hazardous. Side-scan sonar was used to document any visual evidence of active 

 sand movement zones and to help in selecting a location of the intake for the 

 bypassing plant. In addition, the area around the jetties was checked to 

 determine sand transport pathways. 



52. The side-scan sonar inspection of the site was performed using a 

 Klein 500-kHz system with the majority of the images taken at a range of 50 m. 

 The towing vessel was a 26-ft Mako fishing boat (open cabin, with center con- 

 sole), and average towing speed was 2 to 3 knots. Typical fish depth was 4 ft 

 below the surface. Water depth at the site' ranged from 3 to 20 ft, and wave 

 action was negligible inside the inlet and 1 to 2 ft outside the inlet. 



53. The main observations made from the side-scan sonar survey are 

 summarized as follows: 



a. Seaward of the entrance channel and dogleg portion of the north 

 jetty, the bottom is devoid of sand, suggesting that little 

 transport takes place just outside the structure. The sono- 

 graph indicates that a 200-ft width of the bottom seaward of 

 the toe of the dogleg is a clean surface of rough coquina 

 limestone (Figure 29). 



b. Side-scan sonar images of the channel showed that the entrance 

 is devoid of sand from the eastern end of the jetties to a 

 point approximately 300 ft to the west. The remainder of the 

 channel is covered with sand. The shoal formation suggests the 

 majority of the sand appears to come from the north. 



c. The ripple patterns inside the weir have important implications 

 for the design of the deposition basin. Ripples at the seaward 

 and middle portions of the weir are roughly parallel to the 

 weir (Figure 30). A dramatic change in the angle the ripples 

 make with the weir occurs along a line approximately 200 to 

 250 ft from the landward end of the weir. At line A, the angle 

 changes to approximately 45 deg. This phenomenon indicates 

 that the primary wave and current flow direction changes about 

 1/3 of the way along the length of the weir to focus the 

 inshore sediment transport toward the southeast corner of the 

 spit. The region west of line A then becomes a potential loca- 

 tion for the intake of the sand bypassing plant. 



54. Results from the St. Lucie Inlet Study show that side-scan sonar 

 can help determine sand transport paths at the inlet. The information can be 

 valuable in designing sand bypassing systems and has applications for most 

 other coastal projects. 



45 



