East Pass, Florida, Jetties 



55. As part of the MCCP Program effort at East Pass Inlet, Florida 

 (Figure 31), side-scan sonar inspections of the jetties have been conducted. 

 The purpose of this effort is to document changes in the hydraulics and lit- 

 toral patterns of the inlet after closure of the weir section of the west 

 jetty in 1985 and to evaluate the stability and effectiveness of the jetties. 

 The latter objective is accomplished primarily through periodic condition 

 surveys in which side-scan sonar is the principal inspection instrument. 

 Side-scan sonar records have shown that a significant scour hole is developing 

 just southwest of the spur jetty. Apparently, tidal currents along the 

 jetties are creating a zone of localized turbulence, resulting in the scour 

 hole. 



56. Side-scan surveys were conducted using a Klein model 531 -T recorder 

 with a 500-kHz towfish. The survey vessel was a 28-ft outboard powered work 

 boat, which towed the fish at 2-4 knots approximately 6 ft below the surface. 

 Results were improved by attaching the cable at the bow of the vessel to avoid 

 turbulence from the propellers. The majority of the records were obtained 

 using a range of 25 m. 



57. The records clearly showed the toe and the face of the structure. 

 Also apparent were individual stones dislodged from the face and an approxi- 

 mately 1-ft-diam steel pipe, probably lost during dredging operations. Of 

 particular interest are features that require more interpretation of the 

 record. By examination of the initial bottom return and the shadow zones, it 

 is possible to discern a scour-induced trench along a portion of the east 

 jetty and a deeper scour hole at the southwest tip of the spur jetty. 



58. Standard bathymetric surveys around the jetties were also obtained 

 at 200-ft intervals. Comparison of Figures 32a (showing the results of a 

 survey taken in July 1985) and 32b (showing the results of a survey taken in 

 August 1986) illustrates the difficulty of delineating small-scale features at 

 typical bathymetric survey line spacings. Notably, the fathometer survey 

 tracklines miss the center of the scour hole. The principal benefit of a 

 side-scan sonar survey, in this instance, is to identify and locate areas that 

 require closer spacings between bathymetric survey lines. 



48 



