Table 27 



St. George Island Jetties 



St. George Island, Florida, SAM 



Date(s) Construction and Rehabilitation History 



1954- In 1954, local interests cut a channel through St. George Island to 

 1957 provide a direct route to the Gulf from Apalachicola (Figure 43). 

 In April 1957 the Corps completed the existing projects with the 

 construction of two rubble-mound jetties on the Gulf and dredged the 

 channel to a depth of 10 ft (Figure 43, inset). The east and west 

 jetties, 900 and 1,030 ft long, respectively, and spaced 400 ft 

 apart, were built out to the -10 ft mlw contour. Approximately 

 70 ft of the landward end of each jetty flared away from the chan- 

 nel. The design cross section (Figure 43, inset) had a crest width 

 of 14 ft, a crest elevation of +6 ft mlw, and 1V:1.5H side slopes. 

 On the seaward end of each jetty the side slopes changed to IV: 2H 

 via a 100-ft-long transition section. Minimum cover stone sizes 

 were 6 and 10 tons on the trunk and head/transition sections, re- 

 spectively. The core stone weighed from 25 lb to 2 tons, and the 

 2- to 2.5-ft-thick foundation blanket used 15- to 200-lb stone. The 

 stone size was selected using Hudson's slope stability formula, a 

 maximum wave height of 13.7 ft, and a +6 ft mlw storm surge level. 

 Figures 44a and 44b are photographs of the jetties taken before and 

 shortly after the completion. "Keyhole" erosion on the landward 

 side of the jetties (the jetties and the crescentic erosion yielding 

 the silhouette of a giant keyhole) can be seen in the postconstruc- 

 tion photograph (44b) . 



1977- The jetties and channel were surveyed in early 1977. The east jetty 

 1978 showed substantial loss of material over 250-ft section at the sea- 

 ward end, the outer 50 ft was at or below mlw, and the remainder 

 varied from +3 to +5 ft mlw. The landward 350 ft of the east jetty 

 was typically at +5 ft mlw except for the flared portion which was 

 at +3 ft mlw. The west jetty was in good condition except for minor 

 sections and the landward 150 ft which varied from +3.5 to +4.5 ft 

 mlw. In 1978 the jetties were rehabilitated. A total of 4,700 tons 

 of 3- to 6-ton cover stone were placed as required at low sections 

 to bring the jetties up to the previous design elevations. 



1985 The jetties are presently in good condition. The major problem, at 

 present, is the keyhole erosion that has been removing material at 

 an apparently constant rate since jetty construction (and is ex- 

 pected to continue) . The proposed solution is to purchase title to 

 additional land on both sides of the channel. 



78 



