Scraping 



Excavating 



Tunneling 

 beneath 

 seafloor 



Fluidizing 



(sub-seafloor) 



Drag line dredge 

 Trailing cutter 



suction dredge 

 Rock cutter section 



dredge 

 Crust-miner 

 Continuous line 



bucket 

 Clams shell bucket 

 Bucket ladder 



dredge 

 Bucket wheel 



dredge 

 Anchored suction 



dredge 

 Cutterhead suction 



dredge 

 Drilling and blasting 

 Shore entry 

 Artifcial island entry 



Slurrying 

 Leaching 



' Applicable or potentially applicable. 

 " Relative major perturbation. 



SOURCE: Environmental Etfecis Document prepared by U.S. Department of the Interior Regulatory Task Force for Leasing of Minerals Other than Oil, Gas, and Sulphur 

 in the Outer Continental Shelf, unpublished draft, October 1986. 



out overflow. The bucket dredge and the hopper 

 dredge with overflow, however, produce suspended 

 sediments throughout the water column. The mod- 

 ified dustpan dredge appears to suspend more solids 

 than a conventional cutterhead dredge. ^^ 



A typical bucket dredge operation produces a 

 plume of particulates extending about 1,000 feet 

 downcurrent at the surface and about 1,600 feet 

 near the bottom.^* In the immediate vicinity of the 

 operation, the maximum concentration of sediment 

 suspended at the surface should be less than 500 

 mg/1 and should rapidly decrease with distance. 

 Water column concentrations generally should be 

 less than 100 mg/1.^^ When mining stops, the tur- 

 bidity plume will settle rapidly. 



The dispersion of a turbidity plume can be ef- 

 fectively altered by the configuration of the pipe- 



"For more information on dredge designs, see Ch. 4. 



^*W.D. Barnard, Prediction and Control of Dredged Material Dis- 

 persion Around Dredging and Open Water Pipeline Disposal Oper- 

 ations, U.S. Army Engineer Waterways Experiment Station, Vicks- 

 burg, MS, Technical Report DS-78-13, August 1978. 



^^Sediment suspended by a dredge is similar to the amount of dis- 

 turbance produced by a small-scale storm. 



line at the point of discharge (see figure 6-5).'^ Pipe- 

 line angles that minimize water column turbidity 

 (e.g., with a 90-degree angle) produce mud mounds 

 that are thick but cover a minimum area. Con- 

 versely, those that generate the greatest turbidity 

 in the water column disperse widely and produce 

 relatively thin mud mounds of maximum areal ex- 

 tent." 



Many parameters, such as particle setding rates, 

 discharge rate, water depth, current velocities, and 

 the diffusion velocity, all interact to control the size 

 and shape of the turbidity plume. As water cur- 

 rent speed increases, the plume will grow longer. 

 As the dredge size increases or particle setding rates 

 decrease, the plume size will tend to increase^^. Fi- 

 nally, with lower rates of dispersion or particle set- 



"J.R. Schubel et al.. Field Investigations of the Nature, Degree, 

 and Extent of Turbidity Generated by Open Water Pipeline Disposal 

 Operations , U.S. Army Engineer Waterways E.xperiment Station, 

 Technical Report, Vicksburg, MS, D-78-30, July 1978. 



"A general rule of thumb is that, as the height of the redeposited 

 mound decreases by a factor of two, the areal coverage increases by 

 a factorof two. But as the mound height decreases, the amount of 

 wave-induced resuspension of the surface material will also decrease. 



^°In addition, as the diffusion velocity increases for a given current 

 velocity, the plume becomes longer and wider, while the solids con- 

 centrations in the plume decrease. 



