Dredging and Dredged Material Placement

Dredging is a very interdisciplinary field that requires the participation of engineers, scientists, government, regulators, and the public. Ship channel design and shoaling rates determine the basic dredging requirements. The quantities of dredged material are determined from past records, and the planning for dredging projects is based on long-term requirements and hydrographic surveys. There are numerous professional organizations, dredging conferences, and educational opportunities available to the dredging community. Dredges working in the United States must comply with Section 27 of the 1920 Merchant Marine Act (known as the Jones Act), the Foreign Dredge Act of 1906, and the Shipping Act of 1916. There are many dredging companies in the United States and every state has a least one dredging company. The World Dredging Mining and Construction annual dredging directories is the source of data for determining the distribution of dredging companies in the United States. Dredges are classified as hydraulic or mechanical dredges.

The mechanics and dynamics of fluids are important in many aspects of dredging and dredged material placement. This chapter aims to summarize some of the most used fluid mechanics/dynamics concepts that are frequently used in analyzing hydraulic dredges and the placement of dredged material. For the application of dredging, the fluid is a mixture of solids (sands, silts, clays, gravel, and rock) and water, which is called a slurry. In some cases, the slurry may also have entrained gases and organic material. Laminar flow is a flow where the fluid particles move along smooth paths in laminas, or layers, with one-layer gliding smoothly over an adjacent layer. For turbulent flow, the definition of steady flow states that the time mean velocity, density, pressure, and so on do not change with time. One-dimensional flow neglects the variations in velocity, pressure, and temperature, among others that are transverse to the main flow direction.

Hydraulic dredges are commonly classified as large when the diameter of the pump discharge and discharge pipeline are greater than 12 in. (305 mm). Hydraulic dredges perform both phases of the dredging operations (digging and placement). The basic components of a hydraulic dredge are dredge pumps, digging and agitation machinery, and hoisting and hauling equipment. Hydraulic dredges are categorized as hopper, pipeline, cutter suction, dustpan, bucket wheel, and side casting. The agitation dredge suspends the dredged material and lets the natural currents move the sediments to another location in the dredged channel. A water injection dredge lowers its dredge head into the sediments and injects water into the sediment. Small cutter suction and horizontal auger suction dredges are manufactured by VMI Dredges. The Matchbox dredge is a cutter suction dredge with a modified dredge head called the matchbox.

Dredge pumps are designed with sufficient clearances to accommodate occasional gravel, rocks, and other debris passing through the pump without jamming. The fluid pumped is called a slurry, which means the water contains suspended solids (gravel, sand, clay, and silt). The slurry has a variable specific gravity and a highly variable viscosity. Dredge pump characteristics typically are described by curves or graphs that show the variation of head, brake horsepower, and efficiency as a function of volumetric flow rate of water. Dimensional characteristic curves require separate curves for each pump speed, impeller diameter, and specific gravity of mixture. The most common parameter used for evaluating cavitation in pumps is the net positive suction head available produced for a given discharge or flow rate. Large laboratory test facilities are needed to test dredge pumps and develop the pump characteristic curves.

Dredged sediment is composed of solid particles, water, and gas. Sediments are commonly classified as gravel, sand, silt, and clay, depending on the particle size. The International Association of Dredging Companies system for identifying soil types and parameters necessary for evaluating dredgeability are tabulated. A sediment testing method for evaluating the undrained shear strength of sediments is the shear vane test. The two main types of investigations are desk studies and field sampling. All dredging projects and sites are unique, and the geotechnical site investigation is tailored to the needs of the specific project. Dredging by hydraulic methods mixes about 20% solids with 80% water, which causes bulking. The type of sediment, dredging method, and particle size distribution affect the bulking factor. Ship channels, harbors, and ports experience deposition of fine-grained sediments (silts, clays, and organic matter), which can result in a fluid mud layer above the actual channel bottom.

Dredged material from cutter suction dredges is commonly transported by pipeline to dredged material placement/disposal facilities or to beaches for beach nourishment. The first step for evaluating the slurry transport in pipes is to evaluate the major (friction) and minor (valves, swivels, joints, and entrance) losses. The dredging pipeline software described in this chapter follows the procedures described in Wilson for heterogeneous flow. The software needs many fluid and sediment inputs and information on the particle settling velocity, solids concentration by volume, critical velocity, friction factor, and minor loss coefficients. In most cases, the pipelines are long and near horizontal. The effects of inclined pipelines on the suction side of the pump and inclines on the discharge side (near placement area dikes, submerged beach nourishment pipes, and others) are discussed, which include the effects on head loss and critical velocity.

The hydraulic pipeline dredge normally uses a cutter to excavate, loosen, or pulverize the dredged material and make it available to the suction pipe inlet. Basket-type cutters are the most common type of cutter, but other cutters such as the bucket wheel cutter, auger, and disk cutters are also used. Trailing suction hopper dredges lower large drag heads attached to the end of the suction tubes to the channel bottom. The drag heads excavate the sediment by erosion, water jets, blade, or a combination of these and pumps deliver the slurry to the hopper. Dredges such as the dustpan dredge use special dredge heads to remove soft maintenance material. Mechanical dredges such as grapple, clamshell, dipper, and endless chain buckets use different types of buckets to remove the underwater sediments. Merchants have suggested a cutting model for calculating the cutting force for metals breakage. This model assumes that failure is produced by shear stress when orthogonal cutting is exerted.

