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Considerations forEnvironmental Dredging

of Contaminated Sediments

Michael R. Palermo, PhD, PE

Outline

4 Objectives, Definitions4 Site/ Sediment Characterization4 Removal Requirements4 Equipment Characteristics and Selection Factors4 Production and Duration4 Resuspension, Release, and Residual4 Control Measures4 Operational Considerations4 Monitoring and Management4 Conclusions

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Contaminated Sediment RemediationWhat are the Options?

4 Monitored Natural Recoveryη Part of all remediesη May be an integral part of active

remediation4 Dredging

η Need to recognize impacts andlimitations

η Triggers a variety of onshore activities4 Capping

η Can be rapidly implemented withminimal impact

η Need to assess long-termprotectiveness

4 Combinations3

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Navigation versus Environmental Dredging

4 Navigation to maintain depthη Economy/Efficiencyη Effectivenessη Environmental Impact

4 Environmental for Remediationη Environmental Impactη Effectivenessη Economy/Efficiency

Dredging and Disposal

4 Advantagesη Mass removalη Proven technologyη Easily implemented

4 Disadvantagesη Effectiveness reduced by resuspension and releaseη Effectiveness reduced by residualη Disposal is expensive

Objectives of Environmental Dredging

4 Achieve Remediation Goals4 Remove Targeted Sediments within

Horizontal and Vertical Tolerances4 Control Sediment Resuspension,

Contaminant Release, and ResidualSediment

4 Maintain Production/ Efficiency of Removal4 Assure Compatibility with Treatment

and/or Disposal

“Technical Guidelines for Environmental Dredging ofContaminated Sediments”

4 Developed for EPA by USACE4 Detailed technical evaluation

procedures for Env Dred4 Supports EPA 2005 Superfund

Sediment Guidance4 Audience: Project managers,

designers, contractors, and otherstakeholders

4 Published September 2008http://el.erdc.usace.army.mil/elpubs/pdf/trel08-29.pdf

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Dredging is only one part of a remedy

Staging/Transport/Rehandling

Treatment DisposalDredging/Excavation

Site Conditions and SedimentCharacterization

4 Site Conditions (Field Investigations)η Hydrodynamics; Geotech; Bathy;

SS Sonar; Sediment Profiling;Infrastructure; etc.;

4 Sediment Characteristics (Sampling)η Physical – density, GSD, etc.;η Chemical – Conc of COCs to full

depth;4 Define Dredgeability and Removal

Requirementsη Debris removal; Dredging depths;

volumes and volume increases

Equipment Capabilities and Selection

4 Comparative Evaluations, e.g.:η Mechanical vs. Hydraulicη Conventional vs. Specialtyη Operational approaches

4 Wide range of suitable equipment isavailable for environmental dredging

4 No single dredge type is best for allprojects

4 Evaluate/select based on fieldexperience, predictive tools, and fieldtrials as needed

Equipment Characteristics andSelection Factors

4 Sediment Resuspension4 Contaminant release control4 Residual/ Cleanup Level4 Transport by pipeline4 Transport by barge4 Positioning Control4 Maneuverability4 Portability/Access4 Availability4 Debris/ Loose Rock/ Vegetation4 Hardpan/ Rock Bottom4 Sloping Bottom4 Flexibility for Varying Conditions4 Thin Lift/ Residual Removal

4 Production4 Percent solids – need for dewatering4 Vertical Accuracy4 Horizontal Accuracy4 Max Dredging Depth4 Min Dredging Depth

Wire-Supported MechanicalClamshell Or Bucket Dredge

Clamshells

Conventional Bucket Operation

Environmental Bucket

Level Cut

Horizontal Profiling GrabCable Arm

Fixed – Arm Excavatorse.g., Horizontal Profiling Grab

Buckets are Interchangeablefor Size and Purpose

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Backhoes

Smalley Dredge

Dipper Backhoe

Advantages of Mechanical Dredges

4 Can remove hard packed materials4 Can remove debris and debris-laden sediments4 Can work tight areas4 Can operate in deep water4 Can remove sediments with little excess water4 Efficient for transport by barge at long haul distances4 Available in variety of bucket sizes/types4 Can switch from box cut buckets, to toothed buckets,

to smaller buckets, etc.

