater than 12 years. A deep 

(greater than 10 m) pit dredged 3.6 km offshore Coney Island persisted for more than six 

years and had a highly modified infaunal assemblage (Barry A. Vittor & Associates, Inc., 

1999). In deep pits, there may be decreases in dissolved oxygen levels in the water that 

could lead to hypoxic or anoxic conditions (National Research Council, 1995). 

The physical monitoring protocol for “Sediment” was developed for MMS (Research 

Planning, Inc. et al., 2001 and summarized in Nairn et al., 2004) to evaluate the potential 

changes to the sedimentological characteristics of the seabed, including sediment texture 

and total organic content. 

In summary, dredging techniques are needed that will: 1) preserve sediment 

characteristics similar to pre-existing conditions for the surface substrate; and 2) avoid 

creation of anoxic conditions within dredge pits. The review of environmentally friendly 

equipment and approaches should consider these requirements. 

11 
2.3.4 Changes in Bathymetry that can Alter the Wave Climate Reaching 

the Shore, Resulting in Shoreline Changes. 

Excavation of sediments from offshore sand ridges and shoals can result in shoreline 

change in one of two ways: 1) through alterations to the wave transformation pattern, 

changing the waves that reach the shore, in turn modifying the sand transport related 

processes and ultimately changing erosion and accretion patterns; and 2) by interrupting 

or modifying a sand supply pathway from or through the borrow area to the shore. A 

review of the currently identified OCS borrow sites suggests that many of them are not at 

risk from the second impact because they are isolated from the sediment budget of the 

littoral system by large distances and muddy areas (the latter indicating the absence of a 

sand transport pathway). Nevertheless, this will not always be the case. 

MMS has commissioned studies on wave modeling and shoreline erosion at potential 

OCS borrow sites off New Jersey, Virginia, North Carolina, Florida, and Louisiana (see 

Table 2.1). The general conclusion of these site-specific studies is that removal of 

offshore shoals can change the amount of wave energy reaching the shoreline, but no 

significant changes to longshore sediment transport are likely. Two of the four physical 

monitoring protocols developed for MMS (Research Planning, Inc. et al., 2001 and 

summarized in Nairn et al., 2004) were specifically designed to address the potential for 

shoreline impacts. These were the “Waves” and “Shoreline” Protocols. These protocols 

included recommendations on monitoring and numerical modeling to avoid the potential 

for alteration of shoreline erosion and sedimentation patterns.

 

The importance of this concern varied by region. Where the OCS sand bodies were 

close to shore and/or shallow enough to influence the wave climate, there was high 

concern about the potential for increased shoreline erosion as a result of dredging. The 

orientation, depth, and shape of the sand body and borrow areas should be considered in 

evaluating the impact of dredging on wave climate. 

In summary, the potential for shoreline erosion will be determined by site-specific 

modeling studies and long-term shoreline monitoring programs as described in the 

Monitoring Protocols. In other words, these types of impacts will not be specifically 

reviewed as part of this investigation. 

2.3.5 Sedimentation (Burial) Impacts to Adjacent Hard/Live Bottom or 

Other Sensitive Habitats 

Hard/live bottom communities are usually associated with outcroppings of rocks or 

hard fossil substrates that are richly colonized by algae, sponges, hydroids, octocorals, 

stony corals, and other attached species. These areas are important for foraging and 

protection from predation for fish populations, particularly where they occur in sedimentdominated areas. In south Florida, there is particular concern where small sand borrow 

sites occur between hard/live bottom habitats, as well as the Oculina Bank region in both 

12 
State and Federal waters off central Florida. Many hard/live bottom habitats in nearshore 

areas have been severely damaged by sedimentation and burial associated with beach 

nourishment projects (e.g., Lindeman and Snyder, 1999). 

In potential sand borrow sites off central Florida, hard bottom habitat was mapped as 

present along 38 percent of the transects through the southern part of the borrow area 

although previous studies had not reported hard bottom habitat in this area (Byrnes and 

Hammer, 2004). Hard bottom formations identified off central Florida occurred as ledges 

or outcrops of limestone generally arranged in north-south trending outcrops usually 

forming ledges facing west. All hard bottom supported epibiota assemblages of varying 

taxonomic composition (Byrnes and Hammer, 2004). The results of the central Florida 

study highlight the importance of conducting surveys to identify the presence of hard/live 

bottom habitat in sand borrow areas prior to sand mining in regions where there is 

potential for them to occur. 

There are three possible ways that dredging activities in an OCS borrow area may 

result in sedimentation-related impacts to hard/live bottom habitats: 1) through direct 

sedimentation associated with the footprint of the sediment from the overflow of Trailing 

Suction Hopper Dredges (TSHDs) or from the draghead or cutterhead (from Cutter 

Suction Dredge [CSDs] in the latter case); 2) re-suspension and subsequent transport of 

fines (specifically, silt and clay) beyond the initial sedimentation footprint; and 3) 

development of near-bed turbidity plumes or currents that may travel well beyond 

“normal” bounds of a plume sedimentation footprint. Sedimentation affects hard/live 

bottom sessile communities by interfering with photosynthesis, respiration, and feeding. 

In most offshore areas where hard/live bottom habitats occur, the sediment grain size is 

usually in the sand range, and impacts from dredging are likely to be localized and shortterm. Thus, the greatest potential impacts are from sediment deposition, which could bury 

organisms, clog filter-feeding organisms such as sponges, cause corals to expend energy 

producing mucous to clear sediment from their surfaces, and reduce hard surface area 

available for recruitment. NMFS included the following requirement in their Regional 

Biological Opinion for dredging in the Gulf of Mexico (NMFS, 2004): 

Hardground Buffer Zones: All dredging in sand mining areas will be designed to 

ensure that dredging will not occur within a minimum of 400 feet from any 

significant hardground areas or bottom structures that serve as attractants to sea 

turtles for foraging or shelter. NOAA Fisheries considers (for the purposes of this 

Opinion only) a significant hardground in a project area to be one that, over a 

horizontal distance of 150 feet, has an average elevation above the sand of 1.5 feet or 

greater, and has algae growing on it. The COE Districts shall ensure that sand 

mining sites within their Districts are adequately mapped to enable the dredge to 

stay at least 400 feet from these areas. If the COE is uncertain as to what constitutes 

significance, it shall consult with NOAA Fisheries, Habitat Conservation Division 

and NOAA Fisheries, Protected Resources Division for clarification and guidance. 

13 
In summary, it was concluded that dredging methods are needed to make sure that 

significant hard/live bottom habitats are not covered by sediments as a result of dredging 

at sand borrow sites. The review of environmentally friendly equipment and practices 

will, therefore, consider this potential impact. 

2.3.6 Creation of Depressions and Furrows From Removal of Substrate. 

This concern was that, under certain conditions, dredging would affect seabed 

topography or surface roughness enough to interfere with trawling fisheries. Dredging 

activities can result in lowering of the level of the seabed, creation of trailer marks and 

depressions on the seabed, and exposure of hard bottom previously covered by sediment. 

The creation or exposure of these features can lead to difficulties in certain fishing 

activities, in particular trawling, as the trawl gear can potentially become hung up on 

these features (Royal Haskoning, 2004). Figure 2.2 shows an example of dredge scars. 

The images used in this mosaic were collected using a Sea Scan® PC Portable 

Fieldworks PC System with a 600 kHz Towfish at 20 m range and then mosaiced using 

SeaSone Mapper software. The dredging operation shown in this mosaic is in Red Brook 

Harbor, North Falmouth, MA. The images used to create the mosaic were collected by 

John P. Fish of Ocean Star Systems Inc., Cataumet, MA. Figure 2.3 shows the crosssection of a sand borrow site off Miami Beach, comparing pre- and post-dredging bottom

topography. 

There is little information on the magnitude of this potential problem for OCS sand 

borrow sites. MMS has contracted a study entitled “World-Wide Survey of Dredging 

Impacts on Commercial and Recreational Fisheries and Analysis of Available Mitigation 

Measures to Protect and Preserve Resources.” The study includes ethnographic fieldwork 

with commercial and sport fishers and the dredging industry, during which actual data on 

the magnitude of this potential impact will be collected. 

It was concluded that this new MMS study would identify the magnitude of the 

problem and suggest measures appropriate to mitigate potential impacts for this concern. 

Therefore, this impact will not be a focus of this investigation. 

Existing MMS stipulations include: 

• To assure that deep pits and furrows are not created, conduct postdredging hydrographic surveys. 

• The dredged area within the offshore borrow site shall not exceed 

maximum side slopes of 2:1. 

14 
FIGURE 2.2 Dredging scars from a CSD operation in Red Brook Harbor, MA. 

FIGURE 2.3 Pre- and post-dredge surveys borrow pit off Miami Beach (note the 

vertical exaggeration of 12.5 to 1 and dimensions are in feet). 

2.3.7 Short-term Increased Turbidity from Cutterhead or Draghead and 

Overflow (from hopper dredges) that Affects Benthic Communities. 

For TSHDs, increases in turbidity from dredging can be generated at two primary 

sources as shown in Figure 2.4: 1) the draghead; and 2) from the discharge of hopper 

overflow. With CSDs, turbidity is only generated at the bed by the cutterhead. 

15 
Sediments are suspended at the cutterhead or draghead during the process of 

removing sediments from the seafloor. Suspended sediments here are usually confined to 

the immediate vicinity of the cutterhead or draghead and do not reach the surface 

(LaSalle et al., 1991). In sandy substrates typical of OCS sand borrow sites, the extent of 

suspended sediments is likely to be very restricted. The exception would be where a finegrained sediment overburden must be stripped to access the borrow sand. 

Increased turbidity results from overflow discharges from hopper dredges. The 

behavior and persistence of plumes from overflow discharges have been extensively 

studied in the UK where 20 to 80 percent of the dredged material may be discharged 

overboard during screening of gravel deposits (Newell and Seiderer, 2003).

FIGURE 2.4 Hopper dredge sedimentation processes (note this figure shows two S1

sources at overflows from a screening operation; in almost all US dredges the S1 source 

is through the bottom of the hull).

 

Referring to Figure 2.4, the features of sedimentation associated with TSHDs 

dredging sand and gravel deposits are described in the following paragraphs. 

As mentioned above there are two types of sediment sources: S1 from the overflow 

(which for most dredges now is through the bottom of the hull and not directly overboard 

as shown in Figure 2.4); and S2 associated with suspension of sediment at the draghead.

16 
Sediment discharged overboard from the hopper overflow moves faster than would 

be anticipated from simple Gaussian models based on the settlement velocity of

component particles. This is due to high sediment concentration and discharge rate of the 

overflowed material, factors that lead to the development of a density current that moves 

through the water column in a ‘dynamic phase’ of settlement, at least initially. 

As the dynamic plume moves through the water column, sediment is stripped away 

from the plume. The sediment that is stripped away forms a passive plume that is 

advected and dispersed by ambient currents, with the particles settling according to 

Gaussian models. Figure 2.5 shows this process through backscatter measurements from

an Acoustic Doppler Current Profiler (ADCP) as a plume dissipates behind a hopper 

dredge.

In cases of shallow water and/or high discharge rates, the dynamic plume penetrates 

the water column all the way to the bed. In this case, the plume “pancakes” when it 

reaches the bottom. There is recent evidence from UK studies that where pancaking 

occurs, this body of sediment-laden water can travel long distances before dissipating 

through slowing of the lateral current and settling of the sediment. Newell and Seiderer 

(2003) cite an example where the travel distance for sediment-laden near bed water 

exceeded 2.7 km (see Appendix A). This process is captured from the backscatter signal 

of an ADCP in Figure 2.6. While this plume behavior corresponds to an anchor 

(stationary) dredge operation that is particularly conducive to this development, it is also 

possible that this may occur with conventional TSHDs.

In either the case of pancaking or not, sediment that settles on the bed can be 

eventually re-suspended by wave and current action and transported further afield, 

particularly where the sediment is finer than the native sediment.

Newell and Siederer (2003) note that UK studies have shown that, in most cases, 

coarse material up to sand-size particles settles within 200 to 600 m of the point source of 

discharge, depending on depth of water, tidal velocity, and the velocity of flow from the 

discharge pipe (it is noted that many of the areas of study in Britain have much stronger 

tidal currents than along the Atlantic and Gulf coasts of the US, so the 600 m limit noted 

above may be overly conservative in US waters). Although, as noted above, under some

circumstances a highly turbid near-bed flow can develop (i.e. through the pancaking 

process associated with impact of the dynamic plume and the bed) and transport sediment 

at least 2.5 km from a dredge site. 

The pancaking process where the dynamic plume phase is converted to a laterally 

spreading turbidity current upon impact with the bed would appear to be a very important 

one with respect to the potential lateral extent of sedimentation. In order to investigate 

whether the pancaking process is one that might be expected for offshore sand dredging 

in waters of 10 to 30 m depth under MMS jurisdiction, the MMS Plume Model for 

TSHDs (see Baird & Associates, 2004) was applied to evaluate plumes for two of the 

largest TSHDs operating in US waters: the Stuyvesant (8,250 m3

 hopper) and the Liberty 

17 
Island (5,000 m3

 hopper). For these dredges working in medium sand with less than 15% 

fines the overflow rate would range from 5 to 3 m3

/s, respectively for the Stuyvesant and 

the Liberty Island, with solids discharge rates in the range of 200 to 400 kg/s and 120 to 

220 kg/s, again respectively. At these discharge rates, the Plume Model results indicated 

that the dynamic plume phase would easily reach and impact the bed in water depths of 

10 to 30 m. This would also be the case for the smaller TSHDs operating in US waters. 

Whether or not a laterally spreading turbidity current would develop following the 

impact of the dynamic plume with the bed has not been investigated at a theoretical level 

in this project or any others found in the published literature, nor is it represented in any 

numerical models. From the theoretical understanding of turbidity currents, to trigger 

and sustain a laterally spreading turbidity current, the following conditions are required: 

an ongoing supply of water with high sediment concentration (i.e., an unbroken supply 

from the dynamic phase impacting the bed – since the dredge is always moving, this may 

only be sustainable by larger TSHDs on a long run) and sufficient bed slope and/or 

ambient flow condition to sustain the turbidity current (see Parker et al., 1986 and Stacey 

and Bowen, 1988). The ambient current would have to be parallel to the axis of dredging 

for the supply of sediment-laden water from the dynamic plume to remain unbroken. 

This pancaking phenomenon and the related potential for a much larger sedimentation 

footprint (i.e., greater than 2 km) should, therefore, be considered where the ambient 

currents are strong (and parallel to the main axis of dredging) or where the local seabed 

slopes are steep. 

Nevertheless, it has been assumed in the past that the pancaking process of a laterally 

spreading turbidity current does not occur in most cases. In these situations, even fine 

silt-sized particles reach background values within 2 to 2.5 km of discharge, although 

there is a residual ‘signature’ from the dispersing plume at distances of up to 3.5 km, 

which may be attributable to organic matter derived from fragmented benthos discharged 

during the screening process (refer to Figure 2.5). 

Existing MMS stipulations include: 

• Turbidity shall not exceed background levels by more than 29 

Nephelometric Turbidity Units (NTU). If monitoring shows that turbidity 

exceeds the maximum amount allowable, dredging activities shall cease 

immediately and not resume until corrective measures have been taken 

and turbidity has returned to acceptable levels. 

This stipulation is not normally or frequently included by MMS, but it has been invoked 

where an Environmental Assessment analysis indicated some concerns with water quality 

in the project area. The 29 NTU limit (above background levels) is based on waterquality criteria developed for dredging in Florida waters. Apparently, it originally 

evolved from a conversion of the EPA Clean Water value of 50 JTU (Jackson Turbidity 

Units). NTUs 

18 
FIGURE 2.5 Acoustic backscatter images across the plume at varying distances 

downstream of an anchor dredge during loading of a screened cargo at Owers Bank off 

the south coast of UK. Based on Hitchcock and Drucker (1996). The black band at the 

seabed is a data corruption zone that precluded assessment of plume morphology at the 

sediment-water interface. The red signal indicates coarse sand-sized particles; the yellow 

signal indicates the settlement of silt; and the white signal is considered to represent 

organic flocculating material. 

are measured automatically through instruments that measure the scattering of light 

whereas JTUs are based on a visual assessment of the fuzziness of a mark at the bottom

of a clear tube. It is likely that the original value of 50 JTU resulted from limits 

associated with avoiding ecological impact in streams and rivers. In Florida, for typical 

beach nourishment projects, the measurements for turbidity compliance are usually taken 

150 m offshore and no more than 150 m downcurrent of the discharge point within the 

densest portion of any visible turbidity plume. The measurements are taken at the surface, 

mid-depth and near the bottom. Background levels are specified to be measured 1,000 m

upcurrent from the dredging operations. Many investigators have questioned the validity 

of a general limit for all conditions (see Goldberg, 1989). Based on comparison to more 

spatially comprehensive ADCP measurements of turbidity, the point measurements of 

turbidity in space and time have also been argued to add arbitrariness to the evaluation 

(Doug Clarke, personal communication). Surface water quality limits vary by State in a 

range of 5 to 150 NTU above background (depending on the State and the location and 

19 
seasons) but are mostly in the 20 to 50 NTU range (Source: USEPA Office of Water, 

Office of Science and Technology). In most cases, these limits were developed for 

freshwater conditions although are often applied to marine conditions as well. Some

States have separate marine and freshwater limits. Wilber and Clarke (2004) indicate 

that few field studies have actually been completed for the low levels of turbidity that 

exist nearby a dredging operation (much of the data come from acute response tests). In 

summary, there is limited work in defining a scientific limit of elevated turbidity levels 

for offshore marine environments. Also, the application of a single point, single value 

limit is relatively arbitrary and perhaps unsupportable scientifically. 

FIGURE 2.6 Longitudinal section of the sedimentation plume from a dredge loading 

a screened cargo at Owers Bank in 1995. Based on Acoustic Backscatter data from

Hitchcock et al. (2002). The red side of the scale indicates high backscatter levels and the 

blue side of the scale indicates low backscatter levels (depths and Eastings are in meters). 

In the UK and Europe the assessment of dredging impacts related to turbidity is 

almost exclusively focused on sedimentation. There is little concern with the water 

column turbidity levels as it is assumed fish can easily evade areas of higher turbidity 

(Desprez, 2000). 

It is generally assumed by resource managers in the US that, when OCS dredging 

occurs in sandy substrates, turbidity would be short-term and animals in the water 

column would avoid turbid areas. Turbidity might be more of a concern in areas where a 

fine-grained overburden has to be removed to access the coarser sediment below, 

20 
although even in these areas the existence of fine sediment on the bed would suggest a 

relatively high background suspended sediment concentration. However, potential 

impacts to sensitive benthic habitats, such as hard/live bottom habitats, are still of

concern even in sandy substrates. 

In summary, there is a need to determine the potential impacts of plumes from

overflow discharges on sensitive benthic habitats and appropriate methods to reduce 

these impacts where they could be significant. However, it would appear to be generally 

accepted that water column turbidity impacts to marine ecology from dredging operations 

in sandy substrates are not a significant concern. 

2.3.8 Spatial and Seasonal Conflicts Between Dredging and Commercial 

and Recreational Fisheries. 

Dredging in the OCS poses the potential for navigational conflicts with local 

commercial and sport fishers from the presence of operating dredges at the borrow site 

and transit to and from the sand discharge points. Such conflicts may result in diminished 

access to favorable fishing areas and a loss of harvest. Fishery impacts have been 

identified as being of general concern by several groups, and they have also been of 

specific concern in some instances such as in New Jersey with the surf clam fishery. 

Mitigation actions can include lease stipulations, such as avoidance areas, as is done to 

protect archaeological resources or oil and gas infrastructure in the borrow site. 

Mitigation actions can also include institutional activities to facilitate communication and 

cooperation among potentially conflicting entities. Some recommendations made to 

decrease the likelihood of conflicts have included: identifying the most appropriate 

fishery industry liaisons to facilitate communications, providing sufficient advance 

warning of impending dredging activities to fishermen, zoning permitted areas so as to 

protect the most important fishery grounds, avoiding dredging during peak fishing 

seasons, setting up relatively small exclusion zones within the larger permit area to 

shelter sensitive habitats, selection of transit routes that minimize interference with 

fishing activities, choosing those dredging techniques which have the lowest fishery 

impacts, limiting extraction rates, regulating the “at-sea” screening of sediments, and 

effective monitoring of dredging operations to ensure compliance. In the UK, success of 

the majority of mitigation measures related to the commercial fishing industry relies 

heavily on communication between the two industries (Royal Haskoning, 2004). To 

address this concern, MMS has contracted a new study entitled “World-Wide Survey of 

Dredging Impacts on Commercial and Recreational Fisheries and Analysis of Available 

Mitigation Measures to Protect and Preserve Resources.” 

In summary, it was concluded that the new MMS study would address this concern, 

and therefore, it is not a focus of this report. 

21 
2.3.9 Potential to Break an Active or Abandoned Pipeline, Resulting in a 

Release of Petroleum Hydrocarbons 

Throughout the central Gulf of Mexico, numerous pipelines, platforms, wellheads, 

and other related oil and gas infrastructure are present in potential OCS sand borrow 

sites. Given the potential for removal of 3 m or more of sediment in areas near oil and gas 

infrastructure, there is a potential risk that dredging will result in changes to the sediment 

stability and seafloor topography that could lead to damage to existing pipelines and 

structures. The primary mitigation method in practice is to establish no-dredge buffers 

around known infrastructure. However, there are many questions yet to be answered on 

the short- and long-term impacts of sediment removal in the vicinity of oil and gas 

infrastructure, including: How much sediment can be removed from a sand borrow site 

before the surficial integrity of the site is impacted such that the surface collapses and the 

structural integrity of facilities are compromised, especially during storm events? What 

widths are appropriate buffer zones around these facilities to avoid such a compromise? 

MMS is currently conducting a study entitled “Study to Address the Issue of Seafloor 

Stability and the Impact on Oil and Gas Infrastructure in the Gulf of Mexico.” 

In summary, it was concluded that this new study would address this concern. 

Therefore, this topic is not addressed in this assessment of environmentally friendly 

approaches to dredging. 

2.3.10 Collisions with Marine Mammals and Sea Turtles During Vessel 

Operations 

Vessel collisions with endangered whales (Northern right whale, fin whale, and 

humpback whale) are one of the major factors limiting their recovery (NMFS, 1991b,c; 

Reeves et al., 1998). There has never been a report of a whale strike or mortality by a 

hopper dredge in the US (NMFS, 2004), although there is one report of a right whale calf 

mortality resulting from a strike by a dredging vessel in South Africa (C. Slay, Coastwise 

Consulting, Inc., pers. comm., 2004). It is generally thought that hopper dredges move 

slow enough to minimize the risk of a strike with a marine mammal. In areas where 

recreational boating and ship traffic is intense, propeller and collision injuries are not 

uncommon for all sea turtle species. 

Existing stipulations include: 

• If operating in areas of known whale occurrences, observers are required. 

If whales are observed, avoid intentional approaches within 100 yards 

(500 yards for right whales) and slow speeds to less than 4 knots. 

• See stipulations for sea turtles. 

22 
In summary, it was concluded that existing stipulations are adequate for now, and 

this potential impact was not addressed further in the evaluation of environmentally 

friendly approaches to dredging. 

2.3.11 Damage to Archaeological Resources 

The National Historic Preservation Act requires Federal agencies to protect historic 

and cultural resources, which include shipwrecks, historic fortifications, and coastal 

settlements, as well as prehistoric sites that have become submerged due to the global 

and local rise in sea level. As a Federal agency, the MMS must protect the significant 

archaeological and historic sites that may be impacted by its activities. MMS 

requirements for remote-sensing surveys to identify and protect submerged cultural 

resources in lease areas where oil, gas, and sulfur deposits are being exploited in the Gulf 

of Mexico Region were identified in the Notice to Lessees 98-06 (MMS, 1998) and 

revised in Notice to Lessees 2002-G01 on 15 March 2002 (MMS, 2002). MMS 

guidelines for the conduct of archaeological resource remote-sensing surveys address 

three basic issues: survey navigation, survey pattern, and data acquisition instrumentation 

(Notice to Lessees 2002-G01). No requirements have been adopted for the Atlantic OCS 

and slightly different requirements are in place for the Pacific coast (Notice to Lessees 

98-05). 

MMS recently completed a review of its current practices and the development of 

recommendations on dredging methods, protocols, policies, and monitoring requirements 

to minimize impacts on submerged cultural resources (Research Planning, Inc. et al., 

2004). In summary, it was determined that this recent study adequately addressed the 

concerns and recommended approaches needed to minimize potential damages to 

archaeological resources. Therefore, this potential impact has not been considered as part 

of the evaluation of environmentally friendly approaches to dredging. 

2.3.12 Potential Harmful Alteration or Destruction of Essential Fish Habitat 

The 1996 reauthorization of the Magnuson-Stevens Fishery Conservation and 

Management Act required designation and conservation of Essential Fish Habitat (EFH) 

for species managed under existing Fishery Management Plans. EFH is defined as “those 

waters and substrate necessary to fish for spawning, breeding, feeding or growth to 

maturity” [16 U.S.C. § 1801(10)]. Maps of designated EFH for many species cover large 

areas. For example, Figure 2.7 shows the EFH for adult summer flounder as designated 

by the Mid-Atlantic Fishery Council. Clearly, OCS sand borrow sites are often located 

within designated EFH areas. EFH also includes Habitat Areas of Particular Concern 

(HAPCs) that are narrowly focused habitats with demonstrated direct habitat value for 

managed species. 

The potential effects to fisheries from sand dredging are unknown, having been 

identified in most of the environmental impact assessments prepared for OCS sand 

23 
dredging to be minimal or non-existent (Hammer et al., 2003; Louis Berger Group, 1999, 

Byrnes et al., 2003). This assessment was based on the determination that most of the fish 

inhabiting the potential dredge areas were characterized as wide-foraging or migratory, 

spending only part of their life cycle in the dredge borrow area. In addition, the 

ridge/shoal and shelf features identified as potential sand borrow areas are very large in 

geographic extent, extending over kilometers of seafloor and the potential borrow area 

for each dredging event is relatively small. Therefore, it was assumed that the lost or 

altered habitat area, overall, would probably be minimal. 

FIGURE 2.7 Map of the essential fish habitat for adult summer flounder, as 

designated by the Mid-Atlantic Fishery Council. 

24 
 In the literature review conducted by Research Planning, Inc. et al. (2001), they 

found that little was known about the ecological utilization of ridge and shoal features by 

fish species. Anecdotal information suggests that these features are important to fish as 

feeding, staging, or orientation areas during short-term or long-term migrations. To 

address this data gap, MMS is conducting studies of shoal features off Delaware and 

Maryland to determine if they constitute important habitat for fisheries and represent 

essential fish habitat (first study listed in Table 2.1). MMS is also funding a study of the 

utilization of Ship Shoal, Louisiana by shrimp and sea trout, through the Coastal Marine 

Institute at the Louisiana State University. Fish habitat on Sabine and Heald Bank 

offshore Texas are also being evaluated by USGS/BRD.

Although dredging can impact a wide range of types of EFH, at this time most 

existing and upcoming areas of dredging are associated with ridge and shoal features. 

There is a concern that potential impacts from dredging of ridges and shoals may affect 

their morphologic integrity. Hayes and Nairn (2004) identified the issue that repeated 

dredging of these features might lead to the deflation or eventual disappearance of the 

bathymetric feature. They proposed that offshore ridges and shoals are maintained by 

wave-generated sand transport processes, and they hypothesized that lowering of the 

feature below some critical depth would disrupt the processes that maintain the feature. 

To address this concern, MMS is conducting a study entitled “Analysis of Potential 

Biological and Physical Dredging Impacts on Offshore Ridge and Shoal 

Features/Engineering Alternatives and Options to Avoid Adverse Environmental 

Impacts.” Therefore, this study of environmentally friendly approaches will not focus on 

the issue of geomorphic stability of shoal features. Some initial recommendations are 

provided as this topic was discussed at the study workshop. 

2.4 Summary of Potential Impacts and Focus for this Investigation of 

Environmentally Friendly Approaches 

This section provides a summary of the review of various perceived impacts as 

identified through discussions with the regulatory agencies and the literature reviews. It 

provides the focus for the evaluation of environmentally friendly equipment and practices 

presented in Section 5 of this report. Table 2.3 lists the prioritized list of impacts 

together with comments on which impacts will be considered as part of this investigation. 

The key focus of this investigation will be on the following impacts in the order of

priority determined from the discussions with resource managers in the US: 1. loss of 

benthic habitat; 3. changes to substrate characteristics; 5. sedimentation and burial of 

sensitive habitat; and 7. short-term increases in turbidity. The remaining impacts are 

being or have been addressed in detailed studies by MMS (Impacts 4, 6, 8, 9, 10, 11 and 

12) and USACE (Impact 2). 

25 
TABLE 2.3 Summary of perceived impacts and focus for this assessment.

Impact Summary

1. Short-term and cumulative impacts 

from dredging that lead to loss or 

reduced stability of benthic habitats, 

including re-colonization by an altered 

biological community. 

This impact is a key focus for this 

investigation. 

2. Injury and death of special species of 

concern (e.g., sea turtles) from being 

sucked into the draghead during 

dredging operations using hopper 

dredges 

This impact and its mitigation are being 

addressed in detail by the USACE in an 

ongoing study. Therefore, it will not be 

addressed in this investigation. 

3. Changes in the substrate 

characteristics (grain size, dissolved 

oxygen, compaction and organic 

content) that lead to a reduction in 

benthic communities and suitability of

the area for future dredging. 

This impact will be reviewed as part of

this investigation. 

4. Changes in bathymetry that can alter 

the wave climate reaching the shore, 

resulting in shoreline changes. 

This impact was addressed as part of the 

Biological and Physical Monitoring 

Protocols developed for MMS and will 

not be addressed in this investigation. 

5. Sedimentation (burial) impacts to 

adjacent hard/live bottom or other 

sensitive habitats. 

This impact will be reviewed as part of

this investigation. 

6. Creation of depressions and furrows 

from removal of substrate. 

This impact will not be reviewed as part 

of this investigation as it will be 

addressed as part of an ongoing MMS 

study (“World-Wide Survey of Dredging 

Impacts on Commercial and 

Recreational Fisheries and Analysis of 

Available Mitigation Measures to 

Protect and Preserve Resources”). 

7. Impacts from short-term increased 

turbidity from cutterhead or draghead 

and overflow from hopper dredges on 

benthic communities. 

This impact will be reviewed as part of

this investigation. However, it is 

generally accepted that this is not an 

impact of critical concern. 

26 
TABLE 2.3 Cont. 

8. Spatial and seasonal conflicts between 

dredging and commercial and 

recreational fisheries. 

This impact will not be reviewed as part 

of this investigation as it will be 

addressed as part of an ongoing MMS 

study (“World-Wide Survey of Dredging 

Impacts on Commercial and 

Recreational Fisheries and Analysis of 

Available Mitigation Measures to 

Protect and Preserve Resources”). 

9. Potential to cause a break in an active 

or abandoned pipeline, resulting in a 

release of petroleum.

This impact will not be reviewed as part 

of this investigation as it will be 

addressed as part of an ongoing MMS 

study (“Study to Address the Issue of 

Seafloor Stability and the Impact on Oil 

and Gas Infrastructure in the Gulf of 

Mexico”). 

10. Collisions with marine mammals and 

sea turtles during vessel operations. 

Existing stipulations are satisfactory and 

this impact will not be evaluated as part 

of this investigation. 

11. Damage to archaeological resources. This impact will not be reviewed as part 

of this investigation as it has been 

addressed as part of an MMS study 

(Research Planning, Inc. et al., 2004). 

12. Potential harmful alteration or 

destruction of Essential Fish Habitat. 

This impact will not be reviewed in 

detail as part of this investigation as it 

will be addressed as part of an ongoing 

MMS study (Analysis of Potential 

Biological and Physical Dredging 

Impacts on Offshore Ridge and Shoal 

Features/Engineering Alternatives and 

Options to Avoid Adverse 

Environmental Impacts”). Some initial 

recommendations are provided as this 

topic was discussed at the study 

workshop. 

27 
3.0 LITERATURE REVIEW OF ENVIRONMENTALLY-FRIENDLY 

APPROACHES 

3.1 Methodology 

A literature review of environmentally friendly dredging technologies was 

completed in support of this study. 

The review was specifically focused on hydraulic dredging techniques. Mechanical 

dredging approaches were not included in the review as these are almost certainly not 

practical or economic for offshore dredging in Federal waters. As such the primary focus 

for the literature review was on Trailing Suction Hopper Dredges (TSHDs) and Cutter 

Suction Dredges (CSDs). There were some examples of development related to 

Stationary Suction Dredges. Dustpan dredges were also considered in the review. 

A literature review was conducted through the Internet and several information 

databases were identified including government agencies, research institutes and 

universities. One of the most extensive data sources was found at the Delft University of 

Technology in the Netherlands from which a list of approximately 1,200 potential 

documents was retrieved. The abstracts of these documents were read and the number of 

documents was reduced to 60 for full paper reviews. The targeted articles were obtained 

from various online and library sources. A bibliography of all key papers reviewed is 

included as Appendix B. 

Another key source of information was the Dredging Operations and Environmental 

Research (DOER) Program of the Engineering Research and Development Center 

(ERDC) at USACE. Reports, technical notes, bulletins, and research briefs were 

reviewed. It was found that the focus of the USACE research was mostly directed 

toward issues associated with nearshore and, particularly, navigation dredging, where the 

primary interest of the USACE resides. 

All the documents were reviewed and the relevant information related to 

environmental friendly dredging technology was documented and is summarized in this 

section. The summary is subdivided into sections pertaining to the different dredging 

equipment (TSHDs, CSDs and other devices) and monitoring approaches. 

3.2 Trailing Suction Hopper Dredges 

The most likely equipment of choice for offshore dredging for beach nourishment 

sand and aggregates on future MMS projects (i.e., in Federal jurisdiction at least 3 

nautical miles offshore in open water) will be TSHDs. 

A major part of offshore dredging of sands and aggregates in the US and Europe is 

currently undertaken with TSHDs. This type of dredge has been under intense scrutiny by 

28 
the dredging industry lately for two reasons; firstly there has been a large expansion in 

the TSHD market to accommodate the requirements of major reclamation projects in 

South East Asia (Tsurusaki et al., 1988; Evans, 1994), and secondly there have been 

serious efforts in recent years to develop methods of predicting and reducing the effects 

of sediment re-suspension by these dredges, as part of on-going research projects in 

several countries. 

A description of the sources of suspended sediment from TSHD operations was 

provided in Section 2.3.7. The primary source of suspended sediment is the hopper 

overflow. Sediment suspended at the draghead is local in nature and confined to a zone 

close to the bed. The hopper overflow usually produces a dynamic plume phase (where 

highly turbid water forms a turbidity plume or current through the water column), a 

passive phase and, sometimes, a near bed “pancaking” and laterally spreading turbidity 

current phase. “Pancaking” has been used as a metaphor to describe the effect of the 

vertical momentum of the dynamic plume phase impacting the bed and with the 

subsequent transfer of this momentum to spreading in the horizontal plane. 

The effects of overflow as a plume are discussed in Bonetto (1995), Sea Technology 

(1998),Van Dipen (1993), LaSalle et al. (1991), Whiteside et al. (1995), Hirsch et al. 

(1978), and ERDC-TN-DOER-E15. Conventionally, modeling of the passive phase of 

plumes has been performed to assess the impact of overflow (Bonetto, 1995; Whiteside et 

al., 1995; Norem et al., 1990). Only recently have models been developed to evaluate the 

dynamic phase of settling (see Baird & Associates, 2004). 

In terms of vessel design, the most obvious trend is in vessel size. Within a decade 

the maximum hopper size of TSHDs has moved from around 12,000 m3

 to in excess of 

35,000 m3

. This vast increase in size has been accompanied by increased loading 

capacity, particularly at depth. The large vessels themselves have the ability to support 

and deploy long suction pipes and these, together with the addition of underwater pumps 

in the trailing arm, make it possible to dredge sand at high concentration from great 

depth. A by-product of increasing the size of these vessels is that suction pipes and 

hopper dimensions become large. 

The significance of large, low-friction suction pipes are that high concentrations can 

be pumped with greater facility. Large hopper dimensions, combined with the use of a 

single suction pipe rather than the more normal two, allow a considerably better settling 

efficiency to be obtained in the hopper, with a commensurate increase in the retention of 

fine materials. 

As part of the on-going research into the efficiency of hopper systems and the need 

to be able to predict, in a quantitative manner, the re-suspension of sediments caused by 

operating dredges, the industry has recently carried out a number of experiments. The 

first of these was to measure the flow patterns in a large-scale model of a TSHD hopper 

(van Rhee, 2001; Ooijens et al., 2001). This research is being used to obtain a 

29 
fundamental understanding of the hydrodynamic flow processes taking place in a hopper, 

leading to better modeling of hoppers and thence better hopper design. 

At the prototype scale, studies have been conducted to determine overflow losses 

from working TSHDs and the resulting overflow behavior in the near field. At the same

time plumes from dragheads have also been measured (Land et al., 2004). The results of 

these measurements will not only inform current attempts to model sediment releases, but 

will also assist in future design of environmentally friendly dredges. 

In addition to the above advances in design, TSHDs now have high accuracy 

positioning and control systems, allowing them to be operated with considerable 

precision in the dredging area. This in itself makes it possible for more precise zoning 

requirements to be applied without seriously increasing cost-effectiveness. 

Over the last few years, two other areas of development in TSHDs that have been 

adopted almost industry-wide are under hull release of overflow sediment (except for 

screening operations) and the use of anti-turbidity valves (Pennekamp and Quaak, 1990; 

LaSalle et al., 1991; Tsurusaki et al., 1988). The single purpose of these approaches was 

to reduce the extent of suspended sediment plumes generated by the overflow process. 

Dredge equipment manufacturers have developed a closed system where the 

overflow from hopper dredges is re-circulated and used to feed a jet at the draghead to 

loosen the bed (McLellan and Hopman, 2000). The approach is sometime referred to as 

“Green Pipe”. This technique has the advantages of: minimizing the sediment discharged 

through the overflow process; providing for higher load capacity of sediment rather than 

low density water/sediment mixture; and decreasing the pressure drop inside the 

draghead which reduces dredge pulling force. A rule of thumb to guarantee the 

effectiveness of this system is to ensure that the source material to be extracted has a 

density greater than 1,300 grams per liter (g/l). Lower material density tends to remain in 

suspension longer periods of time. Some manufacturers claim a 20% increase in 

efficiency for dredging silty sand with the recirculating system (Francingues et al., 2000). 

In general, this approach has not been adopted by dredging contractors either in Europe 

or the US because the cost of retrofitting existing dredge vessels would not appear to be 

justified by benefits from reduced turbidity in most cases (see Section 5 for further 

discussion). 

An area of intensive development in the US relates to the potential entrainment of

organisms by TSHDs, particularly sea turtles. The three key approaches to mitigating 

this impact are: the specification of environmental windows for dredging operations; 

trawling surveys and relocation; and turtle deflectors for the draghead. The turtle 

deflector is a rigid device that is mounted on the draghead and displaces the turtle outside 

the reach of the suction field (Smits, 1998). A study was carried out at Canaveral harbor 

using this device and it was found effective although more studies were recommended 

since the dredge volume was relatively small (Nelson and Shafer, 1996) and the 

application was limited to relatively shallow waters. The DOER Program is currently 

30 
undertaking additional research and development to refine the design of sea turtle 

deflector dragheads. 

In Harwich Harbour, England, McLellan and Hopman (2000) report that TSHD 

operations were scheduled at intervals during peak tidal ranges to disperse the overflow 

away from the dredge area. Through application of the MMS Plume Model (Baird & 

Associates, 2004), dredging plans for TSHDs could be developed according to 

environmental conditions such that dispersion of overflowed sediment was spatially 

maximized to lower the total sedimentation at any given location, or minimized to reduce 

the footprint in areas adjacent to sensitive habitat. 

3.3 Cutter Suction Dredges 

Less attention was devoted to the review of environmentally friendly developments 

related to equipment and approaches associated with CSDs owing to the fact that their 

application will be less widespread for dredging in the offshore waters under MMS 

jurisdiction, mostly due to the long pumping distances from borrow areas in Federal 

waters to shore (at least 3 miles). Also, sediment plumes generated by CSDs are 

confined to near bed re-suspension around the cutterhead and, therefore, are generally 

much more spatially confined than plumes generated by TSHDs. The impacts related to 

the removal of benthic habitat would be similar to those associated with TSHDs. 

Most environmentally friendly developments related to CSDs are associated with 

modifications to the cutterhead, largely driven by projects to remove contaminated 

sediments from rivers and harbors. As one example, a low turbidity cutterhead was 

designed by Jan de Nul to be mounted on cutter suction dredges to accurately remove 

thin layers of silt, dredge material at in situ density, work in shallow areas, and reduce 

mechanical disturbance of the bed, thus reducing turbidity (McLellan and Hopman, 

2000). 

The environmental disk cutter device was developed by Boskalis and the Delft 

University of Technology. Essentially, it is a disk-shaped cutterhead with a closed, 

adjustable visor system for cutter suction dredges. It operates in a stationary manner and 

can achieve vertical positioning accuracy up to 5 cm. This device is well suited for 

dredging thin layers of sediment and has low spillage of sediment and works with a wide 

variety of sediment mixtures without generating high levels of turbidity. It also 

incorporates a highly accurate positioning and control system (Pennekamp, 1997). One 

of the problems with this type of device is the frequency of blockage of the intake due to 

debris (Smits, 1998). 

A gearbox device has been developed to allow diesel pumps to operate underwater 

on cutter suction dredges, allowing for greater flexibility and eliminating the need for 

electrical pumps. This system is so efficient that one of the largest manufacturers of 

dredge equipment, IHC Holland, employs it on all their standard cutter suction dredges 

31 
(Francingues et al., 2000). The Ellicot Mud Cat dredge uses a similar approach with a 

submerged pump mounted directly behind a horizontal auger. The Mud Cat is primarily 

used for very controlled removal of contaminated fine sediment (The Mud Cat is a 

registered trademark of Baltimore Dredges LLC, see also the USACE report EM 1110-1-

502). 

3.4 Other Dredging Devices and Equipment 

In this section, a range of dredging devices and equipment that are not associated 

with the conventional TSHD and CSD approaches are reviewed. Although some of the 

technologies have only been applied to contaminated sediment removal and 

environmental dredging, their use could be relevant in some site-specific sand/aggregate 

mining activities. 

One class of innovative development relates to the implementation of “remote” 

dredging techniques where the dredge equipment is located near to the seabed. One 

example of this technique is the Punaise approach developed by the PinPoint Dredging 

Company owned by J G Nelis, Ballast Needam and Boskalis (McLellan and Hopman, 

2000). Some of the advantages of being a submerged machine are that it doesn’t 

interfere with navigation and can operate in adverse wave conditions. Also, it does not 

generate a surface plume. Primary limitations are related to the capacity and productivity 

of the approach together with the fact that large and deep dredge pits result from the 

relatively stationary operation of this device, which in many jurisdictions are 

unacceptable due to the associated ecological impact. This dredge has not been used 

recently. It is now considered to be technically too complicated for a reliable, remotely 

controlled operation and is not cost effective compared with other techniques. 

Another example of a remote approach is the Underwater Archimedean Screw 

vehicle, which is a bottom-crawling machine that applies the Archimedean screw 

principle to excavate material ranging from a few centimeters up to 1 m (Smits, 1998). 

This equipment can incorporate a Remotely Operated Vehicle (ROV) that assists in 

guidance, navigation, and control. It uses DGPS for positioning and has an image device 

which shows where the equipment is at all times. This allows for a high degree of vertical 

and horizontal positioning accuracy up to 5 cm. This equipment is mainly used to extract 

fine contaminated sediment. 

The Sweep Dredger is another stationary device but allows for some movement by a 

sweep head similar to a TSHD draghead (Smits, 1998). It can cut layers from 20 to 60 cm

deep with an accuracy of 5 cm in the horizontal. The Sweep Dredger was developed by 

Dredging International (Pennekamp, 1997; Sea Technology, 1998). 

Some alternative methods for dredging include the PNEUMA pump, Dry DREdge 

and Soli-Flo (Romagnoli et al., 1998). These methods have the ability to produce higher 

32 
solids content and less turbidity than hydraulic dredges. The disadvantages are that they 

cannot operate in shallower areas and can be more expensive than traditional dredging 

methods. The PNEUMA uses compressed air to convey sediment through a pipeline. 

The Dry DREdge is a combination of hydraulic and mechanical methods. It drives a 

hydraulic clamshell into the bed and encloses the sediment avoiding spillage and 

turbidity. It captures the sediment with its original moisture content but can only be 

applied to fine sediment extraction in deeper areas. The Soli-Flo with eddy pump 

produces a slurry with high solids content (up to 80% solids) and low turbidity. These 

results can be achieved if the intake nozzle is placed at least 1 to 3 feet deep into the bed 

(Romagnoli et al., 1998). 

Another recent development relates to fluidization techniques. Fluidization 

techniques (also known as Water Injection Dredging or WID) have been utilized as a 

dredging approach where a water jet is directed to the bed to fluidize the sediment that in 

turn is transported away from the dredge area by the current or through the difference in 

densities and/or the bottom slope (Verweij and Winterwerp, 1999). This method has 

only been applied at a commercial scale for fine sediments that are capable of behaving 

as fluid mud. Currently, it would not appear to have a direct application to dredging for 

beach nourishment sand or aggregates. 

3.5 Monitoring Instruments and Approaches 

Monitoring is a key component of dredging projects to ensure an environmentally 

friendly operation and to reduce the impacts significantly (Amann, 1989; Thevenot et al., 

1992). 

The USACE employs an automated dredge contract monitoring system referred to as 

“Silent Inspector” (SI). SI has three components consisting of a Dredge Specific System

(DSS), a Ship Server, and a Shore Server. The DSS collects and displays standard 

information on dredging operations that is then transmitted to the Ship Server. Most 

dredging contractors already have a computer system and sensors onboard for control or 

positioning that can be used as the DSS. The dredging contractor supplies and owns the 

DSS and all associated sensors Figure 3.1 represents a screen shot of the on board menu 

of the DSS from which different functions can be accessed. Figure 3.2 displays the 

Dredge display which shows real time sensor data. Figure 3.3 displays a load summary 

report. Unlike manual load summaries, load summaries in Silent Inspector are 

automatically created from data that is sent directly from the dredge by satellite modem

located on the dredge or uploaded from a zip disk.

The Ship Server acts as the dredge based data archive and report creation center as 

well as performs automated reviews of the data. The Ship Server can produce many 

different reports including dredge location history, volume history, disposal location 

history (Figure 3.4) and operational status. Figure 3.5 displays a screen shot of the 

different plots that can be generated and an example plot is shown in Figure 3.6.

33 
The Shore Server is a larger system operated and maintained by the USACE. SI 

provides information on dredge location history, quantity history, and status of a given 

project. SI helps monitor all aspects of dredge operations from contract compliance to 

assurance that the operation is being performed in an environmentally safe manner 

(Rosati, 1999). Additional information and specifications regarding the SI can be found 

at: http://si.wes.army.mil/ 

Some of the SI capabilities include: 

• Monitors and documents where and when different dredging operations take 

place; 

• 24/7 coverage of operations; 

• Reduces paperwork and contractor reporting duties; 

• Creates detailed production reports; 

• Allows for fast responses to public or environmental concerns; 

• Allows for flexible scheduling of human inspectors; 

• Improves government estimates and planning; 

• Improves project management; 

• Standardizes data collection and reporting; 

• Creates a standard base for dispute resolution and avoidance. 

The Silent Inspector Track-plot viewer provides data and graphics of the dredge 

material disposal aboard hopper dredges. This viewer can be accessed online over a 

public accessible server. The Track-Plot Viewer allows the position of a hopper dredge to 

be monitored for any valid time and location (Figure 3.7). 

In Belgium, the Maritime Schelde Department has standardized a monitoring system

that records real-time data on dredging operations including: location, depth of cut, 

sediment mixture concentration, and several other parameters that help determine the 

performance of the operation (Francingues et al., 2000). 

A dredging project was undertaken in Hong Kong near a sensitive area (coral) and a 

dredging plan was described (Evans, 1994) which takes into account tidal influence, 

overflow control, siltmeter deployment, and weekly dives. 

34 
FIGURE 3.1 Control center window.

FIGURE 3.2 On board dredge display . 

35 
FIGURE 3.3 Load summary display. 

FIGURE 3.4 Dredge disposal history. 

36 
FIGURE 3.5 Plotting options display menu. 

FIGURE 3.6 Plot example (top to bottom: draft, drag depth, volume and 

displacement). 

37 
FIGURE 3.7 Dredge track plot. 

The standard for monitoring turbidity levels around dredging operations in a 

spatially and temporally comprehensive manner is with Acoustic Doppler Current 

Profiler (ADCP) (Land et al., 2004; ERDC-TN-DOER-E15). The ability to get a full 

water column description of turbidity levels along transects through plumes provides the 

ability to build a full understanding of the morphology of dredge plumes. 

Remote sensing approaches such as air photographs and satellite images can be 

analyzed to map surface turbidity, however, this water-surface-biased measure of plumes 

can be misleading and misrepresentative of the actual extent of the plume and 

sedimentation footprint, particularly with the advent of under hull discharge of overflow 

sediment. Evans (1994) utilized SPOT images to observe sediment dispersion at the 

Pearl River and describes the use of color air photography for the evaluation of dredging 

plumes. Optical instruments only provide a measure of turbidity at a single location. 

Some laser approaches (such as Laser In-Situ Scattering and Transmissometry or LISST) 

can provide a description of turbidity through the water column though with a smaller 

range than acoustic techniques such as ADCP (Gartner et al., 2001). 

After conducting a monitoring study of large-scale dredging activities in Hong Kong, 

it was determined that benthic productivity had not been significantly altered (Evans, 

1994). One of the technologies applied in the evaluation of dredge impacts included the 

38 
application of REMOTS (Remote Ecological Monitoring of The Seafloor), which is an 

instrument that samples the first 20 cm of seabed for sediment characteristics and 

biological parameters (Ocean Imaging Systems, 2004). This approach in the US is 

known as Sediment Profile Imaging (SPI) and consists of a prism that is pushed into the 

seabed to take an image of the vertical profile immediately below the bed (see Cutter and 

Diaz, 2000; Cutter et al., 2000). This approach has been recommended as part of the 

Benthos and Fishes Trophic Transfer Protocol for the MMS Physical and Biological 

Monitoring Profiles for offshore dredging projects (see Nairn et al., 2004; Research 

Planning, Inc. et al., 2001). 

3.6 Summary of the Literature Review

The primary focus of environmentally friendly developments in the dredging 

industry has been on a reduction of turbidity levels associated with the dredging process. 

Most recent developments in this area have been driven by contaminated sediment 

remediation projects where the sediment is mostly fine-grained and located in nearshore 

areas. Of these, the developments have mostly focused on modifications to dredge 

cutterheads to significantly reduce the generation of turbidity at the bed. However there 

have also been developments related to hopper dredges operating in offshore areas, and 

these have mostly stemmed from: 1) a need to reduce turbidity levels to address 

regulatory agency concerns and simple visual perception of impacts; and 2) the trend 

towards much larger hopper dredges to serve very large land reclamation projects. 

Other specific environmental impacts that have been addressed through 

modifications to dredge equipment include the development of turtle deflectors for 

TSHDs in order to minimize the potential for turtle takes. This area of development is 

the current focus for a comprehensive program by the USACE DOER program. 

