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: 01-02-CT-85139 

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

Applicable Planning Areas: East and Gulf of Mexico Coasts 

Completion Date of Report: November 2004

Costs: FY 2004: $176,663.85 

Project Manager: Robert B. Nairn, Ph.D., 

Affiliation (Project Manager): W.F. Baird & Associates Ltd. 

Address: 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. 

Description: 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 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. Of the twelve 

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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. 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 and possible re-colonization by an altered biological community; 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 US Army Corps of Engineers 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.

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 recirculating 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 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. 

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 

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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 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 study workshop formed the final phase of the investigation and was attended by 

representatives of: the study team, MMS, US Army Corps of Engineers, 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 are summarized in the following paragraphs for each of the key impacts. 

It is 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. 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 areas considering the 

sedimentation footprint from the dredging operations. This proposed approach should be field 

tested along with a technique to monitor the effectiveness. 

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 Trailing Suction Hopper Dredges and greater 

than 2 m for Cutter Suction Dredges. Maximum pit depths should be determined on a site3 
specific basis through analysis combined with monitoring where necessary. Monitoring may 

assist the development of an appropriate maximum pit depth at borrow deposits that are dredged 

more than once. 

It is likely only 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 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. 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. 

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. 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 AntiTurbidity valve device is widely applied in the US and significantly reduces the size of plumes 

from Trailing Suction Hopper Dredges 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. 

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. This 

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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. 

 

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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