Seasonal and irregular temporary reversals of longshore sand transport 

 which influence the updrift and downdrift shorelines can be studied using 

 tracer which has been initially distributed along a model shoreline and noting 

 its response to waves from various directions. The amount of sand in dead 

 storage updrift of the updrift jetty can be estimated along with the active 

 storage if the wave heights, directions, and durations selected for the 

 testing program are reasonably characteristic of the project site. The tracer 

 is used to simulate a beach adjacent to the proposed inlet structures, and 

 waves approaching from the updrift direction are run to form a fillet. Subse- 

 quently, waves from the downdrift direction are run and changes in planform of 

 the fillet observed. Usually, all of the tracer moved into the fillet by 

 waves from the updrift direction can not be removed from the fillet by waves 

 from the downdrift direction because of the diffraction shadow of the jetties. 

 This sheltering effect can be evaluated from observations of the tracer fillet 

 and its response to variations in the magnitude and direction of incident wave 

 energy. 



3. Jetty Structure Stability . 



Since the cost of large rubble- jetty structures is high, any cost-savings 

 that can be obtained by prudent design and economic optimization should be 

 investigated. Stability tests of various structure cross sections should be 

 pursued if there is any question regarding the structural performance of the 

 jetties under the varying wave and water level conditions prevailing at a 

 site. Generally, jetties are designed for waves with heights limited by 

 shallow depths close to shore (see Sec. 7.12 of the SPM). The limiting design 

 breaker height is thus a function of the maximum water levels that can occur 

 at the jetty site. Maximum wave conditions will prevail during maximum water 

 levels such as occur during hurricanes. If a rubble jetty is designed for 

 given water level and breaker height, greater water levels and breaker heights 

 will result in some damage to the structure. The level of damage and the 

 ability of the jetty cross section to continue its protective function can be 

 assessed in a model. Models of the jetty trunk and head should be subjected 

 to the various conditions of water levels and wave heights characteristic of 

 the site. The model can also be useful to look at the interlocking between 

 the armor and first underlayer and for sizing the underlayers. Also, waves 

 incident on jetties usually propagate roughly along the axis of the structure. 

 Little information is available on rubble structure stability under such wave 

 action. If indicated, stability tests using waves with an oblique angle of 

 approach should be conducted. 



The weir section of a jetty system will overtop during a significant part 

 of the tidal cycle. If the section is built of rubble, unique stability 



problems might arise because of overtopping. Special consideration should be 



given to testing the weir cross section for stability under these conditions. 



Other savings may often be achieved by varying the structure cross section 

 to allow smaller armor units close to shore where breaker heights are lower. 

 Also, the landwardmost part of a jetty may not be subjected to significant 

 levels of wave action because of the accumulation of sand adjacent to the 

 jetty. Decreasing water depths close to shore limit the wave heights to which 

 the nearshore segments of a jetty are subjected; consequently, jetties may be 

 designed for lower wave heights provided sufficient information on the post- 

 construction beach profile adjacent to the jetty is available. Model testing 

 of the various cross sections should be performed. 



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