As part of this review, a range of turbidity levels for different approaches and 

conditions has been compiled based on the work of Pennekamp et al. (1996) and 

Whiteside et al. (1995). The data from these articles are combined and summarized in 

Table 3.1. Pennekamp (1997) reports that the dredging equipment is not the primary 

variable in turbidity levels but instead the key variables are: sediment type; 

hydrodynamic conditions; and the operators and dredging technique application. 

Referring to the discussion in Section 2.4 of this report, it is recalled that the primary 

focus of this assessment of environmentally friendly approaches to dredging is on the 

following impacts (in order of priority as established through discussions with resource 

managers in the US): 1. loss of benthic habitat; 3. changes to substrate characteristics; 5. 

sedimentation and burial of sensitive habitat; and 7. short-term increases in turbidity. 

Once again it is noted that other impacts are being or have been addressed by MMS and 

USACE studies. Of these specific concerns, it has been determined from the literature 

review that most focus has been on Impact 7 (turbidity) and to some extent its related 

Impacts 3 and 5 (changes to sediment characteristics and sedimentation). No specific 

39 
information was found on environmentally friendly approaches to address Impact 1, the 

loss of benthic habitat, aside from monitoring approaches to assess the form (or quality) 

and rate of recovery.

40 
Table 3.1 Summary of turbidity generated by different dredge types taken from Pennekamp et al. (1996)

(1995)2.

Dredge type1 Sample site Production rate (m3/hr) Background turbidity mg/l 

Depth averaged 

turbidity increase 

(C) mg/l 

(on the edge of a 

50 x 50m area 

from dredge point)

Co

Large TSHD Ro tterdam 5,500 75 400 Large TSHD Rotterdam 5,400 40 150 Small TSHD (current =0m/s) Delfzijl 1,750 60 10 Small TSHD Rotterdam 2,170 23 60 Small TSHD (current 0.2m/s) Delfzijl 1,750 70 20 Pneuma dredge system Berghaven Harbour 59 25 0 Dragline with open clamshell Rotterdam 90 20 35 Dragline with open clamshell and silt curtain River Nieuwerkerk 84 35 35 Dragline with watertight clamshell River Nieuwerkerk 166 35 100 

Zierikzee 220 50 90 

Rotterdam 121 20 80 

Dragline with watertight clamshell and silt curtain River Nieuwerkerk 102 35 20 

Zierikzee 204 50 105 

Environmental disk cutter Berghaven Harbour 113 25 0 Auger Delfzijl 300 20 - 50 0 Siltcutter dredge Heusden 115 45 10 Water injection dredge Hellevoetsluis 20 30 Prototype water injection dredge Rotterdam 3,200 45 250 Bed leveler Rotterdam 610 35 60 

Dredge type 2 Sample site Production rate (m3/hr) Background turbidity mg/l Depth average silt concentration at T=10min De

s

TSHD (8,255 m3) Po Toi Hong Kong (1) 13 65 TSHD (8,255 m3) Po Toi Hong Kong (2) 13 55 TSHD (8,255 m3) Po Toi Hong Kong (3) 13 28 

41
4.0 DREDGING INDUSTRY REVIEW 

4.1 Introduction 

 

In order to determine the current state of dredging equipment and dredging 

techniques relative to reducing the impacts enumerated in Section 2, a dredging industry 

review was completed. Dredging publications that annually list the companies that own 

dredging equipment were reviewed. The Dredging Contractors of America was 

contacted to obtain a list of contractors that were in the beach nourishment business and 

recent bids for beach nourishment projects were reviewed. This investigation produced 

the names of four contractors able and willing to undertake beach nourishment projects. 

These contractors had a great deal of experience in beach nourishment projects on the 

East and Gulf Coasts of the United States, the regions under consideration in this study. 

 

As part of the industry review, European dredge contractors were also contacted. It 

was thought that perhaps the contractors would reply to a questionnaire with more 

interest if it originated from an organization based outside of the US. Most dredging 

projects in the US require the dredge hull to be fabricated in the US and the dredging 

company must be controlled by US owners. This eliminates foreign companies from the 

US beach nourishment work. However, we have in the past found several foreign 

dredging companies to be very helpful. The Minerals Management Service agreed to 

form a liaison with a foreign entity that was interested in the environmental aspects of

dredging marine aggregates. Our study partner, Dredging Research Ltd. (DRL), was 

tasked with selecting a suitable organization that was involved in the aggregate mining or 

beach reclamation work outside of the United States. 

 

Once the sponsor organizations were identified, a questionnaire was developed with 

input from all team members. The questionnaire was then sent to each of the identified 

dredging companies in the US and Europe. Responses were received from most and 

follow-up calls were made to clarify responses. The keen interest shown by two of the 

US dredging contractors (Bean Dredging LLC and Great Lakes Dredge and Dock 

Company) lead to their participation in the workshop segment of the study (see Section 5 

for further details). 

4.2 Selection of a European Partner

The Mineral Industry Research Organization (MIRO) is the pre-eminent 

international provider of collaborative research project management to the minerals and 

related industries. MIRO, based in the United Kingdom, works in partnership with 

industry, government, research and service providers to identify, influence, fund, transfer, 

deliver, and communicate information and relevant, innovative research and technology 

development to address the needs of stakeholders in the sector. MIRO’s role contributes 

towards improved communication, safety and environmental performance at all stages of 

the materials life cycle, meeting the challenges of sustainable development and 

increasing the positive image of the minerals sector. 

42 
MIRO was selected to send out a questionnaire to European dredging companies 

because they were, concurrently with MMS’s project, implementing a research project 

entitled “Best Practice Guide to Assessing the Impacts of Aggregate Dredging”. This 

work was being funded through the UK’s Sustainable Land Won and Marine Dredged 

Aggregate Minerals Programme, which is an aggregate minerals research program

established under the terms of reference of the Aggregate Levy Sustainability Fund 

(ALSF), and implemented by the Office of the Deputy Prime Minister. It was considered 

that the European contractors would be more amenable to providing information to a 

European/UK industry supported organization and research initiative, than directly to a 

US governmental organization in a market place to which they only have indirect access. 

4.3 Development of a Questionnaire 

An extensive and detailed review of the perceived environmental impacts of 

dredging together with a literature review of environmentally-friendly approaches to 

dredging were undertaken as summarized in Sections 2 and 3 of this report, respectively. 

These reviews allowed us to frame the questions in a manner that would produce 

thoughtful answers from the dredging contractors. The questions for the most part can be 

grouped into three broad categories: 1) plume related impacts, 2) impacts to benthic 

habitats, and 3) impacts on marine mammals. There are a few additional concerns that 

fall outside these three groups, such as impacts to marine structures or archeologically 

important sites or conflicts with commercial and recreational fishing. 

It was decided to send out two questionnaires, one to companies primarily working 

in the US and the second to companies working outside of the US. The questions are 

essentially the same with some changes in wording for companies whose primary 

business is outside of the US. The Mineral Industry Research Organization’s membership 

has many companies whose business is the offshore mining of aggregates by hopper 

dredge. 

 

As discussed above, the perceived concerns of researchers and staff from Federal 

and State resource agencies in the US were prioritized. These were concerns regarding 

the effects of dredging on the marine biological and physical resources. The focus was on 

dredging on the Outer Continental Shelf for beach nourishment and land reclamation, and 

potentially for aggregates. This list has been updated with information contained in 

stipulations for recent offshore leases. 

 

The concerns were prioritized under twelve headings as described in Section 2 of 

this report. The questions based on these concerns were phrased to obtain information on 

the operational restrictions that the dredge is currently working under. The questions 

were targeted to elicit general and specific information that would be applicable to a wide 

range of possible dredging scenarios relative to environmentally friendly dredging 

equipment and procedures. The List of Questions is provided in Appendix C. This 

appendix includes the cover letter to US contractors (C.1), the questions posed to US 

43 
contractors (C.2) and the questions sent out to European contractors under the auspices of 

MIRO (C.3). 

 

The questionnaire was sent to the four largest U.S dredging companies. These 

companies are equipped to dredge sand in the ocean waters of the Outer Continental 

Shelf. Each has done beach nourishment work. They are all interested in doing beach 

work in the future. We also sent the questions to the US Army Corps of Engineers. The 

Corps owns several hopper dredges although they do not normally provide sand for beach 

work that has been dredged from the OCS, however, they do administer many of the 

contracts associated with beach nourishment in the US. 

 

Our study partner, Dredging Research Limited , arranged to have the joint MMSMIRO foreign version of the questionnaire sent by MIRO to its members. Initially five 

general dredging contractors received the document and three of these responded, being 

Westminster Dredging (BosKalis), Dredging International Ltd and Ballast Ham

Dredging, all subsidiaries of major European dredging contractors. The last of these 

responded somewhat briefly as they were at the time in the throes of being taken over by 

VanOord. Jan de Nul, who’s international division had supplied some useful information 

to the US team in the early days of the study, declined to answer the questionnaire on the 

basis that they were not interested in aggregate dredging in the UK. 

Subsequently, the UK questionnaire was also sent to the main commercial aggregate 

dredging companies, all of which are subsidiaries of mining and construction materials 

groups. None of these responded. However, extracts from the MIRO research project 

relating to the environmental effects of aggregate dredging have been received and these 

have been approved by BMAPA, the organization that represents the major marine 

aggregate extraction companies. 

4.4 Review of Responses 

Overall the responses by the dredging contractors, both US and foreign, were very 

similar. All four US companies and the Corps of Engineers plus several foreign 

companies responded. One US Company, Great Lakes Dredge & Dock Company, gave 

very detailed answers to many of the questions including examples. The Belgian 

company, Jan de Nul, supplied an explanation of the “anti-turbidity valve” and the “green 

pipe system” with graphics, two of the more popular and innovative methods of 

controlling turbidity by equipment modification. 

A matrix of the questions and the answers given by the four US companies, two 

foreign companies and the Corps of Engineers is provided in Appendix C.4. 

44 
4.4.1 Discussion of Responses to Plume Related Impacts 

This section summarizes the responses received from the various dredging 

contractors as they relate to potential plume impacts. 

 

There are several equipment groups that can be used to mine and deliver sand. Under 

the great majority of circumstances the actual mining of the sand on the OCS will be 

done with a Trailing Suction Hopper Dredge. This is because the typical distance 

between the borrow area and the beach area makes the hopper dredge the most 

economical equipment. The hopper dredge also works best in the relatively rougher sea 

and swell conditions offshore. Figure 4.1 is a schematic drawing representing beach 

nourishment scenarios where the numbers represent the following dredging equipment 

combinations: 

1. Cutter suction dredge pumping direct to shore; 

2. Cutter suction dredge loading barges with pump out to shore; 

3. Cutter suction dredge loading barges pumping in front of second cutter suction 

dredge; 

4. Trailing suction hopper dredge digging and unloading through a booster pump to 

shore; 

5. Trailing suction hopper dredge dumping in front of a cutter suction dredge for 

rehandling to shore; 

6. Clamshell dredge loading barges for pump out to shore; 

7. Clamshell dredge loading trucks; 

8. Trailing suction hopper dredge dumping near shore; 

9. Trailing suction hopper dredge pump out to shore; 

10. Trailing suction hopper dredge “rainbowing” to near shore; 

11. Cutter suction dredge loading barges for dumping near shore; 

12. Cutter suction dredge pumping through a spill barge for near shore placement. 

 

The most likely dredging approaches for beach nourishment sand in an OCS borrow 

area are Number 4, where the hopper dredge digs the sand and pumps the sand to the 

beach through a booster and Number 10, where the dredge “rainbows” the material to the 

beach. Approaches 8 and 9 where the hopper dredge dumps or rainbows the sediment 

near the shore are also a possibility, however, these approaches have not had widespread 

use in the US. There may be some instances where a Cutter Suction Dredge may be 

implemented, pumping sand to shore through a pipeline (Number 1 in Figure 4.1). 

In order to understand the terminology in the questions and answers to the 

contractor’s questionnaire, Figure 4.2 provides a sketch of an operating hopper dredge. 

The hopper dredge dragheads loosen the sand and convey the sand/water slurry to the 

onboard pumps that discharge the material into the dredge’s hopper. Some of the material 

settles into the hopper and some of the sediment is overflowed back into the water, in this 

case, through the hull below the keel. 

45 
FIGURE 4.1 Beach nourishment scenarios using different dredging equipment 

combinations (from Randel and Koo, 2003). 

Turbidity Plume

Draghead 

Turbidity Plume

Sediment Overflow

FIGURE 4.2 Hopper dredge in operation showing the sediment overflow and 

draghead plumes (from Jan de Nul – See Appendix C5). 

46 
Unless there are restrictions regarding overflow, the dredge will continue to load 

sand until it reaches its economic load. The economic load is the relationship of the 

loading time versus the loading rate versus the overall cycle time of the dredge. In other 

words, if the sailing (hauling) distance is great, it is usually economically justified to 

spend extra time loading sand and overflowing finer particles. A very small percentage of 

fines below the #200 sieve (i.e., D50 less than 0.06 mm) would be retained in the hopper, 

when a hopper dredge is mining sand on the OCS. The percent of material that overflows 

the hopper is greatly dependent on grain size of the material and the percent of fines. The 

percent of material that can overflow the hopper can vary widely even in the same

borrow area. 

 

Fines that are lost to the sea through overspilling tend to fall to the seabed as a 

density current initially, which generally decays into a passive plume that is moved by 

the ambient currents. Recent observations suggest that a high proportion (85-90%) of the 

total material overspilled in sand dredging operations is not incorporated into the passive 

plume and, thus, never leaves the proximity of the dredging site. 

There has been considerable research into settlement in - and overflow from - the 

hopper of a hopper dredge. Recent research conducted by Ooijens et al. (2001) into 

hopper settlement using large-scale modeling has been used to clarify the hydrodynamic 

flow processes in the hopper. The researchers constructed a test rig, scaled down from a 

working hopper dredge. Their test series showed a process comparable to the 

measurements taken on board a hopper dredge during actual dredging conditions. This 

test rig provides an opportunity to improve hopper design parameters and to develop 

mathematical models to improve the performance of hopper dredges. The rig will also 

make it possible to test different hypotheses that can’t be tested with available 

techniques. 

 

One concern posed to the dredging contractors was the ability of a TSHD to strip an 

overlying area of fine sediment to expose an underlying sand deposit. Borrow areas are 

not normally chosen that have a significant amount of silt overburden. It may be possible 

to dredge sand below a silt layer but this would be highly dependent on the nature of the 

silt. It may be that a cutter suction dredge will be more suitable for a borrow area 

condition with significant overburden or stripping the area of overburden first with 

confined disposal of the silt. 

 

Normally, dredging sand for beach nourishment from an OCS borrow site does not 

produce increased turbidity (i.e., exceeding the background levels) over the often-applied 

State of Florida 29 NTU standard. This is, however, highly dependent on many factors. 

Timing and location of sampling, dilution zone, wind and currents, color of the silt, and 

chemical composition (carbonates) of the fines all influence the turbidity of the overflow. 

The amount of fines in the overflow will increase if an attempt is made to recover sand 

below a layer of fine material. Turbidity from a hopper dredge overflow can be regulated 

by limiting the overflow, to zero, if necessary. It is unlikely a “no overflow” condition 

would be required in an OCS borrow site, as this could have significant impact on the 

47 
cost of dredging the sand. There are dredge systems that recycle some of the overflow 

water and this will be discussed later in Section 4.6.2. Also discussed later in Section 

4.6.1 is the anti-turbidity valve that reduces the air entrained in the overflow. This air 

causes the turbidity to rise to the surface. It is now standard practice to discharge 

overflow through the bottom of the hull. The dredging companies, in their responses, 

were concerned that there is a perception that reducing turbidity is always important. 

However, the resource agencies did not rank this concern at the top of the priority list of 

impacts for most OCS borrow sites (see Section 2). 

 

The layout of the borrow site can have an influence on dredging procedures. It may 

be possible to influence the shape and dispersion of the dredge plume but certain 

parameters of the dredging process itself must be taken into account. The dredge operates 

best if it is moving parallel to the wind, waves, or current. This minimizes the rolling of 

the dredge and loss of bottom contact with the dragheads. In the extreme, requiring a 

dredge to work perpendicular to the sea/wind/current may cause conditions where the 

draghead would slip under the dredge causing an unsafe condition. The best shape for a 

borrow area, all things being equal and geologic conditions being relatively homogenous, 

is for the area to be set out with the primary dredging direction parallel to the major wind, 

waves, and current. Dredging companies do not have much data relative to the patterns of 

sedimentation of material discharged by the dredge. The company’s means of making 

such a measurement is restricted to sounding techniques. 

 

4.4.2 Discussion of Responses to Impacts to Benthic Habitats 

There was considerable concern among the stakeholders relative to the loss of 

benthic community; this was the highest ranked impact (see Section 2). Re-colonization 

rates are being studied, as are changes in the substrate characteristics such as grain size, 

dissolved oxygen, etc. that result from dredging operations. It has been suggested that 

dredging in patterns may speed the re-colonization rate by leaving areas that have 

undisturbed sediment characteristics and undisturbed benthic communities. 

 

The dredging companies were asked - If dredging in zones proved effective in 

increasing the rate of re-colonization, what dredging parameters should be considered?

The dredging contractors thought a dredge area, a minimum of 100 to 200 m wide by 

2000 m long, would be acceptable to the dredge operator. Preferably, the borrow areas 

would be oriented in a direction parallel to the main wind, wave, and current direction. 

The contractors did not have firm policies on zone dredging but seemed amenable to this 

as a possible solution. Some of the contractors work in a modified zone in order to 

manage the borrow area resources. Several contractors mentioned the “Sandpit” study 

currently underway in Europe. 

 

The contractors sidestepped the issue of specific known damage to hard bottom areas 

but these damages are known to have occurred. Everyone agrees that buffer zones should 

be established around hard/live bottom areas. The buffer zones may need to be 

determined on a case-by-case basis. The dredging industry has documented several times 

48 
their ability to track their dredges in real time including the ability to track in x, y, z the 

positions of the dredge’s dragheads (included in their responses to the questionnaire). At 

times, the Corps of Engineers independently tracks contractor dredges on projects with 

the use of their Silent Inspector System. Figure 4.3 provides a screenshot of the dredge 

operator’s display from the Liberty Island TSHD. The MMS has recently developed a 

new stipulation for tracking dredge positions (See Figure 4.4). 

There has been some interest on what the bottom of the sand borrow area may look 

like after dredging has been completed. Figure 2.3 provides an after dredge survey cross 

section of beach nourishment borrow area. 

Plan 

Sensor 

Profile 

FIGURE 4.3 Dredge operators display from the Liberty Island TSHD (courtesy 

of Great Lakes Dredge and Dock Company). 

Stern 

Although the bottom looks rough in this section, we must note the horizontal scale is 

more than 12.5 times the vertical scale. The latest tracks would be visible and parallel. A 

typical track may be 20 feet wide and 3 to 4 feet deep. It is possible to flatten the bottom

further by using navigation dredging techniques but there would be additional cost for 

this refinement. Depending on site conditions, the bottom will tend to smooth as a result 

of waves and currents. Survey data are relatively easy to collect because normally the 

dredge is tended by a survey vessel. Therefore, wherever possible, it is recommended that 

49 
high resolution (multi-beam acoustic is preferable to fully describe the seabed 

topography) bathymetry data be collected and delivered to MMS as suggested in the 

proposed Monitoring Protocols (see Research Planning Inc. et al, 2001). 

STIPULATION NO.____ – Use of Electronic Positioning System on Dredge and Transmittal

of Location and Production Information to the Lessor

Use of Electronic Positioning System and Transmittal of Location Information to MMS: 

In order to ensure the accuracy of the dredge relative to the borrow area specifications denoted in

this lease agreement, during all phases of the offshore operation conducted within the borrow area,

the Lessee will ensure that the dredge is equipped with an on-board differential global positioning 

system (DGPS) capable of maintaining and recording the location of the dredge within an

accuracy range of no more than plus or minus 3 meters. The specific system will be approved by

the MMS prior to the conduct of any dredge procedures within the borrow area. 

Location information (latitude and longitude) in NAD83 must be supplied to the Chief, MMS

Leasing Division (MMS-LD) on a daily basis. The information should be sent to the following

email address: dredgeinfo@mms.gov. 

Submittal of Production and Volume Information to MMS: 

The Lessor has a legal responsibility to ensure the accuracy of cut depths and widths, cut slopes

and site production (sand volumes removed) within the borrow area as specified in the project’s 

operational plan and this lease agreement. This information is routinely collected continuously

throughout the period of dredge operation at a borrow site. The Lessor shall retain all access rights

to all operational data at any time during which dredging is occurring within the designated

Federal borrow area. 

A “certified” summary of all operational, production, and survey activity data will be submitted to

the Chief, MMS-LD on a weekly basis, in a format and method agreed t between the Lessor, the 

Lessee, and the dredge operator prior to the commencement of operations at the borrow site. Any

maps and/or profiles submitted to the Lessor will be provided in digital spatial format compatible

with ArcGIS. Information pertaining to the volume of material removed must be provided with

explanatory text outlining each preceding day’s activities and production values. 

Following completion of all activities within the lease area, the Lessee, in cooperation with the

dredge operator, shall submit to the Lessor, a “certified” copy of the complete operational data set

(dredghead tracklines, cut slope angles, cut depth, etc.), outlining any deviations from the original

operational design plan. This report should be in MS Word format and can be sent to Ms. Renee 

Orr, Chief, MMS Leasing Division, 381 Elden Street, MS 4010, Herndon, Virginia 20170, or by

email to dredgeinfo@mms.gov.

FIGURE 4.4 New MMS stipulation on tracking dredge position. 

4.4.3 Discussion of Impacts to Marine Reptiles and Mammals 

The impact of dredging, particularly hopper dredging, to marine reptiles and mammals is 

a concern of resource agency managers, especially for sea turtles. This impact has been 

studied and mitigation measures have been employed for many years. The Corps of

Engineers, in concert with the dredging industry, has developed a draghead turtle 

deflector device specifically designed to reduce the “takes” of sea turtles. Reportedly, the 

use of this device along with a set of operating requirements has reduced the incidence of 

sea turtle takes. The main requirement is that the draghead must be in contact with the 

bottom while pumping. Most dredging companies rely on the swell compensator to 

50 
maintain contact between the draghead and the bottom. The operating requirements stress 

the need to balance the suction pipe velocities and densities in order to keep from taking 

sea turtles. One of the dredging companies suggested that it would be relatively easy to 

redirect flow away from the draghead when it comes off the bottom but also expressed 

doubt that this would reduce sea turtle takes. The dredging contractors have not 

encountered sea turtles in the vicinity of offshore borrow areas. The dredging contractors, 

also, have not experienced any dredge collisions with marine mammals. 

In addition to the turtle deflector device, there are also seasonal restrictions on when 

dredging can take place that result in the specification of environmental windows. The 

Corps of Engineers generally restricts dredging in channels and harbors to the months of 

December through March, from North Carolina to the tip of Florida. This restriction may 

be expanded both in duration and geographic area. 

 

Seasonal restrictions can, and at times do, cause dislocations in the hopper dredging 

market. This can lead to short-term price increases due to unavailability of equipment. 

These restrictions can also cause projects to be terminated before they are completed due 

to the contractor having difficulties that push completion dates past the dredging window. 

Currently, the seasonal restrictions apply to channels and harbors. Forcing additional 

work into the worst weather conditions causes marginal increase in prices. If this 

restriction should be applied to beach work, the weather risk will be magnified due the 

need to shift discharge pipelines in relatively calm weather. The Corps of Engineers is 

doing further studies on the effectiveness of seasonal restrictions. It seems that from the 

direction these studies are going, there may be a decrease in restrictions and a greater 

emphasis on monitoring. 

Observers on dredges don’t seem to reduce the sea turtle takes. All the US dredging 

companies thought trawling was an effective method for reducing sea turtle takes. 

4.4.4 Discussion of Miscellaneous Issues 

The contractors provided responses to additional miscellaneous questions regarding 

the dredging process. 

 

The contractors didn’t think that conflicts with fishing companies were significant 

and conflicts can be resolved through the permitting process and any mitigation measures 

agreed to at that time. 

 

Once the location of a marine structure or archaeological site is located (typically by 

the owner), the location can be integrated into the dredge operator’s display as an 

avoidance area. A plan is developed to avoid such structures or sites. In Europe, the 

standard security zone is 500 m on either side of the structure. Projects in areas of 

potential archaeological resources warrant extensive pre-dredging investigation to 

determine the precise location of these structures. 

51 
 

The contractors acknowledged that dredging might have adverse impacts on the 

environment. They expressed the desire to cooperate with all concerned parties to 

mitigate, minimize, or eliminate these impacts. They also cautioned against overregulation without scientific need or practicality. In talking to the regulators and the 

dredging companies, it seems that the whole process is moving toward increased 

monitoring and decreased or at least more selective regulation. 

4.5 Follow-Up Conversations with Dredge Contractors 

After the initial review of the contractor’s responses, we contacted the US 

contractors to ask for clarification of some responses and get additional detail. We invited 

all four of the US contractors to attend a Study Workshop held in Washington. The other 

invitees included MMS and the Corps of Engineers. There was spirited discussion of all 

the issues at the Workshop, and the contractors provided valuable commentary. Details 

on the outcome of the Workshop are presented in Section 5. 

 

Follow-up discussions of a general nature were held with some of the European 

dredging contractors. It was clear from these that generally the contractors are reactive in 

a highly regulated market that was somewhat different from the US scene. Much of the 

aggregate dredging is for gravels rather than sand, and the fines content of these zones is 

typically low (less than 5%). Marine reptiles and mammals are not perceived to be a 

problem, as they are rare in these waters, and fisheries concerns are dealt with during the 

statutory licensing procedures. Much of the knowledge held and research into benthic 

impact of such operations in the UK is to be found at CEFAS (Centre for Environment, 

Fisheries and Aquaculture Science), whose publications have been reviewed during this 

project. 

4.6 Discussion of the Anti-Turbidity Valve and the Green Pipe 

The dredging company, Jan de Nul of Belgium, sent us graphics that explain the 

Anti-Turbidity Valve and the Green Pipe System. A more detailed description of these 

approaches and the information provided by Jan de Nul is included in Appendix C.5. 

These two mechanical systems are the most noted in the literature for large-scale 

environmental turbidity control systems. 

4.6.1 Anti-Turbidity Valve 

Figure 4.5 presents the sketch of the anti-turbidity valve. With the standard dredge 

overflow, a large volume of air is mixed with the water and sediment due to the high fall 

height. Below the bottom of the dredge, a density stream of heavier particles moves down 

while an upward air stream occurs. This causes considerable turbulence and increased 

spreading of the dredge plume. 

 

52 
When the dredge system includes an anti-turbidity valve, the valve chokes the 

overflow, which prevents air being mixed with sediment and water leaving the hopper. 

Essentially, this keeps the overflow pipe completely full, reducing the fall height of the 

spoil, minimizing the amount of air entering the spoil. This minimizes the turbulence, 

reducing the visual plume. It is noted that both the size of the plume and the total amount 

of sediment in the plume are reduced through this device (see Appendix C.5). Most of the 

contractors think that this is an effective improvement to reduce turbidity and many 

indicated it is often applied on dredges now. 

FIGURE 4.5 Sketch showing the Anti-Turbidity or Environmental 

Valve (courtesy of Jan de Nul – see Appendix C.5) 

53 
4.6.2 Green Pipe System 

The Green Pipe System utilizes a second pipe system that recycles the overflow 

water back to the draghead (see Figure 4.6). The water is recycled through the suction 

head as process water. Using this system, less overflow drops through the water column. 

This approach is discussed in more detail in Section 3.2 on the literature review. 

 

There were several foreign hopper dredges built in the late 1990s that incorporated 

the Green Pipe System in their design. Recently built dredges have not included this 

feature. The US contractors did not have any experience with recycling the water in this 

manner. They cited large capital costs and increased maintenance cost without a clear 

benefit to the environment, particularly in dredging sand on the OCS. The European 

contractors hold a similar view. The green pipe has been available for use on TSHDs for 

a number of years now and nobody appears to have observed it being used on a project. 

The general opinion seems to be that it increases energy demand and equipment cost, yet 

does little in the way of reducing the total amount of fines lost. In addition, due to the low 

fines nature of many of the European marine dredging sites, there is little call for its use. 

The overflow suspension is 

pumped trough an additional pipe, 

mounted on top of the suction pipe, 

back to the suction head where it is 

used as process water. 

FIGURE 4.6 Sketch showing the “Green Pipe” approach (courtesy of Jan de Nul – 

see Appendix C5). 

54 
5.0 ANALYSIS OF ENVIRONMENTALLY FRIENDLY TECHNOLOGY 

5.1 Introduction 

Section 2 of the report presented a description and discussion of the key ecological 

impacts of offshore dredging. A prioritized list was developed and specific impacts that 

were not being evaluated as part of other ongoing MMS or USACE projects were 

selected for further detailed investigation under this study. 

Sections 3 and 4 of the report presented a summary of the existing and emerging 

environmentally friendly equipment and approaches as identified from the literature and 

industry surveys. 

The next step was to convene a workshop to evaluate the range of environmentally 

friendly equipment and approaches for the various targeted impacts. In addition, the 

workshop provided an opportunity to develop and discuss new approaches that may help 

address the key impacts of concern designated for consideration under this study. 

This section provides a summary of the workshop approach (Section 5.2) together 

with the findings of the workshop on the evaluation of identified and new equipment and 

approaches (Section 5.3). 

5.2 Workshop Approach 

The workshop was organized specifically to consist of a limited number of people to 

ensure active participation by all attendees. Together with the consulting team, 

approximately 20 people participated in the workshop (the invite letter and the list of 

participants are included in Appendices D.1. and D.2, respectively). There were 

representatives from the USACE (ERDC and District offices), MMS staff, other 

consultants to the MMS, and US dredging contractors. 

Input from the natural resource agencies (e.g., USFWS, NOAA Fisheries, State Fish 

and Wildlife agencies) on their concerns associated with the potential impacts of OCS 

sand borrow site dredging was solicited prior to the workshop. In fact, the prioritized 

concerns of the resource agencies formed the framework for the workshop discussions 

among the dredging experts. Key resource managers were asked to comment on the 

detailed minutes of the workshop, as a preliminary review of the workshop results. All 

review comments were positive. 

The workshop was held on April 1 and 2, 2004 in Herndon near the MMS offices. 

An agenda for the workshop is included as Appendix D.3. The morning of the first day 

of the workshop consisted of presentations on information presented in Sections 2, 3 and 

4 of this report. This provided a context for the workshop and focus for the specific 

55 
impact that would be reviewed. The Powerpoint presentation given on the first morning 

is included as Appendix D.4 to this report on a CD. 

The workshop was organized specifically to address existing, emerging and possible 

new environmentally friendly approaches to each of the targeted impacts of concern. 

Most discussion was focused on the targeted concerns defined in Section 2.4 and 

repeated below: 

1. loss of benthic habitat; 

3. changes to substrate characteristics; 

5. sedimentation and burial of sensitive habitat; and 

7. short-term increases in turbidity. 

Some workshop discussion of the issue of alteration or destruction of Essential Fish 

Habitat (Impact 12) is also reported, particularly with respect to the potential impact of 

dredging on the geomorphic integrity of ridge and shoal features. 

For each type of equipment, procedure or approach that was reviewed, the evaluation 

was completed for three criteria: 

1. Appropriateness. The following issues were considered in evaluating the 

appropriateness of an approach: the importance of the impact being addressed; 

whether it was applicable to all settings or just some, and if so, under what 

conditions; 

2. Practicality. This evaluation criterion primarily related to the cost and viability 

of a given type of equipment or technique. It also considered the constraints 

that might be imposed on a given dredging operation. 

3. Effectiveness. Under this criterion the potential success of a given type of 

equipment or approach was assessed. 

5.3 Evaluation of the Key Equipment and Approaches Discussed at the 

Workshop 

This section provides a summary of the evaluation of the key existing and emerging 

environmentally friendly equipment and technologies against the criteria described in 

Section 5.2 for the key impacts discussed at the workshop. While focus was devoted to 

the key impacts noted in Section 5.2, this section provides a summary for each of the 

twelve impacts identified in Section 2, based on discussions with resource managers in 

the US. 

56 
The section is subdivided according to each of the twelve impacts. Under each 

impact more than one type of equipment or approach may be discussed. 

5.3.1 Removal of Benthic Communities 

As noted elsewhere in this report, this was the most important impact to resource 

managers in the US. However, it is also the impact that has received the least attention in 

terms of the development of environmentally friendly approaches. In fact, an 

understanding of the rate of recovery of communities is an area of active scientific 

research and is not well defined, as discussed in Section 2.3.1. 

Only one possible environmentally friendly approach was identified. The possibility 

of creating temporal or spatial refuge areas where the substrate (and the benthic 

community it supports) would be left undisturbed (at least for a significant period of time, 

if not completely) was the one approach developed by the team prior to the workshop. 

The idea is to provide nearby undisturbed areas to promote more rapid re-colonization. 

5.3.1.1 Creation of Temporal or Spatial Refuge Areas (effectively 

“environmental windows at a borrow deposit scale”) 

Appropriateness: 

• This is the most direct, prevalent, and measurable impact of dredging, it is 

appropriate to mitigate in any way possible. 

• Is this necessary if the habitat landscape is ecologically uniform or homogenous?

• It may be more appropriate where there is ecologic diversity (e.g., on a shoal 

feature) where there is a need to preserve certain key habitats/communities in 

specific areas. 

• It may be equally or more important to create temporal refuges whereby areas are 

allowed to recover before dredging the same area again (this requires a spatial 

data base updated with time on timing and extent of each dredging project within 

a borrow area). It also requires knowledge of the time for benthic communities to 

recover, and this is probably best achieved through monitoring. 

• This approach would only be appropriate for types of species where the recolonization process is assisted by close proximity of nearby undisturbed 

communities. 

• An understanding is required of the context of this impact, recruitment and recolonization characteristics with respect to site-specific conditions and species, in 

addition to the influence on higher trophic levels (see proposed approach of MMS 

Physical and Biological Monitoring Protocols). 

• This approach would be most appropriate where recruitment is spatially 

handicapped. 

57 
In summary of the appropriateness of this measure, it would not be a general 

stipulation and instead would be tied to site-specific conditions (ecologic landscape 

and recruitment/re-colonization characteristics). The approach requires testing at 

appropriate locations and should be given consideration where the rate and quality of 

benthic recovery is a critical concern. 

Practicality: 

• From a dredging operation perspective, there is not much added cost to leaving 

un-dredged areas providing the dimensions do not make the dredging operation 

less efficient. For dredging operation efficiency with TSHDs, this requires a 100 

m minimum lane width with a 2 km run length. In other words, this represents a 

minimum corridor for dredging outside which refuge areas could be established 

separating adjacent dredge corridors. 

• While CSDs could achieve a 100 m minimum lane width, it would come at a cost 

owing to the fact that CSD operations usually utilize an idler barge, increasing the 

cut width to a minimum of approximately 200 m. The efficiency and cost of 

pipeline handling operations could be influenced by specifications on line width 

and length. The potential bed disturbance associated with anchor wires in CSD 

operations also needs to be considered (i.e., these would be beyond the 200 m

lane width). 

• Dustpan dredges could achieve a minimum lane width of 100 m without adding 

any cost to the operation. However, as with CSDs, both width and length 

constraints may influence the efficiency of pipeline handling operations. 

• There is site specificity to this as it depends on layout of borrow area and other 

avoidance or exclusion zone considerations. 

• Creating refuge areas (spatial and less so temporal) would limit the overall 

quantity of sediment available within a given deposit (or increase the cost of

geophysical surveys to expand the size of the deposit to allow for protected refuge 

areas), However this is probably not a significant negative consideration with 

respect to the practicality of this approach at most locations. 

In summary, this is not a difficult or costly measure to implement, if appropriate. It 

may be best to determine the optimal approach for creating refuge areas by 

considering the selected equipment, site conditions, and benthic community 

characteristics. 

 

Effectiveness: 

• There are limited or no data on the effectiveness of refuge zones. Monitoring 

should be required when and where this approach is implemented, and some

methods of achieving a BACI (Before, After, Control, Impact) standard and/or 

other performance measures would have to be devised. 

58 
• It would be important to consider that refuge areas would experience significant 

sedimentation, as they are immediately adjacent to the dredging zone. It may be 

appropriate to design the refuge areas such that they are wide enough to provide a 

nearby zone that has not been influenced by sedimentation. This could be 

achieved through application of the MMS Plume model (see Baird & Associates, 

2004). 

• The effectiveness will be directly related to recruitment/re-colonization 

characteristics of benthic communities at the site. 

• The effectiveness of this approach will also require good baseline mapping 

initially and then tracking of the date and location of dredged and refuge areas for 

each dredging operation and borrow site, all within a GIS database held at MMS. 

In summary, the effectiveness of this approach is unknown because it has not been 

specifically or directly evaluated at any location, to our knowledge. Work is required to 

develop a monitoring plan to evaluate the effectiveness of this possible approach. 

Thorough and organized mapping and record keeping in GIS is required to effectively 

implement and test this approach. 

Recommendation on Spatial/Temporal Refuge Areas to Promote Re-Colonization of 

Benthic Communities 

It may be appropriate, practical, and effective to impose spatial or temporal refuge 

areas at locations with one or more of the following characteristics: 1) the presence of a 

unique assemblage of benthic communities; 2) special commercial significance of a 

benthic community in a borrow deposit; 3) at locations where the benthic community is 

spatially limited with respect to recruitment and re-colonization; and 4) where the 

importance of a benthic community within the borrow area is significant for higher 

trophic levels or where this relationship is uncertain. In order to develop a layout of 

refuge areas that is practical and does not significantly influence the cost of the dredging 

operation, the type of dredging equipment and borrow deposit layout should be 

considered. The MMS Plume model should be applied to determine the required size of 

the refuge areas considering the sedimentation footprint from the dredging operations. 

This proposed approach should be field tested along with a technique to monitor the 

effectiveness. For this approach to be effective, an actively updated GIS database is 

required to track dredging and monitoring results.

5.3.2 Entrainment of species of concern 

Existing stipulations being used in MMS leases to protect sea turtles include: 

a. Presence of trained observer(s) for a specified percent of the time who 

follows specific protocols. 

59 
b. Use of a rigid sea turtle deflector, such as the one designed by the 

USACE or similar. 

c. Operation of the dredge in a manner that will reduce the risk of

interaction with any sea turtles that might be present in the dredge 

area. Keep the draghead on the bottom except: 1) when the dredge is 

not in a pumping operation and the suction pumps are turned 

completely off; 2) the dredge is being re-oriented to the next dredge 

line during borrow activities; and 3) the vessel’s safety is at risk. 

d. Dredge equipped with inflow screening baskets (4-inch mesh) to better 

monitor the intake and overflow of the dredged materials for sea 

turtles and their remains. The percent of inflow to be screened varies 

by region from 50-100 percent. 

e. Assessment/relocation trawling to further assess/reduce the potential 

for incidental take during dredging. Trawling is conducted repeatedly 

in front of the dredge as it moves along the track lines. Any turtles 

collected are to be relocated. There are specifications for trawl tow 

time and speed. There may be requirements for flipper tagging and 

genetic analysis of tissue samples from turtles caught during relocation 

trawling. 

f. Filing of detailed reports with the appropriate NOAA office within 30 

days of project completion. 

This impact and its mitigation are being addressed in detail by the USACE in an 

ongoing study. Therefore, it was not discussed in detail at the workshop. 

5.3.3 Changes in Substrate Characteristics 

There are two aspects to this issue: 1) preservation of sediment characteristics 

similar to pre-existing conditions for the surface substrate; and 2) avoidance of anoxic 

conditions within in dredge pits. The primary focus is dredge pits because, in most 

instances on the OCS where sand with a medium grain size is the target, it is likely that 

surrounding areas will also be relatively sandy and thus changes will not be significant 

(i.e., by uncovering sediment with significantly different characteristics or by changing 

the characteristics through sediment overflow during the dredging operation). In 

contrast, the creation of dredge pits of significant depth can lead to the deposition of fine 

sediment (silt and clay) changing the nature of the surface texture. This outcome

occurred in South Carolina (see Van Dolah et al., 1998) and had a direct impact on the 

suitability of the borrow area for future dredging (as desired sand was buried by mud), in 

addition to the direct environmental impacts. 

60 
The approaches reviewed include: 1) limitation of pit depths to a single fixed value 

for all locations; 2) site specific evaluation of pit depth using analytical techniques and 

numerical models. 3) use of monitoring. 

5.3.3.1 Limitation of Pit Depths to a Fixed Value 

The first possible approach is to specify a single maximum pit depth for all locations 

and conditions. 

Appropriateness: 

• From the feasibility of future dredging in a given borrow area, it is desirable to 

minimize the potential for burial of a sandy deposit with fine sediment (i.e., silt 

and clay). 

• It is also desirable to avoid the development of anoxic or hypoxic conditions that 

can impact benthic and fish species. 

• A single maximum pit depth may be inappropriate for all locations. 

• In some locations sedimentation may be low and thus development of a mud layer 

may be slow or imperceptible (and this measure may, in that case, be 

inappropriate). 

• At other locations sedimentation may be high and adjacent areas may feature a 

mud cap over sandy deposits as the natural condition (and this measure may, in 

that case, be inappropriate). 

• In some locations, anoxia may be a prevalent natural condition (i.e., adjacent to 

the Mississippi River delta) and, therefore, it may not be necessary to avoid this 

condition (and this measure may, in that case, be inappropriate). 

In summary, the appropriateness depends on the local seabed sediment and water 

quality characteristics, and whether it is intended to revisit the proposed borrow site for 

future dredging operations. 

Practicality: 

• Providing the pit limit is greater than about 1- 2 m (depending on whether TSHDs 

or CSDs are deployed, respectively) this would be a practical measure with 

respect to typical dredge cut depths. 

• Imposing a maximum pit depth rule would significantly limit the reserves of most 

borrow deposits (i.e., borrow deposits may be significantly deeper or thicker than 

an imposed maximum pit depth). 

61 
Effectiveness: 

• The effectiveness may be limited due to the variability of site-specific conditions. 

For example, while a 4 m pit depth may be satisfactory at some locations, it may 

be too deep at others. 

• Therefore, for a single maximum pit depth rule to be effective, the depth will have 

to be small and thus overly restrictive at many sites (or larger and not effective at 

a large number of sites). 

5.3.3.2 Site Specific Evaluation of Local Sedimentation Potential and 

Dissolved Oxygen Conditions Through the Application of Numerical 

Models or Analytical Methods 

This approach consists of using analytical techniques or numerical models to define 

a site-specific maximum pit depth. 

Appropriateness: 

• From the feasibility of future dredging in a given borrow area, it is desirable to 

minimize the potential for burial of a sandy deposit with fine sediment (i.e., silt 

and clay). 

• It is also desirable to avoid the development of anoxic or hypoxic conditions that 

can impact benthic and fish species. 

Practicality: 

• Providing the pit limit is greater than about 1-2 m (depending on whether TSHDs 

or CSDs are deployed, respectively) this would be a practical measure with 

respect to typical dredge cut depths. 

• Imposing a site-specific maximum pit depth rule would significantly limit the 

reserves of most borrow deposits (i.e., borrow deposits may be significantly 

deeper or thicker than an imposed maximum pit depth). 

• Development of anoxic conditions and prediction of sedimentation requires the 

application of sophisticated numerical models, preferably coupled with sitespecific data. 

Effectiveness: 

• The effectiveness of developing site-specific pit rules will depend on three key 

factors including: 1) the ability of the investigators to apply and interpret 

sophisticated models of complex processes; 2) the availability of data for input to 

these models; and 3) the local conditions as explained below. 

62 
• At some locations where sedimentation potential is low and currents are strong 

(providing a mechanism for flushing to avoid development of anoxic conditions), 

it may be possible to develop a relatively deep maximum depth with a high 

degree of certainty and without the need for sophisticated analysis techniques. 

5.3.3.3 Use of Monitoring to Avoid Development of a Mud Layer and/or 

Anoxic Conditions 

This approach consists of using monitoring to define a site-specific maximum pit 

depth. 

Appropriateness: See Section 5.3.3.2. 

Practicality: 

• Providing the pit limit is greater than about 1-2 m (depending on whether TSHDs 

or CSDs are deployed, respectively), this would be a practical measure with 

respect to typical dredge cut depths. 

• Imposing a site-specific maximum pit depth rule would significantly limit the 

reserves of most borrow deposits (i.e., borrow deposits may be significantly 

deeper or thicker than an imposed maximum pit depth). 

• It would be feasible to monitor for development of anoxia through the 

deployment of instrumentation. 

• It would also be feasible to monitor for the development of a mud layer. 

Effectiveness: 

• The effectiveness of a monitoring approach on its own is limited because once a 

mud layer is observed, or anoxia develops, it would not be possible to reverse the 

situation, at least not without significant cost. 

• It is possible that monitoring following the completion of the initial dredging of a 

borrow deposit (and prior to returning to this deposit) may provide an indication 

of the potential for development of a more severe and unacceptable outcome with 

respect to sedimentation and anoxia. 

Recommendation on Pit Depth Rule: 

A blanket maximum pit depth rule is inappropriate. However, it is appropriate to 

determine a local maximum pit depth to avoid development of a mud cover and/or 

anoxia, providing the limit is greater than 1 m for TSHDs and 2 m for CSDs. Maximum

pit depths should be determined on a site-specific basis through analysis combined with 

monitoring where necessary (as described above). Monitoring may assist the 

63 
development of an appropriate maximum pit depth at borrow deposits that are dredged 

more than once. 

5.3.4 Wave Climate Alterations by Changes in Bathymetry 

Approaches to avoiding or mitigating this impact are addressed in the Physical and 

Biological Monitoring Protocols that have been developed for the MMS and therefore are 

not addressed here. 

5.3.5 Damage to Hard/Live Bottom Habitats 

There are four possible ways that dredging activities in an MMS borrow area may 

result in this impact: 1) direct impact of the dredge vessel or dredge head or anchor wires 

on sensitive hard/live bottom habitat; 2) through direct sedimentation associated with the 

footprint of the sediment from the overflow of TSHDs or from the draghead or 

cutterhead; 3) re-suspension and subsequent transport of fines beyond the initial 

sedimentation footprint; and 4) development of near bed turbidity plumes or currents that 

may travel well beyond “normal” bounds of a plume sedimentation footprint. 

The first concern of direct damage through physical impact has largely been 

addressed through improved maneuverability and better navigation systems on dredge 

vessels. These improvements in dredge positioning have meant that any buffer designed 

to address sedimentation impact will be more than sufficient to address accidental direct 

contact. Therefore, the focus of this review is on potential indirect damage through 

sedimentation. There is overlap between these approaches and those that address 

elevated levels of turbidity as discussed in Section 5.3.7. 

The approaches and equipment reviewed include: 1) stipulation of a blanket buffer 

zone width for all situations; 2) stipulation of region- or habitat-specific buffer zone 

width; 3) use of analysis or numerical modeling to define a site and project specific 

buffer zone width; 4) monitoring turbidity to meet a general stipulation; 5) development 

of site-specific levels based on monitoring of background levels; 6) Green Pipe (or recirculation of overflow to the draghead); and 7) Anti-Turbidity valve. 

5.3.5.1 Implementation of a Blanket Buffer Zone for All Situations 

Appropriateness: 

• It is certainly appropriate to protect adjacent hard/live bottom and other sensitive 

habitat where this habitat has special ecologic significance. 

64 
• A single blanket buffer is inappropriate as it may be overly protective at some (or 

all) locations and insufficient at others. 

Practicality: 

• It may not be practical to develop a single buffer distance that is appropriate for 

all conditions (this would require a consideration of the worst case condition in 

terms of level of suspended sediment generation, degree of advection/dispersion 

and sensitivity of local habitat). 

Effectiveness: 

• In order for these measures to be effective, it is necessary to have some

understanding of lethal or detrimental levels of sedimentation for the most 

sensitive species. 

• It is likely this approach would be ineffective in most conditions (either overly 

conservative or insufficient to protect in others). 

5.3.5.2 Implementation of a Region/Habitat Specific Minimum Buffer Zone 

Width Together with Monitoring at the Sensitive Habitat 

In some jurisdictions, such as Florida (and the Gulf of Mexico), a specific buffer 

zone distance for dredging sand deposits near hardground habitats is specified under the 

Endangered Species Act. NOAA Fisheries Regional Biological Opinion requires a 400 ft 

buffer to protect hardgrounds used by sea turtles for foraging or shelter from sand mining 

offshore Florida. Recognizing that the width of the buffer zone may be insufficient in 

some circumstances, real or near real-time monitoring is performed during dredging 

operations to ensure sedimentation rates do not exceed predefined limits. The monitoring 

technique is in the form of turbidity measurements or sediment traps. Exceedance of 

these pre-defined thresholds triggers direction to the dredging vessel to cease or modify 

operations. Evans (1994) reported on the application of this approach to protect coral 

for a dredging project offshore Hong Kong. 

Appropriateness: 

• It is certainly appropriate to protect adjacent hard/live bottom and other sensitive 

habitat with ecological significance. 

• To a large extent the appropriateness depends on the knowledge of the severity of 

impacts to different habitat types and the uniformity of the habitat of concern (or 

its sensitivity) within the region of application. 

• The appropriateness also depends on whether the conditions at the range of sites 

within the region of application are indeed relatively uniform (i.e., with respect to 

advection/dispersion). 

65 
Practicality: 

• It is practical to specify a minimum buffer zone width. 

• At some locations, the specification of a buffer width may make borrow deposits 

too small to dredge. 

• It is more difficult to perform real-time or near real-time monitoring of 

sedimentation or turbidity, however, with recent advances in underwater 

telemetry techniques this is possible. 

• Sedimentation rates are difficult to measure because of low levels of 

sedimentation, and difficult to separate from background levels. 

Effectiveness: 

• As explained above, the effectiveness will depend on establishing reasonable 

minimum buffer for a geographic area that has: 1) similar level of sensitivity for 

habitat; 2) similar generation of turbidity and sedimentation levels (associated 

with dredge type/operation and borrow deposit sediment characteristics); and 3) 

uniformity in advection/dispersion characteristics. 

• The effectiveness will also depend on the ability of the dredging operation to 

respond to required reduction without significant escalation of costs of the 

operation. 

• This approach has been applied successfully before (see Evans, 1994). 

5.3.5.3 Use of Analysis to Determine a Site-Specific Buffer Zone 

The Plume model that has been developed for MMS to simulate plumes released by 

TSHDs (Baird, 2004) could be applied to pre-determine the potential zone of 

sedimentation impact for the planned dredging operations and accordingly determine a 

suitable and appropriate site-specific buffer zone width. 

Appropriateness: see Section 5.3.5.2. 

Practicality: 

• MMS now has the MMS Plume model for TSHDs to perform this assessment. 

• It would be necessary to determine additional input parameters including: a) 

sediment characteristics [available]; b) specific dredge vessel characteristics 

[available only after a dredging contractor has been retained for the work – but a 

range of representative characteristics could be considered]; c) some indication of 

the possible tracklines of the dredge [a worst case scenario could be estimated]; 

and d) local environmental conditions (waves, currents, etc.) – [may be available 

66 
for some sites but probably would have to be estimated for some where not 

available]. 

Effectiveness: 

• The MMS Plume model has only recently been developed and has not been 

extensively tested against measured data. 

• This would provide at least an initial estimate of the possible sedimentation 

footprint. 

• If applied, this approach should be probably combined with some other method of 

direct measurement. 

