shown on Figure 5-6. Where littoral transport predominates in one direc- 

 tion, any of these types would cause accretion to the updrift shore and 

 erosion of the downdrift shore, unless provision is made for sand bypass- 

 ing. 



At a jettied inlet. Figure 5-6 (Type I), bypassing can normally be 

 performed best by a land-based dredging plant or land vehicles. A floating 

 plant can be used only where the impounding zone is subject to periods of 

 light wave action, or by breaking into the landward part of the impound- 

 ment and dredging behind the beach berm thus leaving a protective barrier 

 for the dredge. Such an operation was performed at Port Hueneme, Cali- 

 fornia, in 1953 (See Section 6.5 SAND BYPASSING.) In any of the types 

 of operations at such a jettied inlet, it is unlikely that bypassing of 

 all of the littoral drift can be attained, and some material will pass 

 around the updrift jetty into the channel, especially after the impound- 

 ing capacity of the jetty has been reached. 



To ensure more complete bypassing of the littoral drift, the combi- 

 nation of the jettied inlet and offshore breakwater. Figure 5-6 (Type II), 

 was developed. In this design, a floating plant works effectively, com- 

 pletely protected by the breakwater and nearly all of the sand moving in- 

 shore of the offshore breakwater is bypassed. Practically no shoaling of 

 the channel by sand would be expected. Although this type is considered 

 the most effective type of improvement for both navigation and sand by- 

 passing, it is also normally the most costly. 



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

 Figure 5-6 (Type III), has been used effectively. Bypassing is performed 

 by a floating plant, although heavy wave action could cause delays when 

 the outer portion of the impoundment is being removed. Nearly all of 

 the sand transported alongshore would be bypassed, either naturally or 

 mechanically, but some shoaling of the navigation channel is likely 

 between dredging operations. 



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

 impounding zone or deposition basin behind the breakwater. Figure 5-6 

 (Type IV) , was designed in an effort to provide for bypassing of the lit- 

 toral 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. Although weir jetties have been construc- 

 ted at three inlets and partly installed at another inlet, none of them 

 has been in operation long enough to provide complete assurances concern- 

 ing their performance. 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. 



5.52 METHODS 



Several techniques have been employed for mechanically bypassing 

 sand at inlets. Sometimes a combination of techniques has proved to be 

 most practicable and economical. The basic methods are: land-based 

 dredging plants, floating dredges, and land-based vehicles. 



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