Estimating production is necessary to determine the required amount of time to complete a dredging project, and it is also critical in the cost estimating process. The production of a cutter suction dredge that has a main pump located at the water surface is calculated using the Jin and Randall software, typical centrifugal pump curves, energy equation, net positive suction head equation, cavitation limits and critical velocity. The production estimation process for a mechanical clamshell dredge is summarized. A simple schematic of hopper dredge or sometimes called a trailing suction hopper dredge is also shown. Trailing suction hopper dredges utilizes large centrifugal pumps to excavate dredged material owing to erosion of sediment as water enters the drag head. The efficient transportation of solid material suspended in liquid or hydraulic transport depends on accurate calculation of the power required to pump the slurry mixture and the rate at which the sediment is removed.

Instrumentation is very important in all phases of dredging. Dredging equipment (e.g., cutter suction pipeline dredges, mechanical dredges, and hopper dredges) all performs more efficiently with instrumentation to measure density, flow velocity, pressures, and dredge position. Differential global positioning systems mounted on dredges record the position of dredges during the complete dredging operation. Magnetic flow meters are a reliable measure of the average velocity of the fluid or slurry moving through the meter. A nuclear density gauge is commonly used to measure the density of the slurry flowing in a dredge pipeline. A production meter combines the outputs from the flow meter and the density gauge and displays the flow and density information on cross-pointer display. Hydrographic software is critical for all dredging operations, whether the dredge is a hopper dredge, cutter suction dredge, or mechanical dredge. Automated monitoring of dredging activities is the main purpose of the Data Quality Management program.

Regulation of the placement of structures, dredging of navigable waterways, and placement/disposal of dredged material within waters of the United States and ocean waters is a complex issue and is a shared responsibility of the Environmental Protection Agency and the US Army Corps of Engineers. The Marine Protection, Research, and Sanctuaries Act, the Clean Water Act, and the National Environmental Policy Act are the major federal statute/laws governing dredging projects, but a number of other federal laws and executive orders must also be considered. Federal agencies prepare an Environmental Impact Statement (EIS) if a proposed major federal action is determined to significantly affect the quality of the human environment. The regulatory requirements for an EIS are more detailed and more rigorous than the requirements for an environmental assessment. Environmental windows are frequently imposed on dredging projects in both coastal and inland waterways.

When dredged material is dredged from waterways, it is typically placed in an open water placement/disposal site, a confined placement/disposal facility, or it is used beneficially (beach nourishment, habitat restoration, land reclamation, wetlands restoration, thin-layer placement, and others). Disposal of dredged material in rivers, lakes, estuaries, and oceans is called open water disposal or open water placement. Open water disposal or placement is accomplished using pipeline dredges, hopper dredges, barges/scows loaded by mechanical or hydraulic dredges, and mechanical dredges. The STFATE model is a commonly used model, and the placement is typically from a hopper dredge or hopper barge/scow, and the dredged material is released through bottom opening doors. Information for dredged material description is input by clicking an arrow icon from the Material Description menu. Information for the disposal operation is input by clicking an arrow icon in the Disposal Description menu.

In the United States, approximately 300 million cubic yards of bottom sediments from various waterways are dredged each year. Contaminated sediments are those that contain chemical substances at concentrations that pose a known or suspected environmental or human health threat. This chapter discusses contaminated sediments that are located at the bottom of water bodies such as ship channels, ports, harbors, rivers, lakes, and oceans. Capping is described along with an example of modeling the cap process. Several examples of dredging and capping projects are described. Dredging and capping are processes that have been used to successfully remediate contaminated sites. The dredging of contaminated sediments is described as environmental dredging. The compatibility of the contaminated dredged material and the cap material is important for a successful capping project. In general, capped disposal sites will be in designated disposal sites that are nondispersive, meaning the sediments will remain inside the disposal site boundaries.

Mechanical and hydraulic dredges conduct dredging of waterways and ports to maintain authorized channel depths and deepen channels for larger vessels. A critical part of any dredging project is estimating the cost. Most dredging contractors have proprietary cost estimating software to develop an estimate for competitive bidding on the dredging projects. This chapter aims to summarize the general approach to estimating. One of the key factors in determining the feasibility of a dredging project is the estimated dredge production rate. The cost estimating software developed by the Center for Dredging Studies uses the Excel platform to estimate production, costs, and time to complete the project. The chapter also presents a comparison of cost estimates for dredging projects using cutter suction dredge, hopper dredge, and mechanical dredge cost estimating software.

This chapter describes the dredge pump cycle as the dredge operates in water and slurry and its effects. Operational limits owing to water hammer (ram-off) and calculation of the step angle are described. Swing width, anchor location, and channel width limitations are discussed. Some common instrument readings and their causes are evaluated. Dredge auxiliary equipment such as anchor booms, spuds, spud carriage, spud and ladder hoists, stern mooring, wire rope, sheaves, ladder pump, and booster pumps are also discussed. Surveying of waterways, lake bottoms and ocean floor is often accomplished using underwater acoustic equipment known as side scan sonar and subbottom profilers. The chapter summarizes possible dredge problems as indicated by dredged instrumentation (pressure gauge, production meter, flow meter, and density gauge). Last, containment barriers and acoustic equipment are described and illustrated.

Dredging research in the United States is conducted by the US Army Corps of Engineers (USACE), academic institutions such as the Center for Dredging Studies at Texas AM University, and private organizations such as GIW Industries, Inc. Test facilities for testing dredge pumps are found at the GIW Industries Laboratories in Groveton, Georgia. Modeling dredging operations and dredged material placement is conducted at the Coastal and Hydraulics Laboratory and Environmental Laboratory in Vicksburg, Mississippi. The simulators give exposure to the automation and controls technology. The simulators are designed for cutter suction dredges. There are seventeen built-in scenarios, or exercises, provided with the simulator software. The exercises are called TEXAM1 through TEXAM17. In addition, there are two dredging magazines published in the United States: World Dredging, Mining, and Construction and International Dredging Review. The Western Dredging Association publishes the Journal of Dredging Engineering annually.