Issue – Debris

Debris/Boulders/Hardpan/Etc.4 Buried obstructions such as debris, boulders, underlying

hardpan, etc. will adversely affect dredging performance

• Slower production, more breakdowns

• More resuspension losses – bucket fill/closure issues

• Higher post-dredging residuals

4 Mechanical dredges are generally capable of removingsediments if debris and obstructions are abundant

4 Dedicated debris-removal operations may be required, especiallyfor large debris

4 MAJOR ISSUE!

Issue – Accuracy/Precision

4 Accuracy/ Precision ~removal of CS withoutremoving clean materialη Positioning only locates

the dredgeheadη Attainable positioning

accuracy now at +/-several inches

4 Accuracy of positioning mayoutstrip that for sedimentcharacterization

Issue - Production

4 Consider both Operating ProductionRate – while dredge is activelyoperating and Sustained Production– over a season

4 Allowable weeks/ year, days/week,hours/day, and other Quality of LifeIssues will determine projectduration/ number of dredgingseasons for a given dredge;

4 Dredging system design; numbers ofbarges, rehandling requirements;other “bottlenecks” may control;

4 Must determine number and sizes ofdredges required;

Issues – Resuspension, Releasesand Residuals

Conceptual Illustration ofEnvironmental Dredging and Processes

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Sediment Resuspension/ Releases

4 All dredges resuspend sediment4 Contaminant releases closely tied to

resuspension4 Field measurement methods are not

consistent4 Resuspension at dredgehead generally

less than 1%4 Increased resuspension for debris

removal4 Place resuspension in context with other

sources4 Resuspension is near field and can be

controlled (at least partially)

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Residuals Characteristics

4 Generated residuals - generallyaccumulate above the dredging cutlinein thin layers at relatively low density;

4 Undisturbed residuals - remain belowthe cutline as higher density sedimentthat may exist as relatively thick layers;

4 Multiple residual layers are possible;4 Differing residuals characteristics may

require different managementapproaches;

Control Measures

4 3Rs evaluations determine need4 Operational controls (timing,

sequencing, operating speeds, etc.)4 Engineered controls (silt curtains,

sheet pile enclosures, etc.)4 Management actions for residuals

(backfill, caps, re-dredging, etc.)4 Evaluate potential effectiveness and

impact on costs, project duration,etc.

Silt Curtains

Mechanical Grab & Barge Operating InsideHanging Curtin

Sheet Pile Enclosure

Air GateAir Gate

Silt CurtainSilt Curtain

Sheet PileSheet Pile Wall Wall

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Residuals ManagementThin Cap, Eco Layer, or Engineered Cap

Cleanup Dredging forResiduals Management

4 Applicable for removal of thin layers of residuals;4 More effective with soft underlying sediments;4 Multiple cleanup passes generally inefficient; so passes

should be limited, with option for residuals cap.

Operating Methods and Strategies

4 Definintion of dredging prisms4 Sequencing of the work (horizontal

and vertical)4 Production cuts, box cuts, layback

slopes, cleanup passes4 Overdredging allowances4 Methods of operation4 Written Operations Plan

Monitoring and Management

• Removal– Interim and final bathy

• Resuspension/Release– ADCP; Turbidity; TSS/COC samples;

Fixed air monitoring stations; etc.;

• Residuals– Grabs or Cores

• Post Construction Monitoring• Written plan with pre-

determined managementactions

Environmental Dredging Conclusions

4 Environmental Dredging is complex, and a technically sounddesign requires an efficient and comprehensive evaluation

4 Implementability, Effectiveness, and Cost should be consideredin determining acceptability of an environmental dredgingdesign

4 Environmental dredging design should be project-specific,sediment-specific, and site-specific; and

4 Dredging design should focus on project goals.

QUESTIONS?

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