Recommendation on Buffer Zones to Protect Sensitive Habitat from Sedimentation 

 

Three approaches were assessed with respect to the stipulation of buffers to protect 

hard/live bottom areas from sedimentation: 1) a blanket buffer; 2) a region/habitat 

specific buffer together with real-time monitoring; and 3) the use of the Plume model to 

assist in the definition of an appropriate site-specific buffer. It is appropriate to apply 

some form of buffer to protect ecologically sensitive hard/live ground habitat from

sedimentation impacts. A blanket buffer for all conditions is inappropriate and would not 

be practical. Real-time monitoring near the limits of pre-defined buffers is now possible 

as an ongoing test of buffer effectiveness and as a trigger to invoke additional mitigation 

measures during dredging operations. The MMS Plume model could be applied to predefine buffers considering site-specific conditions. However, considering the limited 

validation of this model, it should be combined together with monitoring. Specific 

information on acceptable levels of sedimentation is required either through direct 

exposure testing of the site-specific hard/live ground habitat or through background 

station monitoring. 

5.3.5.4 Monitoring Turbidity to Meet a General Stipulation 

In many jurisdictions a general stipulation of a maximum turbidity level is specified. 

For example, the DNR in Florida specifies that turbidity levels cannot exceed 29 NTUs 

above background levels measured at mid-depth at the boundary of a 150 m mixing zone. 

Background levels for beach nourishment projects are typically measured 1,000 m

upcurrent from the dredging operations. More details are presented in Section 2.3.7. 

This approach could be applied to limit turbidity to levels that result in acceptable levels 

of sedimentation in order to protect hard/live bottom habitat. This approach is also 

discussed under Section 5.3.7 as a measure to address the impact of elevated levels of 

water column turbidity. 

67 
Appropriateness: 

• It is certainly appropriate to protect adjacent hard/live bottom and other sensitive 

habitat, where ecologically sensitive. 

• A general stipulation may be inappropriate, as it may not apply to all habitats 

requiring protection. For example, the 29 NTU limit was based on an original 

value of 50 JTU developed to avoid ecological impact in streams and rivers in 

Florida and may or may not be appropriate to protect coral habitat at all locations. 

• A general stipulation on turbidity would not translate to the same sedimentation 

rates at all locations due to variability in environmental conditions (e.g., currents 

and sea bed topography). 

 

Practicality: 

• One difficulty of this measure is associated with where and when the 

measurements are taken to compare to the stipulated maximum turbidity levels. 

Turbidity associated with a dredge plume is highly variable in time and space and 

also background levels are highly variable in time and space. 

• In the study workshop for this project, representatives of leading dredging 

contractors in the US indicated that when dredging for sand in sandy areas the 29 

NTU above background requirement, measured 150 m from the dredge, was not 

difficult to meet (no special measures were required), at least for the way in which 

this limit is currently applied and monitored. 

Effectiveness: 

• This approach will only be effective in those areas where the specified turbidity 

level is low enough to protect the most sensitive habitat in the worst-case 

conditions. 

• In all other areas, this approach will be overly conservative. 

• At the study workshop, USACE representatives indicated that point 

measurements of turbidity seldom provide a reliable estimate of the true range of 

turbidity. This observation was based on their measurements with ADCP giving a 

much more thorough picture of the spatial/temporal variability of turbidity. 

Therefore, it is difficult to measure the temporal and spatial turbidity levels with 

conventional point measurement techniques, profiling techniques such as ADCP 

are more effective. 

• It is difficult to relate turbidity levels directly to sedimentation rates and the 

resulting potential indirect damage to hard/live bottom habitat. At the very least, 

effectiveness in this respect would require ADCP measurements of turbidity 

variability in space and time. 

68 
Recommendation on Turbidity Monitoring to Protect Hard/live Bottom Areas from 

Sedimentation Impacts 

This is a less direct approach than simply monitoring sedimentation rates. Point 

measurements of turbidity (either snapshots or continuous) are difficult to convert 

directly to spatially varying sedimentation rates. Once site- or habitat-specific 

information is developed on acceptable levels of sedimentation, this must be somehow 

translated to acceptable levels of turbidity. More widespread measurements of turbidity 

in time and space (e.g., through the application of ADCP) may be necessary for this 

indirect approach (i.e., linking sedimentation to turbidity levels) to be effective. Data 

from turbidity monitoring may be useful in calibrating or verifying the MMS Plume 

model for definition of buffer zones to protect hard/live bottom areas as discussed above. 

5.3.5.5 Development of Site-Specific Limits Based on Background Levels 

It may be more appropriate to develop limits for sedimentation that are within the 

range of natural background levels. The approach would consist of analyzing measured 

sedimentation rates to determine the maximum levels (peak values and persistence), 

frequency and timing of these events and then to assign appropriate maximum levels 

(with duration) for the dredging operation (see Section 5.3.5.3). The alternative approach 

to developing a site-specific limit for sedimentation rates is to evaluate the direct impacts 

to organisms. This type of work has not been performed in the OCS environment and 

could involve a very extensive research program compared to the method of evaluating 

the characteristics of background sedimentation levels. 

Appropriateness: 

• It is certainly appropriate to protect adjacent hard/live and other sensitive habitat, 

where ecologically sensitive. 

• As dredging events at a given location are generally infrequent (e.g. once every 

two or more years), if repeated at all, the sedimentation generated by such an 

event could be compared to that generated by an extreme storm event with return 

period similar to the frequency of dredging. The rationale is that the natural 

environment would have adapted to avoid or recover from such events in the 

natural system. 

Practicality: 

• The main requirement here is measured sedimentation rates at the proposed 

dredging site. In most proposed borrow sites or areas in OCS waters, this type of 

data are not available. A requirement is that the temporal variation in 

sedimentation rates must be determined (i.e., not only the cumulative 

sedimentation rate for a period of time). Sedimentation traps are one type of 

apparatus that could be applied for these measurements. 

69 
• Where project lead-time permits, it would be practical to acquire at least one or 

two years of data prior to dredging at locations where sedimentation may be a 

concern. The length of the data set required would depend on whether a 

significant sedimentation event, for example related to the passage of a hurricane 

or tropical storm, was captured in the monitoring period. However, few OCS 

projects have lead times of 1-2 years where the exact borrow site is known. 

Effectiveness: 

• This approach relies on the premise that the natural environment will have 

adapted to certain levels of sedimentation, and therefore, this should be a reliable 

approach for preventing negative impacts of sedimentation generated by 

dredging. 

• Understanding the seasonal timing of the natural sedimentation fluctuations and 

the relationship to the seasonal timing of ecological functions of various 

organisms would also have to be evaluated for this approach to be effective. 

• The possibility of the sedimentation generated through the dredging operations 

unacceptably contributing to cumulative impacts would have to be considered to 

evaluate the effectiveness of this proposed approach. 

Recommendation on a Site-Specific Limit Derived from Background Levels 

The premise of this approach is to define a limit that is within the range of natural 

variability. The main practical limitation is the lack of sedimentation data, new 

measurements would have to be made at most locations for a minimum period of one to 

two years prior to dredging. This approach would be appropriate at locations where there 

is justifiable concern for sedimentation impacts to Essential Fish Habitat. 

5.3.5.6 Use of “Green Pipe” (re-circulation of overflow to draghead) 

With the Green Pipe equipment modification, overflow water is fully re-circulated to 

the draghead eliminating all or most overflow. In theory, this significantly reduces the 

size of the plume from dredging (by confining the release of sediment to an area close to 

the bed), and therefore, the sedimentation footprint. However, at the same time, the same

amount of sediment is released thereby concentrating sedimentation related to the release 

or re-suspension of sediment in a smaller area leading to higher sedimentation rates 

within the smaller footprint. 

Appropriateness: see Section 5.3.5.2. 

Practicality: 

• This would require significant overhaul of existing dredges with very significant 

capital investment. 

70 
• It would require additional pumps and greater weight on the vessel, probably 

reducing loading capacity. 

• These costs would be passed on to the consumer as higher unit dredge costs. 

Effectiveness: 

• Sediment balance considerations suggest that no less sediment would be 

“overflowed” or returned to the bed with this approach, the only difference is that 

it would be released close to the bed. 

• There is a concern that this could promote the development of a turbidity current 

near the bed (i.e., due to the high and concentrated sediment loading at the drag 

head with this approach). If a turbidity current were to develop at the bed, the 

sedimentation footprint may extend much further from the borrow site than 

normally expected. 

Recommendation on the Green Pipe Equipment Modification 

In most cases the “Green Pipe” approach is likely unjustified. There may be some

circumstances where it is desirable to confine the sediment loading associated with 

dredging to close to the bed. However, there may be other less costly approaches to 

achieving the same goals. 

5.3.5.7 Use of Anti-Turbidity Valve 

The anti-turbidity valve is a device, which prevents the entrainment of air into the 

hopper and overflow discharge, thus improving the settling characteristics of the 

discharged sediment laden flow (see Section 4.6.1 for more detail). The idea here would 

be that improved settling will lead to a smaller sedimentation footprint, and therefore less 

chance of impacts to hard/live bottom areas located outside the dredge area. The dredge 

contractors that attended the workshop (representatives from Bean Stuyvesant LLC and 

Great Lakes Dredge & Dock Co.) indicated that most TSHDs are now outfitted with this 

device. 

Appropriateness: see Section 5.3.5.2. 

Practicality: 

• According to the dredge contractors that attended the workshop, most TSHDs are 

already equipped with an anti-turbidity valve of some form. 

• For this equipment modification to have been widely implemented it must be a 

relatively straightforward and inexpensive change in relative terms. 

71 
Effectiveness: 

• This approach reduced both the size of the plume (and thus the sedimentation 

footprint) in addition to the total sediment released from the hopper. 

• There is a concern that this could promote the development of a turbidity current 

near the bed (i.e., due to promotion of a higher and more concentrating settling 

process). If a turbidity current were to develop at the bed, the footprint may 

extend much further from the borrow site than normally expected. 

Recommendation on the Anti-Turbidity Valve Device 

It is understood that this device has been widely applied to TSHDs in the US. Where 

it is important to restrict the extent of the sedimentation footprint, such as near to 

hard/live bottom areas, this would be an appropriate device to require on the TSHD. The 

possibility of turbidity current development should be evaluated at sites with strong tidal 

currents or steep slopes in the vicinity of the borrow area (refer to the discussion of 

Section 2.3.7). 

Overall Recommendation on Protecting Sensitive Habitat from Sedimentation 

There is a need to establish field-tested sedimentation limits for different types of 

sensitive habitat. A blanket buffer zone width for all locations is probably unjustified. 

Another way of defining acceptable site-specific sedimentation levels, that may be more 

expedient, is through the monitoring of natural sedimentation rates. Once sedimentation 

limits are established for the local sensitive habitat, the best approach would consist of a 

pre-dredging assessment of the plume sedimentation footprint using the MMS Plume 

model (or equivalent), followed by real-time or near real-time monitoring of 

sedimentation levels as a trigger to invoke additional mitigation measures in the dredging 

operations, as required (see the recommendations summary for buffer zone approached at 

the end of Section 5.3.5.3 for more details). Turbidity monitoring may also be helpful to 

validate the Plume model, however, it is not a suitable replacement for direct monitoring 

of sedimentation. It would be appropriate to require the Anti-Turbidity valve device at 

locations where restricting the sedimentation footprint is important. At almost all 

locations the Green Pipe approach (where the overflow water is re-circulated to the 

draghead) is likely unjustified. At borrow sites with strong tidal currents or steep slopes, 

the possibility of the development of a near-bed turbidity current generated through the 

pancaking effect of the dynamic plume phase should be evaluated (see Section 2.3.7 for 

details). 

72 
5.3.6 Creation of Depressions and Furrows

This was not thought to be an important impact and, therefore, was not discussed in 

any detail. However, the issue of developing pits was addressed under the discussion of 

Impact 3 (see Section 5.3.3). 

5.3.7 Increased Short-Term Turbidity (Turbidity Limits) 

This impact has been partly addressed under Section 5.3.5 above owing to the fact 

that sedimentation is directly related to turbidity levels. Therefore, some of the 

techniques presented in Section 5.3.5 are repeated here with some additions to their direct 

applicability to this concern. 

The approaches and equipment reviewed include: 1) stipulation of a maximum

turbidity level for all locations; 2) stipulation of a site-specific level; 3) Green Pipe (or recirculation of overflow to the draghead); and 4) Anti-Turbidity valve. 

5.3.7.1 Implementation of a General Stipulation 

As discussed under Section 5.3.5.4 related to damage to hard/live bottom habitats, in 

many jurisdictions a general stipulation of a maximum turbidity level is specified. For 

example, the DNR in Florida specifies that turbidity levels cannot exceed 29 NTUs above 

background levels measured at mid-depth at the boundary of a 150 m mixing zone. 

Background levels for beach nourishment projects are typically measured 1,000 m

upcurrent from the dredging operations. More details are presented in Section 2.3.7. 

Appropriateness: 

• In open ocean OCS areas, restrictions on dredging operations are not typically 

required as it is believed that adult fish are sufficiently mobile to avoid dredge 

plumes and levels are almost always sub-lethal and plumes are not persistent or 

frequent enough to have an effect at the sub-lethal level. 

• The concern is generally related to the sedimentation impacts. 

• There may be some locations where high levels of turbidity could result in an 

unacceptable degradation to Essential Fish Habitat. 

Practicality: 

• One difficulty of this measure is associated with where and when the 

measurements are taken to compare to the stipulated maximum turbidity levels. 

Turbidity associated with a dredge plume is highly variable in time and space and 

also background levels are highly variable in time and space. 

73 
• In the study workshop for this project, representatives of leading dredging 

contractors in the US indicated that when dredging for sand in sandy areas the 29 

NTU above background requirement, measured 150 m from the dredge, was not 

difficult to meet (no special measures were required), at least for the way in which 

this limit is currently applied and monitored. 

Effectiveness: 

• This approach will only be effective in those areas where the specified turbidity 

level is low enough to protect the most sensitive habitat in the worst-case 

conditions. 

• In all other areas, this approach will be overly conservative. 

• There has been very little field-testing to determine impacts of elevated water 

column turbidity at sub-lethal levels. At the workshop, the USACE 

representatives argued for more investigation of this issue. Without this 

information, there is no way of knowing how effective limits on turbidity levels 

(at whatever level they are set) are in protecting the environment. 

• At the study workshop, USACE representatives also indicated that point 

measurements of turbidity seldom provide a reliable estimate of the true range of 

turbidity. This observation was based on their measurements with ADCP giving a 

much more thorough picture of the spatial/temporal variability of turbidity. 

Therefore, it is difficult to measure the temporal and spatial turbidity levels with 

conventional point measurement techniques. Profiling techniques such as ADCP 

are more effective. 

Recommendation on a General Stipulation for Turbidity Levels 

The 29 NTU limit above background levels that is sometimes applied to dredging 

operations, particularly in nearshore zones, would not appear to be scientifically justified 

for application to open ocean environments. Little work has been completed on 

understanding the impact of sub-lethal levels of elevated turbidity on fish and other 

organisms. It is believed that adult fish are able to avoid the turbidity plume and that 

other organisms are simply not influenced by this relatively low level of elevation above 

background levels. Point measurements of turbidity levels in space and time are 

probably inaccurate and not representative of the variability of actual levels. 

Nevertheless, the dredging representatives indicated that the 29 NTU limit above 

background measured 150 m from the dredge at mid-depth is not difficult to achieve. 

5.3.7.2 Development of Site Specific Limits Based on Background Levels 

It may be more appropriate to develop limits for turbidity that are within the range of 

natural background levels. The approach would consist of analyzing measured 

74 
suspended sediment data (TSS or NTU) to determine the maximum levels (peak values 

and persistence), frequency and timing of these events and then to assign appropriate 

maximum levels (with duration) for the dredging operation (see Section 5.3.5.3). The 

alternative approach to developing a site-specific limit for turbidity levels is to evaluate 

the direct impacts to organisms. This type of work has not been performed in the OCS 

environment and could involve a very extensive research program compared to the 

method of evaluating the characteristics of background turbidity levels. 

Appropriateness: 

• In open ocean OCS areas restrictions on dredging operations are not typically 

required as it is believed that adult fish are sufficiently mobile to avoid dredge 

plumes and levels are almost always sub-lethal and plumes are not persistent or 

frequent enough to have an effect at the sub-lethal level. 

• The concern is generally related to the sedimentation impacts. 

• There may be some locations where high levels of turbidity could result in an 

unacceptable degradation to Essential Fish Habitat. 

• This approach may be important where a case is made to protect Essential Fish 

Habitat and it is necessary to determine the acceptable levels of turbidity. 

• As dredging events at a given location are generally infrequent (e.g. once every 

two or more years), if repeated at all, the turbidity generated by such an event 

could be compared to that generated by an extreme storm event with return period 

similar to the frequency of dredging. The rationale is that the natural environment 

would have adapted to avoid or recover from such events in the natural system. 

Practicality: 

• The main requirement here is measured TSS or NTU at the proposed dredging 

site. In most proposed borrow sites in OCS waters this type of data is not 

available. This information could be obtained in tandem with velocity data 

through the deployment of ADCP. The backscatter signal from the ADCP, 

together with ground truth TSS measurements consisting of direct water samples, 

can be used to develop a record of turbidity. 

• Where project lead-time permits, it would be practical to acquire at least one or 

two years of data prior to dredging at locations where turbidity may be a concern. 

The length of data set required would depend on whether a significant turbidity 

event, for example related to the passage of a hurricane or tropical storm, was 

captured in the monitoring period. 

Effectiveness: 

• This approach relies on the premise that the natural environment will have 

adapted to certain levels of turbidity, and therefore, this should be a reliable 

approach for preventing negative impacts of turbidity generated by dredging. 

75 
• Understanding the seasonal timing of the natural turbidity fluctuations and the 

relationship to the seasonal timing of ecological functions of various organisms 

would also have to be evaluated for this approach to be effective. 

• The possibility of the turbidity generated through repetitive dredging operations 

unacceptably contributing to cumulative impacts would have to be considered to 

evaluate the effectiveness of this proposed approach. 

Recommendation on a Site Specific Limit Derived from Background Levels 

The premise of this approach is to define a limit that is within the range of natural 

variability. The main practical limitation is the lack of turbidity data. New measurements 

would have to be made at most locations for a minimum period of one to two years prior 

to dredging. This approach would be appropriate at locations where there is justifiable 

concern for turbidity impacts to Essential Fish Habitat. 

5.3.7.3 Use of “Green Pipe” (re-circulation of overflow to draghead) 

With the Green Pipe equipment modification, overflow water is fully re-circulated to 

the draghead eliminating all or most overflow. In theory, this significantly reduces the 

size of the plume from dredging (by confining the release of sediment to an area close to 

the bed). As noted in Section 5.3.5.6, this approach may potentially contribute to 

significantly expanding the actual sedimentation footprint. Nevertheless, it would create 

a significant improvement to water column turbidity levels. 

Appropriateness: 

• In open ocean OCS areas restrictions on dredging operations are not typically 

required as it is believed that adult fish are sufficiently mobile to avoid dredge 

plumes and levels are almost always sub-lethal and plumes are not persistent or 

frequent enough to have an effect at the sub-lethal level. 

• The concern is generally related to the sedimentation impacts. 

• There may be some locations where high levels of turbidity could result in an 

unacceptable degradation to Essential Fish Habitat. 

• Application of this equipment modification would only be justified where a strong 

case is made to eliminate any increase in turbidity above background levels to 

protect Essential Fish Habitat and fish species that were particularly sensitive to 

relatively small increases in turbidity above background levels. 

Practicality: 

• This would require significant overhaul of existing dredges with very significant 

capital investment. 

76 
• It would require additional pumps and greater weight on the vessel, probably 

reducing loading capacity. 

• These costs would be passed on to the consumer as higher unit dredge costs. 

Effectiveness: 

• Measurements provided by the Belgian dredging contractor Jan de Nul showed 

that this approach was very effective at eliminating any plume higher than about 4 

m above the bed. The only source of plume generation with this approach is from

the draghead itself. 

Recommendation on the Green Pipe Equipment Modification 

In most cases the “Green Pipe” approach is likely unjustified. There may be some

circumstances where it is desirable to confine the sediment loading associated with 

dredging to close to the bed. However, there may be other less costly approaches to 

achieving the same goals. 

5.3.7.4 Use of Anti-Turbidity Valve 

The anti-turbidity valve is a device which prevents the entrainment of air into the 

overflow discharge, thus improving the settling characteristics of the discharged 

sediment-laden flow (see Section 4.6.1 for more detail). The idea here would be that 

improved settling will lead to a more confined sediment plume (although the total 

sediment released from the overflow process is not reduced). The dredge contractors that 

attended the workshop (representatives from Bean Stuyvesant LLC and Great Lakes 

Dredge & Dock Co.) indicated that most TSHDs are now outfitted with this device. 

Appropriateness: 

• In open ocean OCS areas restrictions on dredging operations are not typically 

required as it is believed that adult fish are sufficiently mobile to avoid dredge 

plumes and levels are almost always sub-lethal and plumes are not persistent or 

frequent enough to have an effect at the sub-lethal level. 

• The concern is generally related to the sedimentation impacts. 

• There may be some locations where high levels of turbidity could result in an 

unacceptable degradation to Essential Fish Habitat. 

• Application of this equipment modification is justified where a large plume or a 

large sedimentation footprint must be avoided. 

77 
Practicality: 

• According to the dredge contractors that attended the workshop most TSHDs are 

already equipped with an anti-turbidity valve of some form. 

• For this equipment modification to have been widely implemented it must be a 

relatively straightforward and inexpensive change in relative terms. 

Effectiveness: 

• Measurements provided by the Belgian dredging contractor Jan de Nul showed 

that this approach was very effective at reducing the size of the plume and the 

total quantity of sediment released from the hopper during the overflow process 

(see Appendix C.5). 

Recommendation on the Anti-Turbidity Valve Device 

It is understood that this device has been widely applied to TSHDs in the US. Where 

it is important to restrict the extent of the turbidity plume, this would be an appropriate 

device to require on the TSHD. 

Overall Recommendation on Addressing the Impact of Turbidity 

It is generally viewed that elevated levels of turbidity generated from TSHD 

operations in open ocean waters does not represent a significant ecological impact. It is 

believed that adult fish can avoid plumes and that other organisms can survive the sublethal levels of short-term elevated turbidity. A one-size fits all limit of 29 NTUs above 

background levels measured at 150 m from the dredging operation is probably 

scientifically unjustified for the ocean environment. Nevertheless, representatives of the 

dredging industry that attended the workshop indicated that the 29 NTU limit was not 

difficult to achieve. At locations where a more scientifically justified level is required, 

for example where there is a specific ecological concern about turbidity levels, it may be 

possible to develop a site-specific limit based on measurements of turbidity levels over a 

minimum period of one or two years. 

The Anti-Turbidity valve device is widely applied in the US and significantly 

reduces the size of plumes from TSHDs and the total sediment overflowed in the 

discharge process. It would be appropriate to require the use of this device wherever 

turbidity is a concern. The “Green Pipe” approach consisting of re-circulation of the 

overflow water to the draghead eliminates the plume above 4 to 5 m above the seabed 

(i.e. outside of the region of the draghead plume), but it does not reduce the total 

sediment discharged in the overflow process. However, this approach is not included on 

any US dredge vessels and would represent a significant and expensive equipment 

overhaul that would be passed on to the client through higher unit prices and is likely 

unjustified at most locations. 

78 
5.3.8 Commercial and Recreational Fisheries 

This impact was not discussed at any length. The MMS has recently awarded a 

contract to Emu Ltd. of the UK to study this issue and develop recommendations. 

5.3.9 Seafloor Pipeline Breakage and Leakage 

This impact was not discussed in any detail. The MMS has recently awarded a 

contract to Baird & Associates to study this issue and develop recommendations. 

 

5.3.10 Collisions With Marine Mammals and Sea Turtles 

It was concluded that this potential impact is adequately addressed through the 

current MMS stipulation listed below. 

• If operating in areas of known whale occurrences, observers are required. If 

whales are observed, avoid intentional approaches within 100 yards (500 yards 

for right whales) and slow speeds to less than 4 knots. 

5.3.11 Archaeological Resources 

This impact was addressed through the recommendation of approaches to define 

buffers in a recently completed project for MMS (see Research Planning, Inc. et al., 

2004). 

5.3.12 Alteration or Destruction of Essential Fish Habitat 

This impact is currently being addressed in an MMS study by Applied Coastal 

Research Ltd. However, during the workshop there was some discussion of possible 

approaches to address this issue. In addition, subsequent discussions with Chris Spaur of 

the Baltimore District office of the USACE have resulted in other possible approaches to 

addressing this issue. The primary focus is to avoid disrupting the geomorphic integrity 

of ridge and shoal features that have been targeted for dredging. As noted in Section 

2.3.12, there is a concern that removing large quantities of sand from a shoal may disrupt 

the processes that maintain the shoal and trigger the deflation or disappearance of these 

features. 

The approaches reviewed include: 1) dredge depositional areas on the features; 2) 

post-dredge monitoring of changes; 3) complete an assessment of sediment dynamics of 

79 
the shoal feature; 4) limit the sediment removal to some threshold level determined 

through analytical techniques. 

5.3.12.1 Dredge Depositional Areas that are Undergoing Constant Natural 

Recovery (and Burial) in a Physical Sense 

Appropriateness: 

• As some areas of shoals (and the features themselves) may represent unique 

habitat for important and/or commercial species, it will be important to implement 

protective measures. 

• The aggrading section of the shoal is subject to continual deposition and burial of 

surface dwelling communities, therefore, it may have lower ecological importance 

(this needs to be verified on a site-specific basis). 

Practicality: 

• Generally this is the steepest (and thus narrowest or smallest) part of the shoal so 

it limits the area available for dredging. 

• Steepness of the aggrading slope may also cause difficulty for dredging, 

particularly for TSHDs (however this may not typically be a practical constraint 

as slopes are usually in the 1:15 to 1:25 range). 

Effectiveness: 

• The degree to which this measure prevents a possible impact to the geomorphic 

integrity of the future needs to be assessed on a site-specific basis. 

• Some activities that would help evaluate the effectiveness of this approach 

include: numerical modeling of the sediment dynamics and morphodynamics of 

the feature; geomorphic review of the sedimentology and stratigraphy of the 

feature; and monitoring of the changing form of the shoal. 

5.3.12.2 Post-Project Monitoring of Changes 

Appropriateness: 

• As some areas of shoals (and the features themselves) may represent unique 

habitat for important and/or commercial species, it will be important to implement 

protective measures. 

80 
• Post-project monitoring together with other measures may be appropriate, 

however this alone does not provide a projection of possible irreversible changes 

to the morphologic integrity of shoal features. 

Practicality: 

• The Protocols report completed for MMS by Research Planning, Inc. et al., 2001 

(see also Nairn et al., 2004) provides recommendations for monitoring 

bathymetric changes within and adjacent to borrow areas. 

• These approaches are fully practical and based on conventional hydrographic 

surveying approaches. 

Effectiveness: 

• This measure is effective in identifying indirect impacts after the fact, at which 

time it may be too late to avoid an irreversible impact. 

• In some cases, this approach may provide a means of avoiding future additional 

impacts by repeated dredging operations on the same shoal. 

5.3.12.3 Complete a Sediment Dynamics Study of the Shoal to Determine the 

Most Appropriate Areas to Dredge to Avoid or Minimize Impacts 

Appropriateness: 

• As some areas of shoals (and the features themselves) may represent unique 

habitat for important and/or commercial species, it will be important to implement 

protective measures. 

• The wave dynamics and sediment dynamics on shoal features are very complex 

and a site-specific investigation of these processes is fully justified to support the 

development of a dredging plan that minimizes impacts to the morphologic 

integrity of the shoal and to areas adjacent to the borrow area. 

Practicality: 

• The complexity of the wave and sediment dynamics requires the application of 

sophisticated numerical models, and specifically wave models that are capable of 

simulating crossing wave patterns over the crest of the shoals. This process has 

been shown by Hayes and Nairn (2004) to have a key influence on the natural 

maintenance of these features. 

• This class of wave and sediment dynamic models is inappropriate for long-term

applications required to assess geomorphic changes and integrity. Therefore, 

special techniques must be applied to integrate the results of event-based models 

to predict long-term changes. 

81 
Effectiveness: 

• The effectiveness of this measure will be strongly influenced by the experience 

and capabilities of the team to apply the necessary models and address the 

complexities discussed above. 

• In any event, even the best numerical models and the most experienced 

interpretations of these complex processes have limitations and long-term

monitoring is an indispensable component of this measure. 

5.3.12.4 Limit Removal of Sediment to Some Threshold Level (to Avoid 

Deflation or Irreversible Damage to the Morphologic Integrity of the 

Shoal) 

Appropriateness: 

• As some areas of shoals (and the features themselves) may represent unique 

habitat for important and/or commercial species, it will be important to implement 

protective measures. 

• There is a concern (Hayes and Nairn, 2004) that removal of too much sediment 

from a shoal could lead to dramatic deflation of the shoal eliminating most or all 

habitat associated with the shoal feature. 

• How important is the loss of a shoal if there are other shoals nearby? This can 

only be answered on a site-specific basis. 

Practicality: 

• Whether this is a practical approach will depend in part on the total quantity that 

is determined to be available without creating irreversible damage to the shoal 

integrity. 

• Discussions between R. Nairn of Baird & Associates and C. Spaur of the 

Baltimore District office of the USACE, subsequent to the workshop, have 

resulted in the development of some possible guidelines for dredging. From a 

review of shoals offshore Maryland/Delaware on the OCS it was determined that 

the existing features had a wide range of volumes from 6,000,000 to 160,000,000 

m3

. This may suggest that removal of several million cubic m from the larger 

shoals may not impact their geomorphic integrity. Spaur also reviewed the length 

to width ratios of the various shoals and found that the width of the features 

varied in a relatively small range, mostly between 1.6 and 3.2 km. This may 

suggest that the width of the feature is the key dimension and that any dredging 

should be planned to avoid reducing the width below a threshold level, possibly in 

the range of 1.6 km. 

82 
Effectiveness: 

• It is unlikely that the approach of evaluating shoal morphometrics would, on it’s 

own, provide sufficient understanding to determine guidelines for removal of 

sediment at the same time as protecting the geomorphic integrity of the feature. 

• Nevertheless, this approach shows promise in supporting the development of 

appropriate guidelines on where and how much to remove from each shoal. 

Recommendation on Protecting Essential Fish Habitat and, Specifically, Shoal 

Integrity: 

 

There is much to be learned about the processes that maintain the form of shoals, and 

therefore, the potential impacts of dredging sand from these features. Hayes and Nairn 

(2004) have summarized the literature on this topic and suggested a new mechanism for 

the maintenance of OCS shoals; however, the understanding of these features requires 

more investigation. The new mechanism suggested by Hayes and Nairn (2004) consists 

of converging and crossing wave patterns (resulting from refraction processes on either 

side of the shoal) leading to a convergence of sand transport at the crest of the shoal. This 

convergence of sediment transport maintains the shape of the feature and explains why 

they fall into the dominant wave direction and migrate in the direction of the dominant 

wave propagation. This understanding and the development of guidelines for the 

removal of sand through dredging (specifically, how much and where) will require 

several lines of investigation including: a review of shoal morphometrics (as C. Spaur of 

the USACE, Baltimore District has initiated); an investigation of the sedimentology and 

stratigraphy of these features; and numerical modeling of waves, sediment transport and 

morphodynamics. 

83 
6.0 CONCLUSIONS AND RECOMMENDATIONS 

6.1 Conclusions 

This study was commissioned to evaluate on a worldwide basis existing and 

emerging environmentally friendly approaches to dredging for sand and gravel in 

offshore waters on the Outer Continental Shelf. The focus was primarily on Trailing 

Hopper Suction Dredges as these are the most likely vessels of choice for dredging 

operations where the borrow area and the project site requiring the sediment are several 

kilometers or miles apart. However, in some cases Cutter Suction Dredges and Dust Pan 

Dredges may also be utilized so these have also been considered. 

Twelve key impacts were identified and prioritized through discussions with the 

Federal and State resource agencies that are actively dealing with dredging impact issues. 

Each of these impacts is described in detail in Section 2 of this report. The existing stateof-the-knowledge on these impacts was summarized through a literature review by 

Newell and Seiderer (2003) commissioned for this study (see Appendix A) and by 

updating the review of impacts completed by Research Planning, Inc. et al. (2001) and 

earlier MMS studies. 

Of the twelve impacts identified, a short list was developed for detailed investigation 

by focusing on those issues which were not being actively investigated through other 

MMS or other agencies, and which did not have sufficient existing MMS stipulations. 

The list of concerns for focus in this project, in order of priority ranking based on 

discussions with the resource agencies in the US, is: 

1. Short-term and cumulative impacts from dredging that lead to loss of entire 

benthic communities and possible re-colonization by an altered biological 

community; 

3. Changes in the substrate characteristics (grain size, dissolved oxygen, 

compaction and organic content) that lead to a reduction in benthic 

communities and suitability of the area for future dredging; 

5. Sedimentation (burial) impacts to adjacent hard/live bottom or other sensitive 

habitats; and 

7. Impacts from short-term increased turbidity from cutterhead or draghead and 

overflow from hopper dredges on benthic communities. 

Other key concerns such as impacts to turtles (ranked 2), shoreline impacts through 

changes to wave climate (4), spatial and seasonal conflicts with recreational and 

commercial fishermen (8), potential damage to pipelines (9), damage to archeological 

resources (11), and potential harmful alteration or destruction of Essential Fish Habitat 

(12) are being or have been recently addressed in other MMS (and USACE in the case of 

84 
sea turtle impacts) studies. The impact to Essential Fish Habitat was discussed in a 

preliminary manner during the project workshop and some other recommendations were 

developed. Creation of depressions and furrows (ranked 6) was agreed to be of minor 

concern, or covered by Impact 3, which considered the development of dredge pits. The 

existing stipulations for collision with marine mammals (10) were determined to be 

sufficient. 

A review of the range of existing and emerging environmentally friendly techniques 

and approaches to dredging was supported by a literature review and an industry survey. 

The industry survey included both US and European dredging contractors. In general, 

this review found that the US dredging industry is not lagging the European market in 

development of innovative approaches. Two of the key recent developments to address 

dredging impacts in offshore waters, and particularly the size and extent of dredge 

plumes, consisted of the use of an anti-turbidity valve to reduce air entrainment in the 

overflow process and an approach of re-circulating the overflow water to the draghead (a 

“closed system” sometimes referred to as “Green Pipe”), eliminating the plume from the 

upper part of the water column. Both the European and US dredging industries had 

adopted widespread use of the anti-turbidity valve. Neither the European nor US 

dredging industries had adopted the closed system approach to overflow due to capital 

and operational costs and lack of justification to eliminate overflow in the upper part of 

the water column. Another approach that is becoming universally adopted, at least 

within the US market where aggregate dredging and screening are not carried out, is 

below hull release of the hopper overflow. This approach also reduces the size of the 

turbidity plume. 

The key area of difference between the US and European dredging industries was the 

size of hopper dredges. Within a decade in Europe the maximum hopper size of TSHDs 

has moved from around 12,000 m3

 to in excess of 35,000 m3. In contrast, in the US, the 

largest hopper dredges are the Great Lakes Dredge & Dock Liberty Island (5,000 m3

) and 

Bean Stuyvesant (8,360 m3

). With respect to dredging impacts, the primary implication 

of this difference is that almost all of the recent research on hopper design (and 

efficiencies related to the overflow process) has been completed in Europe. However, 

US dredging contractors ultimately benefit from these developments. 

There has also been a tremendous amount of development in dredging equipment 

related to controlling the release of sediment at the dredge head, particularly for projects 

involving the removal of contaminated sediments. These techniques were reviewed and 

discussed as part of this project but do not really contribute to the evaluation of issues 

and techniques appropriate for most OCS dredging operations. 

From the industry survey and the literature review it was apparent that most 

approaches and equipment development has focused on reducing turbidity levels 

associated with overflow from hopper dredges. These various efforts have reduced the 

sedimentation footprint associated with the overflow plume to extending no more than 

about 200 m beyond the dredge area, at least at locations where ocean currents are not 

85 
strong. The success of concentrating the overflow plume may be leading to a new 

problem, at least in some cases, and that is the development of a near bed turbidity 

current. In these cases, a turbidity current consisting of a highly sediment-laden flow can 

travel 100’s of meters up to several kilometers away from the borrow deposit, 

significantly expanding the area of impact. Turbidity currents are triggered under certain 

conditions consisting of a steep seabed slope and/or strong currents (with the dredge 

operating in line with the currents). 

Very little if any development in either equipment or dredging approaches has been 

devoted to the key issue of loss of benthic communities. Some possible approaches, 

consisting of setting aside spatial or temporal refuges, were developed by the study team

for further evaluation. In other cases, where environmentally friendly approaches or 

equipment had not been developed to address particular key impacts, some suggestions 

were generated by the team members for further evaluation. 

The final phase of this study consisted of a workshop to evaluate the various 

environmentally friendly approaches that had been identified under each of the impact 

headings, with particular focus on the ones noted above. The workshop was attended by 

representatives of: the study team, MMS, USACE, and the dredging industry. For each 

type of equipment, procedure or approach that was reviewed, the evaluation was 

completed for three criteria: appropriateness, practicality and effectiveness. The 

recommendations for each key impact are summarized in Section 6.2. 

6.2 Recommendations 

A summary of the recommendations developed through the course of the study 

workshop is presented below for each of the key impacts identified for review. 

6.2.1 Recommendation on Spatial/Temporal Refuge Areas to Promote ReColonization of Benthic Communities 

It may be appropriate, practical and effective to impose spatial or temporal refuge 

areas at locations with one or more of the following characteristics: 1) the presence of a 

unique assemblage of benthic communities; 2) special commercial significance of a 

benthic community in a borrow area; 3) at locations where the benthic community is 

spatially limited with respect to recruitment and re-colonization; and 4) where the 

importance of a benthic community within the borrow area is significant for higher 

trophic levels or where this relationship is uncertain. In order to develop a layout of 

refuge areas that is practical and does not significantly influence the cost of the dredging 

operation, the type of dredging equipment and borrow deposit layout should be 

considered. Some specific dimensions for minimum feasible dredge areas are presented 

in the report as a guideline for developing a feasible layout of dredge and refuge areas. 

The MMS Plume model should be applied to determine the required size of the refuge 

86 
areas considering the sedimentation footprint from the dredging operations. This 

proposed approach should be field tested along with a technique to monitor the 

effectiveness. 

6.2.2 Changes to Substrate Characteristics and Recommendation on a Pit 

Depth Rule 

A blanket maximum pit depth rule is inappropriate. However, it is appropriate to 

determine a local maximum pit depth to avoid development of a mud cover and/or 

anoxia. The minimum practical pit depth would be greater than 1 m from TSHDs and 

greater than 2 m for CSDs. Maximum pit depths should be determined on a site-specific 

basis through analysis combined with monitoring where necessary (as described above). 

Monitoring may assist the development of an appropriate maximum pit depth at borrow 

sites that are dredged more than once. 

6.2.3 Recommendation on Protecting Sensitive Habitat from Sedimentation 

It is appropriate to consider the implementation of these measures at locations where 

there is nearby habitat that is sensitive to sedimentation, such as hard/live bottom areas or 

coral habitat with specific sedimentation sensitive organisms. In these cases, there is a 

need to establish field-tested sedimentation limits for different types of sensitive habitat. 

A blanket buffer zone width for all locations is probably unjustified. Another way of 

defining acceptable site-specific sedimentation levels, that may be more expedient, is 

through the monitoring of natural sedimentation rates. Once sedimentation limits are 

established for the local sensitive habitat, the best approach would consist of a predredging assessment of the plume sedimentation footprint using the MMS Plume model 

(or equivalent), followed by real-time or near real-time monitoring of sedimentation 

levels (for more details see the recommendations summary for buffer zone approaches at 

the end of Section 5.3.5.3). Turbidity monitoring may also be helpful to validate the 

Plume model, however, it is not a suitable replacement for direct monitoring of 

sedimentation. It would be appropriate to require the Anti-Turbidity valve device at 

locations where restricting the sedimentation footprint is important. At almost all 

locations the Green Pipe approach (where the overflow water is re-circulated to the 

draghead) is likely unjustified. At borrow sites with strong tidal currents or steep slopes, 

the possibility of the development of a near-bed turbidity current generated through the 

pancaking effect of the dynamic plume phase should be evaluated (see Section 2.3.7 for 

details). 

87 
6.2.4 Recommendation on Addressing the Impact of Turbidity 

It is generally viewed that elevated levels of turbidity generated from TSHD 

operations in open ocean waters does not represent a significant ecological impact. It is 

believed that adult fish can avoid plumes and that other organisms can survive the sublethal levels of short-term elevated turbidity. A one-size-fits-all limit of 29 NTUs above 

background levels measured at 150 m from the dredging operation is probably 

scientifically unjustified for the ocean environment. Nevertheless, representatives of the 

dredging industry that attended the study workshop indicated that the 29 NTU limit was 

not difficult to achieve. At locations where a more scientifically justified level is 

required, for example where there is a specific ecological concern about turbidity levels, 

it may be possible to develop a site-specific limit based on measurements of turbidity 

levels over a minimum period of one or two years. The Anti-Turbidity valve device is 

widely applied in the US and significantly reduces the size of plumes from TSHDs and 

the total sediment overflowed in the discharge process. It would be appropriate to 

require the use of this device wherever turbidity is a concern. The “Green Pipe” 

approach consisting of re-circulation of the overflow water to the draghead eliminates the 

plume above 4 to 5 m above the seabed (i.e. outside of the region of the draghead plume), 

but it does not reduce the total sediment discharged in the overflow process. However, 

this approach is not included on any US dredge vessels (nor on most European vessels) 

and would represent a significant and expensive equipment overhaul that would be 

passed on to the consumer through higher unit prices and is likely unjustified at most

locations. 

6.2.5 Recommendation on Protecting Essential Fish Habitat, and 

Specifically, Shoal Integrity 

 

There is much to be learned about the processes that maintain the form of shoals, and 

therefore, the potential impacts of dredging sand from these features. Hayes and Nairn 

(2004) have summarized the literature on this topic and suggested a new mechanism for 

the maintenance of OCS shoals (see sect. 5.3.12.4 for an explanation of this new 

mechanism), however, the understanding of these features requires more investigation. 

This understanding and the development of guidelines for the removal of sand through 

dredging (specifically, how much and where) will require several lines of investigation 

including: a review of shoal morphometrics (as C. Spaur of the USACE, Baltimore 

District has initiated); an investigation of the sedimentology and stratigraphy of these 

features; and numerical modeling of waves, sediment transport and morphodynamics. 

88 
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94 
Appendix A 

Literature Review On Ecological 

Impacts Of Dredging

Report By Newell And Seiderer (2003) 

Completed For This Study. 

Appendix 
ECOLOGICAL IMPACTS OF MARINE AGGREGATE 

DREDGING ON SEABED RESOURCES

Prepared For 

Baird Associates, 

627 Lyons Lane, Suite 200, 

Oakville, ONTARIO. Canada L6J 5Z7 

By 

R.C.Newell D.Sc.(Lond.) & L.J.Seiderer Ph.D. 

Marine Ecological Surveys Limited

Trewood Cottage, Steeple Lane, 

St. IVES. Cornwall TR26 2PF. 

Tel: +44 (0)1736-795975 

Fax: +44 (0)1736-793727 

E-Mail: SEASURVEY@aol.com

Web Site: www.marineecologicalsurveys.co.uk

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003
CONTENTS. 

SUMMARY .........................................................................................................1 

1. INTRODUCTION.............................................................................................3 

 1.1. The Dredging Process. ..........................................................................3 

 1.2. The Fate of Overboard Discharges. .........................................................6 

 1.3. Impacts on Biological Resources........................................................... 14 

2. IMPACTS WITHIN THE DREDGING AREA. ..................................................... 16 

3. RATE OF RECOVERY FOLLOWING CESSATION OF DREDGING. ...................... 21 

3.1. Scenario 1. The Sediment Composition of the Deposits Remains 

 Substantially Unaltered following Cessation of Dredging. ............................... 21 

3.2. Scenario 2. The Sediment Composition of the Deposits becomes 

 Sandier following Removal of Coarse Components and Rejection of Fines. ....... 28 

 3.2.1. Short-Term Impacts .............................................................. 28 

 3.2.2. Long-Term Impacts ............................................................... 29 

 3.2.3. Approaches to the Assessment of ‘Recovery’ ............................. 33 

4. IMPACTS OUTSIDE THE BOUNDARIES OF DREDGED AREAS.......................... 34 

5. CUMULATIVE IMPACTS ................................................................................ 37 

6. IMPACTS ON FISHERIES. ............................................................................. 38 

7. ASSIGNING SIGNIFICANCE TO IMPACTS ..................................................... 39 

8. RISK ASSESSMENT ...................................................................................... 41 

 8.1. Risk Assessment for Communities & Habitats. ........................................ 41 

 8.2. Risk Assessment for Fisheries Resources. .............................................. 42 

9. MITIGATION ............................................................................................... 47 

 9.1. Minimise the Dredged Area.................................................................. 49 

 9.2. Manage the Dredging Operation. .......................................................... 49 

 9.3. Liaison & Reporting. ........................................................................... 49 

10. MONITORING. ........................................................................................... 50 

 10.1. Number and Position of Monitoring Sites.............................................. 51 

 10.2. Benthic Infauna. .............................................................................. 53 

 10.2.1. Sampling Methods. .............................................................. 53 

 10.2.2. Laboratory Identification & Analysis........................................ 54 

 10.3. Benthic Epifauna. ............................................................................. 55 

 10.4. Data Recording & Analysis of Results. ................................................. 56 

 10.5. Identification & Mapping of Marine Biotopes. ........................................ 57 

11. REFERENCES.............................................................................................. 58

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003
SUMMARY.

Our assessment of the likely impacts of marine aggregate mining on seabed resources is 

based mainly on the results of impact studies on physical and biological resources in 

relatively shallow water sites of up to 30m depth in European waters. Most of the 

conclusions are therefore strictly applicable mainly to seabed deposits that are subject to 

disturbance by waves and tidal currents, and where the resident organisms are adapted 

to disturbance under natural conditions. We consider the most likely impacts on benthic 

biological resources to be the following:- 

1. The species variety, population density and biomass of benthic fauna is likely to 

be suppressed by as much as 60-90% within dredged areas. This suppression will 

reduce during the recovery process following cessation of dredging, but may be 

significant in coarse deposits for at least half of the overall recovery time, ie. at 

least 6-10 years (see below). In sandier deposits, recovery times are likely to be 

shorter (approximately 2-4 years). 

2. There is likely to be a zone extending for a variable distance outside the dredge 

area (depending on the velocity and direction of the tidal streams at the seabed) 

where deposition and subsequent seabed transport of material discharged 

overboard has an impact on biodiversity, population density and biomass of 

benthos. 

3. Studies recently completed in the North Sea in the vicinity of Licence Area 408 

show that where sand rejected during the screening process has been returned to 

the seabed, areas of fine well-sorted sand extend from the dredged sites along 

the axis of net transport by tidal streams for at least 2km. The distribution of 

these fine sands varies with local seabed current direction, and is consistent with 

the deposition and transport of material rejected during the screening process. 

 

4. The biomass of benthic infauna within an actively dredged zone at this Licence 

Area was suppressed by 82% compared with that at “control” sites well outside 

any impact of dredging. Biomass is suppressed by as much as 66% within the 

areas of fine sand outside the boundaries of the dredge site, and approaches that 

of “control” sites at a distance of 4 km down the axis of net tidal transport to the 

south-east of the dredged sites. Impacts of deposition of sand rejected during the 

dredging and screening process can therefore extend for a considerable distance 

outside the boundaries of a dredge site along the axis of transport of material on 

tidal currents at the seabed. 

5. Some components of the community, such as polychaete worms and small 

crustaceans, are capable of relatively rapid recolonisation and growth following 

cessation of dredging. Recovery of the biomass of some of the long-lived 

components of the equilibrium communities such as larger bivalve species that 

characterise gravel communities could, however, take as much as 15-20 years 

even if the deposits remain of a suitable particle size composition for 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 1
recolonisation. 

6. If removal of coarse material for marine aggregates, and rejection of fine sand 

results in a long-term change in sediment composition towards more sandy 

deposits, then the fauna is likely to revert to one with a low species variety 

dominated by polychaete worms. 

7. Impacts on transitory members of the biological community in the vicinity of the 

dredge sites are less likely than impacts on benthic organisms that cannot evade 

adverse conditions should they occur. Thus although impacts on plankton and fish 

have been reported for long-term exposure experiments in the laboratory, under 

natural conditions most species are likely to evade areas of disturbance or 

turbidity. 

8. We conclude, therefore, that the main impacts of the dredging proposals will be a 

suppression of species diversity, population density and biomass of benthic 

animals within the dredge sites and along the axis of deposition of material 

mobilised by the dredging and screening processes. 

 

9. Benthic communities probably represent an important food resource for fish, but 

we doubt whether losses to the marine food web from the dredged areas would 

result in a detectable impact on the carrying capacity of the waters surrounding a 

dredge site for commercial fish stocks. Of more significance are possible impacts 

on areas of localised or seasonal importance such as spawning grounds and 

nursery grounds for fish and shellfish such as scallop and crab. 

10. Recent proposals for Risk Assessment to marine resources, including invertebrate 

communities of conservation significance and commercial fish stocks, take into 

account both the sensitivity of the resources to the physical impacts of marine 

aggregate dredging, and their actual vulnerability based on the location of the 

resources in relation to the dredge site and contours known physical impact. We 

consider this to be an important approach that allows a full identification of Risk 

to specific environmental resources located near to a dredge site, before possible 

mitigation or remedial measures are considered. 

11. There are currently few practical or cost-effective ways of minimising the impacts 

of marine aggregate dredging within the dredge sites themselves, nor in the 

sedimentation zone which is likely to surround the dredge sites unless restriction 

of discharge of screened material were a commercially-acceptable option. 

Experimental studies are being undertaken to determine whether restoration of 

the seabed surface with a thin layer of gravel could assist in restoration of 

community composition following cessation of dredging, but the results of this 

work have not yet been reported or evaluated as a cost-effective option. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 2
1. INTRODUCTION.

In order to assess the likely impacts of marine aggregate dredging on biological 

resources, it is necessary to summarise the data which are available from impact 

studies, and to clarify the assumptions inherent in the environmental impact assessment 

process. This includes features of the extraction process itself, as well as the key points 

at which an impact on biological resources might be anticipated beyond the immediate 

boundaries of disturbance by the drag head. 

The impact of marine aggregate dredging on seabed resources has been widely reviewed 

and are comparatively well-documented for coastal sites in European waters (see 

Dickson & Lee, 1972; Shelton & Rolfe, 1972; Cruikshank & Hess, 1975; Eden, 1975; 

Millner et al., 1977; de Groot, 1979; Van der Veer et al., 1985; Glasby, 1986; Lart, 

1991, Gajewski & Uscinowicz, 1993; ICES, 1993; Land et al., 1994; Whiteside et al., 

1995; Hitchcock & Drucker, 1996; Newell et al., 1998; Desprez, 2000; van Dalfsen et 

al., 2000; Boyd et al., 2003). There have, however, been several recent studies that 

confirm and amplify what is known about the extent and distribution of material 

discharged during the dredging process, and the impacts that this might have on benthic 

biological resources. 

1.1. THE DREDGING PROCESS.

Most of the sea-going aggregate dredgers are self-contained and use a centrifugal pump 

to lift aggregates from the seabed into a hopper where the material may be screened 

before being transferred to a hold of 5000-8000 tonnes capacity. Where the gravel 

deposits are in a restricted area of seabed, the suction dredger may operate at anchor, a 

method of dredging that can result in pits or depressions in the seabed that can reach as 

much as 20m depth and 75m diameter (Dickson & Lee, 1972; Cruikshank & Hess, 1975). 

