Dredging and the management of dredged material are an important function of Army Corps of Engineers districts. In order to maintain channels and harbors at safe depths, periodic dredging is required. Annually the Corps dredges about 250 million cubic yards of maintenance material from United States waterways.
The US Army Corps of Engineers performs maintenance dredging with its four hopper dredges and also contracts for maintenance dredging by fifteen industry-owned hopper dredges. In addition, the Corps still maintains three small side-casting dredges for coastal work, along with one cutterhead dredge, three dustpan dredges, and one special purpose dredge. The Corps previously operated many more dredges, but increasing pressure from commercial industry obliged the government to "privatize" much of its dredging work.
Private contract dredgers have taken over most of this dredging and are currently responsible for the majority of the federally funded hopper dredging in the US. Although the Corps dredges perform very little of the federally-funded dredging, private contract dredgers have argued that the Corps dredges cut into their business. Ports and shippers counter that the federal presence is necessary to meet capacity needs, provide timely emergency response, and ensure competitive prices.
Construction of new navigation channels involves removal of materials previously undisturbed. Maintenance dredging operations involve the repetitive removal of naturally recurring deposited bottom sediment such as sand, silt, and clays in an existing navigation channel. More than 400 ports and 25,000 miles of navigation channels are dredged throughout the United States to keep traffic operating efficiently.
Channels and harbors are maintained for safe navigation. Without dredging, New York Harbor would be impassable to passenger liners and cargo ships. Periodic maintenance dredging as well as occasional enlarging and deepening of navigation channels is essential to accommodate commercial and recreational vessels. The depths of channels vary with the types of traffic. The navigation channels on the Mississippi and Ohio Rivers are maintained at authorized depths of 9 feet, which is sufficient for the barges and cargo they carry on these rivers. In contrast, the Connecting Channels between the Great Lakes are maintained at depths of 30 feet for the ocean-going ships which carry ore, coal, and other cargos between domestic and international ports. The channels at a particular harbor may have depths up to 30 feet at the entrance with progressively shallower depths as one moves upstream. This is especially common at harbors where commercial navigation is concentrated near the river mouth while recreation traffic extends a distance upstream.
To maintain navigable waterways, approximately 400 million cubic yards of material are dredged in the United States every year. Of this amount, about 60 million cubic yards are placed in ocean waters at more than 100 Environmental Protection Agency approved sites. The other 340 million cubic yards are dredged in coastal and inland waters and placed in a variety of locations, including uplands, beach sites, wetlands construction sites, and riverine sandbars, to name a few.
Dredged material is a term used to describe the material excavated from a river, harbor or lake by a dredge. In the case of maintenance dredging, the material is sediment that has accumulated in the channel bottom since the last time it was dredged. In rivers, these sediments are soils that have been eroded from farmlands, forests, and gardens or washed off city street and carried by the water before depositing in a deepened channel. In the harbors and entrance channels that extend out into the lakes, the sediments are sand and silts that have been carried along the lake shoreline by littoral currents and deposited in the deepened channel. Over 90% of the dredged material is a clean soil that is physically and chemically the same as the soil on a field or in the park. The other ten percent may have contaminants that came from a number of possible sources including urban runoff and sewer overflows.
The national waterway system of the United States is a fundamental component of the nation's economic strength. Comprised of over 12,000 miles of inland waterways and 900 harbors, the system supports over 13 million American jobs. The critical importance of this infrastructure and the competing demands placed on federal tax dollars require that the system's construction and maintenance program be conducted in the most cost effective manner possible. Construction and maintenance of the system is entrusted to the US Army Corps of Engineers. Dredging is managed through contract to private firms and operation of Corps equipment. The nation invests about $800 million annually in channel and harbor dredging.
In 1824 the Supreme Court ruled in Gibbons v. Ogden that federal authority covered interstate commerce including riverine navigation. Shortly thereafter, Congress passed two important laws that, together, marked the beginning of the Corps' continuous involvement in civil works. The General Survey Act authorized the president to have surveys made of routes for roads and canals "of national importance, in a commercial or military point of view, or necessary for the transportation of public mail." The president assigned responsibility for the surveys to the Corps of Engineers. The second act, passed a month later, appropriated $75,000 to improve navigation on the Ohio and Mississippi rivers by removing sandbars, snags, and other obstacles. Subsequently, the act was amended to include other rivers such as the Missouri. This work, too, was given to the Corps of Engineers
The Corps has been involved in regional navigation improvements since the early 1870s. Work to clear the nation's rivers of navigation obstacles continued after the Civil War. In 1871, engineer Major Quincy A. Gillmore chartered a steamer and converted it for suction dredging. Named the Henry Burden, the converted boat was the Corps' first hydraulic dredge, and one of the first in the country. Within 3 years, the government purchased another propeller-driven steamer, the Woodbury, and converted it into a suction dredge to deepen the Cape Fear River below Wilmington, North Carolina. More than half a dozen hydraulic hopper dredges were constructed for the Corps just before the turn of the century.
Dredging technology has evolved to meet the variety of conditions found in the waterway system. Early efforts to keep waterways navigable included manual labor and the use of draft animals for power. With the arrival of steam engines and other mechanical devices, the increasing need for wider and deeper channels soon spawned the development of dredges-ships that were equipped to keep navigation channels open, regardless of difficulty encountered when removing materials. The variety of materials and the need for a variety of placement methods resulted in the many types of dredges that are in use today. An assortment of dredging types exist, each designed for optimum performance under specific circumstances.
