Dredging- Low water levels

by

Daniel Pierson

Underwater excavation is called dredging. After the initial excavation needed to establish a channel, the periodic dredging that must be done to keep it clear and safe for navigation is called maintenance dredging. Once sediments are dredged from the waterway, they are called dredged material.

People have been dredging channels in one way or another since primitive people began to irrigate crops. Until the early 1900s, dredges were crude and barely effective in keeping channels and harbors clean. Keeping the dredge in position in the channel, knowing how deep a channel was being dug, and even making accurate surveys of the completed channel, were a mixture of art and science. Experienced dredge captains and hydrographic surveyors (surveyors of the underwater topography) were able to produce remarkably good results, given the difficulty of their job.

There are numerous types of dredges and methods for the removal of unwanted material within a waterway. Most recently the advent of the long stick excavator has allowed for mechanical removal. Numerous inexperienced individuals have entered into this line of work and unfortunately are not doing a very good job. It takes an exceptional operator to perform work that cannot be seen and most people attempting these projects are inexperienced at best.

.Today, modern dredgers use satellite information and computers to help dig channels. Until the 1970s, dredge captains used celestial navigation and markers placed on the riverbanks to guide their dredges. Now dredge captains use global positioning systems (GPS), which use satellite information to calculate the location of the dredge in the channel. On the dredge, information about the channel, the location of the shoal, and even the position of the dredge in the channel is likely to be displayed on a computer screen while they are working. Using computers to process and display information about the job and the dredge while they are working allows the dredging to be done with great efficiency. It saves time and money, and results in safer navigation channels.

While the onboard instrumentation of modern dredges is computer assisted, the basic excavation methods of dredges have remained the same since the late 1800s. The three main types of dredges are mechanical dredges, hydraulic dredges, and airlift dredges.

Mechanical Dredges – Mechanical dredges remove material by scooping it from the bottom and then placing it onto a waiting barge or into a disposal area. The two most common types of mechanical dredges are dipper dredges and clamshell dredges. They are names for the type of scooping buckets they employ.

Mechanical dredges are rugged and can work in tightly confined areas. The dredge is mounted on a large barge and is not usually self powered, but is towed to the dredged site and secured in place by anchors or anchor piling, called spuds. They are often used in harbors, around docks and piers, and in relatively protected channels, but are not suited for areas of high traffic or rough seas. Usually two or more disposal barges, called dump scows, are used in conjunction with the mechanical dredge. While one barge is being filled, another is being towed to the disposal site. Using numerous barges, work can proceed continuously, only interrupted by changing dump scows or moving the dredge. This makes mechanical dredges particularly well suited for dredging projects where the disposal site is many miles away.

Mechanical dredges work best in consolidated, or hard-packed, materials and can be used to clear rocks and debris. Dredging buckets have difficulty retaining loose, fine material, which can be washed from the bucket as it is raised. Special buckets have been designed for controlling the flow of water and material from buckets and are used when dredging contaminated sediments.

Hydraulic Dredges – Hydraulic dredges work by sucking a mixture of dredged material and water from the channel bottom. The amount of water sucked up with the material is controlled to make the best mixture. Too little water and the dredge will bog down; too much water and the dredge will not be efficient in moving sediment. Pipeline and hopper dredges are the two main types of hydraulic dredges.

A pipeline dredge sucks dredged material through one end, the intake pipe, and then pushes it out the discharge pipeline directly into the disposal site. Because pipeline dredges pump directly to the disposal site, they operate continuously and can be very cost efficient. Most pipeline dredges have a cutterhead on the suction end. A cutterhead is a mechanical device that has rotating blades or teeth to break up or loosen the bottom material so that it can be sucked through the dredge. Some cutterheads are rugged enough to break up rock for removal. Pipeline dredges are mounted (fastened) to barges and are not usually self-powered, but are towed to the dredging site and secured in place by special anchor piling, called spuds.

Cutterhead pipeline dredges work best in large areas with deep shoals, where the cutterhead is buried in the bottom. Water pumped with the dredged material must be contained in the disposal site until the solids settle out. It is then discharged, usually back into the waterway. This method of dredging is not suitable in areas where sediments are contaminated with chemicals that would dissolve in the dredging water and be spread in the environment during discharge.

Because the discharge line for pipeline dredges is usually floated on top of the water, they are not suited to work in rough seas where lines can be broken apart or in high traffic areas where the discharge pipeline can be an obstruction to navigation. If there is a lot of debris in the dredging site, the pumps can clog and impair efficiency.

Hopper dredges are ships with large hoppers, or containment areas, inside. Fitted with powerful pumps, the dredge suctions dredged material from the channel bottom through long intake pipes, called drag arms, and stores it in the hoppers. The water portion of the slurry is drained from the material and is discharged from the vessel during operations. When the hoppers are full, dredging stops and the ship travels to an in-water disposal site, where the dredged material is discharged through the bottom of the ship.

Hopper dredges are well-suited to dredging heavy sands. They can maintain operations in relatively rough seas and because they are mobile, they can be used in high traffic areas. They are often used at ocean entrances, but cannot be used in confined or shallow areas. Hopper dredges can move quickly to disposal sites under their own power, but since the dredging stops during the transit to and from the disposal area, the operation loses efficiency if the haul distance is far.

There are special hydraulic dredges called side-casters and dustpan dredges. Both of these dredges are used to remove loosely compacted, coarse-grained material and place it in areas close to the navigation channel. They are not widely used. The dustpan dredges were specifically developed for jobs on the Mississippi River. Side casting of dredged material, done mainly on some smaller projects, is also limited to certain situations and environments.

