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Dustpan Dredges

The Dustpan Dredge is like a large vacuum cleaner. This type dredge is used on the Mississippi River. The suction head, approximately the width of the dredge, is lowered to the face of the material to be removed. High velocity water jets loosen the material which is then drawn by pump as a slurry through the dredge pipe and floating pipeline where the material is deposited outside of the navigation channel.

The flagship of the Corps' river dredging fleet is the Hurley, a 300-foot-long dustpan dredge capable of removing over 5,000 cubic yards of silt from the river bottom each hour. A dustpan dredge operates like a huge vacuum cleaner. A suction head - over 50 feet wide - is lowered to the soft river bottom. High velocity water jets loosen the material which is then drawn up by powerful pumps, shot through a floating pipeline and deposited up to 1,000 feet away. Several commercially operated cutterhead dredges are also available for service with the Corps. Cutterhead dredges are used mostly in river harbors to remove harder materials from the bottom. The Mississippi River channel is maintained from river mile 599 to 954 to a depth of 9 feet and 300 feet in width. The Dredge Hurley is tasked with this mission and operates 7 days per week and 24 hours per day from June through November.

Dustpan dredges like the Dredge Jadwin are unique to the Mississippi River. They "vacuum" sand and silt accumulated on the river floor. The dredge operation is basically simple. The suction head, about as wide as the hull of of dredge, is lowered to the face of the material to be removed. High velocity water jets in the suction head agitate and loosen the material. The slurry is drawn up by a pump that passes the material along to a floating pipeline and deposited outside the navigation channel.

The Corps' oldest dredge is now also one of its youngest. The dustpan dredge Potter, the U.S. Army Corps of Engineers' last and longest-serving steam-powered dredge, helped keep the Mississippi River open to navigation for almost 70 years. Now she is back home at the St. Louis District Service Base, following a $20 million repowering project by the district and the Corps' Marine Design Center in Philadelphia, contracting with Halter Marine, Inc., of Gulfport, Miss. She departed Halter's New Orleans shipyard Sept. 29 with the same overall profile (sans smokestacks) and capabilities, but with an all-new stern and a diesel-electric power plant.

Named in honor of Brig. Gen. Charles Lewis Potter-Memphis District Engineer from 1900 to 1903 and then President of the Mississippi River Commission from 1920 to 1928-the Potter, originally a steam-powered paddlewheeler, was built for the once-considerable sum of $520,000 and launched in 1932. Originally assigned to the Memphis District and transferred to St. Louis in 1979, she is now the oldest member of the Corps' entire dredging fleet (a year ahead of her sister dredge, the Vicksburg-based Jadwin, which was launched in 1933 and then repowered in 1985).

Dustpan dredging has long been the mode of choice along most of "Old Man River," where sand and fine-grained soils from the Missouri River predominate along the channel bottom. Of the Corps' Mississippi fleet, only the Thompson (St. Paul District) is a cutterhead dredge, better suited to the river's upper reaches. The Potter is authorized to dredge year-round, with its schedule driven by shoaling and also constrained by river stages and environmental seasonal windows. The Corps' river dredging fleet handles approximately half the annual federal channel maintenance along the Mississippi, with contract dredgers accounting for the balance of the work. The crew is organized into three shifts, with two alternating and one off at any given time. The Potter's master, pilot and first mate each lead a shift, so that there is always one officer on board with a master's license. Also split among the three shifts are the second mates and the first and second engineers.

The new equipment is more efficient and state-of-the-art, but the concept of operation remains the same-raised and lowered by a hoisting winch and pulled forward by a pair of hauling winches, the Potter's dustpan cuts a 32-foot-wide swath along the bottom of the river. The dredge pump (a new high-density model that nearly doubles the old capacity) sucks bottom sediment in through the intake pipe, facilitated by the jetting pump, which shoots water through 20 nozzles lining the bottom of the dustpan to stir up the sediment.

Most of the overall dimensions are also intact: length 240 feet, 6 inches, beam 46 feet, hull depth 9 feet 9 inches, draft 7 feet 6 inches, displacement 1,600 light tons.

Everything on the inside - at least everything that makes this dredge run - is new. That is certainly true of the stern, which was completely replaced below the upper decks. In this case the Marine Design Center set forth the functional requirements and then let Halter determine the most cost-effective way to satisfy them. The most visible changes inside involve the boiler room, or rather the space it once occupied. In its place are three Caterpillar 3516B 1,825-kilowatt diesel generators (two for operation and one for standby) that feed a 600-volt main bus. As a result, total horsepower is now up to 2,400 for both propulsion and dredging, compared to 1,800 and 1,400, respectively, before the repowering. The Reliance motor generator set supplies house power, lighting, heating and cooling, compressed air and other utilities through a 480-volt bus.

The biggest labor savings-and 100 percent of the reduction in crew size (from 59 to 50)-were tied to eliminating the boiler room with its plethora of switches, valves and other fully manual controls to operate and maintain. Fortunately, retirements and transfers accounted for all nine positions and no one had to be let go.

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