Dredging and Dredged Material Disposal

Leiden, Netherlands. Dredging spoil is deposited in a transport barge as a pontoon crane dredges a canal. Photo credit: Roel Slootweg

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When new work and maintenance projects require dredging and materials disposal, then specific guidance should be followed for the evaluation of equipment and disposal. This guide is designed to provide an overview of the information that should be considered for these decisions.

Design Considerations

The operation for dredging and dredged material disposal needs to cover both short-term and long-term management goals. Typically, short-term focus is on the channels that are needed for existing navigation, but it doesn’t necessarily need to be based on the project dimensions. Ideally, the dredging process should be done using the best technical options that are both economically feasible and environmentally compatible. On the other hand, long-term goals are focused on the operation and management of the disposal areas.

Preliminary data collection for a dredging and dredged material disposal project should include:

  • Assessing the location as well as the amount that needs to be dredged, based on future needs on the site.
  • Determining the chemical and physical characteristics of the sediments.
  • Identifying potential alternatives for the disposal.
  • Considering the applicable environmental, social, and institutional factors that need to be addressed.
  • Evaluating the dredge plant requirements.

Locations and Quantities for Dredging

When dredging projects are planned, the locations and quantities of material are the most important considerations that need to be addressed. The biggest problem is usually the disposal of the dredged material, which means that long-term projections are essential. Detailed records need to be kept of the quantities and intervals of the dredging, allowing future projections to be identified.

Before the project commences, hydrographic surveys should be used to determine the existing depths as well as the depths that will be attained after the dredging operation. The process requires the use of proper equipment and both vertical and horizontal controls to ensure accurate calculations.

Physical Properties of Sediments

If there is a specialized problem with the excavation or earthmoving operations, then field testing is required to determine the quantities, characteristics, and location of the material that needs to be removed.

Sediment samples need to be obtained down to the depth that will be targeted. At the same time, a pre-dredge survey should be completed. If ongoing dredging is required, then samples should be recurring to ensure the accuracy of the work. Once the characteristics are well known based on multiple samples, then a smaller number of samples can be used for future work. When soft materials are present, then grab sampler (using a bucket or scoop) or push tube sampler (using an open-ended tube that is thrust vertically into the sediment) are obtained.

New Work Sampling

When samples are taken for new work, then conventional boring techniques are usually used. These samples should be obtained in the major work zones for a full representation of site samples. These samples need to go through laboratory testing to determine the proper dredge plant, disposal alternatives, the design of retention dikes and channel slopes, and the estimation of long-term storage capacity when the disposal areas are confined.

These sample tests might include:

  • Natural Water Content Test
  • Plasticity Analysis
  • Specific Gravity Test
  • In Situ Density

Selection of Dredging Equipment

Limitations are placed on equipment depending on the circumstances, although the specifications should be avoided whenever possible to avoid restriction to the competitive bidding process. The goal of equipment selection is to reduce the environmental impact that occurs due to the operation. This protection is an adequate justification for managing the selection and control of the dredging equipment. This topic will be discussed in more detail later in this report.

Disposal Alternatives

Two factors need to be considered when selecting the disposal alternatives: the economics of the disposal operation as well as the environmental impact. According to the Dredged Material Research Program (DMRP), conducted by the U.S. Army Engineer Waterways Experiment Station (WES), there are two conclusions regarding the disposal of dredged material:

  1. No disposal alternative should be assumed to be suitable without proper testing.
  2. Long-term regional planning is required for effective, lasting environmental considerations.

Long-range studies should always be included in regional dredging and disposal management plans, ensuring greater opportunities for reducing project cost, improving environmental protection, and ensuring public acceptance.

United Kingdom. Tipper truck on the beach at Lyme Regis to assist reshaping and dredging of the harbor. Photo credit: Alphotographic

Dredging Equipment and Techniques

Both limitations and advantages for each type of dredge need to be assessed to help with the proper planning and design for the project. The dredging methods vary throughout the country, but there are basically three mechanisms that are used for dredging:

  • Suction Dredging: Removing loose materials using hoppers, dustpans, sidecasters, and hydraulic pipeline plain suction. Most commonly used for maintenance projects.
  • Mechanical Dredging: Removing either compacted, hard materials or loose materials using a dipper, clamshell, or ladder dredge. Can be used for both new work projects as well as maintenance.
  • Combination Dredging: Sometimes, a combination of suction and mechanical dredging is done using cutterheads. Can be used for both new work projects as well as maintenance.