Airlift Dredges – Airlift dredges are special use dredges that raise material from the bottom of the waterway by hydrostatic pressure. They have cylinders that operate like pistons. Material is drawn through the bottom of the cylinder. When it is full, the intake valve closes, trapping the material. Then, compressed air forces the material out through a discharge line to a waiting dump scow or directly to a disposal site. Airlift dredges bring dredged material to the surface with a relatively small amount of water, which is good when environmental contamination is an issue.

Airlift pumps have not been widely used in the United States. They do not typically achieve high production rates, but are well-suited for projects where either site conditions or sediment quality concerns make other dredges inappropriate. They can be used in tight quarters around docks and piers, in rough seas, and in deep water.

Disposal of Dredged Material

Disposal site selection for dredged material is one of the most important and challenging parts of planning a dredging project. The most common dredged material disposal methods are ocean placement, beach nourishment, confined disposal facilities (CDFs), flow-lane and within-banks placement, and capped disposal.

Ocean Placement – Ocean Dredged Material Disposal Sites (ODMDS) are primarily used for material coming from inlets, coastal entrance bars, or main coastal navigation waterways. Typically, in ocean placement, a hopper dredge or towed barge sails to a designated area in the ocean, where the hull (bottom) of the vessel is opened. The dredged sediments drift to the bottom. Only clean dredged material may go to ocean sites.

There are over 100 Corps and EPA designated ODMDS around the coast of the continental United States. On average, about 60 million cubic yards of dredged material is taken to these sites each year. The Corps has developed computer simulation models to help predict the impacts of ocean disposal, which are used by project planners in the site selection and project planning processes.

Beach Nourishment – Beach nourishment is the placement of dredged material on or near the beach, usually to replenish an eroding beach or protect an eroding wetland. The dredged material is generally sand coming from inlets, coastal entrance bars, or main offshore waterways. Both hopper dredges and pipeline dredges can use beach nourishment sites. When hopper dredges place sand offshore along the beach, natural processes carry it onto the beach over a long period of time. Only clean dredged material can be used for beach nourishment. This method of dredged material disposal is considered a beneficial use of dredged sands. It is used in coastal areas all around the country.

Confined Disposal Facilities (CDFs) – In Confined Disposal Facilities (CDFs), dredged material is placed behind dikes, which contain and isolate it from the surrounding environment. There are three types of CDFs: Upland, Shoreline, and Island. A mixture of dredged material and water is pumped into an area that is divided into several smaller areas, called cells. As the water moves between the cells, it slows, the dredged material settles out, and finally, clean water is discharged from the site. The difference in the three types of CDFs is their location. Upland CDFs are on land, above the line of high water and out of wetland areas. Shoreline CDFs are constructed over the sea or lake bottom and are attached to the shoreline on at least one side. Island CDFs are constructed offshore, but in relatively shallow water.

CDFs can be used for any type of dredged material, coarse or fine-grained. Usually, pipeline dredges pump material directly from the dredging site into the CDF, which is the least expensive way to put the material in the site. In special cases, where the CDF is far removed from the dredging site or a pipeline dredge is not used for the dredging, barges or hopper dredges may take dredged material to the site where it is re-pumped into the CDF.

Preparing and caring for a CDF requires a substantial commitment of time and money by local and Federal governmental agencies. Sometimes a CDF can be designed so that there can be other uses for the land during and after the site is used for dredged material disposal. For example, island CDFs in Mobile Bay provide nesting habitat for waterfowl. Upland CDFs along the Columbia River in Oregon and Washington are mined for construction fill. In the Great Lakes, shoreline CDFs have been used to contain and isolate contaminated sediments from the environment.

Flow-lane and Within-banks Placement – Some waterways are in high-energy river systems with rapidly flowing water and strong currents. The energy of the water causes shoals made of coarse sand to form, move along the bottom, and re-form relatively quickly. In these systems, flow-lane or within-banks disposal may be used.

For both methods, dredged material is placed in or along the river that is also subjected to the river’s erosion. They are filled temporarily, until the energy of the river moves the sand out again. With-in banks disposal refers to the temporary use of eroding banks, the river thalweg, sandbars, or man-made islands. It is common on the Mississippi River. Pipeline and dustpan dredges use this disposal option.

Flow-lane disposal of dredged material refers to the placement of materials in water within or adjacent to the navigation channel. It is similar to the “thalweg” disposal on the Mississippi River. (The thalweg of a river is the area where the water has its greatest velocity.) Flow-lane disposal is commonly used in the Columbia River by both hopper and pipeline dredges.

Capped Disposal – Sometimes, but not often, dredged material is contaminated with metals, chemicals, or other substances. If the contamination is bad enough, it might be harmful to the environment to put the material in a typical in-water site or a typical CDF. In these cases special handling will be used to prevent the contaminants from re-entering the environment during dredging and disposal.

The objective of capped in-water disposal is to isolate contaminated material from the environment by capping, or covering, the contaminated material with clean material – usually sand. The contaminated dredged material is placed on a level bottom or in deep pits or bottom depressions. Then clean material is dropped on top. The cap is designed and carefully put over the contaminated sediment to ensure that it stays in place. Caps are designed so that currents, waves, or the burrowing bottom creatures will not erode the protective layer over time. In addition, the caps are continually monitored to look for signs of failure.

Capped disposal for the isolation of contaminated sediment is practiced worldwide. In the United States, capped disposal sites have been used successfully in several places: Long Island Sound, in New Bedford, Massachusetts; Puget Sound in Washington state; the New York Byte; and other locations.

Derek Van Eenenaam

http://michiganshorelineconstructionandpermitting.com

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Dredging- Low water levels