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O & M of Dredge Containment

John Poullain, P.E.


Course Outline

This two-hour online course provides general guidelines and practices for the planning and operation and management of confined dredged material disposal areas. Containment area dikes, effluent weirs and inflow inlets are discussed. While the selection of dredging equipment is important for efficiency and economy in dredging, selection, location, operation and management is of equal or greater importance in determining viability and service life of a project. Consideration must be given to the compatibility of the containment for future land disposal or development. Construction and remedial activities performed at the construction sites must comply with federal, state and local regulations to protect water quality, fish and animal habitat.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:


Intended Audience

This course is intended for civil engineers and planners.


Benefit to Attendees

The student will understand the procedures for planning and operation and maintenance of confined disposal areas for dredge material and the associated benefits and limitations. Design factors include types of dredge material, shape, size, weirs, and inflow inlets of containment areas. Timing of dredge material placement and methods for placement are discussed. The student will also become familiar with the management of dredge flow into and from containment areas by adjusting the weirs, outlets, interior dikes and containment cell arrangements to improve settlement.


Course Introduction

About 70% of the dredge material in the US are placed in aquatic disposal areas, wetlands and open water such as river channels and coastal waters. This course discusses the use of diked containment areas. The types of dredging include hydraulic which pump the slurried dredge material through pipelines and mechanical. Mechanical dredging has existed for over 2000 years and hydraulic dredging (pipeline) has existed since 1850 - when used for the Suez Canal. By about 1900 all present types of dredging were in existence. Another dredging method, agitation, was performed in Holland with paddle wheels powered by water or people. When an unacceptable level of contaminants and chemicals is encountered in dredged material, the material cannot be placed in aquatic disposal without remediation. There are many uses for development of confined disposal throughout the US for port terminals, harbors and waterways. All confined disposal areas to retain suspended solids during dredging disposal operations. The released carrier water must meet effluent solids standards and the confinements must provide adequate storage capacity for present and future disposal needs and land-use goals.

The amount of settlement and solids flowing through the outlet weirs determine the efficiency of containment areas. Factors to consider for efficient containments are ponding depth, shape and size. If the inlet and outlets are not managed correctly the flow through the areas may short-circuit or channel through and carry more solids without adequate settling taking place. Effective settling may require 2-3 feet for ponding and at least 30 minutes for detention. A shape with a 1 to 3 length to width ratio will help to reduce short-circuiting. Unless the containment dikes have rounded corners, a common occurance is inactive or dead spots, which reduce the surface area available for ponding and settlement of solids. Site preparation should include access routes, vegetation removal in most cases and clearing and grading along the dike alignment.

As material is being discharged the dredge pipe should not be a safe distance from the dike interior slopes for erosion protection. If coarse material is desired for other construction the material can be pumped into mounds or it can be diverted with Y connections and placed where needed or stockpiles without interrupting the operation. After material is dredged the goal is to provide for surface drying by removing surface water as fast as permitted. A gentle slope from the inlet to outlet weir aids in the settlement of material. Shallow draft dredges can b operated within a containment area to grade and move material as necessary and to minimize the required dike height.

The type of dredged material will determine its suitability for future development. Both coarse and fine-grained materials may be available. Fine-grained material requires a long time to drain and consolidate. Development of such areas may be limited to recreation uses like parks. The slurry of water and material discharged into the containment is dewatered as the water drains to an outlet weir. It should be noted dredged material contaminants usually fall within acceptable limits, which will allow the material to be used as fill for construction and environmental projects such as marshes and wetlands.

Land disturbed by construction activities causes soil erosion and possible migration of sediments. Sediment contains soil particles and possible petroleum products, metals, chemicals, corrosive acids, pesticides, organics or other pollutants. Offsite and on site borrow sources for containment dikes must be investigated for contaminants. A disposal area construction site must be investigated for a wide range of conditions, including ground water level, surface drainage, topography, and subsurface ground conditions and animal habitat.

