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Dredging Methods and Equipment

John Poullain, P.E.


Course Outline

This four-hour online course provides general guidelines and design considerations used for dredging operations, selection of dredging equipment and disposal of dredged material. The course provides an overview of the types of equipment, their advantages and limitations, the required tests, and the dredging techniques employed. Concerns and precautions for the prevention of adverse effects on the environment and animal habitats and utilization of dredged material for beneficial disposal are considered. Remedial actions and activities performed at the work 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 developing dredging operations, selection of equipment and the benefits and limitations of dredges and techniques employed. Design considerations including the types of dredged material, timing of material placement and methods for placement are discussed. The student will also become familiar with the basic techniques used to construct habitat and measures to avoid adverse results. 


Course Introduction

Dredging basically has two functions, to obtain material, to excavate holes and to provide greater water channel depths. Dredges consist of mechanical and hydraulic types.

Mechanical dredging has existed for over 2000 years and hydraulic dredging (pipeline) originated 1850 for the excavation of the Suez Canal. Basically by about 1900 all the present dredge types were in existence. Another dredging method, agitation, was performed in Holland with a paddle wheel powered by water or people. While most dredged material is placed in aquatic disposal areas such as river channels, confined disposal areas are preferred when conditions are suitable. In cases where the levels of contaminate and chemicals are unacceptable, unconfined aquatic disposal is not suitable. There have been many developmental uses throughout the US especially on the West Coast for new port terminals, harbors and waterways. All confined disposal areas serve to retain suspended solids during disposal operations while releasing the carrier water that will meet effluent solids standards and also to provide adequate storage volume for disposal needs.     

Confined disposal areas produce discharge flows during dredging and dewatering flow as the material settles and dries in a confined disposal area (CDA). Discharges from a CDA may contain contaminants which when released to the waters of a state are regulated by the Clean Water Act Section 404. Discharges also require a Section 401 water quality certification or waste discharge permit from the state. Some states require a National Pollutant Discharge Elimination System (NPDES) permit for toxic waste or chemicals which may be initiated by a local Regional Water Quality Control Board or similar agency.

There are several methods of excavation, which include cutter head suction and hopper suction dredges and mechanical excavators. Cutterhead dredges have suction pipelines to pump the dredged material into disposal areas, barges or hopper barges for longer distance transportation. At least one cutterhead dredge in the US is electrically powered via power cables from shore side substations. The Air Quality Control Board of Los Angeles required stationary the construction equipment to be electrically powered. The all electric dredge has a 34-inch diameter suction pipeline and is powerful enough to pump very long distances without using booster pumps, one of the longest discharge distances was about 18,000 feet. Another type of dredge, the hopper suction dredge, drag suction heads along the ocean floor or marine area to be excavated. The dredge material is deposited in the hopper and transported to disposal areas, making them a good choice for long distanced disposal areas, which usually requires pumping assistance with booster pumps.

Channel excavation may require blasting to break rock masses into more readily excavatable strata. There are a few excavators, which have enough power to breakup rock with out blasting; one of the biggest has perhaps 370,000 lbs. of breakout force. The bucket teeth are used to break into rock gaps in order to reduce a rock mass to smaller pieces for ease in removal and loading. They are similar to the land based excavators and have hydraulically controlled booms and 26 cu yd buckets.

Approximately 350 million cubic yards are dredged from the waterways and shipping channels in the US each year. Striving to achieve greater economy in shipping has led to larger container ships, which in turn require deeper drafts. The port of Los Angeles has excavated channels to provide 81 feet of draft for the larger ships. Some super large ships are called post Panamanian since they exceed the draft of the Panama Canal channel.

An example of a recent beneficial use for dredged material is the Pier 400 of the port of Los Angeles, a 590-acre container terminal that required about 60 million cu yd of dredging for the harbor and ship channels. Cutterhead suction, clamshell and hopper dredges were used. Dredged material used for the port expansion was contained within breakwaters constructed with quarry run rock and 500 lb. to one-ton rock around the perimeter. Several areas were developed into shallow-water habitats to mitigate for the port expansion into bay areas. Some ship channels were dredged to provide 50 to 81 foot drafts. A cutter suction dredge with a 34-inch diameter discharge pipe was used to pump the materials into spill barges. The spill barges in turn placed the material for port landfill via cantilever booms to make the required port landfills.