These pits formed from ‘anchor-dredging’ are likely to be persistent features of the 

seabed for several years except in areas where the sands are mobile (Eden, 1975). In 

such cases, slumping of the sides of the pit and subsequent infilling by fine particles 

transported by tidal currents may lessen the physical impact, restoring the pits to their 

former level. However, this can lead to heavily anoxic sediments within such dredge pits, 

and to colonisation by a community that differs considerably from that in the original 

deposits (Dickson & Lee, 1972; Shelton & Rolfe, 1972; Kaplan et al., 1975; Bonsdorff, 

1983; Hily, 1983; Van der Veer et al., 1985; Hall, 1994). 

The normal process of extraction involves suction dredging whilst the vessel is slowly 

under way. This process of ‘trailer dredging’ results in a series of tracks of 2-3m wide 

and up to 50cm deep (van Moorsel & Waardenberg, 1990; Kenny & Rees, 1994; Boyd et 

al., 2003), although deeper troughs of up to 2m have been recorded from areas where 

the drag head had crossed the area several times. Davies & Hitchcock (1992) reported 

dredge cuts of between 20-55cm depth and 3.0-3.8m width in commercially exploited 

deposits of the Bristol Channel. Somewhat deeper troughs of up to 70cm were reported 

for the Baltic (Gajewski & Uscinowicz, 1993). Desprez (2000) reported furrows up to 5m 

deep separated by crests of shingle in dredged deposits off Dieppe, France. In all these 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 3
cases removal of the surface 0.5m of the seabed is sufficient to eliminate the benthos 

from the deposits. The total depth of removal depends on the intensity of dredging at a 

particular worked site. 

In some instances, in situ gravel deposits are transferred in bulk into the hold for 

subsequent use as beach feed or landfill (see Hess, 1971), but in most instances the 

proportion of sand:gravel in the cargo is adjusted to suit customer requirements by a 

process of screening which can involve rejection of significant quantities of sand 

overboard at the site of dredging. The in situ reserves that are suitable for economic 

exploitation generally range from 15-55% gravel whereas the sand:gravel ratio in the 

final cargo is generally adjusted to between 50:50 and 65:35 depending on customer 

requirements, local geology and ship performance. This implies that 20-80% of the 

material dredged may be rejected overboard during the screening process. 

If we assume a relatively low figure of 30% for material returned to the seabed following 

screening, then a total of 6500te of seabed deposits will be dredged to obtain a 4500te 

cargo load. This process takes 4-6h depending on the type of dredger and nature of the 

seabed deposits and is likely to be associated with a discharge of a minimum of 1,500te 

of fine deposits comprising mainly sand-sized particles from the reject chutes following 

screening. 

Settlement of this reject material can result in a significant ‘overburden’ of sand within 

production licence areas, the deposits then requiring increased screening compared with 

newly-exploited deposits to obtain a suitable commercial cargo. In some coastal areas, 

for example, mass balance studies of emissions from dredgers operating in Production 

Licence areas such as Owers Bank and in relatively sandy deposits of the North Sea off 

Southwold show that much higher proportions of up to 1.7 x the cargo load may need to 

be ‘processed’ by the dredger to obtain a suitable cargo (Hitchcock & Drucker, 1996; 

Newell et al., 1998, 1999). The possibility of significantly higher rejection of screened 

material than the 30% assumed above for typical gravel deposits cannot therefore be 

excluded if an overburden of sand develops after a period of exploitation of the resource. 

Discharge of increased quantities of sand by screening is likely to significantly increase 

the impact of discharged material on benthic biological resources in the immediate 

vicinity of the dredge site, but is unlikely to settle over a wider area than when smaller 

quantities are discharged. 

Progressive removal of the coarse fraction of deposits, and rejection of fine material by 

overboard screening can result in significant changes in particle size composition of the 

deposits both within the dredged area, and along the axis of deposition of material 

discharged overboard by the screening process. This effect is more marked in 

undisturbed environments where screened material is not moved rapidly away from the 

site of deposition by local currents. van Dalfsen et al (2000), for example, reported that 

at a dredge site in the Mediterranean at Costa Daurada, Spain, grain size changed 

considerably as a result of deposition of material from the spillways during the dredging 

operation. After two months, scuba divers observed a 5-20cm thick layer of very fine 

sediment (MD50 = 0.016-0.018mm) on top of the native sand (MD50 = 0.1-0.15mm). 

One year later the fine sediment still formed on average 27% of the sediment by weight 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 4
(Manzanera et al., 1996). Changes in particle size composition of the sediments following 

dredging have also been reported for sediments in relatively shallow water off Dieppe by 

Desprez (2000; for review, see Boyd et al., 2003). 

A second source of loss of material from the dredger during the dredging process is fine 

suspended material which overflows through the spillways once the hold has filled with 

water and screened cargo. In most aggregate areas the fines comprise at least 1-2%, 

and often as much as 8-10% of the deposits. Assuming that all of these fines are 

discharged through the spillways, then even based on the lowest figure of 1-2% silt, the 

mass of silt likely to be discharged during processing of the 6500te of seabed deposits 

required to load a 5000te cargo is likely to be 65-130 tonnes, and it could be a good deal 

higher in some of the dredge sites. 

This fraction dominates the overspill material although there is also a varying component 

of sand that is maintained in suspension by turbulence within the hopper. This can result 

in an obvious visible ‘plume’ which carries for as much as 2-3km down-current astern of 

dredgers operating in deposits that contain significant quantities of silt. 

Plate 1. Typical marine aggregate dredger loading a cargo and discharging screened material from reject 

chutes. Overspill losses from the cargo hold can also be seen. Copyright ©MESL-PhotoLibrary. 

A typical suction trailer dredger operating in the North Sea is shown in Plate 1. 

Overboard losses from the two screening towers and reject chutes can be seen, as well 

as those from the spillways located along the upper parts of the cargo hold. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 5
1.2. THE FATE OF OVERBOARD DISCHARGES.

Most direct studies on the fate of the material discharged through the overboard reject 

chutes suggest that coarse material, including sand-sized particles settle rapidly to the 

seabed as a density current jet. The rate of settlement during this initial ‘dynamic phase’ 

depends on the overflow density, the diameter of the discharge pipe, the water depth, 

the velocity of discharge and the speed of the dredger (Whiteside et al, 1995). During its 

passage through the water column, and following impact on the seabed, the sediment is 

dispersed into the water column and forms a well-defined plume astern of the dredger. 

This second, longer phase has been referred to as the ‘passive phase’ of dispersion by 

Whiteside et al (1995) and starts approximately 10min after discharge. During this 

phase, the material behaves in a relatively simple settling mode according to Stoke’s 

Law, the plume then decaying to background levels after a period of 2-3h. This 

dispersing plume is a clearly visible feature of many marine aggregate dredging 

operations, and is supplemented by losses of fines from the spillways amounting to at 

least 1-2% of the total material dredged. 

Plume generation and decay from dredgers operating at Owers Bank off the south coast 

of UK has been studied by Hitchcock & Dearnaley (1995; see also Hitchcock & Drucker, 

1996). Their results support the view that particles rejected through the screening 

process move rapidly to the seabed. Conventional water sampling techniques used in this 

study suggested that concentrations of sand-sized particles were reduced to background 

levels only 200-300m from the point of release into the water column and that 

concentrations of silt-sized particles are also reduced to background levels of 2-5mg per 

litre over this distance. 

Acoustic backscatter techniques were also used to define the plume morphology in 

relation to distance and time down-current of a dredger operating at anchor and 

rejecting the screened material overboard as described above. These measurements 

suggest that sand settlement occurs in a zone extending up to approximately 600m 

astern of the dredger. Silt-sized particles disappeared from the water column at 

approximately 1800m. Other components, thought to be organic matter, disappear from 

the water column at 2.5 - 3.0km astern of the dredger. 

A series of sections through the outwash plume down-current of the dredger based on 

Hitchcock & Drucker (1996) is shown in Figure 1. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 6
Figure 1. Acoustic Backscatter images across the plume at varying distances downstream of an anchordredger during loading of a screened cargo at Owers Bank off the south coast of UK. Based on Hitchcock & 

Drucker (1996). The black band at the seabed is a data corruption zone which precluded assessment of plume 

morphology at the sediment-water interface. The red signal indicates coarse sand-sized particles; the yellow 

signal indicates the settlement of silt; and the white signal is considered to represent organic flocculating 

material.

Several features of interest emerge from these results:- 

• Although suspended sediment concentrations in the plume were not significantly 

different from background levels beyond 200-300m from the point source of discharge 

using conventional water sampling and optical transmissometer techniques, it is 

possible to track the plume using acoustic backscatter techniques for a distance of up 

to 3.5km. Subsequent studies suggest that this far-field effect reflects the presence of 

organic material derived from benthos fragmented by the dredging and screening 

process (Newell et al., 1999). 

• The sedimenting plume is approximately 200m wide and 3000m long in the region 

where settlement from the water column is occurring. 

• The technique is useful for identifying the zone of settlement from the water column, 

but there is a data corruption zone at the sediment-water interface amounting to 

approximately 6% of the water depth, where further tracking of subsequent deposition 

and movement of material is not possible.

Relative Backscatter (Average all beams) Transects 1- 9

# 1 - 80m astern

# 7 - 1835m astern # 8 - 2640m astern # 9 - 3335m astern

# 4 - 590m astern # 5 - 880m astern # 6 - 1540m astern

# 2 - 205m astern # 3 - 365m astern

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 7
Because the benthic organisms are likely to be mainly affected by heavy sedimentation 

loads on the seabed, direct studies showing the relatively rapid sedimentation of 

particulate material within 600m from the point of discharge have until recently 

suggested that most, if not all impacts of the rejection of screened material are likely to 

be confined to the immediate vicinity of the point of discharge and are unlikely to have a 

potential impact beyond the boundaries of the dredge site itself. 

Recent studies suggest, however, that at some sites the potential impacts of screened 

material are not confined to the relatively small zone of initial sedimentation from the 

water column. Dickson & Rees (1998) deployed a series of ‘mini-POD’ samplers located 

approximately 50cm above the seabed in the vicinity of Area 107 off Skegness in the 

southern North Sea. They have shown that marine aggregate dredging in that area is 

associated with a benthic plume of mobilised sediment that extends along the sedimentwater interface for as much as one tidal excursion (ie., as much as 10km) along the axis 

of the tidal stream in that area (Dickson & Rees, 1998). Simultaneous deployment of a 

mini-POD near the Area 107 South Coast Shipping dredge site and on Race Bank 

approximately 6.5km away from the dredge site in May-June 1995 allowed some 

estimates of the transport of material between the dredge site and adjacent areas. 

 

Figure 2. Mini-POD records showing suspended sediment spikes at dredge Area 107 (magenta) and at Race 

Bank (black) at a distance of 6.5 km from the dredge site in 1995. Redrawn after Dickson & Rees (1998). 

Figure 2 shows the suspended sediment load measured at a mini-POD located on the 

seabed at the site of dredging at Area 107 and at Race Bank approximately 6.5km 

downstream. Each period of dredging activity by the dredger ‘Sand Weaver’ (shown in 

green for the start and red for the stop below the X-axis in Figure 2) is followed by a 

high concentration of suspended sediment at the dredge site mini-POD, followed 

approximately 5h later by the arrival of a smaller spike at Race Bank. The speed of travel 

of the pulse of sediment at the sediment-water interface is therefore approximately 

1.3km per hour. 

A deployment of 4 mini-PODs was then made in June 1996 to record the passage of a 

sediment plume from the site of dredging at Area 107 to Race Bank. The dredger used in 

this experiment was the modern ‘Ham 311'. The results shown in Figure 3 provide the 

first clear and unequivocal evidence of an individual outwash plume passing in sequence 

from one mini-POD to the next across the whole distance from the dredge site to Race 

Bank. 

Date (June)

Suspended Load

4

3

2

1

0

Race Bank

Area 107

Start

Stop

1 34 5 6 2

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 8
Figure 3. Traces from 4 Mini-PODs recording the passage of a benthic sediment plume from a site of dredging 

at Area 107 to Race Bank in June 1996. Redrawn after Dickson & Rees (1998). 

Impacts at the sediment-water interface are of particular significance to marine benthos 

because activities such as larval settlement, irrigation and feeding occur at the surface of 

the seabed. It is therefore of interest to know the particle size spectrum of the material 

carried from the dredge site at the seabed. Dickson & Rees (1998) used event-triggered 

syringe samplers and passive sediment traps to provide a calibration of Miniature Optical 

Backscatter Sensors (MOBS) by gravimetric analysis and analysis of particle size 

spectrum at a mini-POD site within dredge Area 107 and at a site approximately 8.5km 

to the south at Nut and Spanner Buoy on Docking Shoal. This is the furthest site at 

which dredge plumes from Area 107 have been detected at Spring tide conditions. 

Figure 4. Relative Particle Size composition of the benthic boundary plume at the dredge site at 

Area 107, and at Nut & Spanner Buoy 8.5 km to the south of the dredge site. Redrawn after 

Dickson & Rees (1998).

Tim e

Volts

0

1

2

3

0 °4 2 'E

0 °43.5'E

0°4 5 'E

0°48 'E

Race Bank - E

C

Area 107 - B

D

B - Area 107

C

D

E - Race Bank

Start

Stop

16h48 18h48 20h48 22h48 00h48 02h48

0 1 10 100 1000

Particle Size (μm)

2000

4000

6000

8000

0

Relative % by volume

Dredge site Area 107

Nut & Spanner Buoy

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 9
Figure 4 shows that the main difference between the benthic plumes at the dredge site 

in Area 107 and that at the Nut and Spanner site 8.5km to the south is an overall 

reduction in the concentration of material. This probably reflects progressive losses by 

sedimentation to the seabed along the axis of the plume. There is, however, no evidence 

of a relative loss of coarse-sized particles (0.2mm) at the extreme of the plume. This 

suggests a relatively uniform loss of material along the length of the plume, rather than 

a differential settlement based on particle size. It suggests that the generation of a 

seabed plume may be more like a density current generated by material discharged to 

the seabed from the reject chute, rather than a simple settlement of material from a 

dispersing sediment plume. Very little further work has been carried out at other sites to 

investigate whether the results of this important study apply to other dredge areas, and 

the extent to which the scale of the seabed plume is related to local hydrographic 

conditions and the amounts of material rejected by screening within the dredge site. 

These results were used by Dickson & Rees (1998) to show that dredging on Area 107 

had a potential to deliver an extra 50-150mg/l to the near-bottom sediment layer at 

Race Bank during about 7% of the Spring/Neap tidal cycle. The Race Bank is an 

important over-wintering ground for berried brown 

crab (Cancer pagurus) typical landings being reported 

as £300,000 p.a in 1997-8 (Dickson & Rees, 1998). 

Since the sedimented material was carried towards 

this sensitive resource area on the ebb tide, 

agreement with the dredging company to confine 

dredging of Area 107 to the flood tide only, 

successfully minimised potential impacts on biological 

resources of economic significance. 

Cancer pagurus ©MESL-PhotoLibrary 

These direct measurements on the characteristics of near-bed sediment plumes in the 

southern North Sea are supported by a recent re-analysis of the acoustic backscatter 

data originally reported by Hitchcock & 

Drucker (1996) for marine aggregate dredging 

at Owers Bank on the south coast of UK. 

These also provide some evidence of a plume 

at the benthic boundary layer which extends 

along the axis of dispersion outside the zone 

of deposition of sediment from the water 

column Hitchcock et al. (2002). 

Acoustic Backscatter Profiler ©MESL-PhotoLibrary 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 10
Figure 5. Longitudinal section of the sedimentation plume from a dredger loading a screened cargo at Owers 

Bank in 1995. Based on Acoustic Backscatter data from Hitchcock et al, (2002). The red side of the scale 

indicates high backscatter levels and the blue side of the scale indicates low backscatter levels. 

Figure 5 shows a longitudinal section of the sedimentation plume from the dredger ‘City 

of Rochester’ during loading of a screened cargo at Owers Bank in 1995. The near-site 

sedimentation of sand and the subsequent loss of finer material from the water column 

with distance downstream from the dredger corresponds with that shown in Figure 1. 

There is, however, also a strong signal along the benthic boundary layer, indicating the 

presence of a sediment plume located at the sediment-water interface. This extends 

outside the zone of sedimentation from the water column to the limits of measurements 

at 2.7km from the dredger, and may correspond with that described by Dickson & Rees 

(1998) from mini-POD measurements of benthic plumes in the southern North Sea. 

This pattern of sedimentation and subsequent dispersion on the seabed is not confined to 

dredgers discharging screened material overboard during the loading operation. Figure 6 

shows a longitudinal section of the acoustic backscatter profile from a large dredger 

‘Geopotes’ of 8000te hopper capacity loading an ‘all-in’ cargo at Owers Bank in 1995 

(from Hitchcock et al 2002). Again, it is clear that there is an initial rapid sedimentation 

of material from the water column near to the point of discharge, but there is also an 

extended dispersion plume at the sediment-water interface and this extends downstream 

to the limits of measurements at least 4km from the dredge site. 

Easting

505345 506345 507345 508345

85000

85600

86200

Northing

Depth

- 4

- 13

- 22

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 11
Figure 6. Longitudinal section of the Acoustic Backscatter profile from a dredger loading an “all-in” cargo at 

Owers Bank in 1995. Based on Hitchcock et al, (2002). The red side of the scale indicates high backscatter 

levels and the blue side of the scale indicates low backscatter levels. 

Studies recently completed in the North Sea in the vicinity of Licence Area 408 support 

the view that fine sands rejected during the screening process, and mobilised by tidal 

streams, may be transported considerable distances along the seabed outside the 

boundaries of the dredge site. Net sediment transport at Area 408 is to the south-east, 

with local variations due to the influence of topographic features on the tidal streams in 

the west of the survey area. Newell et al (2002) and Evans (2002) have shown that 

areas of fine well-sorted sand with a sorting coefficient of <0.5 phi extend from the 

dredged sites within the Licence Area for at least 2km along the axes of local net 

sediment transport towards the south-east in the survey area. These areas of well-sorted 

fine sand overlay seabed sediments with a more variable composition, and could be 

associated with remobilised material. 

The distribution of sediment with a sorting coefficient of <0.5 phi is superimposed onto 

the seabed morphology of the study area from high resolution side-scan sonar data 

acquired during 2000 in Figure 7. This shows the actively-dredged site where the seabed 

is disturbed by draghead trails, and an area of fine, well-sorted sediment extending 

along the net transport direction to the south east of the dredged site. Since the samples 

reported for Area 408 were taken with a Hamon grab, and therefore represent material 

averaged down to 20-30 cm depth, it is probable that thin surface layers of fine sand 

may extend further along the axis of tidal transport than is recorded from conventional 

grab samples. 

510400 508400 509400 507400

Easting

88300

87300

Northing

-6

-11

-16

-21

Depth

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 12
Figure 7. A section of the Licence Area 408 in the North Sea showing the seabed morphology 

based on seismic interpretation. Superimposed on the map are contours of sorting coefficient for 

seabed sediments with grain size finer than 0.5phi. The currently dredged area is represented by 

the letter A. The letter B represents an area where dredging ceased in 1999. Based on Evans 

(2002). 

Areas of fine sand were also recorded outside the boundaries of zones at Licence Area 

408 where dredging had ceased at least 12 months previously. Such patches of fine sand 

that may be derived from deposition of material rejected and returned to the seabed 

during the screening process thus appear to be relatively persistent, despite the 

movement of sediment that occurs naturally at the seabed. These results support those 

reported by Manzanera et al (1996) for a site at Costa Daurada, Spain. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 13
In summary, all recent information suggests the following features of the dredging and 

sedimentation-dispersion profiles for material released from marine aggregate dredgers 

relevant to the assessment of impact on biological resources on the seabed:- 

• Sedimentation of all components of material discharged overboard is faster than would 

be anticipated from simple Gaussian models based on the settlement velocity of 

component particles. This is due to cohesive properties of the discharged material 

which forms a density current that enters the water column in a ‘dynamic phase’ of 

settlement. 

• Coarse material up to sand-size particles settles within 200-600 m of the point source 

of discharge, depending on depth of water, tidal velocity and the velocity of flow from 

the discharge pipe.

• Even fine silt-sized particles reach background values within 2-2.5km of discharge, 

although there is a residual ‘signature’ from the dispersing plume at distances of up to 

3.5km which may be attributable to organic matter derived from fragmented benthos 

discharged during the screening process. 

• Settlement from the water column in the vicinity of the dredge site is only part of the 

potential impact on benthic resources. There is now some evidence supporting the view 

that recently-sedimented material is mobilised at the sediment-water interface to form 

a benthic plume which can extend for as much as one tidal excursion in each direction 

from the dredge site. 

• This benthic plume is of potential importance as a source of impact and is likely to 

affect a zone of up to one tidal excursion (10km in the North Sea near Area 107) in 

each direction on the Spring tide. The width is likely to be approximately 200m based 

on acoustic backscatter measurements of the sedimented material in coastal dredge 

sites.

1.3. IMPACTS ON BIOLOGICAL RESOURCES.

The impacts of both the dredging process itself and the subsequent deposition of 

material from the dispersing plume on biological resources beyond the boundaries of the 

dredge site are likely to be complex. Impacts are affected not only by physical features 

of the dredge site and adjacent zone of deposition of material rejected by the screening 

process, but also by the nature of the biological communities that naturally occur on the 

seabed. Different components of the biological communities in the vicinity of dredge sites 

are known to have differing thresholds of sensitivity to such deposited material (Sherk, 

1971; Sherk et al., 1974; Moore, 1977; Matsumoto, 1984; Holme & Wilson, 1985: for 

review, see Newell et al., 1998). 

Sediment stability is also known to have an impact both on suitability for recolonisation 

and the type of communities that are initially established in recently sedimented material 

(Holme & Wilson, 1985; Newell et al., 1998) and is a feature of potential impacts that is 

poorly understood. The type of community that inhabits shallow water wave-disturbed 

deposits under natural conditions generally comprises mobile ‘opportunistic’ species with 

a high rate of growth and reproduction. Conversely, communities that occur on more 

stable coarser deposits comprise communities that are dominated by a wide range of 

‘equilibrium’ species that have a slow rate of recolonisation and growth. The rate of 

recolonisation and recovery following cessation of dredging is thus partly dependent on 

the type of substratum and partly on the natural community which is available to 

colonise the deposits (see van Dalfsen et al., 2000; for review, see Newell et al., 1998). 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 14
The nature and scale of impact of aggregate dredging on seabed resources can be 

assessed by addressing the following key issues in relation to the physical features of the 

dredging and screening processes described above:-

• The impact of dredging within the dredged area itself. 

• The rate of recovery following cessation of dredging. 

• The extent of likely impact outside the boundaries of the dredged area. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 15
2. IMPACTS WITHIN THE DREDGING AREA.

 

The impact of dredging on benthic communities within dredged areas varies widely 

depending, among other factors, on the intensity of dredging in a particular area, the 

degree of sediment disturbance and recolonisation by passive transport of adult 

organisms and the intrinsic rate of reproduction, recolonisation and growth of the 

community that normally inhabits the particular deposits (for Reviews, see Newell et al., 

1998; ICES, 2001; Boyd et al., 2003). 

Some examples of the impact of dredging on the species variety, population density 

(number of individuals) and biomass of benthic organisms from a variety of habitats 

ranging from muds in coastal embayments and lagoons, to oyster shell deposits, and to 

sands and gravels are summarised in Table 1. 

Table 1. Table showing the impact of dredging on benthic community composition from various habitats. Based 

on Newell et al, (1998). 

 

LOCALITY HABITAT TYPE % REDUCTION AFTER DREDGING

Species Individuals Biomass

SOURCE

Chesapeake Bay Coastal 

Embayment 

Muds-sands

70 71 65 Pfitzenmeyer, 1970

Goose Creek, 

Long Island, NY

Shallow Lagoon 

Mud

26 79 63-79 Kaplan et al, 1975

Tampa Bay, 

Florida

Oyster shell 40 65 90 Conner & Simon, 1979

Moreton Bay, 

Queensland, 

Australia

Sand 51 46 - Poiner & Kennedy, 

1984

Dieppe, France Sands-gravels 50-70 70-80 80-90 Desprez, 1992 

Klaver Bank, 

Dutch Sector, 

North Sea

Sands-gravels 30 72 80 van Moorsel, 1994

Lowestoft, 

Norfolk, UK

Gravels 62 94 90 Kenny & Rees, 1994

Hong Kong Sands 60 60 - Morton, 1996

Lowestoft, 

Norfolk, UK

Sands-gravels 34 77 92 MESL, 1997

Dieppe, France Sands-gravels 80 90 90 Desprez, 2000 

Bayou Texar, 

Florida 

Mud 55 77 - Lewis et al, 2001 

North Nab, UK Gravels 66 87 80-90 Newell et al, 2001b; 

2003 

Hitchcock et al, 2002 

Area 408, North 

Sea. 

Sandy gravel 0 0 82 Newell et al, 2002 

 

 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 16
Clearly, the extent of impact within a particular dredge site is likely to reflect the net 

balance between the rate of removal of benthos at a particular dredging intensity, and 

the rate of recolonisation of the deposits by inward immigration of adults and settlement 

of juveniles. The rate of recolonisation can be fast in deposits where the benthic 

community is naturally-adapted to high levels of sediment disturbance. 

In a recent study of the composition of benthic communities in the vicinity of an actively 

dredged site at Licence Area 408 in the North Sea, we found that the rate of immigration 

of colonising individuals in the mobile deposits of the study area was evidently in 

equilibrium with the rate of loss by dredging (Newell et al., 2002). Hence we could 

detect no significant difference in population density and species diversity between 

dredged sites and undredged (control) sites. There were, however, major differences in 

body size of individuals within dredged sites compared with those in the surrounding 

deposits, reflecting the small size of the recently-colonised individuals compared with 

those in undredged areas. In most other sites, however, a major suppression of species 

abundance and population density has been reported, as well as changes in species 

composition that may reflect changes in sediment composition following removal of the 

coarse fraction by aggregate dredging (Desprez, 2000; van Dalfsen et al., 2000; Boyd 

et al., 2003; Boyd & Rees, 2003). 

Despite the wide differences in habitat type, and the nature of the benthic communities 

in the dredged areas, it is clear from Table 1 that the dredging process itself can be 

expected to result in a 30-70% reduction in species variety, a 40-95% reduction in the 

number of individuals, and a similar reduction in the biomass of benthic communities in 

the dredged area. 

Generalisations on the impact of dredging on the number of individuals or component 

species is complicated further by the fact that some components of the benthos are likely 

to recolonise dredged deposits faster than others, and to therefore dominate the 

community in the initial phases of the recolonisation process. This process can result in 

a community that is initially dominated by a small variety but large numbers of mobile 

‘opportunistic’ species that are supplemented with time by other more slow-growing 

species that characterise the surrounding deposits. 

REFERENCE SITE SAMPLES

TREATMENT SITE SAMPLES

MARCH (Pre-dredging)

 S2 = 0.03

MAY (Post-dredging)

 S2 = 0.16

AUGUST (Post-dredging)

 S2 = 0.23

DECEMBER (Post-dredging)

 S2 = 0.09

Figure 8. Two-dimensional multidimensional scaling (MDS) ordination for the benthic 

communities in a Norfolk (UK) experimental survey area in March 1992 prior to dredging, and in 

May, August and December 1992. (After Kenny & Rees, 1994). 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 17
Figure 8 shows the output of a multidimensional scaling (MDS) ordination for macrofauna 

sampled before dredging of an experimental dredge site in the North Sea off the East 

coast of UK in March 1992, and in the 7 months after dredging (Kenny & Rees, 1994). 

Their work shows that the community at the site prior to dredging in March 1992 formed 

a small ‘cluster’ on the MDS-ordination. This indicates that the communities sampled 

within the experimental site were similar to one another, and were evidently also similar 

to those at the reference site since they are close to one another on the MDS-ordination 

shown in Figure 8. 

The experimental site was again sampled in May 1992, one month after completion of 

dredging. Figure 8 shows that the communities in all the samples from the dredged site 

were well-separated in the MDS-ordination from those recorded prior to dredging. This 

implies a major change in community composition within the dredged site following 

dredging. The communities within the dredged site were evidently very different from 

one another. This is indicated by the increased variance (S2) between samples and the 

wide spacing of the dredged samples on the MDS-ordination (see also Warwick & Clarke, 

1993). 

Much of the initial process of recolonisation and recovery of the benthic community 

composition at this site off the Norfolk coast was evidently accomplished within 7 months 

following cessation of dredging. Figure 8 shows that the community in the dredged area 

became more similar to those in the surrounding undredged deposits and to those in the 

pre-dredged deposits, and also had a closer internal similarity to one another (S2 

reduced to 0.09) in the months following cessation of dredging. This suggests that many 

of the commoner species present in the deposits prior to dredging in March 1992 had 

recolonised by December 2002. The clear difference from both the reference site and the 

community prior to dredging suggests, however, that many of the rarer components of 

the community had not yet colonised the dredged area in the following 7 months. 

Subsequent studies reported by Kenny & Rees (1996) suggested that the community 

composition at this site was not fully restored even 2 years after dredging, a result that 

has been confirmed in studies by others at several commercially-exploited dredge sites. 

Figure 9. Dominance curves for pooled samples of macrofauna within an anchor-dredged 

site and within a trailer-dredged part of North Nab Production Licence Area 122/3. Based 

on Newell et al, (2001b: see also Hitchcock et al, 2002). 

0

Species Rank

Cumulative Dominance %

80

60

40

20

0 10 100

Anchor - Dredged

Trailer - Dredged

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 18
The relative contribution of the component species to a community can be illustrated by 

means of ‘k-dominance curves’ (see Lambshead et al., 1983). These show the 

cumulative representation of each of the component species (Y-axis) plotted as a 

function of the number of species in the community (X-axis). The k-dominance curves in 

Figure 9 show the relative contribution of component species to the total species 

complement in an anchor-dredged site and in an adjacent trailer-dredged site at North 

Nab Production Licence Area 122/3 to the east of the Isle of Wight. The data show that 

the intensively exploited anchor-dredge site was dominated by one species (Balanus sp) 

which represented almost 80% of the species recorded in the deposits. In contrast, the 

adjacent trailer dredged site supported a wider variety of species which were less 

dominated by one or a few species (from Newell et al., 2001b, 2003; Hitchcock et al.

2002). 

The inference from these results is that the deposits within the anchor-dredged site were 

dominated by species capable of rapid recolonisation by planktonic larvae (see also Van 

Dalfsen et al., 2000) whereas the community in the less intensively exploited trailer 

dredged site had a wider variety of species reflecting the lower rate of removal by the 

drag-head of the dredger. The results show that a relatively low level of exploitation by 

trailer-dredging at the North Nab site had a smaller impact on the diversity of biological 

resources than intensive production by static dredging. 

Recent studies by Boyd & Rees (2003) and Boyd et al (2003) have confirmed and 

amplified the relationship between dredging intensity and impacts on benthic biological 

community composition. Boyd & Rees (2003) have shown that dredging intensity is an 

important determinand of macrofaunal community composition in actively-dredged sites 

in the English Channel to the east of the Isle of Wight, increasing dredging intensity 

resulting in an increase in the proportion of species affected. These impacts appear to 

persist for several years in the relatively stable deposits of the study areas in the English 

Channel, and may be related to restoration of complex features of the deposits such as 

stability of the seabed sediments (see also Kenny & Rees, 1996; Kenny et al., 1998, 

ICES, 2001). 

Boyd et al (2003) studied an area in the English Channel to the east of the Isle of Wight 

at which dredging had ceased 4 years previously. They showed that there were 

significant differences in the macrofaunal assemblages between areas subjected to 

different dredging intensities. As might be anticipated, the area that had been previously 

dredged at a high intensity had a reduced number of species and lower numbers of 

individuals than the surrounding deposits. As in the case of the experimental dredge site 

off the Norfolk coast referred to above, replicate samples taken from the formerly 

heavily-dredged site were dissimilar from one another in terms of species composition - 

a common feature of communities in disturbed habitats (see also Clarke & Warwick, 

1994; Kenny & Rees, 1994, 1996). Correlation analysis suggested that at this site, the 

dominant factor associated with macrofaunal community composition was dredging 

intensity in the area four years previously. Although the precise physical forcing 

functions are unknown at present, it is clear that the time required for restoration of 

community composition may depend on complex features of the seabed and may also 

vary according to the physical conditions to which the resident organisms are adapted. 

In wave-disturbed shallow water environments such as the North Sea, recolonisation by 

opportunistic species is reported to be rapid, with even the biomass of the benthic 

community being restored within 2-4 years (see van Dalfsen et al., 2000, Desprez, 

2000; Desprez & Duhamel, 1993; de Groot, 1979; Kenny et al., 1998; Newell et al., 

1998; Van Moorsel, 1993). In some areas of the North Sea which are subjected to 

natural disturbance of the sediments, and where the benthos comprises mainly small 

mobile (‘opportunistic’) species with a high rate of reproduction and growth, the rate of 

invasion of the deposits even within actively-dredged sites, is sufficiently fast that there 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 19
is no detectable net reduction in species diversity or population density compared with 

non-dredged deposits (Newell et al., 2002, see also Table 1). 

Elsewhere, in deeper waters or where wave energy is lower, dredge tracks may take 3-7 

years to infill and a correspondingly longer time is required before community 

composition approaches that of the undredged deposits (see Boyd et al., 2003). Coarse 

stable deposits are characterised by long-lived and slow-growing components which have 

a slow rate of reproduction (for review, see Newell et al., 1998). The benthic fauna in 

low-energy environments off the Mediterranean coast of Spain, for example, comprises a 

number of slow-growing components and recovery takes longer (van Dalfsen et al., 

2000). In such areas a “footprint” of impact on species variety, population density and 

biomass might be anticipated both within actively dredged sites and in the zone of 

deposition of material rejected during the screening process for several (or many) years 

after cessation of dredging. 

The following section reviews the nature and rate of the recovery process in different 

types of deposits including those which are more typical of the deep water shell gravels 

and coarser deposits that are commonly dredged for marine aggregates in the coastal 

waters of the United Kingdom. This information is then used to predict the likely rates of 

recovery of biological resources following cessation of dredging in coastal deposits of 

different composition. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 20
3. RATE OF RECOVERY FOLLOWING CESSATION OF DREDGING. 

The rate of recovery of biological resources following capital and maintenance dredging, 

disposal of dredged spoils and marine aggregate dredging has been widely studied and 

conforms with well-known general principles of ecological succession. That is, 

communities that inhabit fine semi-liquid and disturbed sediments comprise mobile 

opportunistic species (‘r-strategists’) that have a high rate of recolonisation and which 

can reach high population densities within weeks or months of a catastrophic mortality 

(see MacArthur, 1960; Grassle & Grassle, 1974; Osman, 1977). Conversely, 

communities that inhabit less disturbed deposits of deeper waters or coarse substrata 

have complex associations and are characterised by large slow-growing species that are 

selected for maximum competitive advantage in a habitat that is already crowded. These 

large slow-growing K-selected equilibrium species recolonise only slowly following 

disturbance and may take several (or many) years for recovery of full species 

composition and biomass (MacArthur & Wilson, 1967; Gadgil & Bossert, 1970; McCall, 

1976). These general features of community structure have been reviewed in relation to 

the impacts of marine aggregate dredging by Newell et al (1998). 

The question of ‘recovery’ of biological resources following cessation of dredging is not an 

easy one to define for complex communities whose composition can vary over time, even 

in areas that remain undisturbed. The situation is further complicated by the fact that 

the deposits in an area at which dredging has ceased may not be sufficiently similar 

following dredging to allow recolonisation and establishment of a similar biological 

community to that which occurred prior to removal of the coarse aggregate fraction. 

Estimates of the rate and nature of the recolonisation process may therefore be 

considered under two different scenarios:- 

• The deposits that remain after dredging has ceased are either sufficiently similar 

to the pre-dredge deposits to allow recolonisation by a similar biological 

community immediately, or sediment composition recovers following loss of 

overburden sands by tidal currents over a period of time. 

• The sediment composition of the deposits is permanently altered towards a more 

sandy substratum following removal of the coarse components and deposition of 

fine material rejected during the screening process. 

 

3.1. SCENARIO 1. 

THE SEDIMENT COMPOSITION OF THE DEPOSITS REMAINS SUBSTANTIALLY 

UNALTERED FOLLOWING CESSATION OF DREDGING. 

Table 2 shows the rates of recovery of benthic biological resources following dredging in 

various habitats. We have included semi-liquid muds from freshwater tidal areas and 

have arranged the data along a gradient of increasing environmental stability and 

predictability through estuarine and coastal muds to sands, gravels and reef 

assemblages. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 21
Table 2. Table showing the rates of recovery of the benthic fauna following dredging in various habitats. 

Examples have been arranged along a gradient from disturbed muds of freshwater-tidal estuarine conditions to 

stable reef assemblages. From Newell et al, (1998). 

LOCALITY HABITAT TYPE RECOVERY 

TIME

SOURCE

James River, Virginia Freshwater semi-liquid 

muds

±3 weeks Diaz, 1994

Coos Bay, Oregon Disturbed muds 4 weeks McCauley et al, 1977

Gulf of Cagliari, Sardinia Channel muds 6 months Pagliai et al, 1985

Mobile Bay, Alabama Channel muds 6 months Clarke et al, 1990

Chesapeake Bay Muds-sands 18 months Pfitzenmeyer, 1970

Goose Creek, Long Island, 

NY

Lagoon muds >11 months Kaplan et al, 1975

Klaver Bank, Dutch Sector, 

North Sea

Sands-gravels 1-2 years 

(ex-bivalves)

van Moorsel, 1994

North Sea (Area 408) Sands-gravels 1 year Newell et al, 2002

English Channel (North 

Nab)

Coarse gravel >2 years Newell et al, 2001b 

Hitchcock et al, 2002

Dieppe, France Sands-gravels >2 years Desprez, 1992

Lowestoft, Norfolk, UK Gravels >2 years Kenny & Rees, 1994, 

1996

Dutch Coastal Waters Sands 3 years de Groot, 1979, 1986

Tampa Bay, Florida Oyster shell (complete 

defaunation)

>4 years US Army Corps of 

Engineers, 1974

Tampa Bay, Florida Oyster shell (incomplete 

defaunation)

6-12 months Conner & Simon, 1979

Boca Ciega Bay, Florida Shells-sands 10 years Taylor & Saloman, 1968

Beaufort Sea Sands-gravels 12 years Wright, 1977

Florida Coral reefs >7 years Courtenay et al, 1972

Hawaii Coral reefs >5 years Maragos, 1979

Area 222 Isle of Wight, 

English Channel 

Gravel >4 years Boyd et al, 2003 

Inspection of the data summarised in Table 2 shows that the recovery of the benthic 

fauna in highly-disturbed semi-liquid muds can occur within weeks. This is associated 

with an ability for the resident species to migrate through the surrounding deposits and 

to recolonise disturbed muds as adults (see van Dolah et al., 1984); tidal currents may 

also transport juveniles into the dredged area (see Hall, 1994). However settlement of 

larvae from the plankton is probably of dominant importance in controlling the rate of 

recolonisation and sequence of colonising species in most coastal gravel deposits (Boyd & 

Rees, 2003). 

Inspection of the recolonisation rates reported in the literature and summarised in Table 

2 suggest that a period of 2-4y is a realistic estimate of the time required for recovery of 

species diversity and biomass of the benthic fauna in coastal gravels and sands, 

especially those where the resident organisms are ones that are naturally adapted to 

dynamic conditions in mobile deposits. But this time may be increased to more than 5y 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 22
in coarser deposits where the species diversity is high and where the resident organisms 

are mainly sessile with a relatively slower rate of growth and reproduction. 

The data also suggest that the intensity of dredging may have an influence on the rate of 

recovery of species diversity at some dredge sites. Areas in Tampa Bay, Florida that had 

been dredged for oyster shell, suggest that a period of as much as 10y may be required 

for recovery following complete defaunation whereas a recovery time of only 6-12 

months was required for recovery following partial dredging and incomplete defaunation 

(Benefield, 1976; Conner & Simon, 1979). This suggests that areas of undisturbed 

deposits between dredged furrows may provide a source of colonising species that 

enable faster recovery than might occur solely by larval settlement and growth from 

spawning adults located in the deposits surrounding the dredge site (see also van Dolah 

et al., 1984; van Moorsel, 1993, 1994). 

The likely recolonisation rates for the benthic community of estuarine muds, sands, 

gravels and reef areas have been superimposed onto a generalised colonisation 

succession in Figure 10. This allows some predictions to be made on the rates of 

recovery of deposits following dredging. 

Figure 10. Schematic diagram showing the likely recolonisation rates for the benthic community of 

estuarine muds, sands, gravels and rocky reefs. Based on Newell et al, (1998). 

The fine muds which characterise coastal embayments, estuaries and lagoons are likely 

to be colonised by large populations of a relatively restricted variety of "opportunistic" rselected species which are capable of rapid colonisation within months of space being 

made available for colonisation and growth. Because such deposits are subject to regular 

disturbance under natural conditions, the ecological succession recovers to the 

colonisation phase shown in Figure 10, but does not proceed to the development of Kselected slow-growing "equilibrium" species within the community. Recovery of the 

"normal" community in disturbed deposits such as muds can therefore be achieved 

within months of cessation of dredging, or disposal of spoils. 

ECOTONE POINT

NUMBER of SPECIES

BIOMASS

TIME (years)

ABUNDANCE

COLONISATION 

COMMUNITY

EQUILIBRIUM 

COMMUNITY

TRANSITIONAL 

COMMUNITY

NUMBER of INDIVIDUALS

0.1 1.0 10

% RECOVERY

20

40

60

80

100 ESTUARINE MUDS SANDS REEFS ENVIRONMENTAL DISTURBANCE

GRAVELS

PEAK of OPPORTUNISTS

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 23
The natural communities of gravel and sand deposits, however, contain varying 

proportions of slow-growing, K-selected equilibrium species, depending amongst other 

factors on the degree of disturbance by waves and the speed of tidal currents. In this 

case, the "tail" of the sigmoid recovery curve becomes more pronounced because the 

rarer components of the equilibrium community may take several years to recolonise the 

deposits, even after the main components of the community have become established. 

Where the deposits are sandy, periodic mortality of the long-lived components may 

result in major seasonal changes in community composition such as occurs in the North 

Sea on the Klaver Bank (van Moorsel, 1994), and as has been reported for the 

sediments of Liverpool Bay by Eagle (1975). Under these conditions, the community will 

be held in a transitional state by natural environmental disturbance, and is likely to 

recover within a period of 2-3 years after cessation of dredging. 

As might be expected, the recolonisation sequence is a good deal more complex than 

indicated in Table 3. Studies on the rate of recolonisation of sands and gravels following 

dredging off Norfolk, U.K. by Kenny & Rees (1994, 1996), off Dieppe, France by Desprez 

(2000), as well as at the North Nab site to the east of the Isle of Wight (Newell et al

2001b; Hitchcock et al., 2002) and at Area 408 in the central North Sea (Newell et al., 

2002) show that restoration of species richness and population density is achieved 

relatively rapidly compared with restoration of the biomass. 

Figure 11. Diagram showing the percent recovery of species richness, abundance and 

biomass of benthic fauna following cessation of dredging at a site off Dieppe, France. Based 

on Desprez (2000). 

Figure 11 shows that both species diversity and population density of benthic 

invertebrates was restored in deposits off Dieppe within 16 months after cessation of 

dredging. The biomass values were continuing to increase even 28 months after 

cessation of dredging at the Dieppe site. In contrast, in the North Sea site at Area 408 

restoration of biomass was complete after 12 months (Newell et al., 2002). 

0

16 months

28 months

0

20

40

60

80

100

120

Richness Abundance Biomass

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 24
A generalised scheme showing the likely recovery time for the benthos in shallow water 

coastal deposits, based on data from the North Nab study site is shown in Figure 12. 

RECOVERY OF 

BIOMASS OF 

DEPOSITS

RECOVERY OF 

POPULATION 

DENSITY

RECOVERY OF 

SPECIES 

DIVERSITY

INITIAL COLONISERS

small mobile species

larval recolonisation

ADDITIONAL 

COLONISING 

SPECIES

PROBABLE

RECOVERY

TIME

2 - 10 years

12 months

7 months

1 month

INCREASE IN SIZE

INCREASE IN INDIVIDUALS

INCREASE IN SPECIES

ADDITIONAL 

COLONISING 

SPECIES

Figure 12. Generalised sequence showing the nature and rate of recolonisation of benthic 

macrofauna in coastal deposits following cessation of dredging. This sequence is applicable only to 

mobile sandy gravels. Note that the recovery of long-lived components of the community can take 

more that 10 years in stable coarse deposits. Based on Newell et al, 2001b; see also Hitchcock et 

al, 2002. 

This indicates an initial colonisation by mobile ‘opportunistic’ species within days, or even 

during the dredging process. This can lead to communities dominated by mobile species 

such as the amphipod crustacean Ampelisca sp or 

by species such as barnacles that can rapidly 

recolonise from the plankton, in the initial phases 

of recolonisation of deposits disturbed by 

dredging. This phase is followed by an increasing 

variety of colonising species, an increase in the 

population densities of the component species and 

finally by growth of the individuals which leads to 

restoration of the biomass. 

Ampelisca sp ©MESL-PhotoLibrary 

Obviously, the rate at which recovery of the species diversity occurs is dependent on the 

complexity of the fauna and the inter-relationships that control larval recruitment and 

settlement. Many species do not recolonise regularly, and most require specific physicochemical and biological cues to induce settlement (for review, see Newell, 1979). This 

implies that even if the deposits in a dredge site after cessation of dredging remain 

similar to those prior to dredging, there may be a significant interval before all the 

species components are present in the community. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 25
The community recovery curve for stable reef communities shown in Figure 10 indicates 

that a period of at least 10-years may be required for the process of establishment and 

growth of the long-lived and slow-growing K-selected equilibrium species and for the 

development of the biological interactions which characterise undisturbed deposits. This 

long process of establishment of an equilibrium community reflects partly the time 

required for colonisation by rarer components of the community, but is also influenced 

by the nature and stability of the substratum following cessation of dredging, and the 

time required for complex stabilisation processes involving both physical compaction and 

biological interactions. 

Benthic communities in coarse stones and gravels generally comprise a significant 

proportion of sessile and slow-growing ‘equilibrium’ species 

characteristic of stable substrata. As an example of the 

likely time scale for recolonisation by one of the 

characteristic long-lived components of the community of 

marine gravels, we have recently analysed the sizefrequency distribution and age structure of the dog cockle 

(Glycymeris glycymeris) in a survey which covered much 

of the deposits characteristic of the central part of the 

eastern English Channel (MESL, 2002). This species is 

widely distributed in gravel deposits and is a typical 

component of biotopes described for the central part of the 

English Channel. 

Glycymeris glycymeris ©MESL-PhotoLibrary 

Figure 13a shows the relationship between the age and shell size of Glycymeris 

glycymeris within the area surveyed in August 2001. Shell heights for the population 

ranged from 1-6cm. The age of the shells can be estimated from growth bands and 

indicate that the largest members of the population are as much as 15 years in age. 

Figure 13a. Graph showing the relationship between the shell height (cm) and age (years) 

of the dog cockle Glycymeris glycymeris from deposits in the East Channel Region in August 

2001. Based on MESL, (2002). 

Glycymeris glycymeris Age/Size Data

0

2

4

6

8

10

12

14

16

0123456

Shell height (cm)

Age (years)

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 26
 

Figure 13b. Size-frequency histograms and cumulative curve showing the percentage 

occurrence of different sized Glycymeris glycymeris in deposits of the East Channel Region 

in August 2001. Based on MESL, (2002). 

Figure 13b shows that the dog cockle population comprises a large group of individuals 

of 5cm shell height (ie., approximately 14 years old) and another large proportion at 

2cm shell height (ie., approximately 5 years old). This implies that for this species, there 

was a major recruitment of young cockles 5-6 years prior to the survey in August 2001, 

and another major recruitment approximately 14 years prior to the survey. 

The population structure thus suggests that there is an intermittent pattern of settlement 

by post-larvae of Glycymeris glycymeris. These events evidently occurred mainly in 1987 

and 1996 in the East Channel Region survey area. Furthermore the relatively slow 

growth rate achieved by the oldest cohort in the population suggests that restoration of 

the biomass of this species is likely to require a period of at least 15 years after 

successful settlement by post-larvae from the plankton. 

If we assume that the deposits in the dredged area remain sufficiently similar to those 

prior to dredging to support the original benthic community type, it can be inferred that 

a period of 5-10 years might be required for initial establishment of a population of 

juvenile Glycymeris glycymeris and that a further period of 12-14 years would be 

required for restoration of the biomass of this component of the community. An estimate 

of 17-24 years for restoration of the population density and biomass of the slowestgrowing components of an equilibrium community conforms well with estimates based on 

the curve for reef communities shown in Figure 10. 

Clearly, many of the components of the benthic communities that occur in the sands and 

gravels of the East Channel region will have a shorter life-span and faster growth rate 

than the dog cockle. It is probably safe to assume that at least 50% of the species 

diversity, population density and biomass is likely to be restored within 4-6 years after 

cessation of dredging, with a gradual restoration of the full species complement and 

biomass in the following years.

Glycymeris glycymeris Size Frequency

0

5

10

15

20

25

30

35

40

123456

Shell height (cm)

% Occurrence

0

10

20

30

40

50

60

70

80

90

100

Cumulative %

% occurrence cumulative %

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 27
3.2. SCENARIO 2. 

THE SEDIMENT COMPOSITION OF THE DEPOSITS BECOMES SANDIER 

FOLLOWING REMOVAL OF COARSE COMPONENTS AND REJECTION OF FINES.

3.2.1. Short-Term Impacts.

The interaction between sediment composition and biological communities within a 

dredged area and in surrounding deposits where sedimentation of screened material 

occurs has been studied in relation to a marine aggregates dredging site off Dieppe, 

France by Desprez (2000).The effects of extraction within the boundaries of the dredge 

site itself were a decrease of species richness by 63%, 86% in abundance and 83% in 

biomass. His results also give an important indication of the type of changes in 

community composition that can occur following discharge of screened material to the 

sea bed. 

He reported that the structure of the community had fundamentally changed after 

several years of intensive extraction, with decreased densities of crustaceans, 

echinoderms and bivalves. The population density became 

dominated by errant polychaetes and the biomass by 

echinoderms. On a basis of a survey carried out in 1993, 

the benthic community within the dredged area off Dieppe 

had changed from one of coarse sands characterised by the 

lancelet Branchiostoma to one of fine sands dominated by 

the polychaetes Ophelia borealis, Nephtys cirrosa and 

Spiophanes bombyx, with the heart urchin Echinocardium

as a complementary characteristic species. 

Ophelia borealis ©MESL-PhotoLibrary 

The effects of sand deposition approximately 200m from the site of extraction were 

studied in 1996. The results are of considerable interest because they show that sand 

deposition surrounding the dredge site resulted in a greater impact on the benthic 

biological resources than dredging itself. Presumably this reflects the patchy impact of 

the drag-head within the dredge site, in contrast to the wider and more uniform impact 

of deposition of material rejected from the dredger. 

Table 3. Comparison of the percent composition of the sediments and main population parameters for the 

three sampling areas of the dredging site off Dieppe (in 1996). After Desprez, 2000. 