Dredging may have adverse physical effects on habitat when not properly planned. The following precautions can be taken to minimize the impact of dredging activities:

a. Increased Turbidity from Dredging

Impacts: Damage to coral reefs, fish (gill abrading and coating), and other animals and seagrass beds from resuspended sediments.
Preventative Measures: Avoid cutterhead dredging and restrict dredging and construction activities if located in sensitive resource areas when possible.

b. Disturbance of Habitat and Animals

Impacts: Animal nesting areas and coral reefs are damaged by dredge anchors, cables, pipelines and dredging operations.

Preventative measures: Schedule operations based on time of year, locate and avoid sensitive areas, avoid cutterhead dredging and use sedimentation basins to dewater pumped slurry if necessary.

Vegetation is one of the most commonly used methods for stabilization of containment dikes. Site specific conditions must be considered to use vegetation. Success of vegetation depends on the climate characteristics, time of year, slope grades, site preparation, water and watertable elevation and compatibility of vegetation with these conditions. It is relatively easy to maintain and establish vegetation and properly selected plants and grasses are self-maintaining. Erosion control matting may be necessary to hold the seed and soil in place until the vegetation is established.

Vegetation protects a slope with the roots and exposed branches, stems. Surface flow velocity is reduced and the capacity for infiltration and water withdrawal from the soils is increased. Seedbed preparation, fertilizers, planting dates, rates of application and type of grasses will depend on the region, specific area for planting, time of year and as specified in the design plans. Also there are temporary and permanent plantings. Permanent seeding is typically for periods longer than 12 months with perennial grasses.
Riprap may be necessary for erosion protection from flowing streams, rivers or tidal and wave action at the proposed disposal site. There are several ways to place riprap. It can be mechanically placed along the slope or in wire baskets as a blanket over the slope. Riprap mattresses are relatively flexible and can adjust as changes from settlement or erosion occur. Minor damage can be easily repaired with additional stone to fill settlement or voids from erosion. A rule of thumb for mattress thickness is 1.5 times the thickness of the largest stone being used. Filter fabric or a drainage material is usually placed as an underlayment to protect from loss of fine soils and to allow for water seepage under the riprap.

Course Content

This course is based on Chapters 1 and 7 of the US Army Corps of Engineers Manual, "Confined Disposal of Dredged Material", EM 1110-2-5027, (1987 Edition 19 pages), PDF file.

The link to the above course materials are:

USACE "Confined Disposal of Dredged Material", Chapters 1 - Introduction

USACE "Confined Disposal of Dredged Material", Chapters 7 - Operation and Maintenance of Containment Areas

You need to open or download above documents to study this course.


Course Summary

Urban sprawl has reduced the availability and acceptability of dredge disposal areas, increasing the transport distances for dredge disposal and the cost of dredging. Environmental restrictions have also added to the costs. Among the factors discussed for confined dredge disposal areas are perimeter and interior dikes, weir and inlet locations selection and effects of size and shape. State and federal regulations have to be complied with at construction sites in order to remove any threat to public health or the environment. The construction, type of materials, time of dredging, sources of borrow material and the effects on the physical site conditions are also considered.

Factors considered for economic and productive dredge disposal include:

a. Adequate storage capacity.
b. Containment weir location, size and shape and interior cross dikes. Adjustments in locations and arrangements are made as dredging continues.
c. Disposal operations require monitoring during and after dredging is completed.


Related Links

For additional technical information related to this subject, please refer to:

http://www.dredgingcontractors.com/
The site provides information, news and links for the dredging construction of channels, harbors, and coastal protection.

Also consider US Army Corps of Engineers EM 1110-2-5025, "Dredging and Dredge Material Disposal", Chapter 4, "Containment Area Design" and Appendix C of EM 1110-2-5027, "Example Design Calculations for retention of Solids and Initial Storage".



Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.