offshore movement. A series of nine groins was effective in trapping tracer 

 material, but significant quantities of stone were required. Testing 

 segmented breakwater plans at the -4.6 m depth indicated that shorter segments 

 with shorter gaps produced weaker rip currents and appeared to retain most of 

 the tracer material. However, a large volume of stone was required. Test 

 results with submerged structures at the -3.0 m depth revealed that breaking 

 waves piled water between the breakwater and shore. The seaward return of the 

 water created strong rip currents, resulting in an offshore loss of tracer 

 material. Low sills were placed in the breakwater gaps and were successful in 

 retaining all but small quantities of tracer. Tests with segments located at 

 the -1.5 m contour separated by gaps allowed too much wave transmission into 

 the structure lee. The installation of low sills between breakwater segments 

 appeared to be a viable way of reducing the total wave transmission and would 

 have the least impact on longshore transport seaward of the structure. 

 Figures C12 and C13 show views of the detached breakwaters at the -4.6m and 

 -1.5 m contours in the Imperial Beach model. 



C16 



