Dustpan Dredges
The dustpan dredge is a hydraulic suction dredge that uses a widely flared dredging head along which are mounted pressure water jets. The jets loosen and agitate the sediments which are then captured in the dustpan head as the dredge itself is winched forward into the excavation. This type of dredge was developed by the Corps of Engineers to maintain navigation channels in uncontrolled rivers with bedloads consisting primarily of sand and gravel. The first dustpan dredge was developed to maintain navigation on the Mississippi River during low river stages. A dredge was needed that could operate in shallow water and be large enough to excavate the navigation channel in a reasonably short time. The dustpan dredge operates with a low-head, high-capacity centrifugal pump since the material has to be raised only a few feet above the water surface and pumped a short distance. The dredged material is normally discharged into open water adjacent to the navigation channel through a pipeline usually only 800 to 1000 ft long.
The dustpan dredge maintains navigation channels by making a series of parallel cuts through the shoal areas until the authorized widths and depths are achieved. Typical operation procedures for the dustpan dredge are as follows. The dredge moves to a point about 500 ft upstream of the upper limit of the dredging area and the hauling anchors are set. Two anchors are used. The hauling winch cables attached to the anchors are crossed to provide better maneuverability and control of the vessel while operating in the channel prism. The dredge is then moved downstream to the desired location. The suction head is lowered to the required depth, dredge pump and water jet pumps are turned on, and the dredging commences. The dredge is moved forward by the hauling cables. The rate of movement depends on the materials being dredged, depth of dredging, currents, and wind. In shallow cuts, the advance may be as rapid as 800 ft/hr. When the upstream end of the cut is reached, the suction head is raised and the dredge is moved back downstream to make a parallel cut. This operation is repeated until the desired dredging widths and depths are achieved. The suction head may have to be lowered or raised if obstacles such as boulders, logs, or tree stumps are encountered. Experience with dustpan dredges indicates that the best results are obtained when the height of the cut face does not exceed 6 ft in depth. The dredge is moved outside the channel to let waterborne traffic pass through the area simply by raising the suction head and slacking off on one of the hauling winch cables. The propelling engines can be used to assist in maneuvering the dredge clear of the channel. The vessel is held in position by lowering the suction head or by lowering a spud.
The pipeline system and the rigid ladder used with the dustpan dredge make it effective only in rivers or sheltered waters; it cannot be used in estuaries or bays where significant wave action occurs. Because it has no cutterhead to loosen hard, compact materials, the dustpan dredge is mostly suited for high-volume, loose-material dredging. Dustpan dredges are used to maintain the navigation channel of the uncontrolled open reaches of the Mississippi, Missouri, and Ohio Rivers. Dustpan dredging is principally a low-stage season operation. River channels are surveyed before the end of the high-stage season to determine the location and depths at the river crossings and sandbar formations, and dustpan dredging operations are planned accordingly.
These dredges are high-volume dredges capable of excavating a navigation channel through river sediment in a short time. During FY 71, the dredge Jadwin excavated over 6,200,000 cu yd, with an average production rate of approximately 3600 cu yd/hr.
The dustpan dredge is self-propelled, which enables it to move rapidly over long distances to work at locations where emergencies occur. The attendant plant and pipeline are designed for quick assembly so that work can be started a few hours after arrival at the work site. The dustpan dredge can move rapidly out of the channel to allow traffic to pass and can resume work immediately. The high production rate and design of the dustpan dredge make it possible to rapidly remove sandbar formations and deposits from river crossings so that navigation channels can be maintained with a minimum of interruption to waterborne traffic.
The dustpan dredge was designed for a specific purpose, and for this reason there are certain limitations to its use in other dredging environments. It can dredge only loose materials such as sands and gravels and only in rivers or sheltered waters where little wave action may be expected. The dustpan dredge is not particularly well suited for transporting dredged material long distances to upland disposal sites; pumping distances are limited to about 1000 ft without the use of booster pumps.
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