than its actual dimensions. The probable number of interferences of the tacking sailboats 

 with the axially moving craft are calculated mathematically for various speeds and numbers 

 of craft per hour in channels of various widths. Although a small percentage of such 

 interferences would result in actual collisions, each would require some kind of safety 

 maneuver; the more frequently they occurred the greater would be the likelihood of an 

 accident. The harbor planner must use a considerable amount of judgment in determining 

 the entrance channel width, orientation, and configuration. 



The primary function of a jetty is to protect the entrance channel from waves in the 

 main water body and from littoral drift entering the channel from the flanking beaches. 

 Because the entrance must usually be alined nearly normal to shore, a jetty is usually 

 required on each flank. Occasionally, a natural protection (e.g., a rocky headland) will 

 obviate the need for a jetty on one side or the other. Although the jetties may be alined 

 nearly parallel to the prevailing direction of the waves, occasional waves from other 

 directions must be anticipated. For this reason, jetties must also be designed to serve as 

 breakwaters. The direction limits can be determined by a review of historical records or by 

 wave hindcast and refraction analysis. In small lakes, jetties can often be designed as 

 freestanding sheet-pile structures, but wherever large waves are anticipated, they must be 

 quite massive. Rubble-mound or broad-based concrete construction is usually required to 

 protect entrances from the open ocean or large lakes. 



Since wave energy decreases as the water becomes shallower, considerable savings in 

 construction cost can be affected by making the jetty section progressively smaller from the 

 outer end toward shore (Fig. 26). The outer end must be designed to withstand the full 

 force of a wave from any direction. Rubble mounds may be decreased progressively toward 

 shore, both in height and size of armor stone in accordance with design criteria for 

 breakwaters. Sheet-pile structures require progressively less penetration as they approach 

 shore. Refraction and diffraction analysis (or model study) will be required to determine 

 design wave heights both on the channel side and the exposed flank of each jetty. 



Jetties are usually constructed from the shore end outward for equipment-access 

 purposes, and before channel dredging for dredge-protection and drift -exclusion reasons. If 

 the work is done during wave action, the turbulence at the working end tends to scour a 

 hole in unconsolidated bottom material whether the jetty is of wide-base or sheet-pile 

 construction. Therefore, in a rubble structure, a quarry -waste bedding layer should be 

 placed on the bottom before construction to prevent scouring. If the bottom is soft, this 

 bedding layer should be placed on filter cloth. The outer end of a sheet-pile jetty wiU 

 usually require stabilization against scouring for the same reasons. If construction proceeds 

 from the outer end toward shore, a similar scouring effect will take place at the unfinished 

 inner end, particularly as it nears the shoreline. It is important that scouring be anticipated 

 in the design and accounted for in the cost estimate and the construction procedure. 



Spacing of the jetties must accommodate both the entrance channel and a protective 

 berm of appropriate width on either side of the jetty (Fig. 27). The berm deters 



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