Factors for the Selection of Dredging Equipment

These are the main factors that should be considered when choosing the dredging equipment:

  • Physical characteristics of the site materials
  • Amount of materials that will be dredged
  • Depth of dredging
  • Distance to access the disposal area
  • Site conditions and environment between the site and disposal areas
  • Sediment contamination
  • Disposal method
  • Required production
  • Dredge types that are available

Types of Dredging Equipment

The main types of dredging equipment include:

  • Hopper Dredges: Seagoing ships that are self-propelled and equipped with hoppers (sediment containers), propulsion machinery, dredge pumps, and other special equipment as required. Hopper dredging can be accomplished using three methods: pumping past overflow, agitation dredging, and pumping to overflow. The benefit is the effectiveness and safety in open, rough water. Additionally, the equipment moves economically and quickly using its own power, without obstructing or interfering with traffic. The limitations are that the equipment cannot be used in shallow water, precision is decreased, continuous dredging is not available, and it is not possible to dredge effectively near hardpacked sandbanks or structures (such as piers).
  • Cutterhead Dredges: The most commonly used type of dredging equipment is the hydraulic pipeline cutterhead suction dredge because of its versatility and efficiency. A cutterhead dredge can be used for the excavation of many material types, including silt, clay, gravel, sand, soft rock, and densely packed deposits. Advantages are that the equipment can be used for most types of excavation on both new work and maintenance projects. Continuous dredging is available, helping to improve efficiency and economy. But, there are a few limitations due to the operational problems that can occur when the area has high waves, as well as the lack of self-propelling which requires towboats to move the equipment to dredging locations. Also, areas with rapid currents make it difficult to keep the dredge in position when working upstream. Potential navigation problems might occur when the cutterhead dredge is used in busy, small harbors and waterways.
  • Dustpan Dredges: This hydraulic suction dredge uses mounted pressure water jets that agitate and loosen the sediment. Then, the dustpan head captures the sediments and moves forward into the excavation. Dustpan dredges are most effective for loose-material, high-volume dredging. For example, this type of equipment is used to maintain the navigation channels in the open reaches of the Missouri, Mississippi, and Ohio Rivers. Dustpan dredges are self-propelled, making it possible to cover long distances at a rapid pace. Additionally, the quick assembly makes it possible to use this equipment for emergency situations. Dredging can be completed with minimal interruptions of the waterborne traffic. But, there are a few limitations. This equipment can only be used for loose materials in sheltered waters or rivers and cannot be used in areas with significant wave action, such as bays or estuaries.
  • Sidecasting Dredges: This shallow-draft seagoing vessel was designed with the purpose of removing material from small coastal inlets and bar channels. It is similar to a hopper dredge, but there are no hopper bins included in the equipment. Instead, the dredged material is pumped overboard through an elevated discharge boom. This self-propelled equipment can rapidly move from one location to another so that it can be used for the maintenance of multiple projects regardless of the long distances on the coastline. The equipment dredges while moving over the shoaled area, which means that it needs flotation depths before the work can begin. So, there are times when the schedule needs to be altered based on the high tide periods. Another limitation is that the shallow-draft sidecasting dredge can’t move the same volumes of material that are possible using a hopper dredge.
  • Dipper Dredges: A simple explanation of this equipment is that it is a barge-mounted power shovel. A ladder structure holds a bucket attachment, which is then thrust into the material that needs to be removed. The production rates of dipper dredges vary depending on the bucket capacity and working depth. This equipment is often used when compacted, hard materials need to be excavated. Since the machine is rugged, it is great for glacial till, clay, hardpacked sand, blasted rock material, pilings, foundations, stumps, and other obstructions in the area. The equipment requires minimal room for maneuvering compared to other equipment. Another benefit is that the excavation can be controlled with prevision, helping to reduce the risk of damage to pier and dock foundations when material needs to be removed The scow barges can be used to transport materials over long distances. The limitation of this equipment is the difficulty in holding fine-grained, semi-suspended, soft materials in the bucket.
  • Bucket Dredges: The name of this dredge comes from the bucket apparatus that is used to excavate the material. Various types of buckets can be used depending on the dredging requirements, including clamshell, dragline, and orangepeel This equipment can be used for a variety of excavations, except solid rock and cohesive consolidated sediments. Bucket dredges have the same advantages as dipper dredges, but the capabilities for compact materials and blasted rock are limited. The system minimizes the volume of excess water, helping to improve the operation efficiency for transportation to the disposal area.
  • CURRITUCK Special Purpose Dredge: Used on the same types of projects as sidecasting dredges, and it can also remove material from inlets to allow transportation to eroded beaches located downdrift. This machine is self-propelled and has one hopper that allows easy production monitoring. It is a great choice for shallow-draft inlets, or as a supplement to sidecasting dredges. The limitation is in the smaller hopper capacity, which affects production rate. As such, it is not effective for major navigation channels.

