ill-advised. A designer must keep the ideal weir system as a goal but may 

 need to compensate for the consequences of uncertainties in understanding the 

 processes and data. In some instances designers may need to compromise on 

 bypassing capabilities and design for quantities greater than the net 

 transport. 



III. ELEMENTS OF A WEIR-JETTY SYSTEM 



The design of a weir-jetty system requires that at least six elements of 

 the system be considered: (a) The navigation channel, (b) the jetty struc- 

 tures, (c) the weir structures, (d) the deposition basin, (e) the updrift 

 beach, and (f) the downdrift beach. The design of each of these elements is 

 governed by the hydraulic characteristics of the inlet tides and tidal range, 

 wave and longshore transport climate at the site, the size and type of vessels 

 using the inlet, and the overall inlet geometry. Design factors for each 

 element of the weir-jetty system are discussed below. 



1. Navigation Channel. 



Since the primary purpose of a jetty system is to maintain a fixed navi- 

 gation channel, improvement of navigation conditions at an inlet must be the 

 prime consideration in any weir-jetty design. The depth, width, and alinement 

 of the channel are parameters that need to be established and are usually a 

 compromise between what is needed to serve navigation and what the physical 

 conditions at the site will allow. Channel depth and width are determined by 

 the size, type, and number of vessels which are using or will eventually use 

 the inlet. Constraints are imposed on the depth and width by inlet hydrau- 

 lics. For example, the tidal prism may not be large enough to keep the pro- 

 posed navigation channel open with realistic maintenance dredging efforts and 

 a smaller channel cross section must then be considered. Inlet hydraulics and 

 cross-section stability are discussed by Sorensen (1977). Channel alinement 

 is dictated by navigation requirements as well as local inlet geometry and 

 sedimentation processes. Existing shoals usually establish the most economic 

 jetty alinement and thereby influence the location of the channel. Navigation 

 needs, such as required turning radii and maneuvering areas for safe naviga- 

 tion, also influence channel alinement. The approach direction of prevailing 

 waves is another factor in establishing channel and jetty alinement. If small 

 craft are to be protected from wave action in the channel, the jetties and 

 entrance channel should be alined to afford maximum wave protection. Wave 

 steepening at the seaward end of the jetties caused by opposing ebb tidal 

 currents may also be a critical factor in achieving safe navigation condi- 

 tions. The steepened waves may break over the ocean bar making navigation 

 dangerous. 



2. Jetty Structures . 



Design factors which must be determined for jetty structures are aline- 

 ment, spacing, and structural considerations such as construction type, crest 

 elevation, and structural design. 



Jetty alinement is mostly governed by the geometry of the navigation 

 channel, inlet, and shoals. Designs are usually selected which minimize the 

 overall cost of the structures by making maximum use of shallow water over 

 existing shoals. The cost of rubble structures rises rapidly as the water 

 depth in which they are built increases. Historical data of inlet migra- 

 tion and shoaling patterns should be reviewed to provide information on 

 future tendencies of the navigation channel to migrate. The jetties should be 



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