About 70% of the dredge material in the US are placed in aquatic disposal areas and open water such as river channels and coastal waters. One beneficial use of dredged material is the development of beaches and dunes as an alternative method for dredge disposal. In many coastal states beach erosion threatens developed areas, recreational, cultural and environmental interests. Beaches, dunes and barrier islands are built to protect commercial and residential areas from storm-driven waves and tides and attempt to mimic natural beach and dune creation. Such non-rigid structure construction is an alternative to hard structures like bulkheads, breakwaters and sea walls. As coastal area is lost more open water appears and protection from storms and hurricanes is reduced. Other benefits of beaches and dunes are the creation of wildlife habitats for commercial and recreational purposes and for environmental. 
Increased tropical storm activity, rising sea levels and development along the shorelines has been a concern. Developed beaches require maintenance and beach nourishment because of erosion caused by hurricanes, tropical storms and tides. The state of Florida estimates about 50 % of the state’s beaches have some degree of erosion. Coastal states have developed beach management programs to evaluate beach erosion problems in a manner similar to the erosion, sediment and stormwater control programs employed by other states. They work with other governmental entities: local, state and federal, to protect, preserve and restore eroded beaches and coastal lowlands of the state. Beaches, dunes, boat inlets and inlet sand transfer are included in their studies, monitoring and construction activities.
The type of dredged material will determine its suitability for beach creation. Course or fine-grained materials may be available for beach creation and will determine the need for containment dikes. Beach restoration is typically done by dredging beach quality sand, preferably without any fine sediment and pumping it onto the site. The slurry of water and sand discharged from the dredge pipelines is dewatered as the water drains away. The remaining sand may then be moved about the beach to meet the design grades and profile, or in some cases to form dunes or sand stockpiles. It should be noted dredged material contaminants usually fall within the acceptable limits, which will allow the material to be used as fill for beaches and dunes and for environmental construction such as wetlands and marshes.
Dredging may have adverse physical effects on the habitat when not properly planned. Certain precautions can be taken to minimize the impact of dredging activities especially from offshore dredging. Some of these are:

a. Increase in Turbidity from Dredging
Impacts: Damage to coral reefs, fish (gill abrading and coating) and seagrass beds from resuspended sediments.
Preventative Measures: Use borrow material with low fines, avoid cutterhead dredging and avoid dredging and construction activities in sensitive resource areas.

b. Disturbance of Beach Habitat and Animals
Impacts: Beach 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 as necessary.

c. Offshore Borrow Dredge Sites
Impacts: Beach animals are removed or destroyed by dredging activities. Organic material may accumulate; the effect on animal species will vary by species.
Preventative Measures: Selection of borrow areas, locate sensitive areas, avoid cutterhead dredging, perform shallow versus deep borrow dredging.

 

Course Content

This course is based primarily on Chapters 2, 3 and 4 (pages 4-1 to 4-13) of the US Army Corps of Engineers Manual, “Dredging and Dredged Material Disposal”, EM 1110-2-5025, (1983 Edition, 54 pages), PDF file. The course also based on pages 61-67 of the US Army Engineer Waterway Experiment Station (WES) Technical Report, “Dredge Plant Equipment and Systems Processes – Area 3”, DPR-95-10, (1995 Edition, 7 pages) PDF file. Environmental and water quality issues of dredge discharges are also covered in the US Army Corps of Engineer publication, “Environmental Windows Associated with dredging Operations”, DOER-E2, (1998 Edition, 14 pages), PDF file.

The links to the documents are as follows:

US Army Corps of Engineers Manual is “Dredging and Dredged Material Disposal”, Chapters 2 through 4

Dredge Plant Equipment and Systems Processes – Area 3, pages 61-67

Environmental Windows Associated with Dredging Operation, DOER-E2

Terms

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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 considered for dredging operations are the physical, engineering and chemical characteristics of the sediments and the transport and handling of the material. State and federal regulations have to be complied with at dredging sites in order to remove any threat to public health or the environment. The installation, type of materials, time of placement, types of dredge, the advantages and limitations are considered in the course. 

Factors to consider for the productive disposal of dredge material are:


Related Links

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

a. EM 1110-2-5027, Chapter 4, “Confined Disposal of Dredged Material”.
b. Huston, J. 1970 “Hydraulic Dredging” , Cornell, Cambridge, Mass
c. Appendix C of EM 1110-2-5027, “Example Design Calculations for Retention of Solids and Initial Storage”.

http://www.dep.state.fl.us/beaches/
Florida Department of Environmental Protection, Coastal planning and engineering, includes beach and inlet studies and management policies followed for shore protection.
  
http://www.dredgingcontractors.com/  
The site provides information, news and links for the dredging construction of channels, harbors, beaches, and coastal protection.

http://www.epa.gov/owow/oceans/regulatory/dumped/dredgemgmt.html           
The US EPA site gives a broad overview of dredging, ocean dredging, dredge management and partnerships in dredging, successful projects and protection efforts.


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.