What are dredge spoils?
Dredge spoils are the sand, soil, silt and other organic matter that accumulates on the bottom of a body of water and are removed during dredging. Urban development, farming, natural disasters, tidal influence and other factors contribute to sediment build up in aquatic environments. Understanding the physical and chemical characteristics of a specific sediment is necessary to determine the proper technology, dredge strategy and management options for removing the sediment. The success of any dredge project requires a comprehensive engineering plan and agency approvals in advance of the removal process.
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What is dredging?
Dredging is the process of removing excess sediment, contaminated sediments and debris from aquatic ecosystems. The most common reasons for dredging a water body are:
- To perform navigational dredging for maintaining safe shipping channels, ports, harbors, dams and bridges.
- Environmental dredging to remove excess sediment in sensitive habitats or to remove contaminated sediment containing toxic materials that could adversely affect human health and the environment.
- Flood control and management during high flow conditions through a watershed.
Navigational dredging is an ongoing maintenance initiative that is under the direction of the US Army Corp of Engineers (USACE). Every year, the USACE must oversee the removal and management of millions of cubic yards of dredge spoils. These dredge projects serve a critical role in protecting human life, water treatment systems, property and environment while ensuring safe shipping of goods to the market.
Environmental dredging targets the reduction of additional contaminate exposure by preventing the spread of contaminated sediments to other areas. Contaminants enter waterways from many sources including surface runoff, municipal and industrial discharges, historical industrial practices, chemical spills and other sources. The proper management of contaminated sediments is a complex process. The type and levels of the contamination present will determine the options that are available for beneficial use or disposal. Federal, state or local laws govern the decision-making process for proper management of contaminated sediments.
What are the different types of dredging?
Mechanical Dredging is accomplished by physically removing the sediment and debris from the bottom of the waterbody. The equipment types commonly used include conventional / long stick excavating equipment, clamshell or bucket dredges. Specialized dredge buckets are available to handle all types of sediments and are adaptable to fit unique operational requirements of a project.
Hydraulic Dredging uses specially designed dredge equipment that consists of cutter heads to remove the sediment in a precision manner from the bottom of a water body. The cutter heads can resemble an auger or have a circular head design. Both types make accurate cuts and vacuum up the sediment. Large pumps and a long pipeline move the sediments to barges or to upland processing facilities. Dewatering of sediment in barges, transport to another location for treatment, reuse or disposal are all options to consider when designing a project. Managing dredge spoils in upland facilities can range from complex physical / chemical treatment, dewatering, landfill or disposal in a confined disposal facility (CDF).
Sediment Dewatering Methods
- Geotubes are large membrane fabric bags that hold sediment to allow the solids settle while water slowly drains out over a period of months. The overall effectiveness of the dewatering process will depend on the sediment type. When opening a Geotube it is common for the sediment to need additional drying or stabilization to meet a required specification. The use of lime-based reagents and other products commonly used to condition the sediment to achieve an optimum strength and moisture content. In some large applications, the sediment remains sealed in the Geotubes in an onsite landfill.
- Mechanical equipment of various types is used to physically remove excess water from sediment. Plate and frame filter presses, belt presses, centrifuges and hydrocyclones are options on large-scale dredging projects. Other methods may include pug mills, custom soil-mixing boxes and excavators with an assortment of mixing head attachments for introducing reagents into the dredge spoils.
- Chemicals such as quicklime, cement, Calciment LKD (lime kiln dust) or other products with pozzolanic properties may be required to dry, modify and stabilize unworkable sediments. Chemical technology, when performed separately or in combination with other dewatering processes, will further treat and modify the sediments to achieve soil like characteristics. Clean sediments that are properly conditioned can create opportunities for beneficial use options near most dredge projects.
Managing Dredge Spoils
The list of regulatory compliance requirements is long when it comes to managing dredge spoils. Methods for disposal include onsite and offsite landfills, confined disposal facilities (CDF) and confined aquatic disposal (CAD). CDF’s are special permitted landfills and CAD’s are deep – water disposal areas commonly found in the ocean. CDF’s resemble a landfill in their basic design with exceptions. CDF’s are generally located in close proximity to the dredge site, which minimizes the transportation component. CAD cells are special permitted disposal areas that receive clean sand and material from dredging. Tidal influence and storms along the coastline moves massive volumes of clean sand into channels, ports and harbors. Navigational dredging will remove the clean material and barge the material back to the designated deep-water CAD in the ocean for deposition.
Beneficial Reuse of Dredge Spoils
Every year millions of cubic yards of sediments require dredging for navigational reasons. The vast majority of these sediments are environmentally clean and suitable for reuse. When the physical and chemical properties of the sediment allow for reuse, there can be significant environmental and economic benefits. Some options for repurposing nutrient rich sediments in environmentally sound applications includes restoring degraded farmland, upland restoration, marine habitat projects, beach nourishment, levy improvements, construction fill, mine reclamation, landscape material and many other earthwork projects. There are examples of successful sediment reuse projects around the country, but we have barely scratched the surface on managing clean sediments as a resource.
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