Several types of mechanical dredges are used. Dipper dredges and clam shell dredges are the two most common. Mechanical dredges are rugged and capable of removing hard-packed materials or debris. They can be worked in tight areas and are efficient when large barges are used for long-haul disposal. Mechanical dredges have difficulty retaining loose, fine materials in buckets, do not dredge continuously like pipeline dredges, and may need added controls when handling contaminated sediments. Mechanical dredges place the material into barges for transport to the placement location.
The two primary types of hydraulic dredges are the cutterhead pipeline dredge and the self-propelled hopper dredge. Advantages of cutterhead pipeline dredges include their ability to excavate most materials, to pump directly to a disposal site, to dredge almost continuously, and to dredge some types of rock without blasting. However, cutterhead pipeline dredges have limited capability in rough weather; have difficulty with coarse sand in swift currents; and, for the most part, are not self-propelled. In addition, the necessary pipeline can be an obstruction to navigation and, when handling debris in sediment, the removal efficiency is diminished. Self-propelled hopper dredges can operate in rough water and move quickly to a jobsite under their own power. The dredging operation does not interfere with other traffic. Work progresses quickly and is economical for long haul distances. Hopper dredges are limited to work in deep waters, but they cannot dredge continuously. Excavation is less precise than with other dredges, and this dredge type has difficulty dredging steep banks and consolidated materials.
Specialty dredges such as the dustpan dredge and sidecaster dredge are used to remove loosely compacted coarse-grained material at rapid shoaling sites or in areas where the sediment is needed adjacent to the navigation channel.
From 1906 until 1977, all hopper dredges in the United States were owned and operated by the Corps of Engineers. Almost all non-hopper dredging was and is performed by private industry.
In 1978, Congress directed the Corps to reduce its dredge fleet to the minimum level necessary to meet emergency requirements and ensure that taxpayers' interests were protected. Public Law 95-269, the Minimum Fleet Legislation enacted in April 1978, required the Secretary of the Army to periodically review the capability of the industry dredges and evaluate the configuration of the Corps minimum dredge fleet. Public Law 95-269 called for transferring dredging work to private industry as industry demonstrates its capability to perform the work at reasonable prices in a timely manner.
The Corps of Engineers Minimum Fleet vessels are national assets available at all times to respond to emergency and national defense needs, both CONUS and OCONUS, as determined and directed by the Chief of Engineers and/or the Director of Civil Works. MSC commanders are responsible for ensuring that Minimum Fleet vessels are maintained in a fully operational and ready state, sufficient to respond to emergency and national defense needs both in CONUS and OCONUS, at all times.
The the US Army Corps of Engineers Minimum Dredge Fleet study, focused on hopper dredges, originally began in 1992, but completion was delayed until additional dredging data could be collected. In 1994, the Assistant Secretary of the Army for civil works (ASA(cw)) decided to collect an additional two years of dredging data before reconsidering the minimum hopper dredge fleet configuration.
In a step to develop a final configuration for the US Army Corps of Engineers Minimum Dredge Fleet, in October 1997 the Corps released information describing the suite of options being considered. The paper contained the details of 8 options on the use of the 4 hopper dredges which make up the fleet. The Corps requested comments from representatives of the ports and navigation interests, maritime unions and the dredging industry. The options ranged at one end of the spectrum with maximum use of the 4 Corps hopper dredges, to the other end, with all Corps hopper dredges being placed in a standby/support status and all hopper dredging work offered to industry for bid.
A study released on 12 June 2000 by the Army Corps of Engineers glossed over the results of placing one of its Federal dredges in ready reserve status and recommended additional privatization of Corps dredging. An earlier report, drafted in September of 1999, criticized the privatization experiment as unsuccessful and recommended a reverse policy. The Corps report was mandated by Congress in 1996, as part of legislation placing one Corps hopper dredge, the WHEELER, home-ported in New Orleans, in "ready reserve" status. The report analyses the impact of the semi-retirement of the WHEELER. While the final report released by Corps Headquarters highlights the success of the 1996 policy change and recommends retiring another federal dredge, the draft report recommends that the WHEELER be placed in active duty status and that no other hopper dredge be put in ready reserve. The final Corps report, highlighted the success of placing the WHEELER in ready reserve status and recommended that the McFARLAND, operated out of Philadelphia, also be placed in ready reserve.
West Coast ports rely to a much greater degree on hopper dredging in general and Corps vessels in particular than do their eastern counterparts. Two of the Army dredges, the ESSAYONS and the YAQUINA, operate in the Pacific Northwest. Only one commercial hopper firm is located on the West Coast. It can take from five to twelve weeks to mobilize a dredge from the East Coast to the West. Congress recognized these unique circumstances in the conference report accompanying the 2002 Appropriation Act. Citing limited availability of commercial dredges and the long travel time required to move dredges from the East Coast, the report states that limitations on the Philadelphia-based McFarland were ".not to be considered a precedent for any other Corps of Engineers dredge, especially any dredge operating in the ports and harbors of the Northwest."
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