may allow for a floating plant operating periodically from either the channel, ebb, or flood shoal 

 or the updrift fillet. 



The most significant factor in selecting a system for bypassing sand is its degree of mobility. 

 Mobility reflects the ability of the "plant" to reach different areas of the site as well as react to 

 changing conditions. The three classifications of bypass systems based on mobility are: fixed, 

 mobile, semifixed (or semi-mobile). Each system and its respective components are described 

 in the following paragraphs. 



Fixed systems 



A fixed system is a stationary dredging system that has been designed and built for a specific 

 location. Fixed systems require exacting predictions on littoral transport vectors, transport 

 pathways, and deposition patterns because they are characterized by having the entire plant fixed 

 to one location. Examples include dredge pumps operating on/near the beach or jetty or fixed 

 plants using jet pumps like the innovative facility at the Nerang River entrance in Australia. 

 Fixed system components typically include a suction boom, pumps, discharge line, pump motor, 

 and housing structure. Suction booms contain the open end of the pipe through which sand 

 enters the bypass plant. They are generally constructed out of steel pipe and flexible hose held 

 by a hoist and may also contain fluidizer jets to fluidize sand for pumping. Pumps are usually 

 traditional dredge pumps driven by a motor (diesel or electric) and located in a housing nearby. 

 The discharge line is similar to that used in other dredging systems and can be made of rigid steel 

 or high-density polyethylene (HDPE). An example of one type of fixed plant with the pump 

 housing located on the updrift jetty is shown in Figure 17 (HQUSACE 1991). 



A jet pump could be used with fixed, semifixed, or mobile systems. Jet pumps (also known 

 as eductors) are hydraulically powered pumps with no moving parts that rely on the exchange of 

 momentum to do work. A simple jet pump consists of a reducing nozzle in a section of pipe 

 followed by a mixing chamber with a suction opening and a diffuser (Figure 18). A stream of 

 high-velocity clear water from a supply pump is forced through the nozzle. On exiting the 

 nozzle, the high- velocity jet entrains the surrounding fluid and forces the mixture through the 

 mixing chamber into the diffuser, where velocities slow and pressure energy is recovered. This 

 movement of the fluid into the diffuser creates a negative pressure, inducing flow into the suction 

 opening. When the suction opening is buried in the sand, a slurry of sand and water will be 

 drawn into the jet pump (HQUSACE 1991). 



Jet pump systems are ideally suited for areas where continuous bypassing is needed because 

 they operate at relatively low pumping rates. However, debris can cause particular problems for 

 jet pumps because of the small diameters. The occurrence of debris at a location will be a major 

 factor in the overall success of jet pumps. Similarly, the presence of cohesive material will 

 severely limit the operation of a jet pump system because of the inability to fluidize the material. 

 Finally, a clear water intake is required nearby to provide water to drive the jet pump. This water 

 should be taken from a sheltered region with an absence of shoaling (HQUSACE 1991). 



Chapter 4 Design Criteria 39 