Dredging Area Deposition Area Reference Area

Shingles and gravels 26 11 47

Coarse sands 8 12 34

Fine sands 54 63 18

Very fine sands 19 13 1

Silts 1 1 0

Biomass (g.m-2) 2.4 0.3 6.8

Density (ind.m-2) 810 230 1440

Species richness 44 17 39

 

 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 28
Table 3 shows that significant differences exist between the sediment composition of the 

dredged area compared with that of the surrounding zone of deposition and with a nondredged reference area. There was an increase in fine sands in the deposition area 

compared with either the dredge area or a non-dredged reference area. The biomass of 

invertebrates was lower in the deposition area than in the dredged area, and this in turn 

was lower than in the non-dredged reference area. Table 3 shows that similar differences 

exist between the three areas in terms of both the number of individuals and the 

species richness of benthic fauna. 

 

The community composition of the benthic fauna in the zone of deposition was also 

different from that characteristic of gravels and shingles. The community in the sediment 

deposition zone was dominated by species 

characteristic of fine sands. These included 

the bivalve Tellina pygmaea (29%) and the 

annelid Nephtys cirrosa (22%) along with 

other sand-dwelling species such as the 

polychaetes Scoloplos armiger (3%) and 

Spiophanes bombyx. 

Scoloplos armiger ©MESL-PhotoLibrary 

Species that are characteristic of coarse sands comprised only 1% of the community and 

included the echinoderms Echinocyamus pusillus and 

Amphipholis squamata and species characteristic of 

gravels were absent. 

Echinocyamus pusillus ©MESL-PhotoLibrary 

These results show that the impacts of deposition of material rejected overboard during 

the screening process can have a significant impact on both nature and abundance of 

benthic macrofauna. 

3.2.2. Long-Term Impacts. 

It is well-known that the species variety of benthic communities in mobile sands is often 

sparse compared with stable communities such as occur on coarse gravels and reefs. 

This can be only partially accounted for in terms of differences in the particle size 

composition of the sediments (Seiderer & Newell, 1999; Newell et al., 2001a). 

Differences between the biological communities recorded in sandy deposits and in 

gravels can, however, be used to make some predictions on the type of marine 

communities which might be anticipated if there were a permanent or long-term 

alteration in sediment type associated with the dredging and extraction process and with 

discharge of screened material. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 29
Figure 14. K-dominance curves for the 3 main macrofaunal communities identified by 

multi-variate analysis of the fauna in deposits of Licence Application Areas 458 & 464 

(West Bassurelle). MESL, (1999). 

Figure 14 shows a series of k-dominance curves for the three principal communities of 

benthic macrofauna identified in a baseline survey of West Bassurelle Licence Application 

Areas 458 & 464 (MESL, 1999). Curve A is for a community that characterised sandy 

deposits within the survey area, Curve B characterised communities that occurred in 

gravely sands whilst Curve C represents a community that occurred in coarse gravels 

within the survey area. 

The total number of species recorded for the sandy community (curve A) was 13 species, 

that for the gravely sand was 63 species, and that for the coarser gravel deposits was 

155 species per 0.2 m2 Hamon grab sample (Recalculated from MESL, 1999). One 

species, a paddle worm (Eteone sp) accounted for approximately 22% of the community 

of the sandy and mixed sands and gravels whereas this same species formed a smaller 

proportion of the wide species variety recorded in gravels. 

The sequence of change in species diversity and community composition associated with 

a transition from coarse deposits to ones that are dominated by sand-sized particles is 

shown in Figure 15. The upper part of the figure shows a two-dimensional MultiDimensional Scaling (MDS) ordination for the biological communities of West Bassurelle 

Licence Application Areas 458 & 464. Superimposed on the ordination is the relative 

abundance of gravel-sized particles of 4.0mm diameter and above where the size of the 

symbol represents the relative proportion of gravel at each site. Also shown in Figure 15 

is a list in order of importance of the genera that account for 75% of the similarity of 

each of the three communities of macrofauna identified in the survey area. Cumulative Dominance %

Species rank

0

20

40

60

80

100

1 10 100 1000

C

B

A 13 spp

63 spp

155 spp

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 30
Figure 15. Two-dimensional multi-dimensional scaling (MDS) ordination for the macrofauna 

assemblages in the West Bassurelle Areas 458 & 464. Circles show the relative proportion of 

particles >4mm at each site. The lower part of the figure shows the genera of macrofauna 

accounting for 75% of the similarity of the communities in gravel (Group C), in sandy gravel 

(Group B) and in sandy deposits (Group A). 

Figure 15 shows a wide range of species including polychaetes, crustaceans and 

echinoderms characterised the gravel community (Group C) whereas the mixed sandy 

gravel deposits were characterised by fewer species accounting for 75% of the similarity 

within the community. Finally the Group A community which characterised sandy 

deposits within the survey area, was dominated mainly by a few species of polychaete 

worms. 

These data suggest that if marine aggregate extraction and discharge of screened 

material results in an alteration of sediment composition from coarse deposits towards 

sandy ones, then this is likely to be associated with a decrease in species diversity from 

approximately 155 species to only 13 species per 0.2 m2 within the zone of sediment 

deposition, and by an increased dominance by components of the community such as 

polychaetes that can survive in sandy deposits. 

Stress = 0.21

B

C

A

Sand - Sandy Gravel - Gravel - Coarse Gravel

Eteone

Galathea

Syllis

Prionospio

Echinocyamus

Scalibregma

Lumbrineris

Pom atoceros

Glycera

Notom astus

Pionosyllis

Nemertea

Psamm echinus

Aonides

Odontosyllis

Am pharete

Poecilochaetus

Eunice

Golfingia

Maera

Amphipholis

Typosyllis

Harmothoe

Abietinaria

Hydrallmania

Exogone

Moerella

Eteone

Syllis

Glycera

Eurydice

Glycera

Aonides

Exogone

Echinocyamus

Glycymeris

Notomastus

Nemertea

Nereis

Eteone

Glycera

Pionosyllis

Nemertea

Genera representing 75% of the complement of each community

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Marine Ecological Surveys Limited - October 2003 31
In high-energy habitats such as the North Sea, winnowing of the sediments in the 

dredged site and sediment transport from outside the boundaries of the dredged area 

may result in sufficient restoration of particle size composition of the deposits to allow 

substantial restoration of the benthic community composition following cessation of 

dredging. This appears to be the case at the experimental dredge site studied off the 

east coast of Norfolk, UK by Kenny & Rees (1994. 1996; see Figure 8). At this and most 

dredge sites, however, the gravel-sized fraction is not capable of being mobilised by 

waves and tidal currents at the seabed, so dredge trails and pits left from marine 

aggregate dredging will generally be infilled by fine sand and silt-sized material 

depending on the depth of water and prevalent wind and current conditions (see also 

Dickson & Lee, 1972; McGrorty & Reading, 1984; Millner et al., 1977; van der Veer et 

al., 1985). The seabed deposits in dredged sites are therefore generally unlikely to 

recover to their pre-dredge particle size composition unless this is artificially adjusted by 

deposition of surface aggregate. 

 

The main conclusions currently available from the results of work carried out on 

biological communities within dredge sites in UK waters may be summarised as follows:- 

• The process of dredging can result in a 50-90% reduction of species richness, 

population density and biomass of benthic invertebrates within the boundaries of the 

dredged area. 

• In wave disturbed sites the natural population of benthic organisms comprises 

‘opportunistic’ species that are well-adapted for recolonisation and growth in deposits 

that are disturbed. This leads to rapid recolonisation of deposits that are disturbed by 

dredging to an extent that at some sites the rate of restoration of species richness and 

population density within a dredge site is evidently in equilibrium with the rate of 

removal by dredging. The net impact of dredging within the boundaries of a dredge site 

thus reflects an equilibrium between the rate of colonisation and the intensity of 

dredging at a particular site. 

• Restoration of biomass is achieved by growth of the colonising species. This is always 

slower than initial restoration of species composition and population density. Generally 

a time of 2-4 years is characteristic of restoration of biomass in shallow water 

environments, but a period of 10-20 years may be required for some of the slowest 

growing components of stable ‘equilibrium’ communities which have a wide species 

variety and which are characterised by a slow rate of growth and reproduction. 

• The process of removal of coarse material from a dredge site and the return of sandsized particles following screening can result in long-term changes in the particle size 

composition of the dredged deposits. Studies off Dieppe suggest that the deposition of 

sand from the dredging process results in a greater suppression of species variety, 

population density and biomass than occurs from the dredging itself. This suppression 

of the benthos is associated with significant changes in community composition, leading 

to an impoverished community that is dominated by polychaete worms. 

• Differences in the species richness and community composition is gravels and sands of 

the East Channel Region allow some realistic estimates of the changes which might be 

anticipated in that area if dredging and screening were associated with a long-term 

shift in particle size composition towards more sandy deposits. The results suggest that 

the community would change from one with at least 155 species per 0.2 m2 to one with 

only 13 species per 0.2 m2 and that the sandy community would be dominated by 

polychaetes. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 32
3.2.3. Approaches to the Assessment of ‘Recovery’. 

The results summarised above raise the question of whether it is necessarily a practical 

objective to expect benthic communities to revert to the same community composition to 

that which existed in the deposits prior to dredging. We know that removal of the coarse 

fraction and return of fine sands to the seabed will result in a change in the particle size 

distribution of the deposits in a relatively small area of seabed corresponding with the 

dredged site and zone of deposition and transport of material. It is also well-established 

that sandy deposits tend to have a reduced species diversity compared with more 

complex habitats including cobbles and gravels. It is therefore to be expected that 

benthic communities in sites from which the coarse fraction of the deposits has been 

selectively removed are likely to be different from those prior to dredging. 

One approach to the question of ‘recovery’ of benthic biological resources is to define 

‘recovery’ as the establishment of a community that is capable of maintaining itself, and 

in which at least 80% of the species diversity and biomass has been restored (Newell et 

al., 1998). This implies a substantial restoration of the carrying capacity of the benthic 

food webs leading to fish, even though the precise composition of the benthic community 

may not be identical to that recorded in the pre-dredged system. 

Ellis (1998, 2003) has developed the concept of ‘sustainable ecological succession’ in 

assessing the recovery of seabed biodiversity in relation to mine tailings disposal in fjord 

sediments off Vancouver Island, Canada. This is based on the recognition that:- 

• once ecological succession is established, it will progress to an eventual complex 

and variable climax community in equilibrium with a range of features of the 

habitat unless setback by another perturbation. 

• measures of sustained succession are easier to obtain and clearer to interpret 

than measures of climax community establishment and its range of variability. 

He reported that effective prediction of the time required for sustainable restoration of 

benthic communities at the Island Copper Mine submarine tailings placement site during 

three years of monitoring following closure of the mine could be achieved using two 

criteria:- 

• the numbers of species and the total number of organisms must fall within, or 

above the ranges at unaffected stations (at the Island Copper Mine site the values 

were 20 or more species per 0.15m2, and more than 1000 individuals per m2). 

• several rapidly colonising (opportunistic) species must have sustained themselves 

in large numbers for one or more years (at the Island Copper Mine site values 

were more than 3 species had sustained themselves at a population density of 

>100 per m2 for 2 or more years). 

Although the criteria used to define ‘recovery’ will be different for other locations and 

ecosystems, this does represent a practical approach to defining what is meant by 

‘recovery’ in the context of biological communities that vary in space and time, and 

where the environment may in any case have been significantly modified by dredging 

activities on the seabed. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 33
4. IMPACTS OUTSIDE THE BOUNDARIES OF DREDGED AREAS. 

The effects of deposition of sand-sized particles on the benthic macrofauna 

approximately 200m outside the boundaries of a dredge site off Dieppe, France, based 

on the work of Desprez (2000) have been summarised in Section 3. These agree well 

with the differences between in situ biological communities in sands and gravels 

elsewhere in the East Channel Region, and probably provide a reliable basis for 

predicting the impacts of the dredging and recovery processes within the boundaries of 

individual dredge sites and in the immediate zone of deposition of material rejected 

during the screening process. 

Generalised predictions of the biological impact of discharge of sand-sized particles 

outside the boundaries of dredged sites following screening depend to a large extent on 

whether such material settles over a similar area to that described for shallower waters, 

and the rate at which the sand is transported away from the initial site of deposition. 

Currently our best estimate based on work summarised above is that there is likely to be 

a zone of approximately 200m width and up to 800m length, reflecting a zone of primary 

deposition and extending in each direction from the dredge site along the axis of the tidal 

streams. This material may then be remobilised and transported away from the site of 

initial deposition, with the possibility of impacts on benthic communities where these lie 

in the path of the transported material. 

We can be reasonably confident that long-term changes in sediment composition from 

gravels to more sandy deposits in the immediate zone of sediment deposition 

surrounding a site of dredging and screening will result in a paucity of benthic 

macrofauna and a change in community composition to one that is dominated by 

polychaetes. There is, however, much less information on whether the subsequent 

remobilisation and transport of material from the near-site deposition areas elsewhere 

along the tidal current stream is likely to have an impact on biological communities. 

Many of the macrofauna that live in areas of sediment disturbance are well-adapted to 

burrow back to the surface following initial burial (Schafer, 1972). Studies by Maurer et 

al (1979) showed that some benthic animals could migrate vertically through more than 

30 cm of deposited sediments, and this ability may be widespread even in relatively deep 

waters, as well as in estuarine sediments. Kukert (1991) showed for example, that 

approximately 50% of the macrofauna of the bathyal sea floor of the Santa Catalina 

Basin were able to burrow back to the surface through 4-10cm of rapidly deposited 

sediment. 

More recently, Elliott et al (2001) showed that many estuarine invertebrate species are 

able to survive relatively high rates of sedimentation, the rate of survival being 

dependent on the depth of sediment. As an example, survival of the bivalve Macoma 

balthica was as high as 95% following deposition of 1-7cm depth of mud. Under a single 

deposition of 15cm mud only 66% survived whilst only 25% survived burial by 25cm 

mud. Other studies also summarised in Elliott et al

(2001) suggest that the polychaete Hediste diversicolor, 

and molluscs such as Retusa obtusata and Hydrobia 

ulvae, as well as Oligochaete and Nematode worms are 

capable of migrating up through as much as 20cm of 

dredged material. Survivorship in these species was 

reported to be >90% irrespective of whether the 

sediment was added as a single layer of up to 20cm 

depth, or whether it was deposited at intervals of 4 days 

to give a total of 20cm of consolidated sediment. 

Macoma balthica ©MESL-PhotoLibrary 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 34
A recent study carried out by Newell et al (2002) in the vicinity of a heavily screened 

dredge site in the North Sea at Production Licence Area 408 was partly designed to 

investigate whether there was any evidence of impact of discharge of screened material 

on benthic biological resources surrounding the dredge site. Quantitative estimates were 

made of the species composition, population density and biomass of benthic 

invertebrates at a series of as many as 167 sampling stations in August 2000 including 

areas that had been dredged, those which had been abandoned for known times, and 

along the axis of the tidal streams from these dredge sites. The results showed that 

although the species richness and population density were evidently restored rapidly in 

the mobile deposits of the survey area, the dredged site was characterised by an 82% 

suppression of biomass, reflecting the presence of small colonising species. 

The biomass of benthos in previously-dredged sites that had been abandoned for 12 

months prior to the survey in August 2000, was generally similar to that in “control” 

sites. This suggests that recolonisation and subsequent growth of the benthos was 

substantially complete 12 months after cessation of dredging at this particular site. Of 

particular interest is that “non-dredged” sites outside the boundaries of the dredged 

areas, but within the zone of potential impact of fine sands mobilised by the screening 

process, also show a significant suppression of biomass. The average biomass for such 

sites was 0.4356 g (AFDW) compared with 1.2763 g (AFDW) per 0.1 m2 for the control 

sites. That is, the zone of deposition and transport of material outside the boundaries of 

the dredged sites was associated with a suppression of biomass by approximately 66%. 

The study at Licence Area 408 showed that this zone of suppression of biomass extended 

for as much as 3km to the south-east of the actively dredged site, but for only 100m to 

the north-west. This is consistent with the net south-east transport of sand rejected 

during the screening process. It suggests that even in deposits where the marine 

community is well-adapted to rapid recolonisation and growth, there is evidence of a 

residual “footprint” on the biomass of the benthos for up to 3km from the dredge site. 

This corresponds in general with the zone of settlement and transport of material 

rejected during the screening process (see also Section 1.2.). 

We are not aware of any other studies which provide sufficient information to assess the 

likely far-field impacts of marine aggregate dredging and overboard screening on benthic 

biological resources apart from those cited above for a dredge site off Dieppe (Desprez, 

2000) and that at Area 408 in the North Sea (Newell et al., 2002). Studies adjacent to a 

sand dredge site at Moreton Bay, Queensland, Australia by Poiner & Kennedy (1984) 

have shown, however, that there can be an enrichment of species diversity and 

population density of benthos immediately outside the boundaries of the dredged area. 

They attributed this to the effects of enrichment from organic matter released from the 

sediments during the dredging process. Similar enrichment of benthos along the axis of 

the tidal stream adjacent to a dredged site at North Nab has recently been described by 

Newell et al.(2001b; also Hitchcock 2002), as well as at other sites off the east coast of 

the Isle of Wight in the eastern English Channel (Boyd & Rees, 2003) and close to a 

dredge site at Area 408 in the North Sea (Newell et al., 2002). This may reflect 

settlement of organic matter derived from benthos fragmented during the dredging 

process (Newell et al., 1999), or an impact of organic matter released at the sedimentwater interface. 

The results cited above are for relatively shallow water sites where the sediments are 

subject to natural disturbance and where the fauna is adapted for rapid recolonisation 

and growth. Any assessment of the likely scale of impacts on the stable equilibrium 

communities that characterise the coarse shell gravels and current-swept reefs and 

cobbles elsewhere and in deeper waters is therefore largely anecdotal. Studies of the 

macrofauna of reefs and stones in the central English Channel by Holme & Wilson (1985) 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 35
show that the anemone Urticina felina, various hydroids, and the bryozoan Flustra 

foliacea are able to withstand abrasion by mobilised 

sand and intermittent burial. Urticina felina can 

extend its column to maintain its disc above the sand 

surface and similar behaviour has been described for 

the anemone Anthopleura elegantissima (Taylor & 

Littler, 1982; Littler et al., 1983). 

Flustra foliacea ©MESL-PhotoLibrary 

Holme & Wilson (1985) also showed, however, that areas of abrasion by mobilised sand 

are associated with relatively impoverished epifaunal communities compared with rocks 

and reefs which are not subject to sand scour. Some impacts of mobilised sand on the 

epifaunal communities associated with rocks and reefs in the vicinity of dredge sites are 

therefore to be anticipated, although the nature and scale of any such impacts are 

unclear, particularly in view of the complexity of the faunal associations described for 

some of the adjacent habitats of the eastern English Channel and elsewhere (see 

Sanvicente-Añorve et al. 1996). 

The possible impacts of marine aggregate dredging on biological resources outside the 

boundaries of the dredged area, based on studies in relatively shallow water coastal 

sites, may be summarised as follows:- 

• Best estimates of the size of the zone of sand deposition surrounding a dredged area is 

that the screened material will be deposited as an ellipse of approximately 200m width 

and extending for up to 600-800m in each direction along the length of the tidal stream 

• Within this zone there is likely to be an impoverished fauna in terms of species 

richness, population density and biomass. The community composition is also likely to 

be different from that in gravel deposits and to be dominated by polychaete worms. 

• Little is known of the fate of this material following initial sedimentation. Studies in the 

North Sea and on the south coast of UK show that there is a benthic plume at the 

sediment-water interface that may extend for up to one tidal excursion in each 

direction along the axis of the tidal stream from the site of dredging. But there is very 

little information on whether this is likely to occur at other dredge sites, or the extent 

to which it is related to the return of screened material to the seabed during the 

dredging process. 

• Studies on the benthos associated with a heavily screened site at Production Licence 

Area 408 in the North Sea has established an impact of screened material on the 

biomass of benthic invertebrates up to a distance of approximately 3km along the axis 

of net sediment transport from the dredge area.

• We are unable to provide firm estimates of the possible impacts of abrasion and 

possible intermittent submersion by sand mobilised from the near-site sedimentation 

zone on the relatively complex “equilibrium” communities that characterise stable reefs 

and gravels that occur near to some dredge sites. This is because the impacts on 

community structure are likely to be complex. However the “footprint” of impact on 

biological communities is likely to extend for up to 3km along the axis of transport of 

material from the dredge sites, based on results for the North Sea. 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 36
5. CUMULATIVE IMPACTS. 

Potential cumulative effects of marine aggregate dredging have been reviewed in a 

Report by Oakwood Environmental (1999). This review includes recommendations on 

existing policy framework, a framework for the assessment of cumulative effects on 

marine and coastal environments, and a review of ‘Good Practice Methodology’ for the 

assessment of cumulative impacts on biological and physical resources. The review also 

includes the results of impact assessments for a Pilot Study Area on the south coast of 

U.K. The reader is referred to this review for general aspects of cumulative impacts 

associated with marine aggregate dredging in coastal waters. 

Our best estimate of the likely cumulative impacts of marine aggregate dredging, based 

on what is known for the impacts and rates of recovery for the coastal Production 

Licence areas described in the previous sections is as follows:- 

• Within the boundaries of any one production area, there is likely to be a 50-90% 

reduction in species richness, population density and biomass of marine invertebrates. 

• This impact is likely to extend for a distance of at least 400-600m outside the 

boundaries of each dredge site along the axis of initial settlement of reject material. 

• It is possible that dredging may also be associated with production of a benthic plume 

at the sea bed and extending for up to one tidal cycle along the axis of the tidal 

currents. Studies to date suggest that dredging is associated with a detectable impact 

on the biomass of benthic fauna for up to 3km along the axis of net tidal transport of 

sediments mobilised during the screening process. 

• Recovery of biological community composition is generally initiated by an increase in 

species diversity and population density and is followed only later by restoration of 

biomass. Because the species characteristic of ‘equilibrium’ communities in coarse 

stable deposits have a slow rate of reproduction and growth, the process of restoration 

of community structure and biomass of the benthos is likely to take as much as 15-20 

years for some long-lived components. 

• In its simplest form, therefore the cumulative area of impact of the proposed dredging 

works for any one year can be estimated from the area dredged within each Licence 

Area plus the area likely to be affected outside the boundaries of the dredge site at 

each side of the tidal stream multiplied by the number of sites dredged. 

• The cumulative area impacted over time will be the figure for one year, multiplied by 

the number of years required for recovery. It is likely that the earliest dredged sites will 

be in a stage of partial recovery within months, but species such as the dog cockle 

(Glycymeris glycymeris) may take as much as 15-20 years for colonisation and 

restoration of biomass following cessation of dredging. We estimate that at least 50% 

of the species variety, population density and biomass is likely to be achieved within 4-

6 years after cessation of dredging, even in coarse deposits characterised by 

equilibrium communities. 

• Added to this likely impact zone are the (unknown) potential impacts of seabed 

sediment plumes on both the physical features of the sea bed, and on the biological 

resources including fish eggs. Not enough is known to assess whether such plumes 

have a potential impact on seabed resources. However the fact that benthic plumes 

may extend well outside the boundaries of the immediate deposition zone implies that 

the areas of impact from adjacent dredge sites may overlap one another at some 

stages of the tidal cycle.

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 37
• We have also provided estimates of what might be anticipated if the sediment 

composition of the worked areas and surrounding deposits were to become sandier 

following removal of the coarse components and discharge of fine components by 

overboard screening. Again there is good evidence from both studies of impacts in 

nearshore Production Licence areas, and from communities resident in more stable 

offshore sands and gravels, that discharge of sands has an important impact on both 

the richness and community structure of the benthos.

• Reversion towards a sandy deposit is likely to result in a suppression of biodiversity and 

the replacement of a heterogeneous assemblage of benthic invertebrates to one which 

is dominated by a small species variety comprising mainly polychaetes. The area 

affected in any one year is likely to be similar to that estimated above.

• However the rate of recovery of sandy deposits is faster than in complex ‘equilibrium’ 

communities, so a less-diverse sandy substrate community may come into equilibrium 

with the new environmental conditions within a period of 2-4 years, based on results 

for sandy gravels in the North Sea.

6. IMPACTS ON FISHERIES. 

The impacts of suspended sediments on a wide variety of animals including plankton, 

benthic invertebrates and fish species has been reviewed by Sherk (1971) and Moore 

(1977). Early studies by Loosanoff (1962; see also Collinson & Rees, 1978) showed that 

different species of commercially significant filter-feeding molluscs were differently 

affected by suspended sediment. Subsequent studies by Sherk (1971) and Sherk et al 

(1974) showed that, as in the case of bivalves, fish species have varying tolerances of 

suspended solids, filter-feeding species being more sensitive than deposit-feeders and 

larval forms being more sensitive than adults (see also Matsumoto, 1984). 

Estimates based on trophic food web models suggest that as much as 30% of the total 

exploitable fish yield to man in waters of the North Sea are derived from benthic food 

webs (Steele, 1965; Newell et al., 1998). Indirect effects on fish stocks thus include a 

reduction or alteration in the food available from benthic resources (Daan et al., 1990), 

as well as potential direct effects on vulnerable stages of the life cycle such as the eggs 

and larvae. It should also be noted that a reversion 

from a mixed invertebrate assemblage which is 

characterised by a high species diversity, towards one 

which is dominated by a less diverse community that 

is dominated by polychaetes, may not necessarily 

result in a loss of commercially significant fish stocks 

(Millner et al., 1977). Fish are opportunistic feeders, 

and a relatively uniform food availability comprising 

mainly polychaetes may enhance some stocks such as 

Dover sole (Solea solea) at the expense of others. 

Solea solea ©MESL-PhotoLibrary 

Impacts of Marine Aggregate Dredging

Marine Ecological Surveys Limited - October 2003 38
The likely impacts of marine aggregate dredging on fisheries resources have been 

summarised by Desprez (2000). He concluded that in general, fish are less affected by 

dredging activities than shellfish and other sessile benthic species because fish can evade 

the area of disturbance. However, some fish species, particularly demersal spawners 

such as herring and sandeel, may be vulnerable to damage through smothering of fish 

eggs on the spawning grounds (Westerberg et al., 1996). Changes in the sea bed 

topography caused by dredging and by exposure of oversize material on the sea bed 

may also have an impact on the suitability of the sea bed for subsequent commercial 

fishing activities (de Groot, 1979), although the presence of more heterogeneous 

conditions on the sea bed may also favour the creation of new habitats for epibenthos 

and fish communities (Desprez, 2000). 

The consensus view is that the impacts on commercial fishing activities are mainly 

related to exclusion from traditional fishing grounds, and to potential losses of eggs and 

larvae of demersal species, rather than to damage of commercially significant target 

species. It should however be noted that alteration in sediment composition may have 

complex effects not only on benthic food resources leading to fish, but may also inhibit 

settlement and survival of larvae of commercially significant shellfish. Partly for this 

reason, any ‘Risk Assessment’ for fish and shellfish resources needs to include a wide 

range of potential impacts as described in Section 8. 

7. ASSIGNING SIGNIFICANCE TO IMPACTS. 

Assigning significance to impacts on environmental resources is essentially a subjective 

judgement based on the professional experience and objectivity of those involved. It is 

therefore important to present as much information as possible on the reasons for 

arriving at a particular assessment for each of the environmental resources concerned. 

The following sections summarise the key features of environmental resources that can 

be used in the assessment of potential impacts of marine aggregate dredging. Where 

appropriate, the following criteria should be taken into account:- 

The Extent of Impact: 

! A Small Impact - localised within the immediate dredge site. 

! A Limited Impact - over an area extending up to 1km from the dredge site. 

! A Local Impact - extending up to 5km. 

! A Regional Impact - an impact over a relatively wide area >10km. 

The Duration of Impact:

! A Temporary Impact - existing for less than 1 year.

! A Short-Term Impact - existing for 1-5 years.

! A Medium Term Impact - existing for 5-10 years.

! A Long-Term Impact - existing for more than 10 years.

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The significance ratings assigned to the impacts can be as follows:- 

An Impact is of High Significance if:- 

! the extent is regional 

! the duration is long-term 

! the impact is on species or communities afforded Statutory protection 

! the impact is on resources of high economic or conservation significance 

An Impact is of Moderate Significance if:- 

! the extent is local 

! the duration is medium-term 

! the impact is on species or communities afforded Statutory protection 

! the impact is on resources of high economic or conservation significance 

An Impact is of Low Significance if:- 

! the extent is small or limited 

! the duration is temporary 

! there is unlikely to be an impact on species or communities afforded Statutory 

protection. 

! there is unlikely to be an impact on resources of high economic or conservation 

significance 

An Impact is of No Significance if:- 

! there is no predicted effect on environmental resources.

The significance of the impacts can then be incorporated into a tabular summary of 

impacts and used as the basis of a Risk Assessment for each of the main environmental 

resources located in the vicinity of a particular dredge site. The Risk Assessment process 

takes into account both the sensitivity of the resource in question, and the actual 

vulnerability of the resource based on its distribution in relation to the dredge site.

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8. RISK ASSESSMENT. 

There have been a number of proposals to derive a numerical assessment of ‘Risk’ to 

environmental resources based on what is known of their sensitivity to disturbance, and 

the likely scale and sources of impact by man (Department of the Environment, 1995; 

Department of the Environment, Transport & the Regions, 2000; Associated British 

Ports, 1997). Estimates of Risk for activities such as marine aggregate dredging to 

important economic resources such as fisheries can be derived from a matrix that relates 

the potential sensitivity of species or communities to each component impact of the 

dredging operation and the actual vulnerability at a particular site (Carlin & Rogers, 

2002). In other cases, impacts on resources of conservation significance have been 

related to both biotope sensitivity and recoverability (MarLIN, 2003). 

8.1. RISK ASSESSMENT FOR COMMUNITIES & HABITATS

The sensitivity of particular communities or species to the relatively complex impacts 

imposed by man are not easy to quantify, and often involve a subjective assessment 

based on experience and judgement. In the case of biotopes, sensitivity can be defined 

in terms of species that are considered to be important components of the community as 

follows:- 

T Key Structural Species. Species that provide a distinct habitat that supports an 

associated community. Loss or degradation of the species would result in a loss or 

degradation of the biotope, eg. Sabellaria spinulosa. 

T Key Functional Species. Species maintaining community structure and function through 

interactions with other members of the community (eg by predation, grazing and 

competition). Loss or degradation results in change to the biotope. 

T Important Characterising Species. The species are characteristic of the biotope and are 

important in the classification of the biotope. Loss or degradation would result in a loss of 

the biotope. 

T Important Structural Species. The species which interact with the key or characterising 

species and are important for their viability. Loss of these species may reduce the viability 

of the key, or characterising species. Structural species may prey on epiphytes and 

parasites of the key characterising species. 

 

T Important Functional Species. These are the dominant source of organic matter or 

primary production within the ecosystem. Loss could result in changes in community 

function and structure. 

The following scales have been used to define the sensitivity and recoverability of marine 

biotopes to disturbance by man:- 

Biotope Intolerance Scale (based on MarLIN 2003). 

 

• High Intolerance - Key structural or functional species are likely to be killed and/or the 

habitat is likely to be destroyed. 

• Moderate Intolerance - The populations of key structural or functional species may be 

reduced or degraded, the habitat may be partially destroyed, or the diversity and 

population density of a community may be reduced. 

• Low Intolerance - Key structural or functional species are unlikely to be killed, but the 

viability, diversity and functionality of a community may be reduced. 

• No Intolerance - The factor has no detectable effect on structure and functioning of a 

biotope or the survival and viability of key structural or functional species. 

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Biotope Recoverability Scale (based on MarLIN 2003). 

• None - Recovery is not possible. 

• Very Low - Partial recovery in 10 years, but full recovery time may be at least 25 years, 

or never. 

• Low - Partial recovery in 10 years, but full recovery time up to 25 years. 

• Moderate - Partial recovery within 5 years and full recovery up to 5-10 years. 

• High - Full recovery complete within 5 years. 

• Very High - Full recovery is within 6 months. 

• Immediate - Full recovery within a few days. 

The ‘Intolerance’ and ‘Recoverability’ can then be combined into a single scale that can 

be used to give some indication of the sensitivity of marine communities to 

environmental change. A ‘Sensitive’ community or habitat may then be regarded as one 

that is easily adversely affected by human activity, and is expected to recover only over 

a long period of time. This method of risk assessment has not, so far been widely applied 

to marine habitats and communities. 

8.2. RISK ASSESSMENT FOR FISHERIES RESOURCES

A rather similar approach of combining the perceived ‘sensitivity’ of marine resources 

with the actual vulnerability at any particular site has been proposed for risk assessment 

of Fisheries resources by Carlin & Rogers (2002). They propose a matrix that relates the 

potential sensitivity of particular fish components or stages in the life cycle to the actual 

vulnerability based on the occurrence in a particular area. The scales in this case for both 

sensitivity and vulnerability are:- 

! Very high 

! High 

! Moderate 

! Low 

They propose that the risk assessment for Fisheries should be assessed under the 

following key headings:- 

1. Temporal & Spatial Scale of the Operation. 

2. The Method of Aggregate Extraction. 

3. Plume Effects. 

4. Cumulative Effects. 

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Impacts on the following need to be listed under each of the above headings:- 

 

1. The Benthic Fish Community. 

2. Breeding & Spawning Grounds. 

3. Nursery Grounds. 

4. Over-wintering Grounds. 

5. Migratory Routes. 

6. Reduction in Income. 

The Fisheries Impact Assessment can then be summarised into an Evaluation Protocol as 

outlined by Carlin & Rogers (2002). This comprises a table summarising the potential 

sensitivities of the fisheries resources to potential impacts of marine aggregate dredging. 

The data for a typical North Sea aggregate licence area are summarised in Table 4. 

Table 4. Evaluation Protocol for Fisheries Resource Risk Assessment at a typical aggregate dredging area in the 

southern North Sea.

1. Temporal and Spatial Scale of the Operation

Benthic Community Survey carried out at 38 sites with triplicate samples at 12 sites. Trawl 

samples taken at 14 sites. Benthic community rich with an average of 40 

species and 470 individuals per 0.1 m2. Sabellaria present mainly as isolated 

tubes, but in large quantities possibly forming biogenic reef structures mainly 

well outside boundaries of the dredge site.

Breeding & Spawning 

grounds

Not known to be a spawning ground for commercial fish. Possibly suited for 

herring outside boundaries of the dredge site Likely to be important lobster 

resources and potentially significant pink shrimp spawning grounds to the east 

in deeper water.

Nursery Grounds Not known to be an important nursery ground for any commercial fish or 

shellfish resources.

Over-wintering Grounds Not considered to be an important over-wintering ground for any fish or 

shellfish.

Migratory Routes Not a specific migration route for any commercial species, although pink 

shrimp, herring, roker and several other species that move between deeper 

water and coastal waters to reach breeding and nursery grounds probably 

pass through the area.

Direct Mortality Most fisheries exploitation is by potting for crab and lobster outside the 

boundaries of the dredge site. Risk of direct mortality on these resources is 

zero. 

Reduction of Income Assuming complete exclusion of vessels currently exploiting the general 

vicinity of the dredge site, the losses are estimated to be £120K per year split 

amongst 6 vessels. However we consider the estimate of value of catches is 

either too high (based on the productivity of the seabed elsewhere in the 

region) or represents an unsustainable level of exploitation.

Displacement of Vessels The 6 local vessels reported to operate in the vicinity of the dredge site may 

be displaced from parts of the licence area, depending on zoning agreements. 

However the evidence from the benthic ecology surveys suggests that the 

most important areas for commercial fisheries resources are located well 

outside the boundaries of the dredge site

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2. Method of Aggregate Extraction

Benthic Community The infaunal invertebrates are likely to be removed by suction trailer dredging 

within the dredged zones. The structure of the community suggests that 

recovery will be achieved in 2-4 years after cessation of dredging. The 

temporary loss of potential invertebrate food species is unlikely to affect the 

‘carrying capacity’ for fish because population densities of fish are well below 

historical levels due to commercial fishing pressure. Hence the remaining fish 

stocks are unlikely to be limited by food availability.

Breeding & Spawning 

grounds

The area is not regarded as an important spawning ground for any commercial 

target species. The risk of a direct impact on fisheries resources outside the 

boundaries of the dredge site is negligible.

Nursery Grounds The area is not regarded an important nursery ground.

Over-wintering Grounds The area is not considered an important over-wintering ground.

Migratory Routes There is likely to be some generalised movement of fish and shellfish species 

between shallow coastal waters and deeper waters through the dredge site. 

But this occurs over a wide area and is not specific to the dredge site.

Reduction of Income Trailer hopper dredging results in relatively shallow furrows in the seabed that 

are infilled with sand over time, depending on the rate of transport of material 

on the seabed. Most fisheries exploitation is by potting, although some foreign 

beam trawlers are reported in the area. The method of dredging is unlikely to 

detract from the suitability of the seabed for fisheries exploitation after 

cessation of dredging.

3. Plume Effects

Benthic Community The high gravel content of the seabed resources implies that the amount of 

material returned to the seabed following screening will be relatively small. 

Plumes of dispersing material have been described for some areas extending 

up to 600m along the axis of transport. There is likely to be a temporary 

impact on the benthic community in the immediate vicinity of the dredge site, 

but there is currently little evidence to suggest impacts on the benthic 

community beyond 200m.

Breeding & Spawning 

grounds

There is no evidence that the dredge site, or the deposits potentially affected 

by sediment mobilised by the dredging and screening process are of 

importance as a breeding ground for commercial fish or shellfish.

Nursery Grounds Mobilised material will be moved for a limited distance outside the dredge site 

along the axis of the tidal currents. The only potentially significant nursery 

grounds lie to the east across the axis of the tidal streams. It is considered 

unlikely that dredging in zoned sites within dredge area will result in potential 

sediment transport across the tidal currents into potential nursery ground 

areas, or that the quantities of material will be significant.

Over-wintering Grounds The dredge site does not constitute a known over-wintering area for fish or 

shellfish. The relatively small quantity of material likely to be rejected during 

screening is unlikely to have an effect on over-wintering species.

Migratory Routes The dredge site does not constitute a migratory route for fish or shellfish. The 

relatively small quantity of material likely to be rejected during screening is 

unlikely to have an effect on migratory species.

Reduction of Income The relatively small amount of material likely to be rejected by screening is 

unlikely to have an impact beyond the immediate boundaries of the site being 

dredged. Potential reduction of income (if any) is mainly likely through 

displacement of vessels from actively dredged sites within the dredged area.

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4. Cumulative Effects

Benthic Community The total area under licence for aggregate dredging in the region as a whole 

has been reduced by 50% in recent years, and there are plans for further 

reductions. The risk of cumulative impacts from other dredging activities is 

now significantly less than it was in the past.

Breeding & Spawning 

grounds

The area is not known to be of importance as a breeding or spawning ground 

for any commercial fish or shellfish species. Cumulative effects (if any) will be 

significantly less than in the past.

Nursery Grounds The area is not known to be of importance as a nursery ground for any 

commercial fish or shellfish species. Cumulative effects (if any) will be 

significantly less than in the past.

Over-wintering Grounds The area is not known to be of importance as an over-wintering ground for 

any commercial fish or shellfish species. Cumulative effects (if any) will be 

significantly less than in the past.

Migratory Routes The area is not known to be of importance as migratory route for any 

commercial fish or shellfish species. Cumulative effects (if any) will be 

significantly less than in the past.

Reduction of Income A reduction of income from the combined dredging activities in the area is a 

lower risk than in the past following a 50% reduction of the area under 

Licence in recent years. Part of the proposal is to further reduce the area 

under Licence by progressively relinquishing depleted sites in the adjacent 

dredged areas.

Displacement of Vessels Displacement of fishing vessels can occur if the occupancy of an area by 

dredging increases. The area dredged is, however reduced by more than 50% 

compared with previous years, and is set to be reduced further by 

relinquishment of depleted areas nearby. The risk of displacement is therefore 

now significantly less than in the past.

The second stage in the Risk Assessment is to assign a value to the actual vulnerability 

of the fisheries resources to each of the potential impacts of marine aggregate dredging 

at the particular site in question. This involves a balance of judgement between the 

sensitivity of the resource in question to particular impacts, and the distribution and 

abundance of the resource in relation to the proposed dredge site. This is a subjective 

process, but is based on the data summarised in the Impact Assessment and in Table 4. 

The actual vulnerability of the resources are then allocated to one of the four following 

categories :- Very High, High, Moderate and Low. These are shown for the typical dredge 

site in the southern North Sea in Table 5 and provide the basis for a Risk Assessment 

Matrix. 

Table 5. Fisheries Resources: Risk Assessment Matrix for a typical aggregate dredge site in the southern North 

Sea.

 

1. Temporal & Spatial Scale of the Operation. Actual vulnerability

Potential Sensitivity Very High High Moderate Low

Benthic Community Very High 1

Breeding & Spawning Grounds Very High 1

Nursery Grounds Very High 1

Over-Wintering Grounds Very High 1

Migratory Routes Moderate 1

Direct Mortality Low 1

Reduction in Income High 1

Displacement of Vessels Low 1

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2. Method of Aggregate Extraction. Actual vulnerability

Potential Sensitivity Very High High Moderate Low

Benthic Community High 1

Breeding & Spawning Grounds High 1

NurseryGrounds High 1

Over-Wintering Grounds High 1

Migratory Routes Moderate 1

Reduction in Income High 1

3. Plume Effects. Actual vulnerability

Potential Sensitivity Very High High Moderate Low

Benthic Community Very High 1

Breeding & Spawning Grounds Very High 1

Nursery Grounds Moderate 1

Over-Wintering Grounds Very High 1

Migratory Routes Moderate 1

Reduction in Income High 1

4. Cumulative Effects Actual vulnerability

Potential Sensitivity Very High High Moderate Low

Benthic Community High 1

Breeding & Spawning Grounds Very High 1

Nursery Grounds High 1

Over-Wintering Grounds High 1

Migratory Routes Low 1

Reduction in Income High 1

Displacement of Vessels High 1

The final stage in the Risk Evaluation is a matrix that combines the scores for the 

potential sensitivity of each component of the fisheries resource to each of the potential 

impacts, with the actual vulnerability of those resources at the particular site in question. 

This matrix is shown in Table 6. 

Table 6. Table showing the total scores for actual sensitivity of fisheries resources in relation to their potential 

sensitivity to impacts from marine aggregate dredging at a typical dredge site in the southern North Sea. 

Compiled from Table 5.

 

Potential Actual Sensitivity

Sensitivity VERY HIGH HIGH MODERATE LOW

VERY HIGH 2 6

HIGH 6 6

MODERATE 4

LOW 2 1

This gives the final scores shown in Table 7 for Environmental Risk to the Fisheries 

Resources at the dredge site. 

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Table 7. Table showing the final summed scores for Environmental Risk to fisheries that is likely to be posed by 

marine aggregate dredging at a typical dredge site in the southern North Sea. Compiled from Tables 5 & 6. 

 OVERALL ENVIRONMENTAL 

RISK

SCORE

HIGH 0

HIGH-MEDIUM 2

MEDIUM 6

MEDIUM-LOW 12

LOW 4

NEAR ZERO 3

Table 7 shows that the assessed risk to Fisheries Resources posed by marine aggregates 

dredging for the typical dredge site in the southern North Sea used as an example is 

mainly in the MEDIUM-LOW category. This is similar to Risk Assessments for Fisheries at 

Area 407 in the English Channel to the south of the Isle of Wight, UK, but poses a 

significantly lower assessed risk than at some other Licence Areas where this form of 

evaluation has been carried out (Carlin & Rogers, 2002). 

9. MITIGATION. 

United Kingdom policy guidelines for the marine aggregate dredging industry have been 

summarised in:- 

! Office of the Deputy Prime Minister (ODPM) 2002. Guidance on the Extraction by Dredging 

of Sand, Gravel and Other Minerals from the English Seabed. Marine Minerals Guidance 

Note 1. (ISBN 0 11 7536342 Her Majesty’s Stationery Office 30 pp). 

This gives detailed guidance on Policy Objectives relevant to applications for marine 

aggregate dredging as follows:-

! The careful location of new dredging areas 

! Considering new applications for Dredging Permissions in relation to the findings of an 

Environmental Impact Assessment (EIA). 

! Minimising the total area permitted for dredging. 

! Controlling dredging operations through the use of legally enforceable conditions attached 

to Dredging Permissions. 

! Requiring operators to monitor, as appropriate, the environmental impacts of their 

activities during, and on completion of dredging. 

! Adopting dredging practices that minimise the impact of dredging. 

 

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Several mitigation measures are potentially available to protect environmental resources 

adjacent to dredging areas in coastal waters, and to enhance recolonisation after 

cessation of dredging. Whether these are cost-effective depends on a commercial 

decision on the importance of the resources and the costs and feasibility of mitigation 

measures. Typically these include the following:- 

! Reduction of Overboard Screening. It has been shown that one of the main sources 

of impact on benthic biological resources is the practice of overboard rejection of 

screened material. The fact that this material accumulates within the dredged site 

means that an increasing amount of dredging (and screened discharge) is required as 

the dredge area is exploited over time. Studies cited above show that the deposition of 

screened material on the sea bed may have a more serious impact on sea bed 

resources than dredging itself. The screening process must therefore be viewed as a 

major source of impact on sea bed resources. Obviously the option on whether 

screening is essential depends on a commercial decision whether there is a beneficial 

use for sands on this scale, and the costs implications of bulk transport of ‘all in’ cargo 

compared with screened cargo suited to customer needs.

! Adjustment of the timing of exploitation. Some areas close to the sites of dredging 

may support biological resources of conservation significance, or resources that are of 

importance as a spawning ground or nursery area for commercially significant fish and 

shellfish species. Successful mitigation of potential impacts of seabed plumes on 

breeding areas for crab has been achieved by dredging only when the tidal stream 

transports sediments away from the sensitive area. In other areas, there may be a 

seasonal variation in importance for fish and shellfish. If it is established that some 

parts of the seabed surrounding a potential dredge site are of importance as a habitat 

for fish spawning, and if these areas lie within potential sedimentation plume zones, it 

may be necessary to consider cessation of dredging close to these sensitive areas 

during the breeding season.

! Establishment of ‘Refuge Areas’ to enhance recolonisation. Our review of the 

literature shows that significantly faster recolonisation and recovery rates have been 

reported for shell deposits in which non-dredged areas were left between strips of 

dredged sea bed. The presence of patches of non-dredged deposits within a dredge site 

may assist recolonisation. It should be pointed out, however, that recolonisation by 

planktonic larvae is likely to be of more importance than migration of adults from the 

surrounding seabed in most gravel deposits. In general we view the establishment of 

‘refuge areas’ as a positive proposal, but not one which is seriously likely to represent 

the primary source of recolonising larvae for a particular dredge site. 

In line with Policy Objectives summarised above, proposals to reduce the impacts of 

marine aggregate dredging generally centre on minimisation of the dredged area, and 

the amounts of material likely to be rejected by screening. The principal options for a 

typical marine aggregate dredge site are:- 

• Minimise the dredged area 

• Manage the dredging operations 

• Liaison & reporting 

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9.1. MINIMISE THE DREDGED AREA.

Resources of sufficient depth and quality need to be located within the potential dredge 

site. This will:- 

1. Minimise the area to be dredged 

2. Reduce the amount of screened material returned to the seabed 

3. Minimise direct impacts on benthic biological resources 

4. Allow maximal access for fishing vessels to use the surrounding seabed 

9.2. MANAGE THE DREDGING OPERATIONS.

Dredging operations can be managed with the following objectives:-

1. Avoid formation of depressions on the seabed that might interfere with fishing once 

the site is relinquished 

2. Minimise loading times to avoid interference with other legitimate users of the sea 

3. Resources to be worked to depletion in small zones before moving to a new zone, 

allowing maximal time for recovery of benthic biological resources in each dredged 

zone without further disturbance 

4. The deposits on the seabed to be left in a similar condition to assist recovery of 

biological resources in relinquished areas 

9.3. LIAISON & REPORTING.

As part of the proposals to minimise conflicts with other legitimate uses of the marine 

environment management proposals can include:-

1. Minimise interference with fishing activities by proper liaison protocols with 

Fisheries representatives 

2. Inform other potential users of the area by provision of information on dredging 

activities and other relevant information 

We consider that there are not any practical or cost-effective ways of minimising the 

impacts of marine aggregate dredging within the dredged zones themselves, nor in the 

400-600m primary sedimentation zone, unless a reduction of material returned to the 

seabed following screening were a commercially-acceptable option. It is also unlikely that 

there are cost-effective measures that could be taken to enhance the rate of 

recolonisation and recovery of the complex and slow-growing equilibrium communities 

that typically occur in coarse gravels that characterise many aggregate extraction sites. 

Such communities probably represent an important food resource for fish, but we doubt 

whether losses to the marine food web from the dredged areas, even when summed 

over a 10-20y time period, would result in a detectable impact on the carrying capacity 

of the surrounding seabed for commercial fish stocks. This is because the area dredged 

is very small compared with the feeding habitat available for fish, and partly because 

pressure on commercial fish stocks in UK waters has been so great that currently the 

feeding requirements of the fish population is well below the potential ‘carrying capacity’ 

of the seabed resources for commercial fish species. Of more significance are possible 

impacts on areas of localised or seasonal importance such as spawning grounds and 

nursery areas for fish and shellfish including scallop and crab. This implies that the 

dredging strategy for a particular site needs to take into account the seasonal sensitivity 

of the seabed resources, and the likelihood of ‘Risk’ to those resources bearing in mind 

what is known of the physical impacts of dredging both within the dredge site and on the 

surrounding deposits. 

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10. MONITORING. 

A common objective of monitoring studies has been to assess the nature and sensitivity 

of benthic biological resources to potential disturbance by dredging activities within the 

proposed dredge site, and to identify resources or communities of conservation 

significance that warrant special protection. Most surveys include an assessment of the 

significance of the areas for commercial fishing, and as a breeding or nursery ground for 

young fish. However because of the strong seasonality of fisheries data, and the fact that 

most fisheries investigations require a long series of surveys extending over several 

years, it has not generally been cost-effective or practical to carry out primary fisheries 

investigations as part of the normal environmental impact process. 

The first requirement of the environmental resource survey is to carry out an assessment 

of resources over a relatively wide area to establish the ‘baseline’ conditions prior to 

dredging and to assess the likely ‘risk’ based on what is known of the sensitivity of the 

resources and their location in relation to likely impacts from the dredge site. The most 

common form of survey has been to establish a box-grid of sampling stations which 

generally extends for a distance of at least one tidal excursion at each end of the 

proposed dredging area. It also includes stations located across the axis of the tidal 

stream as well as ‘control’ stations located well outside the boundaries of any likely 

impact of dredging activities within the proposed Production Licence Area. The 

information is then used to predict what might be the impact of dredging activities, and 

to make proposals on how any impacts might be minimised. 

A secondary objective is to identify sites which, because of the sensitivity of their 

communities, or because of their location in relation to the dredge site, can be used as 

‘monitoring’ stations. The main purpose of a monitoring survey is to confirm whether or 

not the predictions of impact made in the initial dredging licence application are correct, 

and to trigger mitigation measures if impacts exceed those accepted as part of the 

project proposal. In general, the number of ‘monitoring’ sites is fewer than that used in 

the baseline survey grid, but there is a need to include an assessment of the variance of 

the samples at each site. Apparent impacts might otherwise merely reflect the wellknown variability, particularly of biological samples, on the sea bed. 

The procedures commonly used in baseline surveys for the marine aggregates industry 

in recent years have been summarised in some detail in a Report on “Procedural 

Guidelines for the Conduct of Benthic Studies at Aggregate Dredging Sites” (Department 

for Environment, Transport & the Regions (DETR) 2001). A further study on the use of 

mapping techniques suitable for the identification of marine biotopes has been reported 

in “Mapping of Gravel Biotopes and an Examination of the Factors Controlling the 

Distribution, Type and Diversity of their Biological Communities” (Brown et al. 2001). 

