NON-TECHNICAL SUMMARY 

Study Title: Review of Existing and Emerging Environmentally Friendly Offshore Dredging 

Technologies

Report Title: Review of Existing and Emerging Environmentally Friendly Offshore Dredging 

Technologies 

Contract Number: 01-03-CT-71516 

Sponsoring OCS Region: Headquarters–Sand and Gravel Unit, Leasing Division

Completion Date of Report: November 2004

Costs: FY 2004: $176,663.85 

Project Manager: Robert B. Nairn, Ph.D., W.F. Baird & Associates Ltd., 2981 Yarmouth 

Greenway, Madison, WI 53711

Principal Investigators: Robert Nairn, Tim Kenny, Fernando Marvan, Jacqueline Michel, Nick 

Bray and Richard Newell

Key Words: sand resources, OCS, dredging, ecological impacts, environmentally friendly 

equipment and approaches

Background: The Minerals Management Service (MMS) is charged with environmentally 

responsible management of Federal Outer Continental Shelf sand and gravel resources, that is, 

those resources lying seaward of the State/Federal boundary. 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. 

Objectives: 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. 

Methods: The project approach was comprised of four main areas of activity: a literature 

review of dredging impacts and existing and emerging environmentally friendly dredging 

technologies; interviews with representatives of the various regulatory agencies at the Federal 

and State level that are responsible for the offshore environment of concern to prioritize areas of 

concern; direct survey of US and overseas dredging contractors; and finally, in a workshop 

setting an evaluation of the various approaches and techniques for their appropriateness, 

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

Significant Results: The focus was primarily on Trailer Hopper Suction Dredges as these are 

the most likely vessels of choice for dredging operations where the borrow deposit and the 

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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. 

A short list of impacts 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. In order of priority ranking based on discussions 

with the resource agencies in the US, the short list included: direct loss of entire benthic 

communities; 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; sedimentation (burial) impacts to adjacent hard/live bottom or other sensitive 

habitats; and, 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, shoreline impacts through changes to wave climate, spatial and seasonal conflicts with 

recreational and commercial fishermen, potential damage to pipelines, damage to archeological 

resources, and potential harmful alteration or destruction of Essential Fish Habitat are being or 

have been recently addressed in other MMS (and USACE in the case of 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. The existing stipulations 

for collision with marine mammals were determined to be sufficient.

The literature and industry review found that he US dredging industry is not lagging the 

European market in development of innovative approaches. 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 Trailing Suction Hopper Dredges 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. 

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 strong. Very little if any development in either 

equipment or dredging approaches has been devoted to the key issue of loss of benthic 

communities, which was identified by resource managers as the most important impact. 

The key recommendation for addressing the loss of benthic habitat was developed by the team as 

part of the study and consists of designating undisturbed refuge areas within a borrow area to 

accelerate benthic re-colonization. It is recommended that this approach should be field-tested 

and evaluated. If successful, it should be implemented at those locations where the benthic 

communities have high ecological importance. 

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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.

Where there is a need to protect hard/live bottom areas or coral habitat from sedimentation, fieldtested sedimentation limits should be established 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, followed by real-time or near realtime monitoring of sedimentation levels. 

It is generally viewed that elevated levels of turbidity generated from Trailing Suction Hopper 

Dredge 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 sub-lethal 

levels of short-term elevated turbidity. Nevertheless, the dredging industry is currently able to 

meet relatively restrictive limits on turbidity levels that have not been scientifically linked to 

impacts in the offshore environment. Some key equipment types and modifications that are 

widely in use both in the US and Europe to limit the size of sediment plumes include the AntiTurbidity Valve and below hull release of hopper overflow. 

 

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. 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. 

Study Products: W.F. Baird & Associates Ltd. and Research Planning Inc. 2004. Review of 

Existing and Emerging Environmentally Friendly Offshore Dredging Technologies. U.S. 

Department of the Interior, Minerals Management Service, Sand and Gravel Unit, Leasing 

Division, Herndon, VA. OCS Report MMS 2004-XXX, XX pp. + appendices. 

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