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Marina Highlights 2007 0320
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Marina Highlights 2007 0320
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6/6/2007 4:08:53 PM
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3/28/2007 4:02:50 PM
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CM City Clerk-City Council
CM City Clerk-City Council - Document Type
Committee Highlights
Document Date (6)
3/20/2007
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_CC Agenda 2007 0402
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\City Clerk\City Council\Agenda Packets\2007\Packet 2007 0402
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<br />,j <br /> <br />/ Gerd Marggraff (510-828-1992) <br />f <br /> <br />System 2: The second system in use is the deflector shield dredging vessel. It uses <br />a specially designed shield that is lowered in the correct arc and distance behind <br />the ships propeller to deflect water flow toward the sediment. It is extremely <br />important that the arc and shield is designed to fit the diameter of the ship's <br />propeller. Propulsion systems used for this dredge are preferably powered by a <br />big diameterlarge pitch propeller fitted with a high reduction gear box. This <br />means the propeller turns slowly but moves the water in an extremely powerful <br />manner, not unlike a tug boat's propeller. The deflecting shield system is used by <br />a company in northern Germany to keep cooling water channels at several power <br />stations at an exact depth of 10-12 ft. This system is especially good for general <br />dredging and pin pointing problem areas. The captain can work his vessel so <br />accurately that he can back up within a few feet of cooling water intake grates to <br />remove sediment and other debris. If the shield is lowered to its maximum <br />depth, the water deflection will actually move the boat in reverse, as common on <br />jet boats and planes as reverse thrusters. We all know the rumble when planes <br />slow down on runways-this is caused by a deflector shield. <br /> <br />System 3: The third system is known as a hydro-injecting dredging vessel. It is the <br />latest development in dredging technology. Nozzles are mounted on a pivoting U- <br />frame on the stern of a tug. A simple craning device lowers the rear end of the <br />frame down to a certain height above the sediment. A la'rge volume, low pressure <br />pump (like those used in cutter-head dredges) injects water out the nozzles <br />approximately 1 }'z-2 ft into sediment and agitating it to saturate the water <br />around it. The current again carries the sediment away. Several of these vessels <br />are used on the Elbe and Weser rivers, creating channels up to 60 ft in depth. <br />Recently, the channel at the ~Ibe had to be deepened by 10 ft to make the <br />channel accessible to even larger Evergreen container vessels. This was <br />completed with no problem in a short amount of time. Admittedly, this dredging <br />depth was done with 150 ft vessel. <br /> <br />All these dredging systems work on the fact that the sediment is carried away by <br />tides, back to the adjacent mud fields where it came from. Around our San <br />Leandro channel, t understand we have a cross current of up to 3 knots. Simple <br /> <br />-z.. <br />
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