Ch. 6— Environmental Considerations • 235 



Figure 6-5.— The Effect of Discharge Angle and Water Current on the Shape and Depth of Redeposited Sediments 



Vertical discharge Horizonal disclnarge 



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If mining ships discharge unwanted sediments through a vertical pipe (left portion of diagram) seafloor deposits will cover 

 a smaller area but to a greater depth than if a horizontal discharge pipe is used (right side of diagram) which results in a large 

 but thin "footprint" of sediments. The movement of water current (bottom of diagram) will similarly expand the area of the 

 seafloor blanketed by sediment but decrease the depth of the deposit overall. 



SOURCE; Modified from W. Barnard, "Prediction and Control of Dredged fvlaterial Dispersion Around Dredging and Open Water Pipeline Disposal Operations," U.S. 

 Army Corps Engineer Waterways Experiment Station, Vicksburg, 1^8, Technical Report OS 78-13, August 1979. 



tling or an increase in water depth, the length of 

 time required for the plume to dissipate after the 

 disposal operation has ceased will increase. 



One method for physically controlling the dis- 

 persion of turbid water is a "silt curtain." (see fig- 

 ure 6-6). A silt curtain is a turbidity barrier that 



extends vertically from the water surface to a speci- 

 fied depth around the area of discharge. At present, 

 silt curtains have limited usefulness; they are not 

 recommended for "operations in the open ocean, 

 in currents exceeding one knot, in areas frequently 

 exposed to high winds and large breaking waves, 

 or around hopper or cutterhead dredges where fre- 



