(4} Determination of channel dimensions based on design vessel 

 data and number of traffic lanes; and 



(5) Other navigation factors such as: 



(a) Relocation of navigation channel to 

 alternative site; 



Cb) Provision for future expansion of 

 channel dimensions; and 



(c) Effects of harbor facilities and layout on 

 channel alignment. 



d. Control Structure Factors . 



(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 main- 

 tenance; and 



(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; and 



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

 (See Section 5.53.) 



f. Maintenance Factor . Dredging will be required, especially if the 

 cross-section area required between the jetties is too large to be main- 

 tained by the currents associated with the tidal prism. 



5.74 EFFECTS ON THE SHORELINE 



Effects of entrance jetties on the shoreline are illustrated by 

 Figure 5-15. 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 seaward to the position of the contour equivalent 

 to project depth of the channel. Accretion takes place updrift from the 

 structures at a rate proportional to the longshore transport rate, and 

 erosion downdrift at about the same rate. The quantity of the accumu- 

 lation depends on the length of the structure and the angle at which the 

 resultant of the natural forces strikes the shore. If the angle that the 

 shoreline of the impounded area makes with the structure is acute, the 



5-48 



