located to take advantage of any beneficial natural processes, provided those 

 processes will continue after project construction. Navigation requirements 

 may also influence alinement. In addition to alining the jetties to provide 

 maximum protection for vessels navigating the inlet, the characteristics and 

 maneuverability of vessels using the inlet may be a factor. 



The distance between jetties is governed by both navigation and hydraulic 

 factors. As with the design of the navigation channel itself, the size of the 

 tidal prism, for the most part, determines the cross-sectional area of the 

 inlet throat and of the channel between jetties. Jetty spacing, therefore, 

 has some influence on the relative dimensions of the channel, i.e., the 

 channel width-to-depth ratio. Jetties spaced too far apart will encourage 

 shoaling with inadequate water depths to serve the vessels for which the chan- 

 nel is intended; jetties too close together may result in channel scouring and 

 cause scour holes which can undermine jetties and eventually require consider- 

 able efforts to prevent a major structural failure. Closely spaced jetties 

 may also endanger vessels by increasing the possibility of collision with 

 other vessels or the jetties and by causing excessive wave steepening over the 

 ocean bar. 



The primary factor influencing the structural design of jetties is the 

 local wave and water level climate. Structural details include establishing 

 the crest elevation, structure type, and the structure cross section. For 

 economic as well as technical reasons, rubble-mound construction is preferred 

 for jetties because the low maintenance required usually results in minimal 

 annual cost, even though other types of construction such as steel or concrete 

 sheet-pile jetties may have a lower initial cost. Rubble structures are con- 

 sidered "flexible" structures. When subjected to waves exceeding their design 

 level, the damage they experience is usually progressive, making repairs rela- 

 tively simple and inexpensive. Rubble structures also continue to provide 

 protection when in a damaged state. Destruction of less flexible structures 

 can be catastrophic with complete loss of function. 



Sheet-pile and caisson-type jetties reflect wave energy and encourage the 

 formation of seaward-moving currents adjacent to the structures. For weir 

 jetties where currents could transport sand away from the weir section into 

 deeper water and eventually into the navigation channel, rubble-mound con- 

 struction is preferred. Wave reflection from the structure is also minimized. 

 The prediction of updrift and downdrift beach planforms is simpler if wave 

 conditions adjacent to the structure are not confused by reflected waves. 

 Rubble jetties also decrease possible adverse effects of reflected waves on 

 navigation conditions. Jetty crest elevation is usually selected to prevent 

 overtopping for some design wave and water level condition. It is generally 

 not possible for a design to preclude overtopping since designing for extreme 

 waves and water levels is usually not economically justified. The effects of 

 exceeding design conditions for wave and water levels should, however, be 

 investigated and the design optimized by balancing the higher initial cost of 

 a more substantial structure against the higher maintenance and repair costs 

 and decreased benefits of a less substantial structure. Some savings may 

 accrue by decreasing armor-stone size near the landward end of the jetties 

 since wave heights are usually depth-limited. Armor near the shore will be 

 subjected to smaller waves or may even be insulated from wave action by the 

 sand that accumulates against it. However, some estimate of scour is neces- 

 sary in order to not underestimate the local wave height which could result in 

 underdesigned armor units. 



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