*************** EXAMPLE PROBLEM 5*************** 



GIVEN ; The beach profile updrift of an inlet where a weir jetty is pro- 

 posed is shown in Figure 33. The weir elevation is at MTL. The normal 

 tidal range is 4.0 feet and the average annual significant breaker height 

 is 3.0 feet. 



Seaward End of Weir 

 _l 



100 200 



300 400 500 

 Distance (ft) 



600 700 800 900 



Figure 33. Weir length as established by average annual breaker 



height (final beach profile after weir construction is 

 assumed to be similar to preconstruction profile). 



FIND : Estimate the length of the weir section and the height of the 



breaker at the end of the weir section at mean high water (MHW). 

 SOLUTION; The water depth in which a 3.0-foot-high wave will break is 



given approximately by the solitary wave theory expression 



H 



b _ 



= 0.78 



(16) 



(The influence of local beach slope on the ratio H,/d^ can be considered 

 by using the design curves in Ch. 7 of the SPM; however, eq. 16 is 

 accurate enough for determining weir length. ) The breaking depth is 



dv = 



M> _ 



0.78 



= 1.28(3.0) = 3.8 feet 



The point where the beach profile is 3.8 feet below MLW locates the 

 approximate seaward end of the weir section. The length of the weir is 

 therefore 480 feet. When the water level is at MHW, the depth at the 

 seaward end of the weir is 3.8 + 4.0 = 7.8 feet. The breaker height in 

 7.8 feet of water (from eq. 16) is 



H b = 0.78d b = 0.78(7.8) = 6.1 feet 

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