d. Con tr ol S tr ucture Fa ctors : 



(1) Determination of jetty length and spacing by considering the 

 navigation, hydraulic, and sedimentation factors. 



(2) Determination of the design wave for structural stability and 

 wave runup and overtopping considering structural damage and maintenance. 



(3) Effects of crest elevation and structure permeability on waves in 

 channel . 



e . Sedimentation Factors: 



(1) Effects of both net and gross longshore transport on method of 

 sand bypassing, size of impoundment area, and channel maintenance. 



(2) Legal aspects of impoundment area and sand bypassing process (see 

 Sec. V,3 of this chapter). 



f. Maintenance Fa ctor: Bypassing and/or channel dredging will usually be 

 required, especially if the cross-sectional area required between the 

 jetties is too large to be maintained by the currents associated with the 

 tidal prism. 



4. Effects on the Shoreline . 



The effects of entrance jetties on the shoreline are illustrated in Figure 

 5-24. A jetty (other than the weir type) interposes a total littoral barrier 

 in that part of the littoral zone between the seaward end of the structure and 

 the limit of wave uprush on the beach. Jetties are sometimes extended sea- 

 ward to the contour position equivalent to the project depth of the channel. 

 Accretion takes place updrift from the structures at a rate proportional to 

 the longshore transport rate, and erosion takes place downdrift at about the 

 same rate. The quantity of the accumulation depends on the length of the 

 structure and the angle at which the resultant of the natural forces strikes 

 the shore. If the angle of the shoreline of the impounded area is acute with 

 the structure, the impounding capacity is less than it would be if the angle 

 were obtuse. Structures that are perpendicular to the shore have a greater 

 impounding capacity for a given length and thus are usually more economical 

 than those at an angle, because perpendicular jetties can be shorter and still 

 reach the same depth. If the angle is acute, channel maintenance will be 

 required sooner due to littoral drift passing around the end of the structure. 

 Planning for jetties at an entrance should include some method of bypassing 

 the littoral drift to eliminate or reduce channel shoaling and erosion of the 

 downdrift shore (see Sec. V of this chapter). 



VIII. BREAKWATERS, SHORE-CONNECTED 



1 . Definition . 



A shore- connected breakwater is a structure that protects a shore area, 

 harbor, anchorage, or basin from waves. Breakwaters for navigation purposes 

 are constructed to create calm water in a harbor area, which provides 

 protection for safe mooring, operating and handling of ships, and harbor 

 facilities. 



5-58 



