some evidence that farther from shore in deeper water near the breaker region, 

 sand moves across the weir in suspension. In the laboratory the amount in 

 suspension appears relatively small in comparison with the amount carried over 

 as bedload near shore. At Perdido Pass, Mississippi, in the Gulf of Mexico, 

 an offshore bar that parallels the updrift beach, and which appears to be 

 continuous across the weir to the channel side of the jetty, suggests that 

 sufficient transport occurs offshore as suspended load to maintain the bar 

 across the weir. Where offshore bars occur, the weir should probably extend 

 seaward beyond the bars. The length of the weir in this case can be deter- 

 mined from normal beach profiles that exist updrift of the proposed jetty away 

 from the influence of the inlet. Seasonal changes in the profiles should be 

 taken into consideration. Figure 32 shows how weir length can be established 

 from beach profiles exhibiting prominent offshore bars. Where offshore bars 

 are not prominent, the seaward end of the weir section should be seaward of 

 the normal breaker location. The profile used to establish breaker location 

 should be the profile expected after construction of the project, not the 

 profile existing at the unimproved inlet. An estimate of the postconstruction 

 profile can be obtained by examining existing profiles updrift of the proposed 

 jetties, far enough away from the inlet to avoid being influenced by tidal 

 currents and localized inlet wave refraction. Based on current knowledge, 

 there are no definitive guidelines for selecting the wave height to determine 

 the breaker depth; however, a first estimate can be obtained by using the 

 average annual significant breaker height at the site. (Values for the 

 average annual breaker height at various U.S. coastal locations are given in 

 Ch. 4 of the SPM. ) Obviously, tidal stage also influences breaker location 

 with respect to the weir. Waves of a given height will break farther seaward 

 from a fixed point on the shore at low tide than at high tide. To ensure that 

 the average annual significant breaker height occurs landward of the seaward 

 end of the weir section, the water level used for the analysis should be 

 MLW. At low tide, waves larger than the average annual significant breaker 

 height break seaward off the end of the weir section; however, at water levels 

 above MLW these larger waves may break adjacent to the weir section. Also, 

 during storms when larger waves occur, storm surge often raises the water 

 level, allowing larger waves to transport suspended sand across the weir. 



100 200 300 400 500 600 700 800 900 

 Distance (ft) 



Figure 32. Weir length in the presence of a well-defined offshore bar. 



51 



