pump is supplied motive water by a 253-kW (340-HP) pump which takes clear water from the 

 inlet. The jet pump discharge supplies a 246-kW (330-HP) booster pump, which then pumps the 

 slurry to the downdrift beach (approximately 457 m with capability to pump as far as 1,070 m). 

 Both pumps and the diesel engines that drive them are located in a pump house well back from 

 the open beach. The Indian River Inlet Bypass Plant is used as a guide for the Shinnecock 

 analysis, because it has a similar design bypass rate (on the order of 76,500 m 3 (100,000 yd 3 / 

 year)) and because this type of semifixed plant provides greater flexibility in reaching a larger 

 area of the updrift fillet and can be better protected during storms by moving the crane and jet 

 pump assembly to the backbeach. 



Several factors that must be considered in analyzing this type of bypass plant include: 



a. Depth of jet pump operation. 



b. Proximity to local structures (east jetty at Shinnecock). 



c. Pipe discharge location (pipe lengths, booster pump requirements, etc.). 



d. Operating time. 



e. System capacity. 



/. System element costs. 



The depth below ambient bottom at which jet pumps operate is generally limited to 7.6 m but 

 may be more specifically limited by the depth where hard or cohesive sediment is found 

 (Richardson and McNair 1981). Because scour holes in the Shinnecock Inlet channel have 

 exceeded -12.2 m mlw, it should be safe to assume that the sand layer reaches at least this depth 

 at the updrift fillet. However, diver observations in the scour hole indicated exposure of some 

 clay sediments. 1 The maximum jet pump operating depth below natural bottom used for design 

 should be 7.6 m. For practical purposes and because the jet pump will be operating at or near the 

 mlw line, assume the 7.6-m depth is also a depth below mlw. 



To minimize the potential for jetty undermining from encroachment of the jet pump crater on 

 the structure, Richardson and McNair (1981) suggest a minimum distance between structure and 

 edge of crater equal to 1.25 x d, where d is the design depth (7.6 m for Shinnecock). The rule of 

 thumb for crater diameter given by Richardson and McNair (1981) is 3 x d. Therefore, at 

 Shinnecock, the crater diameter should not exceed 22.8 m, and the edge of the crater should be 

 no closer than 9.5 m to the base of the jetty. This distance equates to a practical jet pump 

 operational limit of about 21m from the jetty (Figure 23). 



Personal communication, Mr. Fred Anders, New York State Department of State, Albany, NY, 1996. 

 Chapter 5 Bypassing Alternatives 57 



