One example of a fixed system using jet pumps is the Nerang River Entrance Bypass Plant in 

 Australia. This plant uses 10 jet pumps located on a trestle that extends out into the surf. Each 

 jet pump can be operated individually or in combination with other units. A system schematic is 

 shown in Figure 19. 



Submersible pumps may be considered as an alternative to jet pumps in certain situations. 

 These pumps, electrically or hydraulically driven, are lowered directly into the material to be 

 moved. They are relatively small and can be deployed with a minimum of support equipment, 

 and they are less immune to plugging than jet pumps. However, they contain more moving parts 

 and thus require more servicing than jet pumps (HQUSACE 1991). 



Mobile systems 



Mobile systems are designed such that the plant can be moved to different areas within the 

 project site or to other projects. Examples include floating plant (i.e., dredges), movable jet 

 pumps, or land-based dredges (e.g., Crawlcai). The majority of mobile bypass systems use 

 conventional floating dredges (hopper dredges, cutterhead dredges, etc.), which can either be 

 purchased or contracted from dredging companies. Contracting is the usual procedure because of 

 the flexibility of specifying the dredging requirements (volume, location of material, distance to 

 pump, etc.) so that dredging companies can bid based on the available equipment. If a dredge is 

 purchased, the flexibility of dredging a larger volume or pumping a greater distance may be lost 

 because of the permanency of plant ownership (HQUSACE 1991). 



Mobilization and demobilization (mob/demob) is a factor to be addressed when considering 

 using a dredge for bypassing. Mob/demob involves moving the equipment to the project site, 

 transport, and placement of pipe and other equipment (crew boats, tugs, and/or barges). The 

 costs of mob/demob can be a significant portion of overall project costs, with some estimates 

 varying between 10 and 40 percent of total costs (HQUSACE 1991). 



Bypass frequency should also be considered when evaluating a mobile system. Quantities 

 large enough to require yearly bypassing may make the purchase of a dredge more feasible, 

 though the mob/demob costs would be repeated every year. By dredging larger volumes every 

 2 or 3 years, mob/demob costs can be amortized, and depending on the operational costs, annual 

 dredging costs may be reduced. 



Mobile systems sometimes use deposition basins to accumulate littoral material for 

 bypassing at a later time. Some deposition basins are linked to a weir jetty, which allows littoral 

 transport to move along the beach and into the basin located inside the inlet. This material can 

 then be periodically dredged and the material placed downdrift. Figure 20 shows a typical weir 

 jetty bypass system. Deposition basins may also be located in the inlet throat so material from 

 the gross transport (i.e., from both directions) can be collected. As the basin fills, periodic 

 dredging can remove the material in a manner similar to traditional channel maintenance 

 (HQUSACE 1991). 



40 Chapter 4 Design Criteria 



