The effects of channel edge slumping on the residual slope of 

 the channel sides will be a function of the sediment particle size and 

 roughness, local current velocities, and other factors. To avoid 

 excessive slumping and therefore the high expense of maintenance 

 dredging, channel sides should be initially dredged to a stable slope 

 (or final "angle of repose") during the initial operation, the exact 

 cut depending on specific geohydrological conditions. Excessively 

 steep channel edges may lead to unnecessary loss of adjacent vital 

 habitat areas, such as shellfish or grass beds. 



Dredge Type : Dredges are classified as mechanical types or hydraulic 

 types (Table 21 , Figure 8). A third type, the "suction/mechanical" 

 dredge, is a variation on the hydraulic dredge which has a cutterhead 

 or rotating bit attached to the mouth of the suction line to loosen 

 compacted sediments and rock. 



A major functional difference between mechanical and hydraulic 

 dredging is the amount of water contained in the dredged material. 

 Mechanical dredges collect bottom material at or near the saturation 

 level of the submerged soil, while hydraulic dredges add water to 

 facilitate transportation of the dredged material. Water is added to 

 create a slurry which can be pumped to the disposal area. The size of 

 particles of materials being dredged and the distance being pumped 

 govern the amount of water used to create the slurry. Disposal of the 

 slurry mixture causes particles to settle according to grain size, 

 whereas mechanical dredging has no arrangement of particle types. The 

 use of hydraulic dredges can greatly speed up the dredging process as 

 larger quantities of material can be moved in a shorter period of time 

 compared to mechanical dredges. Suction dredging, suitable only for 

 removal of loose material, produces the least turbidity of all dredging 

 activities, if properly operated [31]. However, the suction dredge has 

 a high potential for ecologic disturbance at the terminus of the 

 effluent pipeline, where the spoil (a slurry of usually about 20 percent 

 sediment and 80 percent water) is discharged. The effects may be severe 

 if the spoil contains a high proportion of contaminants: organic 

 matter, nutrients, toxics, heavy metals, and fine sediments. Impacts 

 are reduced if spoil areas are diked to confine and settle the slurry 

 material . 



The mechanical dredge is less desirable from an environmental 

 point of view because material is washed freely from the bucket as it 

 is raised from the bottom. This increases turbidity, which can in turn 

 reduce dissolved oxygen, release toxic chemicals, reduce light 

 penetration, clog the gill structures of organisms, and destroy 

 microorganisms usually in the immediate project vicinity. The material 

 moved must be disposed of within the length of the dredge boom 



86 



