special flushing system to keep the line from clogging when the pumps are shut 

 down. 



The point of discharge on the downdrift side of the littoral barrier may 

 be of critical importance. Although the point is not critical in an area with 

 unidirectional longshore transport, in areas with transport reversal periods, 

 sons of the material at the point of discharge is transported back toward the 

 littoral barrier or into the inlet. This reverse transport should be kept to 

 a minimvE to reduce channel maintenance and, where transport reversals occur, 

 a detailed study must be made of the distribution of littoral forces downdrift 

 of the barrier. Tidal currents toward the inlet may frequently predominate 

 over other forces and produce a strong movement of material toward the down- 

 drift jetty or into the inlet, particularly if no downdrift jetty is included 

 in the plan. In this case, the best discharge point will be a point on the 

 shore just beyond the influence of the downdrift jetty and the littoral forces 

 that tend to move material in an updrift direction. The establishment of the 

 point requires the use of statistical wave data, wave refraction and diffrac- 

 tion diagrams, and data on nearshore tidal currents. Such currents may some- 

 times dominate the littoral processes immediately downdrift of the littoral 

 barrier. Alternative points of discharge nearer the barrier may also be con- 

 sidered, using groins to impede updrift movement of material at the discharge 

 point. Such alternative considerations are of value in determining the most 

 economical discharge point. 



b. Floating Dredges . The operation of floating dredges may be classified 

 in two general categories: hydraulic and mechanical. Hydraulic dredges 

 include the suction pipeline dredge, with a plain suction or with a cutterhead 

 for digging in hard material, and the self-propelled hopper dredge. Mechan- 

 ical types include the dipper and bucket dredges. 



The pipeline dredges employ a discharge pipeline to transport dredged 

 material to the point of discharge or area of placement; booster pumps may be 

 used in this line if required. The standard hopper dredge, whose bins are 

 filled hydraulically, usually discharges by dumping the dredged material out 

 of the bottom of the bins. This type of dredge requires disposal areas with 

 enough depth to allow dumping. The hopper dredge is not suitable for bypass- 

 ing operations unless it discharges in an area where the material may be 

 rehandled by another type of dredge or it is equipped to pump the material 

 ashore. Since about 1960, a number of hopper dredges have been equipped to 

 pump the material from their bins, greatly increasing their importance in 

 bypassing operations. 



Mechanical dredges require auxiliary equipment (such as dump scows, con- 

 veyors, and eductors) to transport material to the area of placement. Equip- 

 ment and techniques for transporting sand are continually being improved; 

 therefore, incorporating a mechanical-type dredge to bypass material may 

 be most favorable in some cases. In considering a floating dredge for a 

 bypassing operation, each type of dredge plant must be evaluated. The 

 evaluation should include: first, the feasibility of using various types of 

 floating dredges; second, the details of the operation; and finally, the 

 economics to determine which floating plant will transfer the material at the 

 least unit cost. Since local site conditions vary, factors to be considered 

 for each type of floating plant cannot be standardized. Some of the more 

 important factors to evaluate are discussed below. 



5-32 