The following sections outline a general procedure for establishment of a benthic 

biological monitoring survey for a particular Production Licence area. However it is 

recommended that reference is made to the above Reports for much background 

information on suitable survey gear, analytical methodology and recommendations for 

reporting of data. A general procedure based on common practice to date is given below. 

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10.1. NUMBER AND POSITION OF MONITORING SITES. 

Most studies on the benthic communities of sands and gravels suggest that a minimum 

of 2-3 replicate samples of 0.1 m2 are required to identify the majority of the species 

present in sandy deposits of the North Sea whereas at least five replicates are necessary 

for more complex communities that characterise stable substrata such as rocks and 

cobbles (see Newell et al., 2001a & 2003). Clarke & 

Green (1988), Warwick & Clarke (1996) and more 

recently Somerfield & Gage (2000) have addressed 

the problem of the number of replicates and the 

appropriate spacing between samples to 

accommodate the patchiness that occurs in the 

macrofauna of marine deposits. 

Retrieval of 0.1m2 Hamon grab ©MESL-PhotoLibrary 

Somerfield & Gage (2000) studied the macrofauna of Scottish sea lochs and concluded 

that there was a need to understand and guard against the problem of 

‘pseudoreplication’ (Hurlbert, 1984) in the design of a sampling strategy. They 

recommended that samples to be taken as replicates should be taken at least 40 m 

apart. By ensuring this, an investigator is unlikely to collect samples more similar to one 

another than they should be and reduce the chances of concluding that a difference 

exists when it does not. 

We have shown above that there is evidence of an impact of marine aggregate dredging 

on benthic biological resources within a dredged site, and that an impact has been 

reported for the zone of deposition of material rejected during the screening process. 

This zone is likely to be between 400-600m on each side of the dredge site along the 

axis of the tidal streams. We have also shown that there is evidence for the presence of 

a benthic sediment plume which may extend as much as one tidal excursion along the 

axis of the tidal streams on each side of the dredge site. 

The distribution of sample sites for a ‘monitoring’ survey should thus take into account 

the need for replicate samples to assess variance, and the need to cover an area of sea 

bed that extends for up to one tidal excursion on each side of the proposed dredge site. 

‘Control’ sites situated perpendicular to the axis of flow of material from the dredge site 

are also required to assess the natural variations in benthic biological resources against 

which any impacts need to be assessed. 

Figure 16 shows a schematic diagram of an idealised dredge site together with 

recommended positions and numbers of replicates for monitoring sites both within the 

dredge site and along the axis of probable dispersion of material from the dredge site. 

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Marine Ecological Surveys Limited - October 2003 51
Figure 16. Monitoring survey plan for an hypothetical dredge site. The sites of benthic grab 

sampling stations including monitoring and control sites (spots) are shown, together with trawl 

sites for epibenthos (arrows). 

Ecologists will have their own preferences, but the following general recommendations 

can be made on a basis of Figure 16:- 

1. In order to monitor impacts within the dredge site itself, we recommend at least 15 

sample sites. This is because it is difficult to locate dredge tracks within a trailer dredge 

site and one needs to have sufficient samples to locate impact areas within the 

Production Licence area. The data from these samples can be pooled to give an estimate 

of impact and variance of benthic communities within the dredge site as a whole. 

2. Monitoring stations have been arranged at 1000m, 4000m and 8000m on each side of 

the dredge site along the axis of the tidal streams. Each monitoring station comprises a 

target area of 200m, within which 5 replicate samples can be taken at distances of 40m 

to avoid the problem of ‘pseudoreplication’ of patchy organisms referred to above. The 

results from these samples can be used to express mean population characteristics and 

variance between samples at each site in relation to distance from the dredge site. 

3. Control stations have also been indicated across the main axis of the tidal stream. It is 

considered unlikely that these will be impacted by the dredge site shown in Figure 16, 

but clearly the position will need to be located away from possible impacts of adjacent 

Production Licence Areas. Note that we have indicated a minimum of triplicate samples 

in these ‘control’ stations, but that this might need to be increased to 5 replicates if the 

substrate comprises a gravel with rich fauna. 

4. The epifauna comprises organisms that live on the surface of the sea bed and which are 

not sampled quantitatively with conventional grabs. We recommend a series of single 

epibenthic trawl samples should be taken within the dredge site and at each of the 

monitoring stations. These sample site are indicated by arrows in Figure 16. 

8000m 4000m 1000m 1000m 4000m 8000m

Dredge Site

Control

Control Control

Control

Axis of Tidal Flow

Flood & Ebb tidal Excursion

Transect sampling station, 5 replicate samples

spaced approximately 40m apart

Epibenthic trawl station

Benthic Grab Sample

Monitoring survey plan for an hypothetical dredge site

Dredge Site

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5. There is a serious lack of information on the size and scale of the sedimentation 

processes and no information on the extent to which screened material may accumulate 

on the sea bed, or whether benthic turbidity layers occur at the sediment-water interface 

in relatively deep water sites. We consider that monitoring of the quantities and fate of 

overboard screened material is an important component of the monitoring programme, 

and that direct measurements of plume characteristics by seabed mini-pods coupled with 

side scan sonar and appropriate visual techniques could make a significant contribution 

to our understanding of the likely scale of impact of this project on biological resources 

surrounding aggregate dredge sites.

The generalised scheme for a monitoring survey of biological resources thus involves 57 

grab samples and 12 epibenthic trawl samples. Clearly the number of stations may be 

reduced if it is shown that the impact along the axis of the tidal streams is significantly 

less than one tidal excursion. All evidence from dynamic environments is that any impact 

on benthic biological resources is confined to distances of up to 400-600m from the 

dredge site, corresponding with the zone of deposition from the discharge plume. But in 

view of the unknown impact of a probable benthic plume at the sediment:water 

interface, monitoring should be carried out for a full tidal excursion until the limits of 

impact are established for the dredge site at a particular location. 

 

10.2. BENTHIC INFAUNA

10.2.1. Sampling Methods. 

The benthic infauna are commonly sampled with either a 0.2m2 or, more recently a 

0.1m2 Hamon grab. Use of this grab has the 

advantage that loss of material by ‘washout’ from 

the jaws experienced with conventional grabs is 

reduced (Holme & McIntyre, 1984; Sips & 

Waardenburg, 1989; Kenny & Rees, 1994; van 

Moorsel, 1994; DETR, 2001). The larger 0.2m2 

grab takes approximately 16 litres of sediment 

whereas the smaller 0.1m2 grab takes 

approximately 9 litres of sediment. The latter is 

easier to sort and has been increasingly used in 

recent studies of the benthos. 

0.1m2 Hamon grab ©MESL-PhotoLibrary 

The samples taken with the grab may be sieved aboard the survey vessel to separate the 

macrofauna from the bulk of the deposits. However it is often preferable to carry out the 

entire separation of the macrofauna from the sediment sample ashore, rather than 

partially at sea as recommended in the Guidelines referred to above. 

This is for the following reasons:-

1. Collection of samples at sea often involves long hours in rough sea conditions and poor 

lighting. Samples are often taken at night because larger survey vessels operate a 24h 

shift system. This reduces the effectiveness of initial sorting aboard the survey vessel. 

2. Sorting at sea can significantly reduce the rate of sample collection. Because of the costs of 

a survey vessel and the short weather ‘window’ often available for sampling, the time 

taken for collection of samples at sea needs to be kept to an absolute minimum. 

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3. If sorted alive aboard the survey vessel, many of the benthic infauna (especially the 

polychaetes) can escape collection through the apertures of the sieve mesh. 

4. Although transport of the whole sample ashore for sorting involves a significant amount of 

material, the quantitative extraction of the fauna is maximised under ideal lighting in the 

laboratory.

Once the sediment sample has been taken with the grab, a small sub-sample is taken for 

particle size analysis of the sediments. The remainder is 

transferred to a 15 litre bucket, agitated with formalin 

and sealed with a lid before being transported to the 

laboratory for subsequent separation and identification of 

the macrofauna. 

Sediment sub-sampling ©MESL-PhotoLibrary 

10.2.2. Laboratory Identification and Analysis.

On arrival at the laboratory, the samples are thoroughly washed with tap water and the 

supernatant poured through a 1mm mesh sieve to collect 

the smaller macrofauna. The residual sediment is then 

washed on to a 1mm mesh sieve and the larger stones 

and shells retained on the sieve are carefully sorted by 

hand to remove the larger macrofauna and organisms 

attached to stones and pebbles. 

Sorting on a 1mm mesh sieve ©MESL-PhotoLibrary 

The biological material is then preserved in methanol for subsequent separation in to 

major faunal groups, identification and enumeration. Biomass is commonly expressed as 

ash-free dry weight (AFDW) for marine organisms. However measurement of the ashfree dry weight results in destruction of the 

samples which are not therefore available for 

subsequent Quality Assurance control if required. 

The normal procedure is therefore to measure the 

blotted wet weight of either the individual species 

or the main phyletic groups in the sample, and to 

estimate the AFDW from standard conversion 

figures. A reference collection is kept and the 

identified material is retained for use in Quality 

Assurance procedures if required. 

Identification & enumeration in the laboratory ©MESL-PhotoLibrary 

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10.3. BENTHIC EPIFAUNA.

Although the methods used for sampling the benthic infauna are similar for most of the 

baseline surveys carried out as part of the 

Environmental Impact Assessment process for 

marine aggregate dredging areas, very few have 

used compatible methods for analysis of the motile 

epifauna. In some cases samples have been taken 

with a scallop dredge, in others a small 2m beam 

trawl has been used. Few have quantified the 

catches sufficiently to allow use of multivariate 

statistical techniques to assess community structure. 

CEFAS 2m beam trawl ©MESL-PhotoLibrary 

Recent studies by Ellis & Rogers (2001) show, however, that analysis of the community 

composition of the epifauna sampled with a commercial sized beam trawl can give an 

insight into this important component of the benthos. Furthermore, the data include both 

fish and epibenthos, and thus give information on the epibenthic community as a whole 

as well as information on commercially exploitable fish stocks at the time of the survey. 

Some recommendations on the suitability of gear to sample the epibenthos is also given 

in the Guidelines referred to above (DETR, 2001). 

We recommend that a series of trawl samples should be taken in the survey area as part 

of a comprehensive monitoring survey that includes both benthic infauna and the 

epibenthic community as a whole. The length of haul needs to 

be adjusted to obtain a representative sample of the mobile 

epifauna, without obtaining so much catch that sub-sampling 

is required. Generally a haul of 5-10 minutes yields sufficient 

sample for macrofauna analysis. The entire sample should be 

transferred to a sealed bucket, immersed with seawater and 

preserved in formalin for separation and analysis ashore. The 

position of the deployment and hauling of the net from the sea 

bed should be recorded and used to estimate the numbers of 

epifauna per unit area trawled. 

Cod end sample release ©MESL-PhotoLibrary 

On arrival at the laboratory, the main components of the macrofauna are washed over a 

1mm mesh sieve, identified and weighed. These 

data are not quantitatively compatible with those 

for the infauna collected with a Hamon Grab. But 

the data are suitable for community analysis and 

can be used to establish spatial and temporal 

variations in community structure using similar 

methods to those suitable for the benthic infauna. 

Weighing epibenthic macrofauna ©MESL-PhotoLibrary 

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Three points are worth noting in relation to the use of trawl surveys to study the 

distribution and abundance of the epibenthos:-

1. The nature of the catch varies a good deal with the type of gear used and the way it is 

rigged, as well as with the speed of the vessel and other factors. A trawl suited to capture 

of small epibenthic invertebrates will not capture larger more mobile fish species. Therefore 

although a beam trawl rigged with a fine mesh net will give compatible results within one 

survey area for the epibenthic invertebrates, it will seriously under-record fish. If fish 

catches are of primary interest, it is recommended that a small commercial otter trawl or 

beam trawl is used, and that the by-catch of invertebrates is used for analysis of epibenthic 

invertebrates. 

2. The epibenthos and fish vary a great deal seasonally. In contrast to the benthic infauna, it 

is probably necessary to take several epibenthic trawl surveys per year to establish the 

variation in baseline conditions that occurs in the absence of impact by man. 

3. If surveys are carried out using a small mesh suitable for capture of undersize fish, 

Fisheries Regulations may apply. 

10.4. DATA RECORDING & ANALYSIS OF RESULTS.

The analytical procedure for interpretation of community structure in marine benthos is 

now widely agreed and adopted in all of the baseline surveys carried out in UK waters. 

These are reviewed in some detail in the Guidelines (DETR, 2001). A recommended 

procedure for the recording format is as follows:- 

1. The species identification should be recorded in a standard format using Picton B.E. & 

Howson, C.M. (1999). The Species Directory of the Marine Fauna and Flora of the 

British Isles & Surrounding Seas. CD-Rom version. Published by The Marine 

Conservation Society and The Ulster Museum. (ISBN 0-948150-11-4). Note that the 

coding in this latest version differs somewhat from the 1997 hard copy. 

2. Data for the sediment characteristics, positions of the sampling stations, species 

variety, population density, biomass and all other relevant physical and biological data 

obtained in the survey should be recorded on an EXCEL format. 

3. The analysis of sediment types and biological data should be carried out using 

multivariate methods or compatible methods. The most convenient and purposedesigned statistical software package for use in marine benthic surveys in the UK is the 

Plymouth Routines in Multivariate Research (PRIMER) version v5 (Clarke & Gorley, 

2001). This version has additional features that allow calculation of biodiversity indices 

based on ‘taxonomic distinctness’ of the species comprising a quantitative sample or 

species list. These indices have statistical properties that are robust in relation to 

variations in sampling effort and may therefore be considerably more useful than some 

of the more conventional indices used in earlier studies. Other software packages 

including TWINSPAN & DECORANA and, more recently, CANOCO for Windows are also 

widely used for ordinating multivariate species data and for providing insight into the 

structure of biological communities and their relationship with environmental 

determinands (see Hill & Gauch, 1980; ter Braak & Šmilauer, 2002). 

4. The presentation of the results should be in electronic format using a suitable GIS 

system. We have found MapInfo 7.0 to be well-suited for coastal surveys as it allows 

data from dredging vessels and bathymetric data to be superimposed on to the results 

of baseline and monitoring surveys. Obviously use of an EXCEL spreadsheet in the 

Report allows other software packages such as ArcView to be used without difficulty. 

5. The full Report should be made available on CD-ROM as a PDF file. This allows 

electronic transmission without the Report being inappropriately modified. 

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10.5. IDENTIFICATION AND MAPPING OF MARINE BIOTOPES. 

Because of the need to identify and manage coastal zone resources, there has been 

some interest in the development of methods for the rapid assessment and mapping of 

marine biotopes in recent years. These have varied from relatively crude assessment of 

the main types of sea bed substrata, coupled with sporadic assessment of communities 

associated with the main sediment types to a sophisticated combination of methods 

including grab sampling, acoustic ground discrimination systems (AGDS) such as RoxAnn 

and QTC-View, sidescan sonar and photographic methods (see Brown et al., 2001). 

The use of these methods is outside the scope of this Review. However several points 

are worth noting in the context of monitoring marine aggregate dredging and other 

impacts of man on marine resources. 

1. Remote sensing systems have obvious attractions for overall habitat mapping for 

management of coastal resources. However it is clear from most detailed benthic surveys 

that community composition can vary significantly within particular substrate types. 

Consequently fine-grained resolution of marine community structure is likely to require 

relatively large numbers of epibenthic and grab samples to ‘ground-truth’ the remote 

sensing data. Brown et al (2001) therefore emphasise the need for survey techniques to be 

used in combination when producing high-resolution biotope maps of an area. In particular 

they caution against the use of acoustic methods in isolation as a tool for predicting 

biological and physical traits on the sea bed. 

2. Such systems (used in conjunction with several other techniques) have some potential 

value in baseline surveys, especially in areas where rocky outcrops and reefs prevent 

suitable sampling by grabs or trawls. 

3. It is unlikely that remote systems, even when used in combination, will provide sufficient 

discrimination to monitor changes in community composition associated with potential ‘farfield’ impacts of marine aggregate dredging. Neither is it likely that they could be used as 

an effective tool to quantitatively demonstrate the nature and rate of recovery in marine 

deposits after cessation of dredging. They may, however, be useful in establishing the 

gross near-site impacts of dredging and overboard screening on physical and biological 

resources on the sea bed. 

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Appendix B 

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For The Literature Review Of Existing 

And Emerging Environmentally 

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Stevens, B.G. 1981. Dredging Related Mortality of Dugeness Crabs Associated with 

Four Dredges Operating in Grays Harbor, Washington, Report No. DA-79-45, 

Washington Department of Fisheries and US Army Engineer District, Seattle, 

Seattle, WA. 

Thevenot, M.M., Prickett, T.L. and Kraus, N.C. 1992. Tylers Beach, Virginia, 

Dredged Material Plume Monitoring Project 27 Sep to 4 Oct 1991. USACE 

Report No. DRP-92-7, WES. 

Tsurusaki, K., Iwasaki, T. and Arita, M. 1988. Seabed Sand Mining in Japan. Marine 

Mining, Vol. 7, pp. 49-67. 

Uren, M..J. 1988. The Marine Sand and Gravel Dredging Industry of the United 

Kingdom. Marine Mining, Vol. 7, pp. 69-88. 

USACE CERC 1980. Ecological Evaluation of a Beach Nourishment Project at 

Hallandale, FL, Volume Evaluation of Fish Populations Adjacent to Borrow 

Areas of Beach Nourishment . CERC report MR No. 80-1. 

USACE WES. 1990. Engineering Design And Environmental Assessment of 

Dredged Material Overflow from Hydraulically Filled Hopper Barges in Mobile 

Bay Alabama. USACE report D-90-4. 

Appendix 
Van Dipen, H., Hunt, A., Van Raalte, G.H. and Pennekamp, J.G.S. 1993. Dredging 

and the Environment: new Developments from the Netherlands. Bulletin de 

PIANC, Vol. 167 No. 80 pp. 29-37. 

Van Drimmelen, N.J. and Loevendie, N.J. 1988. Modern Dredging Methods and 

their Environmental Aspects. CEDA Dredging Days, No. 7, pp 1. 

Van Rhee, C. 2001. Numerical Simulation of the Sedimentation Process in a Trailing

Suction Hopper Dredge. Dredging for Prosperity: Achieving Social and 

Economic Benefits: World Dredging Congress and Exhibition/ WODA, Vol.2 

no. 6. 

Verweij, J.F. and Winterwerp, J.C. 1999. Environmental Impact of Water Injection 

Dredging. Proceedings CEDA Dredging days 99, 18-19 Nov 1999 Amsterdam,

the Netherlands, pp. 175-189. 

WDCM. 1995. Overview of Dredging International's Environmental Operations. 

World Dredging Mining and Construction, Vol. 31, No. 8, pg. 8. 

Whiteside, P.G.D., Ooms, K. and Postma, G.M. 1995. Generation and Decay of 

Sediment Plumes from Sand Dredging Overflow. World Dredging Congress 

Vol. 2, pp. 877- 892. 

Williams, S.J., Reid. J.M. and Manheim. F.T. 2003. A Bibliography of Selected 

References to US Marine Sand and Gravel Mineral Resources, US Geological 

Survey Open-File Report 03-300. 

Windom, H.L. 1976. Environmental Aspects of Dredging in the Coastal Zone. CRC 

Critical Reviews in Environmental Control. New York. 

 Zhou, Q.Y. 2001. Environmental Impact Assessment of Dredging Projects in the 

Yangtze Estuary. Dredging for Prosperity: Achieving Social and Economic 

Benefits: World Dredging Congress and Exhibition/ WODA, Vol.1, no. 8. 

Appendix 
Appendix C 

Dredging Industry Survey 

Questions And Responses 

• Appendix C1 - Cover Letter to US Dredging 

Contractors 

• Appendix C2 - List of Questions For US 

Dredging Contractors 

• Appendix C3 - MIRO Sponsored Questions To 

European Dredging Contractors 

• Appendix C4 - Summary of All Industry 

Responses 

• Appendix C5 - Document and Powerpoint 

Presentation Provided by 

Jan de Nul 

Appendix 
Appendix C1 

Cover Letter To 

US Dredging Contractors 

Appendix 
February 4, 2004 

Dredging Contractor 

1234 Easy Digging Way 

Anytown, World 

Gentlemen: 

W.F. Baird & Associates has been retained by the U.S. Department of the Interior, 

Mineral Management Service (MMS) to conduct studies relative to the mining of sand 

for beach nourishment and construction aggregates. The borrow areas are located on the 

Outer Continental Shelf (OCS) under Federal jurisdiction. 

The United States (U. S.) Government, and specifically, the MMS, a bureau within the U. 

S. Department of the Interior, has jurisdiction over all mineral resources on the Federal 

OCS. The MMS has the authority to convey, on a noncompetitive basis, the rights to 

OCS sand, gravel, or shell resources for shore protection, beach or wetlands restoration 

projects, or for use in construction projects funded in whole or part or authorized by the 

Federal Government. MMS has provided Federal sand for beach nourishment projects in 

New Jersey, Maryland, Virginia, Florida, South Carolina, and Louisiana. 

Offshore sand-dredging for beach nourishment projects employ hydraulic dredges almost 

exclusively and are normally either cutterhead or hopper dredges. The process may result 

in adverse effects on various components of the marine or coastal environment. 

The offshore-dredging industry is constantly changing as the industry strives to make 

operations more efficient. New advances in offshore-dredging technology are leading to 

more environmentally-sensitive offshore operations. Researchers are actively increasing 

the knowledge base relative to physical processes involved in dredging procedures. 

Physical and mathematical modeling of these processes is being conducted with the aim 

to reduce the negative environmental aspects (biological and physical) associated with 

the offshore removal of sand. New engineering technologies currently used overseas are 

now contemplated for use in U.S. waters. 
Page 2 

As the Federal agency responsible for regulation of OCS sand resources, the MMS must 

ensure that sand and gravel dredging operations conducted under its jurisdiction are 

conducted in a safe and environmentally-sound manner. This may, in some instances, 

entail the required use of particular dredging equipment or techniques. Thus, MMS must 

have sound knowledge of the most current dredging technologies available.

The objective of the study is to review and analyze dredging equipment and projects on a 

worldwide basis to identify both existing and emerging dredging technologies that aim to 

lessen or avoid potential adverse effects on the offshore biological and physical 

environment.

Contacts were made with Federal and State natural resources agency staff and others 

involved in research on the impacts of dredging, studies of the life history of special 

species of concern, and permit approvals to determine the direct and indirect impacts that 

are of greatest concern for dredging operations in the OCS. Recent MMS-sponsored 

reports and environmental assessments on dredging impacts in the OCS were also 

reviewed. This identification of the perceived environmental impacts of greatest concern 

will be used to evaluate the advances in dredging techniques and equipment to measure 

their success in reducing the degree of such impacts. 

The prioritized list of perceived concerns from OCS dredging operations on marine 

biological and physical resources is shown below. For the concerns that are currently 

being addressed by stipulations in the MMS lease for a site, the stipulations are 

summarized for ease of review.

1. Short-term and cumulative impacts from dredging that lead to loss or reduced 

stability of benthic habitats, including recolonization by an altered biological 

community. All resource managers raised this concern. The greatest concern 

was in known benthic-associated fishery areas, such as the surf clam fishery 

off New Jersey and the shrimp fishery in the Gulf of Mexico. There was less 
Page 3 

concern in areas of general biological productivity or dynamic processes, such 

as in South Carolina. 

2. Injury and death of special species of concern (e.g., Sea Turtles) from being 

sucked into the draghead or cutterhead during dredging operations. This 

concern was raised by every Federal agency with management responsibility 

for T&E species. Most agency staff thought the existing stipulations were 

effective, but even a single “take” was considered a significant impact. 

Existing stipulations include: 

a. Presence of a trained observer following specific protocols. 

b. Use of a rigid Sea Turtle deflector (i.e., one designed by NRDC or 

similar.) 

c. Operation of the dredge in a manner that will reduce the risk of 

interaction with any Sea Turtles, which may be present in the dredge 

area. Keep the draghead on the bottom except: 1) when the dredge is 

not in a pumping operation and the suction pumps are turned 

completely off; 2) when the dredge is being reoriented to the next 

dredge line during borrow activities; and 3) the vessel’s safety is at 

risk. 

d. Dredge equipped with inflow screening baskets (4-inch mesh) to better 

monitor the intake and overflow of the dredged materials for Sea 

Turtles and their remains. These screens should sample at least 70% of 

the overflow area and should be installed at the applicable area. 

e. Assessment/relocation trawling to further assess/reduce the potential 

for incidental takes during dredging. Trawling is conducted repeatedly 

in front of the dredge as it moves along the track lines. Any turtles 

collected are to be relocated. 
Page 4 

3. Changes in the substrate characteristics (grain size, dissolved oxygen, 

compaction and organic content) that lead to a reduction in benthic 

communities AND suitability of the area for future dredging. This concern 

was identified in South Carolina where 3-to 40m of sediment was removed. 

The depressions persisted for many years and filled with fine-grained 

sediments. 

4. Changes in bathymetry that can alter the wave climate reaching the shore. The 

importance of this concern varied by region. Where the OCS sand bodies were 

close to shore and/or shallow enough to influence the wave climate, there was 

high concern regarding the potential for increased shoreline erosion. The 

orientation, depth, and shape of the sand body and borrow areas should be 

considered in evaluating the impact of dredging on wave climate. 

5. Damage to hardbottom habitats: physical damage during dredging; burial by 

suspended sediment during dredging; and altered sediment processes that 

could bury hardbottom. This issue was of concern when dredging smaller 

sand bodies in between hardbottom habitat, even though these areas are 

supposed to be avoided. 

6. Creation of depressions and furrows from removal of substrate. Though MMS 

has a “no pits” stipulation, there was still concern that furrows might interfere 

with bottom fishing. At least one responder thought that the furrows acted as 

recruitment sources, supporting the idea of leaving strips of undredged areas. 

Existing stipulations include: 

a. To assure that deep pits and furrows are not created, conduct postdredging hydrographic surveys. 
Page 5 

7. Short-term increased turbidity from cutter head on benthic species. Most 

responders assumed that OCS dredging occurred in sandy substrates, thus 

turbidity would be short-term and animals would avoid turbid areas. However, 

turbidity might be more of a concern in areas where a fine-grained overburden 

has to be removed to access the coarser sediment. 

Existing stipulations include: 

a. Turbidity shall not exceed background levels by more than 29 

Nephelometric Turbidity Units. If monitoring shows that turbidity 

exceeds the maximum amount allowable, dredging activities shall 

cease immediately and not resume until corrective measures have been 

taken and turbidity has returned to acceptable levels. 

8. Spatial and seasonal conflicts between dredging and commercial and 

recreational fisheries. 

9. Potential to break an active or abandoned pipeline, resulting in a release of 

petroleum. 

10. Collisions with marine mammals and Sea Turtles during vessel operations. 

Existing stipulations include (and are probably adequate): 

a. If operating in areas of known whale occurrences, observers are 

required. If whales are observed, avoid intentional approaches within 

100 yards and slow speeds to less than 4 knots. 

b. See stipulations for Sea Turtles. 
Page 6 

11. Damage to archaeological resources.

Existing stipulations include: 

a. Identification (by remote sensing) and avoidance of potential 

archaeological site locations (minimum avoidance ratios around 

potential site.) 

Our current study is focused on Atlantic and Gulf Coast sand borrow sites. The sites 

range from 5 kilometers to 20 kilometers offshore. The water depth at these sites varies 

from 5 meters to 25 meters deep. The material to be dredged for borrow is assumed to be 

sand with an average grain-size of 0.30 mm and less than 10% passing the 200 sieve. As 

a part of this study, we are reviewing the current scientific data to determine which of the 

perceived concerns enumerated above are real and need to be addressed. 

With this letter, we intend to inform the dredging industry of this study and request 

comments from the industry. This study is to focus on new and emerging technologies 

and operational techniques that are designed to reduce the degree of adverse impacts to 

the environment from dredging offshore borrow sites. 

We would greatly appreciate your answering the enclosed questions by February 24. 

Please return your comments to me at WF Baird & Associates, 2981 Yarmouth 

Greenway, Madison, Wisconsin, 53711 or email me at tkenny@baird.com. Feel free to 

include any comments you feel are germane. 

Sincerely, 

W. F. Baird & Associates 

Thomas F. Kenny 

enc: as stated 

File No. 10687
Appendix C2 

List Of Questions 

For US Dredging Contractors 

Appendix 
Please reply by February 24. 

We appreciate you taking the time to answer our questions concerning new and 

emerging technology and techniques designed to reduce the adverse environmental 

impacts of dredging sand in Federal offshore borrow areas. Some of the questions are of a 

scientific nature and are asked to see if you have any information that will contribute to 

the database 

For the following questions, please give an estimate of cost differential due to 

environmental restrictions where appropriate?

Plume Related Impacts

Much of the perceived concerns were due to the plume resulting from hopper overflow 

and the bottom agitation at the draghead. 

1) What percent of material overflows the hopper while digging sandy, low-silt 

content material, assuming a 10 km sailing distance to pump ashore?

2) Can a hopper dredge mine sand from below a 1-meter silt overburden without 

removing the overburden? Does this result in significantly increased material

overflow and consequently an increase in turbidity? 

3) In mining sand with a low-silt content, with a turbidity requirement not to 

exceed 29 NTUs above background, is it necessary to take special measures to 

meet this maximum turbidity requirement?

4) What measures do you employ to minimize turbidity?

5) If you use measures such as recycling overflow water back to the draghead, is 

there a reduction in dredging production?

6) Have you completed research on passive and dynamic plume processes 

associated with overflow and is this information publicly available? 

7) Do wind, wave, and/or current forces offshore determine the direction the 

dredge works? What are the consequences of dredging perpendicular to the 

current in order to influence the shape and dispersion of the dredge plume? Do 

1 
you have any data to demonstrate the direction and rate of deposition of 

material discharged by the dredger during dredging operations? 

8) Given a mandate to reduce turbidity, what are the most cost effective ways of 

accomplishing the reduction? We understand this is a question of degree. 

Please explain the consequences to us. 

9) What are your views on requiring overflow to be discharged below the hull?

Impacts to Benthic Habitats

Considerable concern was expressed relative to the loss of benthic community. 

Recolonization rates are being studied, as are changes in substrate characteristics 

such as grain size, dissolved oxygen, etc. It has been suggested that dredging in patterns 

may speed the recolonization rate by leaving “refuge” areas (that have undisturbed 

sediment characteristics and undisturbed benthic communities). 

10) If there is a stipulation in the specifications that required that only 70% of a 

borrow area can be used and the unused portion cannot be on the boundaries, 

what would be the most efficient use of the area? What is the minimum width 

cut that a hopper can dig efficiently? The reason for this proposed stipulation 

is that the benthic community will recolonize faster if the area is dredged with 

intermittent non-dredged areas. Do you have any comparative data to show

whether dredging in strips to leave recolonizing adults in the dredge site 

enhances recovery rates compared with sites where all the surface deposits are 

removed?

11) Are you aware of any damage to hard bottoms caused by dredging including 

covering by sediment? If yes, was a buffer or exclusion zone applied and was 

it sufficient? 

12) Are your dredges capable of tracking and recording the position of each 

draghead? Have you done tracking relative to a buffer zone? Would you have 

a problem providing this information to the regulatory agencies?

2 
13) When offshore sand dredging is completed for a beach project, what does the 

bottom look like? Are there draghead tracks, (width and depth?) throughout 

the area? Are the tracks parallel or crossing? Can you provide examples to us 

of high resolution mapping of pre- and post-dredging seabed conditions for 

offshore dredging with TSHD?

14) When mining sand off shore, is the dredge tended by a survey boat?

There is an ongoing concern with marine mammal/dredge collisions and entrainment of 

Sea Turtles.

15) Other than turtles, has your dredge ever been in a collision with a marine 

mammal? Do marine mammals have a tendency to swim near an operating 

dredge?

16) When appropriate, does your dredge use a draghead designed to reduce the 

probability of entraining sea turtles? Is this use mandated by the Owner? Does 

the use of these dragheads reduce the productivity of the dredge? Is the 

modified draghead effective? Do you have any recommended changes to the 

design of the turtle deflector? Do you have any recommendations on 

operating techniques to avoid entraining turtles during offshore dredging 

operations?

17) What effect do the seasonal requirements restricting dredging due to the 

proximity of turtles have on the overall annual dredging schedule?

18) Does your dredge have a system to reduce pressure/flow at the draghead when 

the draghead is off the bottom? How does it work? 

19) How effective are observers and trawling to reducing turtle takes?

3 
Additional questions

20) What has been your experience dredging in a fishing ground? Has any 

fisherman or commercial fishing company complained about any aspect of the 

dredging process? Did you modify your operation to accommodate the 

fisherman?

21) What measures do you, the dredge operator, take to insure that the dredge 

does not damage underwater pipelines and cables, or archaeological 

resources?

22) Some operating companies have a policy of dredging localized zones to 

exhaustion before moving to further zones within the dredge area. This assists 

management of the resource, but it also helps to minimize occupation of 

seabed and allow maximum time for recovery of seabed resources. Does your 

company have a policy of zoned dredging, and what are your reasons for 

dredging policy? Do you have any information that documents the impact of 

your dredging operations on marine organisms in specific dredge sites? Do 

you have any information on the rates of recovery of biological resources at 

your sites following cessation of dredging?

23) Do you have any comments, general or specific, regarding dredging 

equipment and procedures and the reduction of adverse impacts on the 

environment?

4 
Appendix C3 

MIRO Sponsored Questions 

To European Dredging Contractors 

Appendix 
Minerals Management Service 

A REVIEW OF EXISTING AND EMERGING ENVIRONMENTALLYFRIENDLY OFFSHORE DREDGING TECHNOLOGIES 

We appreciate you taking the time to answer our questions concerning new and emerging 

technology and techniques designed to reduce the adverse environmental impacts of 

dredging sand and gravels in offshore borrow areas. 

For any appropriate question below, could you give an estimate of cost increase due to the 

application of environmental restrictions? 

A. PLUME RELATED IMPACTS 

Many of the perceived concerns are due to the plume resulting from hopper overflow and 

the bottom agitation at the draghead. 

No. Question Response 

1 What percent of material 

overflows the hopper while 

digging sandy, low silt content 

(5% less than 63 micron) 

material, assuming a 10 km

sailing distance to pump 

ashore 

2 Can a hopper dredger excavate 

sand from below a 1-meter silt 

overburden without removing 

the overburden? Does this 

result in significantly increased 

overflow losses and 

consequently an increase in 

turbidity? 

Mi n er al I n du str y Re sear ch Or gani sat i o n
No. Question Response 

3 In mining sand with low silt 

content (5% less than 63 

micron), with a turbidity 

requirement not to exceed 29 

NTUs above background, is it 

necessary to take special 

measures to meet this 

maximum turbidity 

requirement? 

4 What measures do you employ 

to minimize turbidity? 

5 If you use measures such as 

recycling overflow water back 

to the draghead, is there a 

reduction in dredging 

production? 

6 Have you completed research 

on passive and dynamic plume 

processes associated with 

overflow and is this 

information publicly available?

7 Do wind, wave, and/or current 

forces in licensed exploitation 

areas determine the direction 

the dredger works? If so, at 

what level do they begin to 

influence operations? 

8 What are the consequences of 

dredging perpendicular to the 

current in order to influence 

the shape and dispersion of the 

dredge plume? 

9 Given a mandate to reduce 

turbidity, what are the most 

cost effective ways of 

accomplishing the reduction? 

We understand this is a 

question of degree. Please 

explain the consequences to us.

10 What are your views on 

requiring overflow to be 

discharged below the hull? 
B. IMPACTS TO BENTHIC HABITATS 

Considerable concern has been expressed relating to the loss of benthic community. 

Recolonization rates are being studied, as are changes in substrate characteristics such as 

grain size, dissolved oxygen, etc. It has been suggested that dredging in patterns may speed 

the recolonization rate by leaving “refuge” areas (that have undisturbed sediment 

characteristics and undisturbed benthic communities). 

No. Question Response 

11 If there is a stipulation in the 

specifications that required that 

only 70% of a borrow area can 

be used and the unused portion 

cannot be on the boundaries, 

what would be the most 

efficient use of the area? What 

is the minimum width of cut 

that a hopper dredger can dig 

efficiently? The reason for this 

proposed stipulation is that the 

benthic community will 

recolonize faster if the area is 

dredged with intermittent nondredged areas. Assume a 

borrow area 1000 meters x 

2000 meters. 

12 Are you aware of any damage 

to hard sea bed caused by 

dredging, including covering 

by sediment? If yes, was a 

buffer or exclusion zone 

applied and was it sufficient? 

13 Are your dredgers capable of 

tracking and recording the 

position of each draghead? 

Would you have a problem 

providing this information to 

the regulatory agencies? 
No. Question Response 

14 When sand dredging is 

completed for a beach project, 

what does the bottom of the 

dredging area look like? Are 

there draghead tracks, (width 

and depth?) throughout the 

area? Are the tracks 

continuous or crossing? Can 

you provide examples to us of 

high resolution mapping of 

pre- and post-dredging seabed 

conditions for offshore 

dredging with TSHD? 

15 When mining sand, is the 

dredger tended by a survey 

boat? 

There is an ongoing concern with marine mammal dredge collisions. 

C. IMPACTS ON MARINE MAMMALS 

No. Question Response 

16 Has your dredger ever been hit 

by a marine mammal or turtle? 

Do marine mammals or turtles 

have a tendency to swim near 

an operating dredge? 

17 When appropriate, does your 

dredger use a draghead 

designed to reduce the 

probability of hurting 

mammals or turtles? Is this use 

mandated by the Owner? Does 

the use of these dragheads 

reduce the productivity of the 

dredger? Is the modified 

draghead effective? 

18 What effect do the seasonal 

requirements restricting 

dredging due to the proximity 

of mammals or turtles have on 

the overall annual dredging 

schedule? 
No. Question Response 

19 Does your dredger have a 

system to reduce pressure/flow 

at the draghead when the 

draghead is off the bottom? 

How does it work? 

20 How effective are observers 

and trawling to reducing 

mammal or turtle takes? 

D. ADDITIONAL QUESTIONS 

No. Question Response 

21 What has been you experience 

dredging in a fishing ground? 

Has any fisherman or 

commercial fishing company 

complained about any aspect 

of the dredging process? Did 

you modify your operation to 

accommodate the fisherman? 

22 What measures do you, the 

dredger operator, take to insure 

that the dredger does not 

damage underwater pipelines 

and cables? 

23 Do you have any comments, 

general or specific, regarding 

dredging equipment and 

procedures and the reduction 

of adverse impacts on the 

environment? 
No. Question Response 

24 Some operating Companies 

have a policy of dredging 

localized zones to exhaustion 

before moving to further zones 

within the Licensed dredge 

area. This assists management 

of the resource, but is also 

stated to minimize occupation 

of seabed and allow maximal 

time for recovery of seabed 

resources. Does your Company 

have a policy of zoned 

dredging, and what are your 

reasons for dredging policy? 

25 Do you have any information 

that documents the impact of 

your dredging operations on 

marine organisms in specific 

dredge sites? 

26 Do you have any data to 

demonstrate the direction and 

rate of deposition of material 

discharged by the dredger 

during dredging operations? 

Do you have information on 

the amounts of material 

discharged overboard as a 

proportion of that removed 

from the seabed? 

27 Do you have any information 

on the rates of recovery of 

biological resources at your 

sites following cessation of 

dredging? 

28 Do you have any comparative 

data to show whether dredging 

in strips to leave recolonising 

adults in the dredge site 

enhances recovery rates 

compared with sites where all 

the surface deposits are 

removed? 
No. Question Response 

29 What steps do you take to 

identify archaeological 

material in the dredged 

material? What steps do you 

take if such material is 

observed? 

30 How do you determine the 

width of any ‘exclusion zone’ 

that might be required to 

protect archaeological or 

biological resources of 

conservation significance 

within your dredge site? 

31 Have your dredging operations 

been subjected to any form of 

environmental window / 

seasonal constraint related to 

the natural marine 

environment, i.e. fish 

spawning/migration? If so, 

what is the main basis for such 

restrictions? 

32 How have post-dredging 

conditions been monitored as 

part of your operations? How 

have these results been 

disseminated following 

analysis? 
Contact details of your organisation 

Your Organisation 

Your Name 

Your Position 

Your Contact Details 

Contact details for return of the questionnaire 

R N Bray 

Dredging Research Ltd 

High Pines 

Hoe Lane 

Peaslake 

Surrey GU5 9SW 

United Kingdom 

 

Telephone: +44 1306 730867 

Fax: +44 1306 730882 

E-mail: nickbray@drl.com

RETURN DATE: End January 2004 
Appendix C4 

Summary Of 

All Industry Responses 

Appendix 
Question 

Number 

Question 

Plume Related Great Lakes Dredge & Dock Bean Stuyvesant Weeks Marine Manson 

1. What percent of material 

overflows the hopper whiledigging sandy, low-silt content material, assuming a 10 kmsailing distance to pump ashore?

We have not conducted an exhaustive study of this because it is not generallyrequired. In dredging sand for beach projects or for other uses it is usuallydesirable to have a coarser productthan a finer product. Therefore, unless there are restrictions regarding overflow, loading typically continues until the dredge obtains an economic load measured in displacement of the vessel. If silt or very fine-grained sands are part of the materials being dredged a considerable quantity may overflow. We have not conducted specific studies of this although I would suspect that the Corps of Engineers or others havestudied or modeled these items overthe years for various purposes. In rule of thumb type thought it is verydifficult when loading sand to the ships maximum tonnage to retain more thana few % of particles smaller than the 200 sieve (ie. .076mm). Sands with grain sizes between .076mm and 150mm are not well retained in a hopper. As the grain size increases they become more easily retained. Sands in the range of 150- 200mmbecome increasing retained within the hopper and above 200 mm I believe that most are retained. Other factors impacting overflow losses are loading concentration, flow rate, method of loading, hopper size, hopper layout / configuration. On a beach project in San Diego with a 3600 cy hopper dredge we experienced retention rates of between 16 and 100%. The majority of loads retained between 40 and 65% of the quantity discharged into the hopper. The average retention percentages and grain size distributions were asfollows: Retention D15mm D50mm D85mm

90%. 3 .56 .9 60% na .2 na 31 na .09 na

The percentage of materialoverflowed is the result ofmany different factors, the mostimportant of which is the % fines (i.e. silts and clays) in the material being dredged. The other factors that come into play are the overall grain size distribution, the overflowingmethod (overflow type andlocation) and the duration ofoverflowing. The greater thesailing distance the longer the overflowing duration in order to achieve an economic load, The % overflow increases with increasing overflow duration. 

Unable to answer with any accuracy due to a lack of information provided. Overflowlosses are a function of several variables. The terms “sandy” and “low-silt content” is used todescribe the material. A grainsize distribution curve is needed to determine both the grain size and the percentage of fines thatare to be expected in arepresentative sample. Sand isclassified as having a grain sizebetween .06mm and 2mm.Obviously, the higher the concentration of fines, the greater the losses will be. Additionally, it is impossible todetermine the percentage of silts from the information providedwithout a grain-size distribution curve, although, one couldexpect that most if not all of the silts will be lost via overflow. Sailing distance noted in the question has no bearing onoverflow losses expected. 

Material less than 200 siwill be lost in the overfloas the dredge loads. 

1 
Question 

Number 

Question 

Plume Related Great Lakes Dredge & Dock Bean Stuyvesant Weeks Marine Manson 

With respect to sailing distances, generally, the greater the distance thelonger the vessel will overflow attempting to achieve its maximumload. 10 kms is a relatively short distance and therefore the vessel may depart the borrow site with somewhat less than a full load. I have attached a research paper fromMTI Holland on overflow losses. Please reference 04-06 Research onHopper Settlement.

2. Can a hopper dredge mine sand

from below a 1-meter silt overburden without removingthe overburden?Does this result in significantly increased material overflow and consequently an increase in turbidity?

Depending on the nature of the silt it may not be necessary to strip an area of silt before dredging the sand below it as the drag heads will penetrate through some of the silt. However it is likely that in dredging an area over time that the silt will eventually pass through the pumps and be returned as overflow. 

Where this silt settles will depend oncurrents and sea conditions in the area. Much of it may settle back very close to where it was originally dredged andmay even be dredged and overflowedmany times during subsequent dredge passes through the same area. Turbidity will be related to the volumeof silt overflowing. If overflow occurs near the surface it will be more visible than if it overflows at a lower elevation such as through overflows which discharge beneath the hull. However since hopper dredges normally dredge when underway to avoid potholing, the ships propulsion propellers may tend tospread the turbidity out even making it more visible at the surface. Air entrained in overflow water columns may also tend to bring turbidity to the surface increasing its visibility.

This is possible but generally leads to a significant increase in the turbidity created during dredging as part of theoverburden material is suckedin during the dredging process. In addition the characteristics of the silts must be such (very soft and low density) that the draghead be able to penetrate the overburden by means of its own weight. Yes, as explained above, some overburden will get mixed intothe mixture leading to more fines being overflowed. Theseadditional fines will lead tomore hindered settling and a higher percentage of fine sands being overflowed. 

A hopper dredge is not suited toremove material below a layer of silt without removing the overburden first. If theoverburden must be removed first, any overflow of material will create increased turbidity. 

It is likely to increase turbidity by entraining thsilt along with sand as dragheads work by erosi

3. In mining sand with a low-silt 

content, with a turbidity requirement not to exceed 29 NTUs above background, is itnecessary to take special 

It can require measures to meet a 29 NTU standard. Generally the only very successful means is to restrict overflow time. This can result inreducing the load size dredged. This 

This is highly dependent uponthe exact location and timing of the turbidity sampling relative to the working area and dredge. In low-silt material the settling

Again, the percentage of silt encountered will greatly affectthe turbidity created during dredging operations. Onespecial measure which could be 

For 29 NTU, with a low material, no special precautions are necessar

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measures to meet this maximum turbidity requirement?

can have severe economic effect on a project. Properties such as color of silt andother items may also have significanteffect on measurement of turbidity. In near shore borrow areas, thebackground NTU level is highly influenced by wind and currentconditions on a day-to-day basis. Our understanding of the 29 NTUstandard is that it came originally fromState of Florida Statutes for industrial discharges. What science the 29 NTU standard was originally based on I donot know. I am also unaware of any studies that indicate that 29 NTU’s is a significant level in effecting species at an offshore dredging site. It may be that levels significantly higher within a limited area or over a short duration may have little effect on organisms. 

of the fines is generally quick enough to remain withinturbidity requirements. 

implemented to reduce, to the point of not exceeding turbidity is to reduce time overflowing. A second measure would be toeliminate overflowing entirely. A third measure would be torecycle overflow water into the drag arm jet system.

4. What measures do you employ 

to minimize turbidity?

Typically turbidity at offshore borrow areas has not been a big problem. Primarily the borrow areas utilizedhave been picked because they are known to contain primarily clean sandwith low silt or clay content. Since the primary uses of the borrowed materials have been for beach nourishment and for construction aggregates, sites havebeen chosen which have qualitymaterials. Also in an offshoreenvironment the dispersal zone for turbidity is large enough that mixingoccurs rapidly.Dredging in more confined areas suchas channels, very near shore or in baysand rivers can be more problematic. Dredging near known reef areas ornear highly sensitive environmental areas has increased immensely. In recent years we have performed several projects in the Dade County, Florida area where the borrow sites were in close proximity of reefs. TheDepartment of EnvironmentalRegulation Miami (DERM) has closelymonitored impacts to the reefs duringdredging and has worked with contractors to modify dredging

By far the most effective is to reduce the overflowing time if necessary to nil, This reduces or eliminates the turbidity produced by the pumping process but costs can as much as double due to reduced loads. There also exist various‘limited overflow’ systems which modify the dredgingprocess in such a way that thedimensions and characteristics of any turbidity plume are reduced. 

Reduction of time pumping material past overflow or elimination of time pumping past overflow are two measures that will minimize turbidity. 

Limit overflow as necessary, depending onpermit conditions. 

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techniques as needed to minimize impacts. This has included limitingwork in certain areas of the borrow site periodically, reducing overflow time when necessary, and even changing the boundaries of the borrow site as actualconditions require. 

5. If you use measures such as 

recycling overflow water backto the draghead, is there a reduction in dredgingproduction?

We have not recycled overflow water back to the drag head because it has not been necessary to meet the needs todate. The costs of equipping orretrofitting a dredge to recycle all the excess water would be very expensive. For example if a dredge loadsapproximately 30% solids, you wouldhave to have a system which mustrecycle as much as 70% of its flow back to the drag head. This would takean additional piping and pumpingsystem not much smaller than the existing dredge pump loadingequipment.It might be accomplished without a large loss of pump production, there would be a huge Capital cost inequipping the dredge and significantlyhigher wear and maintenance costs for the equipment. Additionally the weight of the additional equipment would reduce the tonnage available for the Cargo and therefore reduce the loadsize causing an increase in the number of loads to complete a project. The maintenance and wear to the additional equipment would also add somewhatto downtime for the dredge thereby also negatively impacting the production. There were several foreign vessels built in the late 90’s equipped withrecycling systems. However, recent US and Foreign new builds have notincluded this equipment on the vessels. 

By recycling part of theoverflow water the volume of suspended sediments discharged is reduced as part ofthem are recirculated. Thiswon’t affect productions insands but will lead to increasedcosts due to the wear and tear of those components involvedin the recirculation.

Typically no, however, there isan increase in wear to the jet water systems. 

It is impractical to recycall overflow back todragheads. It will result silt being recirculated ancould increase turbidity oa smaller area. 

6. Have you completed research

on passive and dynamic plume processes associated with overflow and is thisinformation publicly available?

We have not conducted research. We have provided turbidity-sampling results to the owner of a project. However I know that the Corps engineering station (WES) inVicksburg has done studies and someresearch on hopper overflow and Turbidity. This information should all 

Studies such as thosementioned are generally carried out by large research institutes that develop the theoretical models and then turn to leadingdredging firms such as Bean Stuyvesant for the collecting offield measurements. 

The Norfolk District COE inconjunction with VIMS (Virginia Institute of MarineSciences) did an extensive research project on this subject in 1988 at the deepening of the York Spit Channel project. 

Have not done any resea

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be available. DERM may have somespecific information on the projects,which they have monitored which they would be willing to share. 

A good outline of this type of studies can be found in the Report CIRLA C547 ‘Scopingthe assessment of sediment plumes arising from dredging’.

7. Do wind, wave, and/or current

forces offshore determine the direction the dredge works?What are the consequences of dredging perpendicular to thecurrent in order to influence the shape and dispersion of the dredge plume?Do you have any data todemonstrate the direction andrate of deposition of material discharged by the dredger during dredging operations?

Yes, typically it is best to dredgestemming the current and seas. The ship is much more easily controlled inthis manner. If dredging broadside toheavy seas the ships will roll causingdifficulty in maintaining bottomcontact with the drag heads as they aredropped and snatched off the bottomwith each roll of the ship. The ship’s hull also tends to set over the downcurrent drag head and away from the up current drag head. This forces the drag tender to continuously raise each head and reset it on the seabed, which impacts loading production. It is much more difficult to maintain a course if seas, current, or swells are effecting the ship from a beamdirection. We have worked in suchconditions with difficulty and at times have been unable to continue dredgingoperations when we certainly could have if the borrow site layout was conducive to stemming currents, seas or winds. We do not posses any data showing the direction and rate of deposition of the overflow plume. However, the WES or DERM may possess suchinformation.