Dredge Locations in the United States

Public Law 95-269 set by Congress established that Corps would maintain a dredging fleet that is sufficient for both emergency and national defense situations, at home and abroad. This fleet is maintained with the latest industry technology while prioritizing efficiency at the same time.

An inventory of the dredge equipment in the United States is published annually. So, designers can reference this list when exploring options for dredge types that are available in a given area.

Agitation Dredging Techniques

Sometimes agitation dredging is required when the bottom material needs to be removed for an area with equipment that raises the material in the water column so that the currents can carry the materials away from the project site. If agitation dredging is used, then specific recommendations and guidelines should be followed.

Various equipment can be used for this process, including prop-wash, hopper dredge agitation, vertical mixers or air bubbles, drag beams or rakes, or water jets. The objective is to remove the material from a selected area. Plans need to consider the environmental, economic, and technical feasibility for the site.

Two phases are needed for agitation dredging:

  1. Equipment that suspends the bottom sediments
  2. Currents that transport the sediment material away from the site

This process takes advantage of the natural currents in the area. But, there are times when the dredging equipment also augments the natural currents.

Here are a few common types of agitation dredging that might be considered:

  • Hopper Dredge Agitation: The hopper needs to have adequate mobility, pumping rates, and overflow provisions. This process can perform the same maintenance function as conventional hopper dredging. But, several conditions need to be met: sediments should be loosely consolidated and fine-grained, currents must be strong enough to move the sediments from the area, and there is no acceptable level of environmental impact from the dredging. Since the currents are used instead of equipment, the costs are typically lower compared to other methods. But, this technique can only be used for the maintenance of smaller areas; hopper dredge agitation is not sufficient for the maintenance of a larger area. Also, the equipment is not suitable for areas that are deemed to be environmentally sensitive.
  • Prop-Wash Agitation: Propeller-generated currents are directed at the bottom shoal material, causing the sediments to suspend in the water so that they can be carried away by natural and augmented currents. It is common for this type of agitation to occur unintentionally when vessels move through the waterways. But, the technique can be used intentionally in river mouths, coastal harbors, and estuaries. Prop-wash agitation is usually 40% – 90% cheaper per cubic yard compared to conventional dredging techniques. This method is limited to areas with little or no wave action, and the water depth should be less than four times the draft of the agitation vessel. Also, it only works when the sediments are loose silt, clay, or sand.
  • Rakes and Drag Beams: These devices are pulled over the bottom to loosen the bottom material mechanically. As the material is raised, it can be carried away by the natural currents. Rakes and drag beams are only effective when the natural current is strong enough to move the sediment. Even though the drag beams and rakes don’t create an augmented current, sometimes the towing vessel helps with prop-washing.

Disposal Alternatives

In addition to selecting the proper dredging equipment, it is also important to consider the options for disposal alternative, especially from an environmental standpoint. Three common disposal alternatives might be used:

  • Open-water disposal
  • Confined disposal
  • Habitat development

There are unique considerations for each of these selections. So, both environmental and economic factors need to be evaluated. Environmental consequences might occur due to the chemical environment and the short- and long-term physical changes that occur. Bacteriological activity is associated with the dredged material, as well as other parameters such as oxidation-reduction conditions, pH, and salinity.

Properties of the sediment can influence the way the dredged materials react. Typically, the material is high in organic matter, which means that it is both chemically and biologically active. If contamination occurs, then the sediments could release toxic substances causing undesirable disposal conditions. Initially, most contaminated sediments are near neutral pH, although some compounds can be acidic which increases the risk of the release of toxic metals. As a result, these conditions create a high environmental risk.

Methods of Characterizing Pollution Potential

Common methods to characterize the potential of pollution include:

  • Bioassay: To determine the way the contaminant(s) will affect biological organisms.
  • Water Column Chemistry: When the chemical constituents are unequally distributed and remain dissolved in the water, it can cause an impact on the balance in the environment. Testing needs to be completed to determine this impact.
  • Total or Bulk Sediment Chemistry: Tests that show the similarity between the dredged sediments and the sediments at the proposed site of disposal.

Dredged Material Dispersion at the Discharge Site

  • Water Column Turbidity: The suspended solid levels can have varying ranges. The highest concentrations are near the discharge operation, with rapidly decreasing levels as the distance increases downstream.
  • Fluid Mud: A minor amount of fine-grained dredged material slurry is discharged and dispersed as a turbidity plume. But, some of it descends rapidly in the disposal area and accumulates to create a low-gradient fluid mud mound.
  • Mounding: When the bottom slopes are not great enough to maintain mudflows, then the fluid mud will begin to consolidate. The accumulation at the discharge point is known as mounding.
  • Special Circumstances: Knowing the anticipated behavior of the discharged material improves the control of the material dispersion at the disposal site.

