of the littoral reservoir and reduce the possibility of land- locking the 

 plant. Mobility of a land-based dredging plant may overcome some defi- 

 ciencies of a fixed plant, but this has not been demonstrated in field 

 installations. It seems unlikely that such a mobile plant would be able 

 to bypass all material when the rate of arrival at the site is high. 

 Therefore, some material would be lost around the barrier. 



b. Discharge Line Consideratioas . The best alignment of the dis- 

 charge line from the fixed plant to the downdrift side of the littoral 

 barrier or inlet is controlled by local conditions. The discharge line 

 must traverse a channel maintained for vessel traffic, and a floating 

 discharge line is impracticable. If the line is positioned on the channel 

 bottom, allowance must be made for protection of the line against damage 

 by pitching ships and by maintenance dredging of the channel. Also, a sub- 

 merged line may need a special flushing system designed 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. The discharge point is not critical in an 

 area with unidirectional longshore transport. However, in areas with trans- 

 port reversals, some of the material at the point of discharge is trans- 

 ported back toward the littoral barrier or into the inlet during periods 

 of transport reversal. This should be kept to a minimum 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 downdrift 

 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 littoral 

 forces tending to move material in an updrift direction. Establishment 

 of this point requires the use of statistical wave data, wave refraction 

 and diffraction diagrams, and data on nearshore tidal currents. If such 

 currents are present, they may sometimes dominate the littoral processes 

 immediately downdrift of the littoral barrier. Alternative points of dis- 

 charge nearer the barrier may also be considered, 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. 



5.522 Floating Dredges . The operation of floating dredges may be classi- 

 fied in two general categories, hydraulic and mechanical. Hydraulic 

 dredges include the suction pipeline dredge with plain suction or with 

 cutter-head for digging in hard material, and the self-propelled hopper 

 dredge. Mechanical types include the dipper and bucket dredges. 



The pipeline dredges employ a discharge pipeline to transport dredged 

 material to the point or area of placement. Booster pumps may be used in 

 this line if required by distance to discharge point. The standard hopper 

 dredge, although its bins are filled hydraulically, usually discharges by 

 dumping the dredged material out of the bottom of the bins. This type of 



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