Wind and wave conditions only determine the dredgingdirection when they reach borderline magnitudes. At this point the captains orient their dredgers parallel to the wavedirections in order to avoid excessive roiling of the vessel. If strong currents are alsoencountered a compromise is reached that leads to minimumrolling while avoiding damageto the suction pipes. Dredging perpendicular to a current requires the vessel tosail at an angle to the dredgingdirection. The greater thecurrent the greater the angle. This can lead to the dragheadsslipping under the vessel with the consequent increased risk of damage to the equipment.Though there have been a number of attempts to modelthis process, to the best of our knowledge there is no hard data on this subject. This is in great part due to the considerable difficulties involved in trackingand recording an ever expanding plume withsuspended solids barely abovebackground levels. 

All three elements (wind, waveand current) have an impact on a hopper dredges ability tomaneuver and maintain position while dredging. As such, the dredge will often dredge in different orientations to thewinds, waves, and currents, asconditions (both physical and environmental) will allow. A consequence of dredgingperpendicular to the current isthe vessels possible restrictedability to maneuver and stay oncourse during dredgingoperations depending upon the speed of the currentencountered. Yes but only some bathmetry, change of which may be attributable to other factors.

The dredge typically woagainst current or sea conditions. Perpendiculaapproach requires thedredge to “crab” which clessen production by 10 20 %. 

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8. Given a mandate to reduce 

turbidity, what are the most cost effective ways of accomplishing the reduction?We understand this is a question of degree. Please explain the consequences to us. 

A concern that we have is that therewill always be a perception that reducing turbidity is important. This can easily translate into a mandate even though scientifically some turbidity can be easily tolerated.The most effective way to reduce turbidity is to not allow overflow. However when dredging sand, this will seriously reduce the volume of sandthe ship carries on each load. In somecompact sands the loading density ofthe slurry may be as low as 10%. Obviously, if a ship can carry for example 2400 cubic yards of sand in a 3600 cubic yard hopper but is restricted to no overflow and therefore can only haul 360 cubic yards (10 %)of its hopper capacity there is a huge impact as it will have to make almost 7times the number of cycles to obtainthe same quantity. Even if loose coarse sand can be loaded at 50 % solids in the slurry it would nearly double the number of loads with a commensurate cost increase. A reduction in the time of overflow will also lessen the turbidity somewhat, with less but still likely very significantimpact. The loading time reduction necessary to bring turbidity intoacceptable levels will be dependent on the character of the materials being dredged, the conditions encountered at the borrow site (ie: currents, seas, water depths, background turbidity levels, proximity and character of environmental resources to beprotected), as well as the levels and duration of turbidity produced. These will be very difficult to model and predict in advance of actual work and therefore will cause extreme risk andperhaps contingency in bids on

Eliminating overfloweliminates virtually all turbidity but at a very high cost. Limited overflowing (e.g. I hour) putsan effective ceiling on turbidity at reduced cost. These costs are inversely proportional to the duration permitted. Use of ‘limited overflow’ systems would have some cost impact depending on the method chosen. 

The most cost effective way toreduce turbidity during dredgingoperations is probably to refit the vessel to be able to recycle overflow water and not limitpumping times to the point of reaching overflow. 

The real way to limit turbidity is to reduce overflow time. This resuin a lesser load being carwhich increases the cost proportionately. For example, most dredges reach overflow at about minutes and full loads inabout 60 minutes. If adredge can carry 3000cy material it could load 50in 10 minutes and 3000c60. If the remainder of thcycle (sailing ,discharginand sailing) amounts to 1minutes, then the dredge deliver 500cy in 160 minutes or 3000cy in 21minutes. Assuming a cos$2,000. Per hour, this resin a cost of $10.66/cy wno overflow compared to$2.33/cy with overflowallowed. The above are jhypothetical numbers. 

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projects. 

Requiring overflow to be discharged

beneath the bottom of the hull will reduce turbidity in the upper water column. Some systems can be employed at a cost to reduce the amount of air that is entrained in theoverflow. This reduction of air thathelps lift some of the turbidity to the surface will further lessen turbidity in the upper water column, althoughcommensurately it may increase the turbidity in the lower water column as the sediments will be spreadthroughout a lower volume ofseawater. This system is called ananti-turbidity valve. Re-circulation of water back to the drag head may also have an impact inreducing the visible turbidity in the upper water column. However theparticles that cause the turbidity still are not retained in the hopper andeventually are returned to the sea. Typically multiple dredging passes are made in a borrow site and these particles will be moved around withinthe area regardless of what technique is employed. While it may reduce visible turbidity at the surface, it may causeincreased turbidity concentration in the lower water column or at the sea floor. I have attached a paper from MTI Holland on overflow systems. Please reference 04-09 Overflow Design. 

9. What are your views on 

requiring overflow to be discharged below the hull?

This is the normal practice within the industry except where the goal is todeliberately agitate material in order tohave currents carry the material away. For example at the Mississippi River Southwest Pass, the Corps deliberately uses agitation dredging and requires that overflow occur at or near the water surface. 

This is a simple and effective way of reducing turbidity byreducing the settling distance and settling time of any sediments released through the overflow. All Bean Stuyvesanthoppers are equipped accordingly. 

Unsure if this question means discharging at the drag head or at the bottom of a thru hull overflow system (as opposed toa water surface overflow system). 

It’s not a problem. 

Question 

Impacts to Benthic 

Habitats

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8 

10. If there is a stipulation in the 

specifications that required that only 70% of a borrow area can be used and the unused portion cannot be on the boundaries, what would be the mostefficient use of the area?What is the minimum width cutthat a hopper can dig efficiently? The reason for thisproposed stipulation is that thebenthic community will recolonize faster if the area isdredged with intermittent nondredged areas. Do you have any comparative data to show whether dredgingin strips to leave recolonizingadults in the dredge site enhances recovery rates compared with sites where all the surface deposits areremoved?

In general, borrow sites ofapproximately 6000 feet in length are ideal. They allow the hopper dredge to trail at 2kts, making one pass in each direction to obtain a full load. If only 70% of the site could be utilized, a ‘nodredge’ zone in the middle of the area would be preferred, as shown in the following drawing. 

6000lf

150 ft

No Dredging Zone 

The minimum width of the dredginglanes should be 150 feet. This will allow positioning of both drag heads within the dredge area at all times and eliminate any 1-arm dredging. We do not have any data with respect to the effect of strip dredging onrecolonizing adults. However, WESand DERM may have such information 

It would be viable to define dredging areas interspaced withnon-dredging areas so long as the work areas have the necessary characteristics not tolimit the standard dredgingprocess. To this end each dredging area should beroughly rectangular with itslong axis aligned parallel to the predominant currents and aminimum width of 500 ft at the maximum dredging depth. Ideally this area should also be at least 5000 ft long. An area less than 300 ft wide (at the maximum dredgingdepth) would lead to reducedproductions due to increasedmaneuvering as well as leadingto an inefficient removal of all the available sands. The reason for this proposed stipulation is that the benthic community will recolonize faster if the area is dredged with intermittent non-dredged areas. It is our understanding thatthere is currently a study being carried out in Europe under the name ‘Sandplt’ that addresses this issue. The DutchBijlcswaterstaat-NL has alsolooked into this and a conference was held on the issue in 2003 (see EMSAGG2003 conference). 

There is not enough information to answer this question. What is the total surface area (length and width) that “70%” would be used. There is an enormousdifference between using 70% ofa borrow area that is 500’ X 5,000’ and 70% of a borrow area that is 5,000 X 50,000’. The minimum width that a dredgecan dig efficiently is dependenton wind, wave, and current, butin general, a hopper dredge of300’ length and 55’ width can dredge efficiently in a cut width of 300 – 500 feet. We have nodate for the final question on this item.

The best way to work a swould be in rectangular strips, typically 300-500with by 5,000-6,000’lengths. No data is availafrom Manson.

11. Are you aware of any damage 

to hard bottoms caused by

There have certainly been cases, although the technology of monitoring

Beyond the damages caused byone of our competitors in the 

No. When pumping silty borrow material directly from a 

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dredging including covering by sediment?If yes, was a buffer orexclusion zone applied and was it sufficient?

has improved dramatically over the years. I cited earlier the projects inDade County Florida. These wereCorps projects that were closely monitored by DERM. Partneringbetween the Corps, DERM and the Contractor has been very successful atminimizing impacts. The borrow areas were first established with designated buffer distances from hard bottoms and then these buffers were modified as necessary during the project in order to account for actual conditions encountered 

very sensitive habitats of southFlorida we are unaware of anyadditional impacts arising fromdredging operations. In the case of Florida we do not recall if buffer zones were used. None the less, it is evident that due to the heavy activity taking place in the dredging area and the nature of the dredging process there isalways a high probability ofencountering noticeable sideeffects in the contiguous areas. As a result, based on the specific characteristics of eachsituation, buffer zones should be defined where limited impact of dredging isacceptable. 

borrow site, the concern is greater, however on suchprojects we have never seen itoccur. 

12. Are your dredges capable of

tracking and recording the position of each draghead?Have you done tracking relative to a buffer zone?Would you have a problemproviding this information to the regulatory agencies?

Yes, tracking and recording ofpositional information is standard operating procedure for our TSHDoperations. Great Lakes’ TSHD fleet isequipped with real-time monitoringelectronics and positioning softwareenabling us to effectively position our dredges at all times. Because the systems incorporate project control criteria and real-time dredge orientation our operational staff can efficiently plan and manage of precise dredging operations. Each of our TSHD’s use a combination of the following electronic equipment for dredge position monitoring. • Navigation / Guidance System. - 

Compaq Workstation with Great Lakes’ Hopper PositioningSoftware. 

• DGPS Receiver. - Trimble 

Navigation 4000 GPS receiver (or like) using RTCM corrections from US Coast Guard Beacon Transmitter or site specific Reference Stations. 

• Electronic Water Level Receiver. 

- Valeport VTM-710 or HazenHTG 5000. 

• Heading Sensor. - Sperry Marine 

MK-37VT Gyro Compass or like.

• Drag Head Depth Sensors. - 

Yes, all our hopper dredgersare equipped with thiscapability. It is often the situation that we must track the draghead position relative to a knownfeature or area, be that a bufferzone, shipwreck or submerged pipeline. No problem, this is pretty muchstandard procedure. In additionthe USACE often uses a ‘silent inspector system’ to track this kind of information independently of our own systems. 

Yes, the dredge guidance systemand Silent Inspector System is capable of tracking and recording the position (X,Y) and elevation (Z) of each drag head. Drag head and vessel tracking has been performed on everyproject for several years. Information can be provided uprequest. 

We typically track and record position of dredgedraghead and other parameters and furnish into COE on almost every project. 

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Various sensors use

d to attain

depth readings can include:differential pressure o

bservator

device on t

he drag hea

d a

nd 

gimbal, angle measurement devices, a

n

d h

ull mounted vessel

draft / press

ure sensors. 

Great Lakes’ hopper positionin

g

syste

m provides real-time displays of 

the dredge and contoured c

hannel plans

for reference b

y the dredge o

perators.

Dre

dge sensor information such as

DGPS information, hea

ding, dra

g hea

d

depth, tide, p

u

mp pro

duction

informatio

n, etc, is co

mpiled for the 

syste

m through several interfaces and

Pro

gram Logic Controllers (

PLC). In

ad

dition, relevant dred

ge data, such as 

dredge and drag

head coor

dinates, is 

displayed and 

may be store

d at userspecified intervals. 

See Figure 1 Attac

hed.

Examples of t

he t

ype data stored 

inclu

de: 

Loa

d No., Date m

m/dd/y

y

1/27/98,Ti

me 24

hr hh:

m

m:ss, Northi

n

g

of GPS Ant., Easting of GPS Ant., PortDra

g Hea

d Depth, Stb

d Drag Head

Depth, Fwd.

Draft, Aft Draft, Port

Density, Stbd Density, Port Velocity, Stbd Velocity, Port Pump RPM, Stbd Pump RPM, Tide Ft., Gyro, Speed, Port Gi

mbal Depth, Stb

d Gi

mbal

Depth, Easting of Bow, Northin

g of

Bow, Easting of Port Drag Head, Northing of Port Drag Hea

d, Easting of

Stbd

Drag Hea

d, Northi

n

g of Stbd

Dra

g Hea

d. 

At user-defined intervals the data is written to the hard drive of the positioning syste

ms co

mp

uter in

com

ma separated form. For e

xample: 

135, 12/19/03, 00:00:15, 171929.66, 929651.55, -36.3, -36.8, 18.9, 20.3, 1.00, 1.00, 0.00, 0.00, 218, 200, 5.76, 152.2, 6.92, 15.7, 16.3, 171874.00, 929815.30, 172078.80, 929759.80, 172121.50, 929829.10 

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Dredge operations staff utilize these track data files to pinpoint where the dredge has traveled throughout the previous days operations. As shown inFigure 2, the dredge’s draghead locations and depth is compiled into a track plot indicating the dredge area limits, project stationing, etc. To the question “Have you donetracking relative to a buffer zone?” Yes our TSHD positioning / trackingsystems track relative to any user defined zone provided the “bufferzone” means some area calling for special dredging or non-dredgingareas. Lastly, to the question “Would youhave a problem providing thisinformation to the regulatory agencies?” No, we would not. Theprocess of providing information toregulatory agencies is standard practice on every project administered by theU.S. Army Corps of Engineers. As youmay know, a majority of our domestic operations are for the Corps so such a requirements is expected. See Figure 2 Attached.

13. When offshore sand dredging is 

completed for a beach project, what does the bottom look like?Are there draghead tracks, (width and depth?) throughout the area?Are the tracks parallel or crossing?Can you provide examples tous of high resolution mappingof pre- and post-dredgingseabed conditions for offshore dredging with TSHD?

 I have attached before and after dredging plan view maps and cross section overlays of the Miami Beach / Sunny Isles North Borrow Area takenin late 2001 and summer of 2002. The cross sections clearly show drag headtracks and multiple pass trenches. These tracks / trenches are typically 10 to 20 feet wide and 3 to 4 feet deep. See Miami Beach Survey Data attached.

After offshore dredging in sands the bottom is generally somewhat more uneven than before dredging. The average roughness (i.e. difference between the highest and lowestspots in an area) will depend onthe original layer thickness to be removed and thecharacteristics of the borrow area (length, width, currents) 

A high quality survey would

allow individual tracks to berecognized though currents and wave action would steadilysmoothen the bottom until it is difficult to distinguish from the surface encountered before dredging As mentioned previously, a narrow borrowarea will lead to parallel tracks and a higher roughness. 

Unsure what the bottom lookslike at completion of the project. 

Offshore borrow areas whave tracks. The size and condition will depend ondimensions off the borropit and how the vesselworks. 

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Though rarely done in borrowarea, this roughness can be reduced at the cost of diminished productions. That information can beprovided to you separately as it will entail very large tiles. 

14. When mining sand off shore, is

the dredge tended by a surveyboat?

Normally, there is a survey boat utilized. Most project owners require at least before project and after project surveys. Contractors typically will survey for their own purposes inmanaging the resource. This vessel istypically employed on a 12 hour/day basis 

The availability of a survey boat will generally depend onthe contract specifications and the necessary level of follow up. The regularity of the surveys may vary between twice daily or only for beforeand after dredging surveys. 

A Survey/Crew boat is assigned to each dredge during all projects. 

Yes a survey boat is used

15. Other than turtles, has your

dredge ever been in a collisionwith a marine mammal?Do marine mammals have a tendency to swim near anoperating dredge?

We have never noted any collisionswith marine mammals, nor am I aware of any collisions between a hopper dredge and a marine mammal in the United States. I did hear several years ago that a dredge off of South Africa collided with a whale, and there certainly are documented collisions of ships with Marine mammals. However there is quite a difference between an ocean going ship sailing at 20-30 knots and most hopper dredges that typically attain maximum speeds of 10-15 knots. It should be noted turtle takes occur from contact with the dredge drag headat or near the sea floor not due to contact from the hull of a sailing dredge. We recently saw a permit thatrestricted dredge sailing speed whenturtles were in the area. Whenchecking with the permit agency wewere told that they didn’t request the restriction or think that it was necessary but that it had been in the proposed application. Such a restriction put in by an uninformedperson can have a huge impact on a project. We have not noted that marine mammals are attracted to dredges. 

To the best of our knowledge this has never happened. Inorder to reduce this risk we generally employ biologicalobservers to carry out aconstant lookout for such fauna in the areas of operation and if any arc spotted we modify our operations to minimize the risk of collision. Based on experience the onlymammals with a tendency toswim near a dredge aredolphins and seals. The former tend to escort sailing ships of any kind and the latter are known for their curiosity and tendency to sunbath on floatinglines and auxiliary equipment. 

No incidents with other marinemammals. In Ft. Pierce, Manatees have been seen in the vicinity of the dredge while it was dredging and transiting to/from the dig area. 

No, Not in our experienc

16. When appropriate, does your 

dredge use a draghead designed to reduce the probability ofentraining sea turtles?Is this use mandated by the

The following is the Corp turtle exclusion devise specification from ourpresent dredging project in Kings BayGeorgia. Hopper Dredge Operation:

Yes. In most cases. Yes, in many cases it does. 

When mandated by contractualrequirements, the dredges dragheads are outfitted with NMFS designed and approved TED’s (Turtle Excluder Devices). The use of TED’s does reduce 

We use draghead deflecton most projects. The bechange would be to allowthe deflector to float on tbottom.

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Owner?Does the use of these dragheadsreduce the productivity of the dredge?Is the modified dragheadeffective?Do you have any recommended changes to the design of the turtle deflector?Do you have any recommendations on operating techniques to avoid entrainingturtles during offshore dredgingoperations?

(1) The Contractor shall operate the hopper dredge to minimize the possibility of taking sea turtles and tocomply with the requirements stated in the Incidental Take Statement providedby the National Marine Fisheries Service in their Biological Opinion. (2) The turtle deflector device and inflow screens shall be maintained inoperational condition for the entire dredging operation. (3) When initiating dredging, suctionthrough the drag heads shall be allowed just long enough to prime the pumps, then the drag heads must be placed firmly on the bottom. Whenlifting the drag heads from the bottom,suction through the drag heads shall beallowed just long enough to clear the lines, and then must cease. Pumping water through the drag heads shall cease while maneuvering or during travel to/from the disposal area. (Information Only Note: Optimal suction pipe densities and velocities occur when the deflector is operated properly. If the required dredging section includes compacted fine sands or stiff clays, a properly configuredarrangement of teeth may enhance dredge efficiency, which reduces total dredging hours, and "turtle takes." Theoperation of a drag head with teeth must be monitored for each dredged section to insure that excessive material is not forced into the suctionline. When excess high-density material enters the suction line, suction velocities drop to extremely low levels causing conditions for plugging of the suction pipe. Dredge operators shouldconfigure and operate their equipmentto eliminate all low level suction velocities. Pipe plugging in the past was easily corrected, when low suction velocities occurred, by raising the draghead off the bottom until the suction velocities increased to an appropriate level. Pipe plugging cannot becorrected by raising the drag head off the bottom. Arrangements of teeth and/or the reconfiguration of teeth should be made during the dredging

Though no hard evidence is yet available, all signs indicate a greatly reduced incidence of sea turtle takes, It might bequestioned if this is due to the effectiveness of the design or a drop in turtle density but anecdotal evidence suggests a slight increase in turtle numbers.  No, none at this time thoughwe have made some practicalmodifications to reduce wear and tear. Our present operating procedures for eliminating takes apply equally well toinshore and offshore activities. 

productivity. Uncertain if the TED is or is not effective as it is designed. No recommendations on changes to design oroperating techniques. 

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process to optimize the suctionvelocities.) (4) Raising the drag head off the botto

m to increase suction

velocities is 

not acceptable. The primary a

djustment 

for 

providing ad

ditional mixin

g water 

to the suction line should be throu

g

h

water 

ports. To ins

ure that s

uction 

velocities do not dro

p below 

appropriate levels, the Contractor'spersonnel shall monitor production meters througho

ut the job and adj

ust 

primarily the nu

mber and opening

sizes of water ports. Water

port 

openi

n

gs on top of the dra

g head or on

raised stand pipes above the dra

g hea

d

shall be scree

ned before they are 

utilized on the dredging

project. If a 

dre

dge section includes sandy shoals

on o

ne e

nd of 

a tract line a

nd mud sediments on the 

other end of the tract line, the Contractor shall adjust the e

quipment 

to eliminate drag head pic

k-ups to clear 

the suction line. (5) Near the completion of each pay

ment section, the Contractor shall

perform sufficient s

urveys to 

accurately depict those portions of the acceptance section requiri

ng cleanup. 

The Contractor shall keep the dra

g

head b

uried a

minimu

m of 6 inches i

n

the sediment at all ti

mes. Although the 

over depth prism isnot the required dre

dgi

n

g prism, the 

Contractor shall achieve the required prism by removi

n

g t

he material from

the allowable over depth pris

m.

(6) Durin

g turning o

perations the 

pumps must either be shut off orreduced in speed to the point where nosuction

velocity or vacuu

m e

xists. 

(7) T

hese operational procedures are

intended to stress t

he i

mportance of

balancing the suction pipe densities and velocities in order to

keep fro

m

taking sea turtles. T

he Contractor s

hall

develop a written o

perational plan to

minimize turtle takes and submit it as part of the En

vironmental Protection 

Plan. (8) T

he Contractor must comply with 

all requirements of this specificatio

n

14 
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Plume Related Great Lakes Dredge & Dock Bean Stuyvesant Weeks Marine Manson 

and the Contractor's accepted Environmental Protection Plan. The contents of this specification and the Contractor's Environmental Protection Plan shall be shared with all applicable crew members of the hopper dredge. The use to TED’s is mandated by the Owner and dredging permit. The use of TED’s negatively impacts the productivity of the dredge. The leading edge of the TED is held a minimum of 6 inches in the seabed and in effect plows sand away from the drag head visor. Occasionally, the TED will plow into the seabed, burying the drag and slowing thevessel. When shell or gravel is encountered,the inflow screens plug up and havecaused hopper deck piping plugs. There is no hard data on the effectiveness of TED’s. There is no way of knowing what the number oftakes would be if TED’s were notemployed. Operating a hopper dredgewith one drag head TED equipped and one head unequipped has not beentested. The only testing we are aware of was done by Scripts Institute andinvolved the use of concrete turtles. Our recommendation would be toconduct dredging operations during the low turtle population season. Wetypically only encounter turtles in near shore borrow areas or shippingchannels and do not have manyproblems when working in offshore borrow sites. Please reference attached TED Operational checklist.

17. What effect do the seasonal 

requirements restrictingdredging due to the proximityof turtles have on the overall annual dredging schedule?

The seasonal effects have had a very large impact on the hopper dredgingindustry. Currently there are seasonal restrictions enforced by the US ArmyCorps on their projects in the South Atlantic area. For maintenance andnew work channel dredging it effects the Harbors from North Carolina south 

The use of seasonal restrictions, either due to turt1es or otherfactors, lead to a disproportionate volume of the total annual dredging workbeing restricted to a limited time period. As a result of this there is a lack of dredging 

Seasonal requirements increase the cost of our services someyears, and some years it doesnot. 

It bunches some of the work. 

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through the East Coast of Florida. The allowable operating windows in these areas run from December through March with some of the Ports restrictedto Dec15-February. There is also movement to extend these restrictionsto Virginia on the North and into theGulf of Mexico. This concentrates dredgingrequirements in these areas to a short winter time period. This if coupledwith a considerable amount of new work, heavy dredging needs on theMississippi River which sometimes occur during this period, or with otherheavy demand elsewhere can occasionally cause a short termdemand for hopper dredges which strains capacity. However, during the remainder of the year there can be serious overcapacity with a lot of idle plant. During the past few years there was overcapacity and considerable idle plant even during this time periodhowever.Of more concern to contractors is thatwith such tight contract periods there isoften little or no flexibility inscheduling equipment. If a dredgeunexpectedly has a significant problemrequiring repair they can be hard pressed to meet the schedules. With windows it no longer is a matter ofbeing a few days late and perhaps having some penalties, there can be a risk that the project won’t be completed.As regards maintenance and deepening work, there is an effect that these winter months are the worst weather months. This can moderately lower time efficiencies somewhat and make surveying to monitor projects more problematic. However, if and when, beach projects are restricted to thiswindow these effects are much more severe because the laying and moving of pipelines offshore and connecting tooffshore pumpout connections requires much better sea conditions to be

capacity (i.e. equipment) during part of the year leading to higher prices and. sometimes insufficient capacity tocomplete all projects. This is inthe interests of neither the clients nor the contractors, both of whom would benefit from a reduced overlap in the seasonal restrictions of different regions. 

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accomplished safely. Waiting for calmer seas to safely place, repair or move an offshore pipeline can cause weeks of delay to a project. 

18. Does your dredge have a

system to reduce pressure/flow at the draghead when the draghead is off the bottom?How does it work?

When the drag head is raised off the bottom as can be evidenced by the seating of theswell compensator ram, the drag tender manually reduces the dredge pump speed. Raising the drag head off theseabed is typically not a problem inborrow sites since there is generally plenty of material to dredge above the specified maximum depth, if one isstipulated. In channel dredging where a grade elevation must be achieved and the contractor is not paid for excess dredging, drags do occasionally hang off above the seabed 

In the event that the draghead comes off the bottom we havevarious technical solutions thatwould apply. One possibility is to redirect the flow so that it doesn’t come through the draghead, Another more attractive possibility is to stop the flow completely. Both these solutions would require relatively minor adjustments tothe equipment. None the less, the effectiveness of a reducedflow in reducing turtle takes is questionable. The system determines if the draghead is off the bottom bymeans of sensors on the draghead and swell compensation system. The flow is then interrupted by use of valves or the pump control. 

No such system exists on our dredges. 

No 

19. How effective are observers

and trawling to reducing turtle takes?

There is no relation between observersand the number of turtle takes. Observers may occasionally see turtles on the surface, but turtle takes occur onthe seabed. Observers inspect the dragheads, inflow and outflow screens anddocument / identify any takes. We believe trawling can be aneffective method to reduce the number of turtle takes. Great Lakes was involved on an emergency contract in Canaveral where trawling was performed by 2 vessels on a 24 hour /day basis for 3 days prior to the start ofdredging and in front of the vessel during dredging operations. Canaveralis one of the most populated turtle areas and no turtles were taken duringthe dredging operations 

The only contribution of‘observers’ is to inspect the turtle cages at the hopper inflows for any evidence of a take. In addition, if turtles are spotted at the surface you may draw the conclusion that thereis a high density of turtles inthe vicinity. This often triggers the requirement for turtle trawling. Trawling follows special guidelines (e.g. net type and size) and often lead to the capture of turtles in the generalarea which reduces the density of turtles in the vicinity of the dredging though by no means does it eliminate the risk of turtle takes. 

Observers have not proven toreduce turtle takes. Trawlinghas proven to be effective in some instances. 

Trawling when working channel seems to help. Observers just record results. 

Additional 

Questions 

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Plume Related Great Lakes Dredge & Dock Bean Stuyvesant Weeks Marine Manson 

20. What has been your experience

dredging in a fishing ground?Has any fisherman orcommercial fishing company complained about any aspect of the dredging process?Did you modify your operation to accommodate the fisherman?

We have conducted hopper-dredgingoperations near identified fishing grounds. The majority of problemsinvolve contact with nets or traps within the dredging area or the disposalroute. In some cases, the disposal route was modified to avoid concentrations of nets / traps.The Portland Corp District has investigated the impact of dredging onthe commercial crabbing industry inseveral West Coast Ports.

Designated dredging areas arealways very clear-cut and it isvery rare that permission be given to operate in a designated fishing ground. In such an event it would be the responsibility of the authorities to communicate to thefishermen that such a permission had been given and to deal with any conflicts of interest that might arise from it. Fishermen, despite a lack of evidence, often feel threatenedif dredging is carried out in orclose to their fishing grounds.Contrary to these complaints, the release of additional nutrients into the water columnoften attracts shoals of small fish and game fish leading to an improvement in the local sports fishing. In some cases small changes have been made to theoperating methods in order toresolve a potential conflict withfishermen, The responsibility for any additional costs arisingfrom these was determinedfrom the contract. Our company takes a pro-activestance with the fishing industry. Any conflict of interest is usually resolved prior to the issue of any permission todredge and mitigation measures adopted where appropriate. Fishing liaison officers we often employed at particularlysensitive sites. 

No experience dredging in a fishing ground. Haveexperienced isolated encounters with crap pots, lobster traps, and fishnets in the North East area, which has resulted in complaints from commercial fishermen who claimed to have lost equipmentdue to our operations. 

No real problem as most work is done in navigatiochannels. 

21. What measures do you, the 

dredge operator, take to insurethat the dredge does not damage underwater pipelinesand cables, or archaeological resources?

As shown in previous examples, our TSHD positioning displays providereal-time dredge orientation at all times. Using this system, operationsstaff can integrate pipeline locations, archeological resources, cables, etc., into the heads up displays such that the dredges operator can avoid such hazards. In Figure 3, a screen shot of our positioning system displays, the 

Assuming that accurate of suchobstacles are known with some accuracy, The coordinates of each obstacle, assuming that they are known with someaccuracy, are used as a basis for defining a no-dredging zone which is input into the onboardcomputer system. The dimensions of this zone are 

With respect to pipelines or cables, we make every effort tocontact the owners of them and request detailed location andelevation information. We also ask them to mark the location of their utilities (sometimes marked with buoys..etc) and give themthe option of placing their own representative on board to

Notify owners and obtaininformation on locations that cross the work area. 

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area highlighted in BLACK is an avoidance area. In this instance it is for shallow depths that could ground the ship but the same application is indicative to any predefined caution area or obstruction. See Attached Figure.

As a precautionary measure, someprojects warrant hydrographic surveyinvestigation prior to dredgingactivates to determine the locations ofunderwater obstructions. Such surveys are supported with a compliment of equipment that could include CesiumMagnetometers, side scan sonar, cable tracking devices, or high resolution multibeam / swath bathymetry systems. In most US government contracts, such underwater pipelines and cables, or archeological resourcesare previously located. Once the underwater obstruction locations are verified, operations staff integrates the information into the dredges positioning systems for dredge operator reference throughout dredgingactivities. Further, project meetings are held prior to dredging activities todiscuss the plan for avoiding suchobstructions. 

adapted to the local operating conditions to include sufficientsafety margin, both in horizontal and/or verticaldirection. This no-dredgingzone then shows up on the operator’s screen and in certain cases activates a proximity alarm signal. Depending on the level of automation the draghead will be hoistedautomatically if coming withinthe safety zone. The dimensions of these safetyzones are based on risk and will generally be in the order of50m – 100m in the proximityof cables or pipelines. 

witness our operations while dredging over or in the vicinityof their property. Should the pipeline or cable crossing be shown on the contract drawings, or the owner provide sufficientlocation information (X,Y, Z), that data is used to plot the utility on the dredge guidance screen so both the operator navigating the vessel and the drag tender operating the dredging gear can visually see where the utility is located. Typically, the customer (COE, State, or Private) will give uswritten direction to dredge over the utility, lift the dredging gear while navigating over the utility while dredging, or avoid the area completely where the utility islocated (buffer zone provided). Archaeological resources typically are noted on the contract drawing with an avoidance buffer zone placed around it. These noted areas are also put on the navigation screen and are avoided. 

22. Some operating companies 

have a policy of dredginglocalized zones to exhaustion before moving to further zones within the dredge area. This assists management of the resource, but it also helps tominimize occupation of seabedand allow maximum time forrecovery of seabed resources. Does your company have a policy of zoned dredging, and what are your reasons for dredging policy?Do you have any information that documents the impact of your dredging operations onmarine organisms in specific dredge sites?

Our standard operating procedure is todredge in specified lanes. In this waywe can move the dredge to an adjacentlane while surveys and volume computations are run to check progress and output in the initial dredging lane. This procedure is also beneficialduring clean up dredging and helps limit over dredging. We do not have any information inhouse on the impacts of lane dredgingon marine organisms. WES andDERM have conducted extensive monitoring studies of impacts tomarine organisms and rates of recoveryin borrow areas and should becontacted. 

Our company does not have a firm policy on this subject. Dredging strategies are projectspecific and aim to achieve the best possible economic and environmental situation.The collection of suchinformation is normally done by the project client independently of the dredgingcontractor. In addition these studies often extend well beyond the completion ofdredging making it difficult tofollow up on. As mentioned above, the results of these studies are not alwayseasy to come by, depend on

We have no “policy” regarding zone dredging as you note.Given a borrow area to dredge, we follow the contract specifications which typically give directions for materialremoval. Should no directivesbe given, we typically seek tofind areas of the borrow area which have the best production and dredge that area to exhaustion before moving on toless productive areas of the borrow area. No additional information available regarding additional questions listed.

Our work plans are typicdictated by the owner as what areas we work. All info concerning biologicresources would be accomplished by the permitting agency. 

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Do you have any information on the rates of recovery ofbiological resources at your sites following cessation of dredging?

many different factors and arevery variable in their results. The European studies mentioned in the answer toquestion 10 showed recoveryperiods that ranged between a few months and a few years.

23. Do you have any comments, 

general or specific, regarding dredging equipment andprocedures and the reduction ofadverse impacts on the environment?

While special precautions areobviously necessary when working inproximity of highly sensitive resources, these need to be addressed on a project specific basis. Often mandating restrictions and procedures is unnecessary and based on “feel good” perceptions rather than scientific need or practicality. We should becautious about over regulating a situation where the benefits are not well founded. 

It is essential that all parties involved pool their knowledge in an open discussion toachieve a balanced result thatbest achieves the interests of all. This process is also essential in building the mutual trust and understanding thatwill ensure the successful solution of any hurdles that might arise. Our company recognizes the adverse impacts that dredging might have.Every effort is made tominimize the impacts. Mitigation might include seasonal restrictions, minimization of impacted areaand developing site specific procedures just to name a few.It is in the interest of all todefine a feasible methodologythat allows often critical projects to proceed without delay.

We have dredged +/- 50,000,000cubic yards of sand fromoffshore borrow areas for beach nourishment in the past 10 years. We have damaged (1) utility cable, taken 0 turtles fromborrow areas and have notdamaged the environment or archeological resources to thebest of our knowledge. 

A typical hopper dredgeoperation in clean sand iprobably the least disrupto the environment. 

20 
Appendix C5 

Document And 

Powerpoint Presentation 

Provided By Jan de Nul 

Appendix 
JAN DE NUL

Group of Companies 

3er Congreso Argentino de Ingenieria Portuaria 5-6-7/11/2003 

13/01/05 1/ 11 
INDEX 

INDEX 2

SLIDES 3

1. INTRO 3 

2. COMPANY AND ENVIRONMENT 4

3. DREDGE TYPES 4

4. TSHD ‘CRISTOFORO COLOMBO’ 5

5. LOW TURBIDITY VALVE 6

6. ‘GREEN PIPE’ 6 

7. FILLING THE HOPPER 6

8. THE OVERFLOW FUNNEL 6

9. THE SECOND DREDGE PUMP 6

10. THE RE-CIRCULATION PIPE 6

11. THE SUCTION HEAD 6

12. SUCTION HEAD RE-ENTRY 6

13. PROCESS WATER 7

14. ADDITIONAL FILLING 7

15. MINIMUM TURBIDITY 7

16. LTD ‘DIRK MARTENS’ 7

17. LOW TURBIDITY DREDGE HEAD 7

18. TSHD ‘CRISTOFORO COLOMBO’ : TURBIDITY PLUME 8

19. THE DREDGING PROCESS 8

20. FIRST PHASE OF THE DREDGE CYCLE 8

21. LOADING WITHOUT OVERFLOW 8

22. ADDITIONAL LOADING WITH STANDARD OVERFLOW 8

23. PLUME GENERATION WHEN LOADING WITH STANDARD OVERFLOW 8 

24. ADDITIONAL LOADING WITH LOW TURBIDITY VALVE 9

25. PLUME GENERATION WHEN LOADING WITH LOW TURBIDITY VALVE 9 

26. ADDITIONAL LOADING WITH ‘GREEN PIPE’ 9

27. PLUME GENERATION WHEN LOADING WITH ‘GREEN PIPE’ 9

28. TURBIDITY PLUME DISPERSION COMPARISON 9

29. DREDGE TECHNIQUE COMPARISON 10

30. BACKGROUND TURBIDITY 10

31. CONCLUSIONS 10

32. OPERATIONS WITH LTD ‘DIRK MARTENS’ 11

33. GENERATED TURBIDITY 11

34. SECONDARY SOURCES 11

35. BACKGROUND TURBIDITY 11

36. CONCLUSIONS 11

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SLIDES 

1. Intro 

2. 

3. 

4. 

JAN DE NUL Group is one of the mayor dredging contractors of the world, leading the 

market with the most modern fleet of dredging equipment. During the last 10 years JDN 

expanded its fleet with 13 newly built units. Some of these new dredging vessels have been 

milestones in the industry. In 1992 the JFJ DE NUL, a trailing suction hopper dredge with a 

hopper capacity of 11.750 m3, the biggest at that time in the world, was commissioned and 

deployed at the construction of the century, as it was called at these times: The Chep Lap Kok 

Airport Platform in Hong Kong. 

The largest added vessel is a trailing suction hopper dredge with a hopper capacity of 33.000 

m3: the Vasco Da Gama. Today the vessel is foreseen to be extended to 44.000 m3 hopper 

hold, to become again the largest trailing hopper of the world. It is clear that these new vessels 

have added to the companies’ growth thanks to their size and new technologies which were 

implemented. The size of these equipment was unthinkable 10 years ago. Large reclamation 

works in the Far East and Europe would not have been executed by dredgers without these 

mega dredgers. Today almost the whole world fleet of jumbo and mega dredgers, constructed 

over the last years is operating on large reclamation works in Singapore, Hong Kong, Korea 

and Taiwan. It would require too much time to talk about the technological developments 

which helped built these vessels. 

The South American dredging market normally has no requirement for these large dredges, 

which have been built mainly for dredging sand at sea, transport it over considerable distances 

in open sea and pump it over several km into reclamation areas. South America, and Argentina 

in particular, with the relative shallow waterways and no need for large reclamation, will not 

see these mega dredgers deployed in the near future. 

Actually the group has a still 5 dredges under construction, 2 sister vessels with a capacity of 

11.300 m3, another 2 sister vessels with a 4.400 m3 hopper and one new cutter suction dredge, 

which will be again the most powerful of the world in its kind. These 5 dredges will come into 

operation at the end of this year, while the next 4 split hoppers are already ordered and should 

be delivered in 2005. 

Another main development in the dredging technology is the evolution in alternative dredging 

methods for environmental sensitive projects. The need for environmentally acceptable 

solutions in dredging is something, which evolved all over the world, and will be more seen in 

the near future, also in South America and Argentina. Therefore I will give an overview of 

some new dredging methods developed by our company over the last years. Monitoring 

campaigns have demonstrated their efficiency and limitations. 

As an environmentally conscious international company, Jan De Nul is involved in 

dredging projects around the world with strict requirements concerning removal and 

3er Congreso Argentino de Ingenieria Portuaria 5-6-7/11/2003 

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spreading of pollutants. A major policy of Jan De Nul is translated in a continuous strive to 

develop more efficient and environment friendlier dredging techniques. 

5. Company and Environment 

As environmental considerations continue to become more and more important throughout 

the world there are continuing calls to reduce the effects of dredging operations on the 

water column and surrounding marine environment. Dredging causes particulate 

suspension when certain soil types are disturbed and this can effect not only the balance in 

the water column, but also the sea bed environment following subsequent settlement of 

suspended particles. In particular whilst removing layers of contaminated or polluted silt 

for subsequent treatment processing, particulate suspension should be reduced to a 

minimum.

Jan De Nul Company recognises both the necessary stringent constraints now being placed 

on projects involving dredging works and the effect that dredging operations can, in some

circumstances have on the marine environment. 

6. Dredge types 

At present two categories of dredges were equipped with special facilities enabling to work 

in accordance with strict environmental requirements. 

In the next sections the performance of a trailing hopper dredge and a stationary dredge are 

discussed with special attention to the impact on turbidity. 

What is turbidity ? 

Turbidity is a measure for the reduction of the transparency of a liquid due to the presence 

of non dissolved particles. 

A bundle of light beamed into a liquid will be attenuated when dissolved elements in the 

liquid cause a change of colour and will be dispersed if the liquid contains non dissolved 

particles. 

Different approaches to measure the turbidity have been developed. Depending on the 

approach the turbidity will be expressed in different units. International norm ISO 7027 

explains 4 methods to determine the water turbidity : 2 semi-quantitative and 2 

quantitative methods. 

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Semi-quantitative method 1 : A clear graduated glass tube with a black mark on a white 

background at the end is used to evaluate the liquid transparency by measuring the 

height of the liquid (cm) when the mark fades 

Semi-quantitative method 2 : A white round plate which is immersed into the water until it 

becomes hardly visible. The depth is measured to 1cm accurate when less then one 

meter and to 10 cm when more then one meter. 

Quantitative method 1 : An optical sensor measures the intensity of the dispersed 

(backscatter) light with a cell, immersed in a fluid with non dissolved particles. The 

turbidity is given in nephelometric turbidity units (NTU).

Quantitative method 2 : An optical sensor measures the intensity of the attenuated light 

(transmissiometer) with a cell, immersed in a fluid with non dissolved particles. The 

turbidity is given in nephelometric attenuation turbidity units (NAU). 

Different methods use different units and depending on the used calibration fluid different 

measuring units are adapted. The Jackson turbidity meter used to be the standard tool for 

turbidity measurement and Jackson turbidity units (JTU) have been the standard for a long 

time. At present optical sensors are calibrated using formazin calibration fluid and FTU 

(formazin turbidity units) have become the new standard. No general correlation between 

the different unit systems can be established. 

Formazin is a chemical solution and is prepared as follows : 

- dissolve 10 g hexamethylenetetramine in water and dilute to 100 ml 

- dissolve 1g hydrazinesulphate (poisonous) in water and dilute to 100 ml

- mix 5ml from both dilutions and further dilute to 100 ml to obtain a 400 FTU solution 

The suspended solids concentration is the dry weight of sediment devided by the weight of 

sample (expressed in ppm) or by the volume of sample in liters (expressed in mg/l). 

The terms turbidity and suspended solids concentration are similar but not equal.

7. TSHD ‘Cristoforo Colombo’ 

This modern dredge of Jan De Nul’s versatile fleet combines following dredging 

techniques : 

 standard dredging without overflow : dredging with one or two suction pipes until 

the overflow level is reached in the hopper 

 standard dredging with overflow : continue loading the hopper barge with higher 

density while process water leaves the hopper by standard overflow 

 low turbidity valve in the overflow funnel : an adjustable valve in the overflow 

funnel chokes the flow in such a way that no air is taken down with the suspension 

leaving the hopper 

 ‘green pipe’ : the overflow suspension is pumped trough an additional pipe, mounted 

on top of the suction pipe, back to the suction head where it is used as process water 

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8. Low turbidity valve 

When using the dredge technique with the low turbidity valve, without ‘green pipe’. An 

adjustable valve chokes the flow in such a way, that no air is taken down with process 

water leaving the hopper. The result is a density stream, causing a minimum of turbulence, 

taking the excessive material back to the sea bottom. 

Without low turbidity valve (standard overflow), a big volume of air is taken with the 

overflow water due to the big fall height. Underneath the vessel a density stream of 

haevier particles moves down and at the same time an upward airstream occurs. These 

opposite actions cause a lot of turbulence with spreading of the plume as a result. 

9. ‘Green pipe’ 

Using the ‘green pipe’ feature , a recycling pipe is mounted on the dredge pipe. When the 

overflow level is reached in the hopper, the overflow suspension is pumped through this

recycling pipe , back to the suction head where it is used as process water. As positive 

result the overflow water does not fall through the complete watercolumn to the bottom. 

10. Filling the hopper 

Activating the ‘green pipe’ : the hopper is filled up to the overflow level. 

11. The overflow funnel 

Activating the ‘green pipe’ : the process water flows into the overflow funnel. 

12. The second dredge pump 

Activating the ‘green pipe’ : the process water flows into the second dredge pump. 

13. The re-circulation pipe 

Activating the ‘green pipe’ : the process water is pumped trough the re-circulation pipe. 

14. The suction head 

Activating the ‘green pipe’ : the process is pumped back to the suction head. 

15. Suction head re-entry 

Activating the ‘green pipe’ : the process water re-enters the suction head directly or trough 

the jet pipes.

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16. Process water 

Activating the ‘green pipe’ : the process water flows into the overflow funnel is recirculated. 

17. Additional filling 

Activating the ‘green pipe’ : additional filling of the hopper is enabled. 

18. Minimum turbidity

Activating the ‘green pipe’ : during additional filling of the hopper minimal turbidity is

generated. 

19. LTD ‘Dirk Martens’ 

Focused on strict environmental performance criteria in mind a dredge with the following 

features was developed: 

- able to remove thin layers of silt with high accuracy reducing the overdredged 

volume and the mixing of clean soil with slightly polluted silt 

- dredge at in situ density for optimal utilisation of the barge capacity 

- dredge in shallow areas 

- minimise the mechanical disturbance to reduce the turbidity generation and the 

mobilisation of pollutants 

- extended automation and monitoring of the dredging process 

20. Low turbidity dredge head 

To cope with these challenges Jan De Nul developed a special suction head : the so called

‘Low Turbidity Dredge Head’ (LTDH). The sweephead for LTD (low turbidity dredge) 

‘Dirk Martens’ has been designed to dredge at maximum productivity and accuracy and 

with minimum disturbance to the environment. 

The sweephead has two inlets and works without additional mechanical movements. A 

hydraulic valve in the head opens the inlet towards the dredging direction while the shape 

of the contact surfaces ensures optimal sediment transport. Process water is minimal, 

which means that the material is pumped at almost in situ density. 

Additional sensors and instruments allow for better process monitoring and more accurate 

dredging. 

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21. TSHD ‘Cristoforo Colombo’ : turbidity plume 

During dredge operations conducted with different dredging techniques, the performance

related to the turbidity criterion was established. 

Using the standard dredging technique as reference the following techniques were 

monitored : 

- standard dredging without overflow 

- standard dredging with overflow 

- environmental valve in the overflow funnel 

- ‘green pipe’ 

22. The dredging process 

The turbidity generated during the dredge operations using the different dredging method 

was monitored starting from an empty hopper. 

23. First phase of the dredge cycle 

During the first fase of dredge cycle, independent of the adapted dredging technique, 

material is pumped into the hopper without overflow. 

The suction head always causes some disturbance near the interface between dredged and 

non-dredged material and a small amount of solids is brought into suspension near the 

bottom. 

24. Loading without overflow

The particles are brought in suspension at the suction head disperse in a plume close to the 

seabed and settle down again within a relative short time. 

25. Additional loading with standard overflow 

Once the overflow level is reached, with the overflow water, a big volume of air is taken 

due to the big fall height. Underneath the vessel a density stream of heavier particles

moves down and at the same time an upward airstream occurs. These opposite actions 

cause a lot of turbulence with spreading of the plume as a result. 

26. Plume generation when loading with standard overflow

While the dredge trails on the vessel’s propellers pass this area only seconds later. The 

energy of the revolving propellers spread the particles at high speed in all directions. 

The turbidity plume stretches now from the surface to the seabed and settlement is as 

follows : the density stream of quasi unstirred and heavier particles and the slow 

settlement of stirred material and fine particles. 

3er Congreso Argentino de Ingenieria Portuaria 5-6-7/11/2003 

13/01/05 8/ 11 
27. Additional loading with low turbidity valve 

In contradiction with the standard cycle, almost no air is taken with the overflow water as a 

result of the choking effect of the environmental valve. Underneath the vessel the density 

stream is almost not disturbed and the overflow suspension sinks to the seabed. 

28. Plume generation when loading with low turbidity valve 

A small fraction, mainly fine particles, is caught in the turbulence around the vessel. This 

fraction is spread out when the vessel’s propellers pass and settles slowly afterwards. 

29. Additional loading with ‘green pipe’ 

The overflow suspension is pumped back to the suction head where it is recycled almost

integral as process water. The fraction leaving the suction head contains a suspension

which is mixed during the dredge process by passing through suction pipe, dredge pump, 

hopper, discharge pump and discharge pipe and generates a turbidity increase near the 

suction head. 

30. Plume generation when loading with ‘green pipe’ 

Due to the mixing process the particles will stay longer in suspension compared to the 

particles brought in suspension during the cycle without overflow. 

31. Turbidity plume dispersion comparison 

The graph shows cross sections of the turbitity plume for each dredge technique at an

interval of 5 minutes. High sediment concentrations are dark, low concertrations are light. 

Comparing the plumes created with standard overflow and low turbidity valve : 

- the plume width for the low turbidity valve is narrower 

- the particles settle much faster from the upper section for the low turbidity valve 

Comparing the ‘green pipe’ and no overflow cycles with these cycles: 

- the turbidity stays close to the bottom and the plume never reaches the water surface 

3er Congreso Argentino de Ingenieria Portuaria 5-6-7/11/2003 

13/01/05 9/ 11 
32. Dredge technique comparison 

3⁄4 When using the environmental valve in the hopper overflow the quantity of solids 

staying in suspension long enough to spread out through current effects is reduced to 

41 percent of the quantity generated during the standard cycle. 

3⁄4 With the ‘green pipe’ cycle the quantity of solids in suspension is reduced to 21

percent compared to the standard cycle. Apart from this the generated plume is 

situated just above the seabed which reduces the settlement time considerably. 

3⁄4 Dredging without overflow reduces the generated turbidity to 13 percent of the 

quantity measured for the standard cycle. Everything happens very close to the seabed 

which further reduces the duration of the settlement process. 

33. Background turbidity 

The selection of the dredge technique to be adapted on a specific project will not only 

depend on turbidity-related performance. 

Many ports are located near river estuaries where huge quantities of silt carried from 

upstream have settled. Exposed to hydro-meteorological forces, particles will be brought 

in suspension during periods of strong current or wave action and will settle down again

during calm sea conditions. 

The displayed example shows the tidal background turbidity at the cross entrance of the 

Port of Zeebruges, located near the Scheldt estuary. The suspended solids concentrations 

during periods of strong current raised to such a level that a different colour scale had to be 

used. Dark brown for the dredge induced turbidity plumes indicated 300 mg/l or more, for 

the background turbidity dark brown indicates concentrations higher than 1000 mg/l. 

The graph on the right illustrates the contributing parameters even better. The brown line 

shows the background concentration of suspended solids on a time axis, the other lines 

show the impact of the different dredging methods. 

34. Conclusions 

3⁄4 Plume generation relative to the plume generated during the dredge cycle with standard 

overflow is reduced to 41 % by using the low turbidity valve. Working with the ‘green

pipe’ will generate 21 % and dredging without overflow will reduce the turbidity

generation to 13 % compared to the standard dredging cycle.

3⁄4 Application of alternative dredging techniques is not relevant if the additional turbidity 

generated during the dredging works is only a fraction of the background turbidity.

3er Congreso Argentino de Ingenieria Portuaria 5-6-7/11/2003 

13/01/05 10/ 11 
35. Operations with LTD ‘Dirk Martens’ 

To evaluate the performance of this dredging technique the following aspects were 

considered : 

- generated turbidity 

- secondary sources 

- background turbidity 

36. Generated turbidity

During the dredging operations with low turbidity dredge ‘Dirk Martens’ the turbidity

increase at 100 m from the dredge location amounted to 16 mg/l average and 43 mg/l

maximum. 150 m away from the dredge location these figures were respectively 11 and 31 

mg/l. At 200 m from the dredge location the amounts further reduced to 6 and 23 mg/l. 

37. Secondary sources 

In shallow areas with a silty top layer on the bottom the suspended solids concentration 

increase caused by secondary sources (e.g. a manoeuvring vessel) easily exceeds 150 mg/l. 