Environmental Impacts

The environment can be impacted in a variety of ways due to the dredging, such as:

  • Water column contaminants
  • Turbidity
  • Bottom organism community disruption
  • Migration of organisms

These environmental factors need to be considered, with plans to accommodate the recovery of the site as needed. It can take weeks, months, or even years for full recovery, depending on the biology of the plants and animals affected by the dredging. Methods that use more natural techniques, such as using waves or currents for the shifting of the materials, tend to have a smaller effect than the more disruptive techniques.

One primary mechanism of recolonization is larval recruitment and migration, which means that recovery will be rapid when the disposal timing matches the seasonal increase in larval abundance and biological activity. There are situations where it makes sense to do the dredging on a seasonal basis to minimize environmental impact.

Dredging Contaminants

The process of dredging and disposal don’t bring new contaminants to the environment. Instead, the sediments are redistributed to the natural depository of contaminants that come from other sources. Contamination levels vary depending on the site, animal presence, and anticipated bioaccumulation.

Common types of contamination include:

  • Organochlorine compounds such as DDT, dieldrin, and polychlorinated biphenyls (PCB’s), which are man-made and not soluble in surface waters.
  • “Oil and Grease,” referring to natural and contaminant sediments that are fat soluble. These contaminants are often present due to spillage or chronic pollution from industrial and municipal causes.
  • Ammonia, although the levels are typically below the concentrations that cause environmental concern.

Overview of Open-Water Disposal

It is usually a straightforward process to predict the physical effects of dredging and disposal. For example, these effects might include the removal and burial of organisms at the disposal sites. There is a potential of recolonization of opportunistic species which aren’t normally the dominant site species.

Organisms are often resistant to the effects of suspended sediment in the water, such as aquatic plant beds and coral reefs. One of the most dangerous environmental concerns is the formation of fluid muds in disposal.

Most laboratory studies look at the worst-case scenarios for environmental impact, and it has been found that relatively short-term exposure to sediments and contaminants result in minimal impact. But, chemical testing and biological evaluation of the dredged material should always be done to determine any site-specific factors that need to be considered.

Confined Dredged Material Disposal

The design of the containment area will vary depending on site-specific considerations. Two objectives should be achieved:

  1. Adequate storage capacity based on the anticipated dredging requirements
  2. Maintaining the highest efficiency in retaining solids during the operation to match effluent suspended solids requirements.

The major components of a contaminant area usually include a tract of land, which is designed with a confined surface area that is surrounded by dikes. Hydraulics are used to pump the dredged channel sediments into the containment area. Most of the solids settle out of suspension and occupy a specific storage volume.

The planning and design of containment areas first require a complete evaluation of the dredging plan, including estimates of the frequencies, volume, location, and types of material. Field investigations are important to characterize the material accurately. Additionally, these investigations can be completed to define the conditions of the foundation and gather samples for laboratory testing.

Evaluation of Long-Term Storage Capacity

When the containment area is designed for single use, then long-term storage capacity estimates are not required. But, if recurring maintenance work is necessary, then the long-term storage capacity needs to be estimated for the service life of the facility.

Evaluation factors for long-term storage might include:

  • Sedimentation and self-weight consolidation that results in gains in storage capacity
  • Additional settlement due to compressible foundation soils that are consolidated
  • Time-consolidation relationship based on the fineness of the materials
  • Settlement of the containing dikes

Containment Area Operation and Management

In considering a proper containment area, it is important to have the necessary ponding for the retention of suspended solids. The weir crest elevation can be controlled to maintain adequate ponding depth during the dredging operation. This effort improves the maximum possible efficiency available in the containment area.

It is essential that periodic site inspections and ongoing site management are of the highest priority during the dredging operation. After the dredging is complete, the continued site management is required for drying and consolidation to maximize containment storage capacity and minimize the risk of pond water.

Thin-lift placement is a strategy that might be used to help with long-term storage capacity. This natural drying process increases potential capacity through active dewatering and disposal reuse management programs. Removing the excess water can help with storage area volume by promoting shrinkage and consolidation of the dredged material. Trenching and underdrainage are two dewatering methods that can be used to improve surface drainage.

Also, the benefits of disposal area reuse shouldn’t be overlooked. When the dewatered fine-grained material and coarse-grained material are removed from the site, then partial or total reuse of the disposal area is available. This strategy basically turns the disposal area into a transfer station, where the dredged materials are collected, processed, and then moved to another location for inland disposal or productive use. Examples of productive use include:

  • Construction or landfill material
  • Surface mine reclamation
  • Material for sanitary landfill cover
  • Enhancement of agricultural land

These containment areas can sometimes be used productively for recreational, industrial, or waterway-related sites depending on the needs in the area.

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