Hydraulic Fill Manual: For Dredging and Reclamation Works (Curnet Publication)
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In addition, site characteristics may influence how removal can best be accomplished.
Inadequate site and sediment characterization for environmental dredging can potentially result in delays, higher costs, unacceptable environmental impacts, and failure to meet cleanup levels and remediation goals. The data collected must be adequate to either determine whether removal should be selected as a remedy or to design a removal remedy. The timing and staging of the site characterization can also affect results. For example, during the early stages of an RI, there is less certainty as to which of the detected chemicals are COCs that require remediation.
Therefore, the scheduling of site characterization often must be adapted based upon new information. These results determine the nature and extent of sediment contamination, inform remedy selection, and support remedial design. At many sites, a multi-phased characterization effort beginning during the RI and continuing into the FS and remedial design stage may be appropriate.
The characterization must collect adequate site data to support decisions required during critical stages of the remediation process. Sediment stability is not critical in the evaluation of removal as a remedial approach. In areas where sediments are unstable, however, natural disturbances would likely lead to significantly increased contaminant mobility and risk. These areas, therefore, may be good candidates for an active remedy such as removal.
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The net deposition rate The amount of material deposited per unit time or volume flow. Residuals cover or backfilling, described in Section 6. Note that removal can result in creation of depressions in the sediment bed and therefore net deposition rates immediately following removal can be greater than rates prior to removal.
Erosional potential is not critical in the selection of removal as a remedial technology.
Zones where erosion of the sediment bed would likely increase contaminant mobility and risks may be good candidates for engineered containment or removal, as long as erosion of dredge residuals are not a concern. Sediments with relatively low bulk density less than roughly 0. The potential for resuspension, which is further discussed in Section 6. Site bathymetry The measurement of or the information from water depth at various places in a body of water.
Generally, removal becomes increasingly more challenging as water depth increases. Removal experience to date has been limited to depths of about 50 ft or shallower; however, removal in water depths up to 75 ft is possible for instance, using hydraulic dredge equipment with a ladder pump configuration or cable mounted buckets. Removal of contaminated sediment in water deeper than about 75 ft is generally impractical. Note that as water depth increases productivity can decrease, releases to the water column can increase, and the accuracy of removal can decrease. Physical isolation controls for example, silt curtains or rigid containment such as interlocking sheet piling also have practical depth limitations for installation and effective operation generally limited to about 20 ft of water or less.
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Mechanical dredges using fixed arm buckets are generally limited to about 20 ft of water unless a long-stick arm is used, which reduces the capacity of the bucket. Alternatively, shallow water can also restrict access for hydraulic and mechanical dredges by not providing sufficient draft for the equipment being used.
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Water shallower than 3 to 4 ft may limit access and removal to form a channel may be needed to facilitate access. Excavation is generally restricted to zones with shallow water depths typically less than 10 ft where the removal area can be isolated and dewatered such as shoreline excavation or lower flow streams that can be bypassed. Areas to be dewatered generally must be small enough to accommodate the dewatering operations. Larger areas and deeper water zones may still be considered for excavation in certain circumstances, but special engineering considerations may be needed, which complicate implementation and increase construction duration and cost.
All infrastructure bridges, pilings, piers, utilities and even shoreline structures adjoining the removal areas must be evaluated for stability before, during, and after removal. An adequate factor of safety should be built into the assessment. Safety offsets leaving a buffer between the infrastructure and the removal area or stabilization measures are often specified to avoid disturbance to the structures.
Sediment located under structures such as piers may make removal impractical. For example, hydraulic dredges have limited access, maneuverability, and functionality to set cables and anchors to work around structures. A crane with a cable-mounted bucket has height requirements that can limit access, while fixed arm buckets can provide better accuracy in bucket placement and have the ability to reach under some structures.
Excavation generally poses concerns for shoreline slope and structure stability. Greater concerns for infrastructure integrity arise for deeper excavations, and structures and underwater utilities may limit effective containment, isolation, and dewatering of the removal area. In some cases removal and relocation of infrastructure may accommodate sediment removal, but in other cases moving the infrastructure may not be practical and may preclude sediment removal. The presence of a hard bottom can limit effective containment during removal if sheet piling is contemplated , depending on the composition and configuration of the hard bottom.
Contaminated sediment overlying bedrock or glacial till may impede some dredging equipment. Contaminated sediment lodged in crevices in bedrock can be impractical to remove. For hydraulic dredging, the presence of a hard bottom underlying the contaminated sediment limits over-dredging into a relatively clean surface and can also increase the magnitude of generated residuals and undisturbed residuals.
Hydraulic Fill Manual
For mechanical dredges, the presence of hard bottom typically leads to greater amounts of generated residuals and resuspension, due to over-dredging difficulties and the higher energy required to remove the consolidated underlying material. On the other hand, a hard bottom below contaminated sediment tends to limit over-excavation of material.
Attempting to re-dredge residuals on top of a hard bottom using either mechanical or hydraulic dredges has been shown to be less effective in reducing contaminant concentrations, but plain suction dredges can more effectively capture generated sediments and residuals from a hard bottom. Mechanical leverage of an excavator during excavation results in more accurate removal and can remove hard material with less sediment loss.
Both large and small debris can slow some dredging equipment.
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Hydraulic dredges have inherent limits to the size of material that can be removed and are designed to only pass debris smaller than the diameter of the inlet pipe. As a result, a separate mechanical debris removal operation is often used to clear the area of large debris, logs, boulders, and cables prior to hydraulic dredging. Mechanical dredges are better suited to removing debris prior to sediment removal, but they also have some limitations depending on the specific equipment being used.
For example, debris can become lodged in the bucket and allow sediment to discharge to the water body, thereby increasing turbidity.
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Special equipment may be needed to clear the debris. Debris removal activities, however, may disrupt the sediment structure and promote sediment erosion. In general, the presence of debris tends to result in increased resuspension and generation of residuals and, consequently, reduced production. Zones with extensive debris make removal less effective, and in some cases may make removal impractical.
Excavation techniques can generally accommodate debris removal without an increase in resuspension, release, and residuals. Hydrodynamic characteristics such as water velocities, water depth changes tides and waves can affect the performance of removal operations. Experience has shown that higher water velocities can increase the release and transport of contaminants due to resuspension both initial resuspension as well as resuspension of generated residuals and can also affect the implementability of resuspension control technologies.
Waves greater than 2 ft, currents greater than 1.
The use of rigid resuspension containment structures, such as sheet piling, can also cause secondary effects such as flood rise and create the potential for erosion due to channel conveyance constrictions. This effect may also arise adjacent to isolation systems used for excavation. Excavation can be designed to accommodate a range of hydrodynamic conditions to mitigate concerns for resuspension, erosion of residuals, and release of contaminants.
The design should consider the potential for containment over-topping events and potential for releases, as well as effects on production rate. Sloping bathymetry of more than a few percent can affect removal operations. Each type of removal equipment has varying suitability to remove contaminated sediment on a slope. Navigational dredging equipment and operators are usually accustomed to performing removal operations to achieve a relatively flat bottom. Advances in equipment and operational procedures for environmental dredging, however, can now leave a more contoured bottom bathymetry after removal.
Steeper slopes can complicate dredging.