downdrift beach completes the bypassing operation. The location of the sand 

 trap in sheltered waters allows a dredge to operate during any season in all 

 but the most severe wave conditions. 



To ensure more complete bypassing of the littoral drift, the combination 

 of the jettied inlet and an offshore breakwater (Fig. 5-8, type III) was 

 developed. In this design, a floating plant works effectively, completely 

 protected by the breakwater and most of the sand moving inshore of the off- 

 shore breakwater is bypassed. Practically no shoaling of the channel would 

 be expected. Although this type is considered the most effective type of 

 improvement for both navigation and sand bypassing, it is also normally the 

 most costly. 



The shore-connected breakwater with impoundment at its seaward end (Fig. 

 5-8, type IV) has been used effectively. Bypassing is performed by a floating 

 plant, but heavy wave action could cause delays during the removal of the 

 outer part of the impoundment. Most of the sand transported alongshore would 

 be bypassed, either naturally or mechanically, but some shoaling of the navi- 

 gation channel is likely between dredging operations. 



The shore-connected breakwater or jetty with a low sill or weir and an 

 impounding zone or deposition basin behind the breakwater (Fig. 5-8, type V) 

 was designed to provide bypassing of the littoral drift moving inshore of the 

 seaward end of the weir by a floating plant, thus not permitting any of that 

 part of the littoral drift to shoal the navigation channel. A successful 

 bypassing operation at Hillsboro Inlet, Florida (Hodges, 1955), where a basin 

 behind a natural rock ledge is dredged periodically, formed the basis of this 

 design. 



Over the past 15 years the weir jetty bypassing concept has been shown to 

 be an effective means of bypassing a part of the littoral drift. Although the 

 performance of the first weir jetty systems, like any new concept, was not 

 always as expected, recent advancements in their design criteria and in the 

 understanding of their functional behavior have transformed the weir jetty 

 concept into one of the most feasible methods of bypassing littoral drift. 

 The methodology for weir jetty design is discussed by Weggel (1981). 



2. Methods . 



Several techniques have been employed for mechanically bypassing sand at 

 inlets, with a combination of techniques proven to be the most practicable 

 and economical. The basic methods of operation include (a) land-based dredg- 

 ing plants, (b) floating dredges, and (c) land-based vehicles. 



a . Land -Base d Dredging Plants . 



(1) Plant Considerat ions. During this operation, a dredging plant is 

 fixed in position near the beach from which the sand transported alongshore is 

 to be intercepted as it moves within reach of the plant. Currently, these 

 plants are of the pump type and operate basically as an ordinary suction 

 dredge. Most plants are positioned on an existing structure; however, some 

 are on an independent foundation. A few movable plants located on piers or 

 jetties with the capability of dredging along the length and on both sides 

 have been built in the United States and abroad. Such plants have a much 



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