A dredge plume is a cloud of debris that forms as a result of dredging. Such plumes usually begin either at the bottom where the dredging takes place, or at the surface from either overflow from the dredging equipment or dumping of the dredged material in a different location.
Dredging is the operation of removing material from one part of the water environment and relocating it to another. In all but a few situations the excavation is undertaken by a specialist floating plant, known as a dredger. Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material that has some value or use, or to create a greater depth of water.
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A primary reason for dredging is the creation, deepening, or widening of shipping lanes in or near harbors, especially those that service large ships. Such work, however, is often subject to governmental regulation, one element of which concerns the effects dredge plumes might have on the surrounding environment. At least two groups have a stake in dredge plumes: governments and dredging companies. Determining the effects of dredge plumes requires understanding their dynamics and interaction with the area's biodiversity.
A harbor or harbour is a sheltered body of water where ships, boats, and barges can be docked. The term harbor is often used interchangeably with port, which is a man-made facility built for loading and unloading vessels and dropping off and picking up passengers. Ports usually include one or more harbors. Alexandria Port in Egypt is an example of a port with two harbors.
As an example, imagine dredging using a clamshell dredger, which deposits the dredged material on a barge that takes its load to a separate area and dumps it overboard. In this scenario, a dredge plume can be expected to begin at the bottom as the clamshell bucket stirs up loose material. As it rises through the water column to the surface, any material not fully contained by the bucket also creates a plume. At the dumping site, a dredge plume begins at the surface as the material enters the water column from above. [1]
Once begun, a dredge plume develops and dissipates. These processes depend on a number of factors—such as the rate at which dredgers release material into the water column, and exactly where they release it. Another factor is the types of dredged materials. A dredge plume of coarse sand has different properties and dynamics than a similar plume of fine sand. Dredge plumes usually contain different constituents, each of which affects the plume's character. For example, denser material settles faster than less dense material. A fourth factor is the degree to which the plume's material clumps together to form larger particles, a process known as flocculation. Still another factor is water movement where the plume is introduced, e.g., currents, tides, waves, and turbulence. [2]
Flocculation, in the field of chemistry, is a process in which colloids come out of suspension in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no formation of a cake, since all the flocs are in the suspension.
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.
Dredge plume evolution—how large it becomes, where it moves, how long it lasts, where its constituent materials settle—can be modeled, albeit imperfectly. Examples of such models include DREDGEMAP, [3] the MMS Dredge Plume Model and the Delft3D Suite. The result of years of research and engineering, such models are important tools to help determine if the expected effects of proposed dredging are within acceptable governmental limits.
A plume's evolution determines its effects on marine plants and animals. For example, the total suspended solids in the plume affects the amount of light that can pass through it, and this may harm marine life, which needs a certain amount of light to thrive. Similarly, the plume's constituents can alter the water column's salinity or oxygen content—and these changes can harm certain organisms. Also, should the plume's constituents settle in places ill-suited to their presence, e.g., where fish would normally lay their eggs, [1] a necessary element of an ecosystem may be disrupted.
Total suspended solids (TSS) is the dry-weight of suspended particles, that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus. It is a water quality parameter used to assess the quality of a specimen of any type of water or water body, ocean water for example, or wastewater after treatment in a wastewater treatment plant. It is listed as a conventional pollutant in the U.S. Clean Water Act. Total dissolved solids is another parameter acquired through a separate analysis which is also used to determine water quality based on the total substances that are fully dissolved within the water, rather than undissolved suspended particles.
Salinity is the saltiness or amount of salt dissolved in a body of water, called saline water. This is usually measured in . Salinity is an important factor in determining many aspects of the chemistry of natural waters and of biological processes within it, and is a thermodynamic state variable that, along with temperature and pressure, governs physical characteristics like the density and heat capacity of the water.
An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one-another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.