38. Background turbidity 

In silty shallow port areas where current speeds remain low 10 to 50 mg/l suspended solids 

are measured for currents lower than 0.1 m/s, 70 to 150 mg/l for current speeds between 

0.1 and 0.3 m/s. 

39. Conclusions 

With the development of the low turbidity head of ‘Dirk Martens’ the amount of particles 

brought into suspension during the actual dredging has been reduced to such a level that, 

especially in shallow areas, secondary sources and background variations generate more 

turbidity than the dredgehead. 

3er Congreso Argentino de Ingenieria Portuaria 5-6-7/11/2003 

13/01/05 11/ 11 


Environmental considerations become m

and more important throughout the wor

As an enviroconscious incompany, a Jan De Nul nin a continuodevelop morenvironmendredging tec
Trailing hopper dredge :

-

low turbidity valve

-

green pipe

Stationary dredge :

-

low turbidity dredge head
Multi - Functional Trailing Suction Hopper 

‘Cristoforo Colombo’Features following dredging techniques :

- Standard dredging without overflow- Standard dredging with overflow- Low Turbidity Valve in overflow fun- Environmental dredging with ‘Green 
Low Turbidity Valve

An adjustable valve in the overflow funnel chokes the flow in such a way that no air is taken down with the suspension leaving the hopper.
‘Green Pipe’

The overflow suspension is pumped trough an additional pipe, mounted on top of the suction pipe, back to the suction head where it is used as process water.
Activating the ‘Green Pipe’

the hopper is to the overflo
Activating the ‘Green Pipe’

process water fthe overflow fu
Activating the ‘Green Pipe’

to the second dpump ...
Activating the ‘Green Pipe’

and is pumpean additional 
Activating the ‘Green Pipe’

back to the suhead ... 
Activating the ‘Green Pipe’

where it re-ensuction head 
Activating the ‘Green Pipe’

and is re-circprocess water 
Activating the ‘Green Pipe’

which enables afilling of the the 
Activating the ‘Green Pipe’

with minimal generation.
Low Turbidity Dredge ‘Dirk Martens’

Features :

-

accurate removal of thin layers of sil

-

dredge at in situ density

-

dredge in shallow areas 

-

minimise

the mechanical disturbance 
‘Low Turbidity Dredge Head’

-

two inlets

-

a hydraulic valve opens the inlet t

dredging direction

-

the shape of the contact surfaces e

optimal sediment transport 
Turbidity plume generated during dredging 

operations with TSHD ‘Cristoforo

Colombo

Compared dredging techniques :

-

Standard dredging without overflow

-

Standard dredging with overflow

-

Low Turbidity Valve in overflow fun

-

Environmental dredging with ‘Green 
The dredging process :

-

commencing from an empty hopper
- dredged material is pumped into the hoppe

turbidity plume
- filling the hopper (loading without overflow

turbidity plume
- additional loading with standard overflow

turbidity plume dredge head

turbidity plume overflow

continue loading
- plume generation while loading with standard 

turbidity plume dredge head

turbidity plume overflow

continue loading

New parameters : - upward airstream- propeller impact
- additional loading with low turbidity valve

turbidity plume dredge head

turbidity plume overflow

continue loading
- plume generation while loading with low turbi

turbidity plume dredge head

turbidity plume overflow

continue loading

New parameters : - density stream 
- additional loading with ‘green pipe’

turbidity plume dredge head and ‘green pipe’

continue loading
- plume generation while loading with ‘green pip

turbidity plume dredge head and ‘green pipe’

continue loading

New parameters : - recirculation fluid 
Comparison turbidity plume dispersion :

Standard overflowLow turbidity valve‘Green pipe’No overflow

Elapsed time : 00:30 05:00 10:00 
Comparison dredge techniques :

Comparison dredge techniques

0

102030405060708090

100

0:00:00 0:05:00 0:10:00 0:15:00 0:20:00 0:25:00 0:30:00 0:35:00 0:4

Time elapsed since plu

me generation

Relative plume

StandardEnvironmeGreen pipNo overflo
Background turbidity :

HW -

4:00

Zeebrugge - CDNBImportance of turbiditycompared to backgrounear the dredge plume 

0

200400600800

100012001400160018002000

10:00 10:30 11:00 

Time 

Average turbidity (mg/l)

HW -

3:00

HW -

2:00

HW -

1:00

HWHW + 1:00HW + 2:00HW + 3:00
Conclusions :

• Plume generation relative to the plume generated d

cycle with standard overflow :

- low turbitiy valve : 41 %-

‘green pipe’ : 21 %

- without overflow : 13 %

• Application of alternative dredging techniques is no

additional turbidity generated during the dredging a fraction of the background turbidity .
Turbidity plume generated during dredging 

operations with LTD ‘Dirk Martens’Performance evaluation aspects :

-

Generated turbidity

-

Secondary sources

-

Background turbidity 
Generated turbidity :

Turbidity increase :

- 100 m from dredge location : average 16 mg/l - maximu- 150 m from dredge location : average 11 mg/l - maximu- 200 m from dredge location : average 6 mg/l - maximu
Secondary sources :

The turbidity increase caused by a manoeuvring vesseeasily exceeds 150 mg/l.
Background turbidity :

10 - 50 mg/l durinlow current speed

70 - 150 mg/l during higher current speed
Conclusions :

With this dredging technique the amount of particles brougduring the actual dredging has been reduced to such a level shallow areas, secondary sources and background variationturbidity then the dredgehead.
Appendix D 

Workshop Details 

• Appendix D1 - Invite Letter To Attendees 

• Appendix D2 - List of Attendees 

• Appendix D3 - Workshop Agenda 

• Appendix D4 - Powerpoint Presentation Given At 

The Beginning Of The Workshop 

(See Enclosed CD) 

• Appendix D5 - Meeting Summary Notes 

By Jim Clausner, USACE 

Appendix 
Appendix D1 

Invite Letter 

To Attendees 

Appendix 
W.F. Baird & Associates Ltd. 

2981 Yarmouth Greenway 

Madison, WI 53711 

Telephone: 608-273-0592 

Fax: 608-273-2010

 March 22, 2004 

Baird Re: MMS Contract 0103CT71516, Review of Existing & Emerging Environmentally 

Friendly Offshore Dredging Technologies 

Dear Sir/Madam:

You are hereby invited to attend a workshop on the above noted project sponsored by 

MMS. Most of you will have already been informed of the workshop by Barry Drucker 

at MMS. The workshop will be held at the Days Hotel and Conference Center in 

Herndon, Virginia. We are planning sessions for the morning and afternoon of Thursday, 

April 1st and the morning of Friday, April 2nd. We will have time on Friday afternoon for 

further discussion if issues develop that need additional time. The hotel has set aside 

rooms for those attending from out of town. Reservations can be made by calling the 

hotel at 703-471-6700 and referencing the Baird conference. These reservations must be 

made by March 25th to take advantage of the conference rate ($109.00/day). The Hotel 

offers a complimentary 24-hour shuttle from Washington Dulles Airport. 

Attached is a brief outline of the study with a tentative Workshop Agenda. We intend a 

full discussion on all agenda items but mostly focusing on Item 6, Item 7 and Item 8. 

 

We look forward to your participation. Do not hesitate to contact us if you have any 

questions. 

Sincerely, 

W.F. Baird & Associates Ltd. 

Rob Nairn, Ph.D., P.Eng. 

Principal 

File No. 10687 

Innovation, Excellence & Service 

Oceans, Lakes & Rivers
Appendix D2 

List of 

Attendees 

Appendix 
Name Affiliation Telephone Email 

Barry Drucker MMS 703-787-1296 barry.drucker@mms.gov 

Chris Spaur USACE 410-962-6134 christopher.c.spaur@usace.army.mil 

Tony Giordano MMS 703-787-1283 anthony.giordano@mms.gov 

Will Waskes MMS 703-787-1287 will.waskes@mms.gov 

Bill Hanson GLDD 630-574-3469 whhanson@gldd.com 

Bill Pagendarm GLDD 630-574-2990 wfpagendarm@gldd.com 

Nick Bray Dredging Research, Ltd. 011 44 1483 860 731 nickbray@drl.com 

Jim Clausner USACE/ERDC 601-634-2009 James.E.Clausner@erde.usace.army.mil 

Doug Clarke USACE/ERDC 601-634-3770 Douglas.G.Clarke@erdc.usace.army.mil 

Shawn Alam MMS 703-787-1690 Shawn.Alam@mms.gov 

Maureen Bornholdt MMS 703-787-1300 maureen.bornholdt@mms.gov 

Keith Good MMS 703-787-1052 keith.good@mms.gov 

Roger Amato MMS 703-787-1282 roger.amato@mms.gov 

Ancil Taylor Bean Stuyvesant 504-587-8600 ataylor@efbean.com 

Rob Nairn Baird & Associates 905-845-5385 rnairn@baird.com 

Tim Kenny Baird & Associates 608-273-0592 tkenny@baird.com 

Jacqui Michel Research Planning, Inc. (803) 256-7322 x 329 jmichel@researchplanning.com 
Appendix D3 

Workshop Agenda 

Appendix 
WORKSHOP AGENDA 

Review of Existing and Emerging Environmentally 

Friendly Offshore Dredging Technologies 

April 1st and 2nd, 2004 

Sponsored by Minerals Management Service, US Dept of the Interior 

W.F. Baird & Associates has been retained by the U.S. Department of the Interior, 

Minerals Management Service (MMS) to conduct studies relative to the mining of sand 

for beach nourishment and construction aggregates. The borrow areas are located on the 

Outer Continental Shelf (OCS) under Federal jurisdiction. 

The United States Government, and specifically, the MMS, a bureau within the U.S. 

Department of the Interior, has jurisdiction over all mineral resources on the Federal 

OCS. The MMS has the authority to convey, on a noncompetitive basis, the rights to 

OCS sand, gravel, or shell resources for shore protection, beach or wetlands restoration 

projects, or for use in construction projects funded in whole or part or authorized by the 

Federal Government. MMS has provided Federal sand for beach nourishment projects in 

New Jersey, Maryland, Virginia, Florida, South Carolina, and Louisiana. 

Offshore sand-dredging for beach nourishment projects employ hydraulic dredges almost 

exclusively and are normally either cutterhead or hopper dredges. The process may result 

in adverse effects on various components of the marine or coastal environment. 

The offshore-dredging industry is constantly changing as the industry strives to make 

operations more efficient. New advances in offshore-dredging technology are leading to 

more environmentally-sensitive offshore operations. Researchers are actively increasing 

the knowledge base relative to physical processes involved in dredging procedures. 

Physical and mathematical modeling of these processes is being conducted with the aim 

to reduce the negative environmental aspects (biological and physical) associated with 

the offshore removal of sand. New dredging technologies being used overseas are 

contemplated for work on the OCS. 

As the Federal agency responsible for regulation of OCS sand resources, the MMS must 

ensure that sand and gravel dredging operations conducted under its jurisdiction are 

conducted in a safe and environmentally-sound manner. This may, in some instances, 

entail the required use of particular dredging equipment or techniques. Thus, MMS must 

have sound knowledge of the most current dredging technologies available.

The objective of the study is to review and analyze dredging equipment and projects on a 

worldwide basis to identify both existing and emerging dredging technologies that aim to 

lessen or avoid potential adverse effects on the offshore biological and physical 

environment.

Contacts were made with Federal and State natural resources agency staff and others 

involved in research on the impacts of dredging, studies of the life history of special 

species of concern, and permit approvals to determine the direct and indirect impacts that 
are of greatest concern for dredging operations in the OCS. Recent MMS-sponsored 

reports and environmental assessments on dredging impacts in the OCS were also 

reviewed. This identification of the perceived environmental impacts of greatest concern 

will be used to evaluate the advances in dredging techniques and equipment to measure 

their success in reducing the degree of such impacts. 

The prioritized list of perceived concerns from OCS dredging operations on marine 

biological and physical resources is shown below.

1. Short-term and cumulative impacts from dredging that lead to loss or reduced 

stability of benthic habitats, including recolonization by an altered biological 

community. All resource managers raised this concern. The greatest concern 

was in known benthic-associated fishery areas, such as the surf clam fishery 

off New Jersey and the shrimp fishery in the Gulf of Mexico. There was less 

concern in areas of general biological productivity or dynamic processes, such 

as in South Carolina. 

2. Injury and death of special species of concern (e.g., Sea Turtles) from being 

sucked into the draghead or cutterhead during dredging operations. This 

concern was raised by every Federal agency with management responsibility 

for T&E species. Most agency staff thought the existing stipulations were 

effective, but even a single “take” was considered a significant impact. 

3. Changes in the substrate characteristics (grain size, dissolved oxygen, 

compaction and organic content) that lead to a reduction in benthic 

communities AND suitability of the area for future dredging. This concern 

was identified in South Carolina where 3- to 40 meters of sediment was 

removed. The depressions persisted for many years and filled with finegrained sediments. 

4. Changes in bathymetry that can alter the wave climate reaching the shore. The 

importance of this concern varied by region. Where the OCS sand bodies were 

close to shore and/or shallow enough to influence the wave climate, there was 

high concern regarding the potential for increased shoreline erosion. The 

orientation, depth, and shape of the sand body and borrow areas should be 

considered in evaluating the impact of dredging on wave climate. 

5. Damage to hardbottom habitats: physical damage during dredging; burial by 

suspended sediment during dredging; and altered sediment processes that 
could bury hardbottom. This issue was of concern when dredging smaller sand 

bodies in between hardbottom habitat, even though these areas are supposed 

to be avoided. 

6. Creation of depressions and furrows from removal of substrate. Though MMS 

has a “no pits” stipulation, there was still concern that furrows might interfere 

with bottom fishing. At least one responder thought that the furrows acted as 

recruitment sources, supporting the idea of leaving strips of undredged areas. 

7. Short-term increased turbidity from cutterhead or draghead and overflow from 

hoppers on benthic species. Most responders assumed that OCS dredging 

occurred in sandy substrates, thus turbidity would be short-term and animals 

would avoid turbid areas. However, turbidity might be more of a concern in 

areas where a fine-grained overburden has to be removed to access the coarser 

sediment. This impact could also include burial by sedimentation from fallout 

of the plume. 

8. Spatial and seasonal conflicts between dredging and commercial and 

recreational fisheries. 

9. Potential to break an active or abandoned pipeline, resulting in a release of 

petroleum. 

10. Collisions with marine mammals and Sea Turtles during vessel operations. 

11. Damage to archaeological resources.

Our current study is focused on Atlantic and Gulf Coast sand borrow sites. The sites 

range from 5 kilometers to 20 kilometers offshore. The water depth at these sites varies 

from 5 meters to 25 meters deep. The material to be dredged for borrow is assumed to be 

sand with an average grain-size of 0.30 mm and less than 10% passing the 200 sieve. As 

a part of this study, we are reviewing the current scientific data to determine which of the 

perceived concerns enumerated above are real and need to be addressed. 

We have distributed a survey to the Dredging Contractors in the United States and 

Europe, who mine sand from offshore borrow sites for aggregates and beach 

renourishment. We have also sent the survey to the Corps of Engineers Hopper Dredge 

Operations in Portland. The questions for the survey were based on the information 

developed during the literature review portion of the study. There are twenty-three 

questions in the survey meant to elicit comments relative to the perceived impacts 
enumerated above. The questions fall into the broad categories of plume related impacts, 

impacts to benthic habitats, dredge/marine mammal collisions and related questions. 

Please see attached list of questions. We anticipate receiving comments from the 

Contractors and Corps within the next week. Once the comments are received they will 

be summarized in preparation for the Workshop. 

The Workshop is intended to provide a forum for the interested parties to review the data 

and opinions collected to date. It is hoped a discussion of the issues will provide a clear 

direction for the remainder of the study. The goal is to provide MMS with 

recommendations on the feasibility and performance of existing and emerging dredging 

technologies that reduce the adverse environmental impacts of dredging on the Outer 

Continental Shelf. 

A tentative agenda for the Workshop follows. Once we have reviewed the Contractor 

survey results, we will allocate times to the agenda that reflect the relative interest in the 

different issues. There will be a full discussion of all agenda items with a focus on Items 

6, 7, and 8. 

 

AGENDA 

1. Introduce background of the project and objectives 

2. Review of Perceived Environmental Impacts of Dredging (US perspective) 

3. Summary of Known Impacts Related to Offshore Dredging (US/UK/International 

perspective) 

4. Current Efforts to Mitigate by Stipulation (US Perspective) 

5. Review of Contractor’s Survey (US, European and Worldwide) 

6. Environmentally Friendly Technologies Related to Offshore Dredging (from 

Contractors Survey and Literature Survey) 

7. Assessment of Effectiveness of New Technology for Offshore Work 

8. Recommendations – Which technologies are feasible and effective, what is the cost 

implication of these, if any. Methods to implement promising technologies. 
Appendix D4 

Powerpoint Presentation Given 

At The Beginning Of The Workshop 

(See Enclosed CD) 

Appendix 
Baird – RPI – DRL - MES

Review of Existing and Em

Environmentally-Friendly O

Dredging Technologi

Workshop to Develop Recomme

Workshop to Develop Recomme
Baird – RPI – DRL - MES

Workshop Outline Workshop Outline

 Introduction to project and object Review perceived impacts Literature survey and questionnai Current efforts to mitigate by 

stipulation

 Appropriateness, effectiveness an

practicality

 Recommendations and summary

1
Baird – RPI – DRL - MES

Team Members and Worksho

Team Members and Worksho

Participants Participants

 Baird & Associates Research Planning, Inc. Dredging Research Ltd Marine Ecological Surveys MMS, USACE, dredging con

consultants
Baird – RPI – DRL - MES

Goals and Objectives Goals and Objectives

 Evaluate dredging equipment and 

worldwide basis to identify existiemerging technologies that aim to avoid potential adverse impacts obiological and physical environm

 Develop recommendations for an 

implementation strategy for promequipment or approaches
Baird – RPI – DRL - MES

Understanding the Impacts Understanding the Impacts

 UK/Worldwide and US state 

understanding reviews

 Real vs. perceived is really a 

sensitivity of receptor to impa

 Knowledge is rapidly develop

impacts (mostly through monmodeling)
Baird – RPI – DRL - MES

Physical and Biological Environmen
Baird – RPI – DRL - MES

Physical and Biological Impacts
Baird – RPI – DRL - MES

Environmentally Environmentally-Friendly Te

Friendly Te

 Literature surveys (scientific 

industry periodicals, etc.) werin US and UK (latter with wofocus) 

 Two questionnaires were issu

the US (MMS sponsored) and from UK (MIRO sponsored)
Baird – RPI – DRL - MES

Stipulations Current and Futu

Stipulations Current and Futu

 Review existing legislation, s

guidelines and best practices, overseas

 The primary focus of current 

in the US are measures to promammals, pipelines and arch 

 This project will provide 

recommendations to revise, r
Baird – RPI – DRL - MES

Evaluation of Possible Techn

Evaluation of Possible Techn

 Appropriateness, effectiveness an Appropriate: consider sensitivity 

to impact and relative improveme

 Effectiveness: how well does the 

measure work?

 Practicality: feasibility of implem

to the dredging process (capital anmaintenance) 
Baird – RPI – DRL - MES

Summary and Recommendati

Summary and Recommendati

 Matrix of physical and biolog

and related environmentally-fapproaches

 Summary of appropriateness, 

effectiveness and practicality 

 Propose guidelines and stipul
Baird – RPI – DRL - MES

Purpose of this Workshop Purpose of this Workshop

 Discussion of real vs. perceiv Discussion of environmentall

technologies and approaches

 Evaluation of appropriateness

effectiveness and practicality and new technologies/approa
Baird – RPI – DRL - MES

Framework for EvaluatingEnvironmental Impacts from OSC Dredging

1. KNOW impacts are small or recovery 

action is necessary

2. KNOW impacts are not small or recov

Take actions to minimize impacts and recovery

3. UNCERTAIN if impacts are small or 

quick – Take actions, monitor results, 
Baird – RPI – DRL - MES

PERCEIVED ENVIRONM

IMPACTS FROM OCS DR

ON MARINE BIOLOGICA

PHYSICAL RESOUR
Baird – RPI – DRL - MES

Impact # 1: Benthic Habitats

 Short-term and cumulative impac

dredging that lead to reduced biolproductivity of benthic communit

 All resource managers raised this c

greatest concern was in known benfishery areas, such as the surf clam Jersey and the shrimp fishery in the Less concern in areas of general biproductivity or dynamic processes.
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES

Impact # 2 : Sea Turtles

Loggerhead - ThreatenedGreen - Endangered Endangered populatio

Florida; others are listed as 

Leatherback - Endangered Endangered

Kemp’s ridley - Endangered Endangered

Hawksbill - Endangered Endangered
Green

Ha

Leatherback
Kemp

Loggerhead
Kemp's Ridley Kemp's Ridley Hawksbill Loggerhead Loggerhead Green 
Baird – RPI – DRL - MES

Impact # 2 Turtles: Documen

Sea Turtle “Takes” 1995-2003 (Channel DGalveston 31New Orleans 39 (most in MRMobile 0 (only requir

and screen

Jacksonville 6NMFS: even with observers/deflectors/re

documented takes = 50% of actual
Baird – RPI – DRL - MES

Impact # 2 Turtles: Documen

Sea Turtle “Takes” from Sand DredgingBouge Bank, NC 5 12/01-04/02Bouge Bank, NC 1 2003Myrtle Beach, SC 11 1997-99 Canaveral Shoals 1 2001None reported in the Gulf of Mexico
Baird – RPI – DRL - MES

Impact # 3: Changes in SubstCharacteristics

 Changes in the substrate characteristics 

dissolved oxygen, compaction and orgalead to a reduction in benthic communisuitability of the area for future dredgin

 This concern was identified in Sout

where 3-4 meters of sediment were depressions persisted for many yeawith fine-grained sediments (van Do
Baird – RPI – DRL - MES

Impact # 4: Bathymetry Chan

 Changes in bathymetry that can a

climate reaching the shore. 

 The importance of this concern vari

Where the OCS sand bodies were and/or shallow enough to influence climate, there was high concern abofor increased shoreline erosion. 

 Modeling studies have been used t

impacts; need to determine what ch“significant”
Baird – RPI – DRL - MES

Impact # 5: Hardbottom Habi

 Damage to hardbottom habitats: Physic

during dredging; burial by suspended sdredging; and altered sediment processhardbottom. 

 Of concern when dredging sand in hardbo

 Highest along the Florida coast  Growing awareness that hardbottom ha

common along the mid-Atlantic coast, tOCS sand borrow sites
Baird – RPI – DRL - MES

Impact # 6: Depressions and Fu

 Creation of depressions and furro

removal of substrate that might inbottom fisheries

 Rate of infilling by sedimenta

slumping of the sides will be 

 No existing data on whether 

actually been impacted
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES

Impact # 7: Turbidity

 Short-term increased turbidity fro

or draghead and overflow from hbenthic species. 

 Assume that OCS dredging occurs 

substrates, thus turbidity is short-teMay be of concern where a fine-graoverburden has to be removed

 Extensive studies of sand and grav

UK
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES

Far Field Plume Impacts Far Field Plume Impacts
Baird – RPI – DRL - MES

Race Bank is 6.5 km from Area 107 Dredge 50 - 150 mg/l elevation 7% of time
Baird – RPI – DRL - MES

Evidence of a Turbidity Current Evidence of a Turbidity Current
Baird – RPI – DRL - MES

MMS Plume Model MMS Plume Model
Baird – RPI – DRL - MES

MMS Plume Model MMS Plume Model
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES

Plume Model Output Plume Model Output

Current Direction

Accum. (m) 

Boundary oBorrow Are

Direction of Dredging Operation

Sediment Footprint Outside of Borrow Bo

0 100 200 300m

Scale
Baird – RPI – DRL - MES

Turbidity Turbidity – Quantity vs. Disp

Quantity vs. Disp

 Total amount of overflow Extent of dispersion
Baird – RPI – DRL - MES

Turbidity and Sedimentation 

Turbidity and Sedimentation 

Conundrum Conundrum

 Reduce dispersion - increase 

dynamic plume and pancakincurrent – far field influence

 Increase dispersion - larger p

sedimentation is thinner, wid
Baird – RPI – DRL - MES

Impact # 8: Dredging - Fishin

 Spatial and seasonal conflicts betw

and commercial and recreational 

 No data on degree of significanc Could be prevented by coordina

fisheries groups and notifications dredging
Baird – RPI – DRL - MES

Impact # 9: Structural Damag

to O & G

 Potential to cause structural dama

gas infrastructure by direct contacstabilization, and erosion

 Great concern in the Gulf of Mex Planned MMS study to address 
Baird – RPI – DRL - MES

Impact # 10: Mammal/Turtle

Collisions

 Collisions with marine mammals 

during vessel operations

 Existing stipulations include (probably 

If operating in areas of known whalobservers are required. If whales aravoid intentional approaches within right whales) and slow speeds to lesSee stipulations for sea turtles.
Baird – RPI – DRL - MES

Impact # 11: Archaeological

Resources

 Damage to archaeological resourc

- Structural damage from direct con- Soil de-stabilization leading to exp- Burial (not a technical problem, bu

recreational access)
Baird – RPI – DRL - MES

OCS Study MMS 2004 OCS Study MMS 2004-0

http://www.m http://www.mms.gov/sandandgravel sandandgravel/OtherG

OtherG
Baird – RPI – DRL - MES

Archaeological Study

Recommendations

1.Implement a GIS-based data 

strategy

2.Refine and test baseline studi

define the potential archaeoloresource base

3.Require state-of-the-art mean

and identifying those resourc
Baird – RPI – DRL - MES

Archaeological Study

Recommendations

4.Develop a scientific basis for 

for resource protection, based 

 Uncertainty in the resource loc Accuracy of dredge positioning Ultimate stable slope Prevention of full pedestals
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES

Guideline for Dredging to P

Pedestal Formation
Baird – RPI – DRL - MES

Archaeological StudyRecommendations

5. Dredging operations

 Require DGPS positioning equ

tracking software

 Require plots of actual dredge 

buffer zones
Baird – RPI – DRL - MES

Archaeological Study Recommendations

6.Require monitoring during dr

(high-potential areas):

 Onboard monitor Random monitoring of sedimen

beaches

 Test the relic landform hypothe

7.Post-dredging, document effe

buffers and slope stability of 
Baird – RPI – DRL - MES

Impact # 12: EFH Impacts

 Change in shoal shape that degrad

 Limited data on potential degree 

greatest concern is cumulative, limpacts

 Current MMS study to assess st

and potential impacts of dredgin
Baird – RPI – DRL - MES

Unique Habitats Associated with Shoals
Baird – RPI – DRL - MES

Ridgesand Shoals –TechnicalBackground
Baird – RPI – DRL - MES

Ridges and Shoals – TechnicaApproach
Baird – RPI – DRL - MES

Literature Review Literature Review

 TU Delft, Texas A&M, U W

CISTI, British Library

 USACE DOER Program World Dredging, Internationa

Review, Terra et Aqua, DredConstruction 
Baird – RPI – DRL - MES

24 check

Assessment Of Short Term Environmental Impacts On Dredging InA Tropical Estuary

Cbalchand A.N and K.Rasheed 2000 Terra et Aqua num

25

Ocean Wave Attenuation Due To Soft Seafloor Sediments(Personal Interest) Kraft L.M et al 1990 Marine geotechno

26

check/send rob

An Approach To The Physics And Modeling Of Submarine Flowslides Norem et al 1990 Marine Geotechno

27 check Environmental Protection Spurs Dredging Technology XXXXXX 1998 Sea technology, V

28

Navigation In Marine Protected Areas: National And InternationalLaw Spadi F 20000 Ocean developme

29 ? Environmental Considerations To Channel Dredging Ghobrial F 1987 Coastal Zone 87 V30 check

Biological Effects Of Marine Sand Mining And Fill Placement ForBeach Replenishment: Lessons For Other Uses Hurme A.K 1988 Marine mining Vo

31 check Fisheries Interests And Ocean Mining Scarratt D.J 1987 Marine mining Vo32 check

A Process For Setting, Managing And Monitoring EnvironmentalWindows For Dredging Projects NRC US 2002 Transportation res

33 x

Engineering Design And Environmental Assessment Of Dredged Material Overflow From Hydraulically Filled Hopper Barges In USACE WES 1990 USACE report D-9

34 x

Ecological evaluation of a beach nourishment project at hallandale,FL, Volume I evaluation of fish populations adjacent to barrow USACE CERC 1980 CERC report MR 

35 si

A Common Sense Plan For Prevention Of Overflows: Applicable ToAll Rivers With Sandy Channels Shaughnessy M ------------ ------------------------

36 Need To Get37 ?

Practical Criteria To Assess Dredging Methods On EnvironmentalAspects Arts T 1993 CEDA Dredging d

38 1 Modern Dredging Methods And Their Environmental Aspects

Van Drimelen N.J and 

Loevendine N.J 1988 CEDA Dredging d

39 yes Environmental Impact Of Water Injection Dredging Verweij, JF, Winterwerp, JC 1999 CEDA Dredging d40 1

Environmental Dredging Technology In Close Cooperation WithLocal Partner Shipyards Pflug J. and Ohlig, F 2000 Int Conf/ Exhibitio

41 x

Environmental Impact Assessment Of Dredging Project In The Yagtze Estuary Zhou Q.Y. 2001

Dredging for prospcongress and exh

42 3

Numerical Simulation Of The Sedimentation Process In A Trailing Suction Hopper Dredge Van Rhee C 2001

Dredging for prospcongress and exh

43 3

SSFATE (Suspended Sediment Fate), A Model Of SedimentMovement From Dredging Operations Anderson E etal 2001

Dredging for prospcongress and exh

44 yes One Mans View Of Dredging Equipment 2020 Greener, G.E. 1994 Dredging and dred45 yes Dredging And The Environment Van Diepen H et al 1993 Bulletin de PIANC46 3 The Environment Friendly IHC Cutter ------------- 1992 Ports and dredgin

47 yes

Environmental Dredging: New Techniques From Europe And The US ------------- 1995 Port Engineering 

48 yes Environmental Friendly Dredging Techniques In The Netherlands ------------- 1997 Port Engineering 49 yes

Sedimentation Engineering Techniques For EnvironmentallyFriendly Dredging Kirby R 1994 Underwater Techn
Baird – RPI – DRL - MES

Literature Review - Environmentally Friendly Dredging TechnologyMineral Management Service 2004

Title Authors date Source

2 si

Proceedings Of The National Workshop On Methods To MinimizeDredging Impacts On Sea Turtles

Dickerson Dena.D & NelsonDavid A. 1988 USACE report EL-90-5, USAED, Jacksonville FL

3 ?

Sand Waves; Engineering Considerations And Dredging Techniques Alexander Michael P 1990 USACE report HL-90-17, USACE Washington DC

4 x

Benthic Community Response To Dredging Borrow Pits, PanamaCity Beach, Florida Saloman Carl H et al 1982

USACE report MR 82-3, Coastal engineering researFort Belvoir, VA 22060

5 x

The Ecological Impact Of Beach Nourishment With DredgedMaterials On The Intertidal Zone At Bouge Banks, NC

Francis J Reilly and VincentJ Bells 1983

USACE report MR 83-3 , Coastal engineering reseaFort Belvoir, VA 22060

6 beach

Tylers beach, Virginia, dredged material plume monitoring project27 Sep to 4 Oct 1991 Michelle M Thevenot et al 1992 USACE report DRP-92-7, WES

7 x

Seasonal Restrictions On Dredging: An Approach Toward IssueResolution Mark W La Salle 1992 USACE report D-92-1, Environmental laboratory WE

8 si

A Framework For Assessing The Need For Seasonal RestrictionsOn Dredging And Disposal Operations Mark W La Salle et al 1991 USACE report D-91-1, Environmental Laboratory W

9 si

Effectiveness Of Sea Turtle Deflecting Hopper Dredge Draghead InPort Canaveral Entrance Channel, FL

David A Nelson and Deborah J. Shafer 1996 USACE report D-96-3, WES EEDP (Environmental 

10 x Effects Of Dredging And Disposal On Aquatic Organisms Nina D Hirsch et al 1978 USACE report DS-78-5, US Army Washington DC11 x Environmental Effects Of Dredging ---------------------- 1983 ---------------------12 rob

Patterns Of Succession In Benthic Infaunal CommunitiesFollowing Dredging And Dredged Material Disposal In Monterey John S Oliver 1977 USACE D-77-27

13 si

Natural Gas Pipeline Rupture And Fire During Dredging Of TigerPass, Louisiana NTSB 1996 National Transportation Safety board report NSTB/P

14 x

The Marine San And Gravel Dredging Industry Of The UnitedKingdom Uren M.J 1988 Marine mining, Vol 7 pp 69-88

15 quick Seabed Sand Mining In Japan Tsurusaki K. et al 1988 Marine mining, Vol 7 pp 49-6716 si

Effects Of Marine Mining Dredge Spoils On Eggs And Larvae Of ACommercially Important Species Of Fish, The Mahimahi Jokiel P.L 1989 Marine mining, Vol 8 pp 303-315

17 si The Red Sea Pilot Project: Lessons For Future Ocean Mining Amann H. 1989 Marine Mining, Vol 8 pp 1-2218 si

Liquefaction In The Coastal Environment: An Analysis Of CaseHistories Chaney R C 1991 Marine Geotechnology Vol 10 pp 343-370

19 check Impact on the Environment of turbidity caused by dredging

Pennekamp J G S and M.PQuaak 1990 Terra et Aqua Number 42

20 x Assessment Of Offshore Sand And Gravel For Dredging Selby I and Ooms K 1996 Terra et aqua Number 6421 si Turbidity Caused By Dredging; Viewed In Perspective Pennekamp J G S et al 1996 Terra et Aqua number 6422 rob has

Effects Of Dredging And Dumping On The Marine Environment OfHong Kong Evans N. C. 1994 Terra et Aqua number 57

23 si

Dispersion In The Marine Environment Of Turbidity GeneratedOverflow Bonetto E 1995 Terra et Aqua number 58
Baird – RPI – DRL - MES

50 x Marine Aggregate Dredging In The UK: A Review Singleton G.H. 2001 Underwater techn51 3 Assessment Of The Impact On Seafloor Features In INDEX Area Sharma R 2000 Marine Georesour52 x Protecting Manatees During Blasting Roeder D.A 1984 World dredging, V53 yes Overview Of Dredging International's Environmental Operations 1995 World dredging m54 yes

Innovations In Dredging Technology: Equipment, Operations, AndManagement Francisques, NR et al 2000

Proc. Western dreA&M dredging se

55 3 Reflections Made By The Dredging Industry Boer, P 1999

Proc. Oresund Linand lessons on en

56 yes The Future Challenges Of Environmental Dredging Romagnoli, R et al 1998

WODCON: World Vol 2 pg 651

57 yes

Generation And Decay Of Sediment Plumes From Sand Dredging Overflow Ooms, K and Postma , GM 1995 World dredging co

58 yes Environmental Effects Of Dredging, The United States Experience Herbich J.B. 1985 Dock and harbour 59 yes

Investigation Of Benthic And Surface Plumes Associated WithMarine Aggregates Mining In The UK

Hitchcock D.R and DruckerB.R. 1996 Oceanology Intern

60

Automated inspection tool "silent inspector" undergoes field testing James Rosati 1999 Dredging research 

61

Inovations in dredging technology: Equipment, Operations and management

McLellan N.T and HopmanR.J. 2000 USACE ERDC TR

62

Environmental aspects of dredging, Machines Methods and Mitigation 1998 IADC /CEDA

63 yes Bibliography of Selected References to US Marine Sand & Gravel S. Jeffress Williams 2003 U.S. Geological S

64 yes

Sand Transport and Morphology of Offshore Sand Mining SANDPIT Jun-01

65 yes

National Practices and Regulation in the Extraction of Sand &Gravel

66 yes

Research Brief: Environmebtal Effects of Near Shore Placement ofDredged Material D.G. Clarke et al Nov-01 U.S. ERDC

67 yes

A Process For Setting, Managing And Monitoring EnvironmentalWindows For Dredging Projects

68

Review of current state of knowledge of the impacts of marine aggregate extraction - a U.K. perspective Boyd,S.E. et al. Feb. 2002 EMSAGG Confere

69

Assessing the impact of sand extraction on shore stability: projectfor a methodological framework Cayocca, F. Feb. 2002 EMSAGG Confere

70

Aspects of sediment disturbance associated with Marine Aggregate Dredging Hitchcock D.R 1997 University of Wale

71 Legislative and environmental development in the Netherlands Stolk, A Feb. 2002 EMSAGG Confere72 Section 5 & 6 MIRO Recommendation for Best Practice From Nick Bray Minerals Industry 73 Research on Hopper settlement using large-scale modeling S.C. Ooijens, et al MTI Holland74 Hopper Overflow System Training Institute f
Baird – RPI – DRL - MES

Literature Review – TechnoloApproaches 

 Hopper settlement and overfl

modifications)

 Draft Best Practices – mitigat

monitoring (Mineral Industry Organization)

 Innovations in dredging equip

technology 
Baird – RPI – DRL - MES

Literature Review – TechnoloApproaches 

 Machines, Methods and Mitig Endangered Species Biologic SANDPIT – EC research init
Baird – RPI – DRL - MES
Baird – RPI – DRL - MES

Draghead with turtle deflecto
Baird – RPI – DRL - MES

Questionnaires Questionnaires

 Two questionnaires MIRO an

sponsored

 Bean, Great Lakes, Manson, 

USACE (Portland District), W(Boskalis), Dredging InternatOord, Jan de Nul, Rhode NieBMAPA 
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Questions To Dredging ContrResponsesPlume Related Impacts

Much of the perceived concernthe plume resulting from hoppeand the bottom agitation at the 
Baird – RPI – DRL - MES

Plume Related Impacts

1.What percent of material ove

hopper while digging sandy, content material, assuming a sailing distance to pump asho

 All sediment below 200 sieve 

is not retained, some losses 0mm depending on loading ratdesign (our estimates: 7 to 17
Baird – RPI – DRL - MES

Plume Related Impacts

2. Can a hopper dredge mine s

below a 1 m silt overburden removing the overburden? Dresult in significantly increaoverflow and consequently in turbidity?

 Depends on the density of t

silt
Baird – RPI – DRL - MES

Plume Related Impacts

3. In mining sand with a low-s

with a turbidity requiremenexceed 29 NTUs (about 50 

background, is it necessary special measures to meet thturbidity requirement?

 Depends on silt content and 

measured (among other factinfluencing overflow)

 Not generally used by MMS
Baird – RPI – DRL - MES

Plume Related Impacts

4. What measures do you emp

minimize turbidity?

 Turbidity in offshore borrow 

not been a problem (borrow generally feature clean sand
Baird – RPI – DRL - MES

Plume Related Impacts

5. If you use measures such as r

overflow water back to the drthere a reduction in dredging 

 Requires dredge refit, would 

cost, reduce carrying capacity increase maintenance

 Most recently built dredges (U

Europe) do not have this featu
Baird – RPI – DRL - MES

Plume Related Impacts

6. Have you completed research 

and dynamic plume processewith overflow and is this infopublicly available? 

 No, but referred to CIRIA pu

C547 and Norfolk USACE anresearch study at York Spit C
Baird – RPI – DRL - MES

Plume Related Impacts

7. Do wind, wave, and/or current forces o

the direction the dredge works? What aconsequences of dredging perpendiculaorder to influence the shape and disperplume? Do you have any data to demondirection and rate of deposition of mateby the dredger during dredging operati

 Best when headed into current and/or p

direction

 Problem with down-current drag arm p

hull, also need to continually raise drag with turtles

 Pre/post dredging surveys (insufficient 
Baird – RPI – DRL - MES

Plume Related Impacts

8. Given a mandate to reduce turbid

the most cost effective ways of acthe reduction? We understand thiof degree. Please explain the con

 Responded turbidity has not been 

the extent that would require spec

 If necessary, could reduce overflo

loading (after 10 min of 60 min lo1/6 load) – US multi-purpose TSH

 Aggregate dredges in UK use low

with lower loss rate (longer loadin
Baird – RPI – DRL - MES

Plume Related Impacts

9. What are your views on requiring 

be discharged below the hull?

 All respondents indicated this wa

practice in the industry
Baird – RPI – DRL - MES

Impacts to Benthic Habitats

Considerable concern was expressed loss of benthic community. Re-coloare being studied, as are changes in characteristics such as grain size, dioxygen, etc. It has been suggested thpatterns may speed the re-colonizatileaving “refuge” areas (that have unsediment characteristics and undistucommunities).
Baird – RPI – DRL - MES

Impacts to Benthic Habitats

10. If there is a stipulation in the specifica

required that only 70% of a borrow arand the unused portion cannot be on twhat would be the most efficient use 

What is the minimum width cut that a hop

efficiently? The reason for this propothat the benthic community will recolarea is dredged with intermittent non-

Do you have any comparative data to show 

dredging in strips to leave recolonizindredge site enhances recovery rates cosites where all the surface deposits are 
Baird – RPI – DRL - MES

Plume Related Impacts

 Ideal borrow area dimensions to l

areas are (assuming 3,000 cy hopdrag arms is about 75 ft):

 Length: 5000 to 6000 ft (one turn on Minimum Lane Width: 250 to 600 ft These should be considered in deter

borrow area and design of refuge zo

 One respondent indicated refuge str

100 m wide at the top

 Several referred to the SANDPIT st
Baird – RPI – DRL - MES

Impacts to Benthic Habitats

11. Are you aware of any damage to h

caused by dredging including covsediment? If yes, was a buffer or applied and was it sufficient?

 There have been some instances, b

monitoring of turbidity and sedimreduced impact considerably (DER
Baird – RPI – DRL - MES

Impacts to Benthic Habitats

12. Are your dredges capable of trac

recording the position of each dyou done tracking relative to a bWould you have a problem provinformation to the regulatory ag

 All dredges in the US have this 

would provide information
Baird – RPI – DRL - MES

Impacts to Benthic Habitats

13. When offshore sand dredging is 

a beach project, what does the blike? Are there draghead tracksdepth?) throughout the area? Arparallel or crossing? Can you pexamples to us of high resolutiopre- and post-dredging seabed coffshore dredging with TSHD?

 GLDD provided survey of track

is 10 to 20 ft wide and 3 to 4 ft 

 Bed will naturally smooth with 
Baird – RPI – DRL - MES

Impacts to Benthic Habitats

14. When mining sand off shore, is 

tended by a survey boat?

 Yes
Baird – RPI – DRL - MES

Sea Turtles

There is an ongoing concern with mmammal/dredge collisions and entraTurtles.
Baird – RPI – DRL - MES

Sea Turtles

15. Other than turtles, has your dred

in a collision with a marine mammarine mammals have a tendennear an operating dredge?

 No contact reported with any ot
Baird – RPI – DRL - MES

Sea Turtles

16. When appropriate, does your dredge 

designed to reduce the probability of turtles? Is this use mandated by the Ouse of these dragheads reduce the prodredge? Is the modified draghead effhave any recommended changes to thturtle deflector? Do you have any recon operating techniques to avoid entrduring offshore dredging operations?

 All use Turtle Deflectors, may reduc

(but not a strong opinion), small chanmade to address wear & tear
Baird – RPI – DRL - MES

Sea Turtles

17. What effect do the seasonal require

restricting dredging due to the proxhave on the overall annual dredgin

 Large impact on inshore hopper dr

but not yet an influence on offshor
Baird – RPI – DRL - MES

Sea Turtles

18. Does you dredge have a system to 

pressure/flow at the draghead when is off the bottom? How does it wor

 One possibility to redirect flow so 

come through draghead or stop flo(more restrictive than the current s
Baird – RPI – DRL - MES

Sea Turtles

19. How effective are observers and tr

turtle takes?

 Observers document turtle takes bu

directly influence reduction of take

 Trawling may be helpful in some c

but does not eliminate risk

 ERDC – perceived as effective und

turtle abundances in channels
Baird – RPI – DRL - MES

Additional Questions

20. What has been your experience dredg

ground? Has any fisherman or commecompany complained about any aspecprocess? Did you modify your operataccommodate the fisherman?

 There are some problems with fisherm

be addressed through coordination (prmeeting) and notification 
Baird – RPI – DRL - MES

Additional Questions

21. What measures do you, the dredge op

insure that the dredge does not damagpipelines and cables, or archaeologica

 Structures and buffers show up on nav

(MMS requires infrastructure is accurthe lessee)
Baird – RPI – DRL - MES

Additional Questions

22. Some operating companies have a policy 

localized zones to exhaustion before movzones within the dredge area. This assists the resource, but it also helps to minimizseabed and allow maximum time for recoresources.

 Does your company have a policy of zon

what are your reasons for dredging policy

 Do you have any information that docum

your dredging operations on marine orgadredge sites?

 Do you have any information on the rate

biological resources at your sites followindredging? 
Baird – RPI – DRL - MES

Additional Questions

 Some companies dredge in lanes Another responded they follow s If no specification the contractor 

material first

 No direct information on recove Latest in UK: recently there is c

opinion on whether to dredge lohigh intensity (exhaustive – histthought to be the best approach)
Baird – RPI – DRL - MES

Sea Turtles

23. Do you have any comments, gener

regarding dredging equipment and the reduction of adverse impacts on environment?

 Regulations and guidelines should 

scientific need and practicality

 One respondent indicated no turtle

50,000,000 cy of offshore dredged nourishment in the last ten years
Baird – RPI – DRL - MES

Low Turbidity Valve

An adjustable valve in the overflow funnel chokes the flow in such a way that no air is taken down with the suspension leaving the hopper.
Baird – RPI – DRL - MES

‘Green Pipe’

The overflow suspension is pumped through an additional pipe, mounted on top of the suction pipe, back to the suction head where it is used as process water.
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

the hopper is to the overflo
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

process water the overflow fu
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

to the second dpump ...
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

and is pumpean additional 
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

back to the suhead ... 
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

where it re-ensuction head 
Baird – RPI – DRL - MES

Activating the ‘Green Pipe’

and is re-circprocess water 
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Activating the ‘Green Pipe’

which enables afilling of the ho
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Activating the ‘Green Pipe’

with minimal generation.
Baird – RPI – DRL - MES

The dredging process:- commencing from an empty hoppe
Baird – RPI – DRL - MES

- dredged material is pumped into the hoppe

turbidity plume
Baird – RPI – DRL - MES

- filling the hopper (loading without overflow

turbidity plume
Baird – RPI – DRL - MES

- additional loading with standard overflow

turbidity plume dredge head

turbidity plume overflow

continue loading
Baird – RPI – DRL - MES

- plume generation while loading with standard 

turbidity plume dredge head

turbidity plume overflow

continue loading

New parameters: - upward airstream - propeller impact
Baird – RPI – DRL - MES

- additional loading with low turbidity valve

turbidity plume dredge head

turbidity plume overflow

continue loading
Baird – RPI – DRL - MES

- plume generation while loading with low turbi

turbidity plume dredge head

turbidity plume overflow

continue loading

New parameters : - density stream 
Baird – RPI – DRL - MES

- additional loading with ‘green pipe’

turbidity plume dredge head and ‘green pipe’

continue loading
Baird – RPI – DRL - MES

- plume generation while loading with ‘green pip

turbidity plume dredge head and ‘green pipe’

continue loading

New parameters: - recirculation fluid 
Baird – RPI – DRL - MES

Comparison of turbidity plume dispersion:

Standard overflowLow turbidity valve‘Green pipe’No overflow

Elapsed time: 00:30 05:00 10:00 
Baird – RPI – DRL - MES

Comparison dredge techniques:

Comparison dredge techniques

0

102030405060708090

100

0:00:00 0:05:00 0:10:00 0:15:00 0:20:00 0:25:00 0:30:00 

Time elapsed since plume generation

Relative plume
Baird – RPI – DRL - MES

Background turbidity:

Zeebrugge - CDNImportance of turcompared to bacnear the dredge p

0

200400600800

100012001400160018002000

10:00 10:30 11:0

Average turbidity (mg/l)

HW - 4:00HW - 3:00HW - 2:00HW - 1:00HWHW + 1:00HW + 2:00HW + 3:00
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Conclusions:

• Plume generation relative to the plume g

during the dredge cycle with standard ov

- low turbidity valve: 41 %-

‘green pipe’: 21 %

- without overflow: 13 %

• Application of alternative dredging techrelevant if the additional turbidity generadredging works is only a fraction of the bturbidity
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CURRENT AND POTEN

EFFORTS TO MITIGAT

STIPULATIONS

(US Perspective)
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Impact # 1: Benthic Habitats

 Possible stipulations:

 Leave strips of undredged areas to 

recruitment sources

 Refuge areas within dredging  Exclusion zones to be avoide

during pre-dredging studies
Baird – RPI – DRL - MES

Impact #2: Turtles

 Existing stipulations include:

a. Presence of trained observer(s)50-1

who follow specific protocols.

b. Use of state-of –the-art a rigid sea tuc. Keep the draghead on the bottom ex

1) when the dredge is not in a pumpiand the suction pumps are turned co2) when the dredge is being re-oriendredge line during borrow activities3) the vessel’s safety is at risk
Baird – RPI – DRL - MES

Impact #2: Turtles

 Existing stipulations include:

d. Dredge equipped with inflow screen

(variable mesh sizes) to better monitoverflow of the dredged materials fotheir remains. Screens sample 50-100% of the overshould be installed at the applicable 
Baird – RPI – DRL - MES

Impact #2: Turtles

 Existing stipulations include:

e. Assessment/relocation trawling to fu

assess/reduce the potential for inciddredging.There are many details on:- Trawl tow time- Trawl speed- Handling and relocation of capture- Ancillary data collection for turtle 
Baird – RPI – DRL - MES

Impact #2 Turtles

 Existing stipulations include:

f. Minimal lighting on dredges and pum

within 3 nm of nesting beaches- to reduce disorientation of females 

g. Seasonal windows when dredging is 

- In the GOM, hopper dredging shalbetween December 1 and March 31, abundance is lowest throughout Gulf 

NOTE: no windows for OCS dredging
Baird – RPI – DRL - MES

Impact # 3: Changes in SubstCharacteristics

 Changes in the substrate characteristics 

dissolved oxygen, compaction and orgalead to a reduction in benthic communisuitability of the area for future dredgin

 Possible stipulations include: 

 In sandy substrates, specify maximu

depths 

 Modeling studies to predict rate of i

acceptable depths
Baird – RPI – DRL - MES

Impact # 5: Hardbottom Habi

 Damage to hardbottom habitats: Physic

during dredging; burial by suspended sdredging; and altered sediment processhardbottom. 

 Existing stipulations include: 

 buffers of 60-120 m (but driven by 

concerns)
Baird – RPI – DRL - MES

Impact # 6: Pits

 Creation of depressions and furro

removal of substrate. 

 Existing stipulations include:

 To assure that deep pits and fur

created, conduct post-dredging surveys

 Slopes are not to exceed 2:1
Baird – RPI – DRL - MES

Impact # 7: Turbidity

 Short-term increased turbidity 

cutterhead or draghead and ohoppers on benthic species

 Existing stipulations include:

Turbidity > 29 NTUs beyond 150 mshows turbidity exceedences, dredgshall cease immediately and not rescorrective measures have been takehas returned to acceptable levels.
Baird – RPI – DRL - MES

Impact # 7: Turbidity

 May be more of a perception prob

sand borrow sites

 What is the footprint for sand ove There are mitigation measures to 

overflow but are they necessary fborrow sites?

 See data from UK studies
Baird – RPI – DRL - MES

Impact # 12: EFH Impacts

 Change in shoal shape that degrad

Existing stipulations include:

 Detailed bathymetric surveys prior 

immediately following dredging, andetermine the amount of recovery a

 Surficial sediments be removed from 

practicable. Avoid shoal crest and awhich are generally more productiv