Table 14 

 Jacksonville Harbor Jetties 

 Jacksonville, Florida, SAJ 



Date(s) Construction and Rehabilitation History 



1879- The original design consisted of two jetties, a 9,400-f t-long north 

 1895 jetty, and a 6, 800-f t-long south jetty, which were to converge at 

 their outer ends to a distance of about 1,600 to 1,800 ft (Fig- 

 ure 22, present location map). The outer 2,000 ft of the jetties 

 would have a crest elevation at midtide level, and the inner por- 

 tions would be at -3 ft mlw. The purpose of the jetties was to 

 maintain a 15-ft-deep channel via the natural scour action that was 

 expected to occur once the jetties were completed. The principal 

 method of construction was placement of one to several courses 

 (layers) of log and brush mattress (Figure 23a) . Each layer was 

 sunk and weighted down by placing a 12- to 15-in-thick layer of rip- 

 rap stone. Once a firm foundation of mattresses was created, the 

 remainder of the section geometry was built up with larger sized 

 riprap stone. This method of construction was used at several other 

 locations on the east coast during the late-1800's and early 1900 's. 

 The underlying concept of the method was that a supporting layer of 

 material was required prior to stone placement since it was expected 

 that direct placement of stone would sink into the "soft" bottom. 

 Thus, without a supporting mattress, large amounts of stone would be 

 required to provide a solid base. Many problems were encountered 

 with this method, principally because of the methods of early con- 

 struction, the dynamics of the natural bottom (scour and deposi- 

 tion) , and destruction of the mattresses by the teredo (a wood- 

 boring marine mollusk) . The north jetty was completed in 1892 to a 

 length of 10,930 ft at a total cost of $411,000. In 1893 the south 

 jetty was extended 2,900 ft, to a total length of 11,300 ft, using 

 15,900 tons of 1- to 6-ton stone and 123,000 tons of 15- to 400-lb 

 stone. The south jetty was completed in 1895 at a total cost of 

 $993,000. Although there were no in situ section geometry details 

 found for either jetty, it appears that both had been built up to 

 approximately mlw. 



1897- During this period both jetties were raised above mhw (+4.9 ft), 

 1928 numerous repairs were made to the jetties, the north jetty was ex- 

 tended seaward 2,070 ft, and the channel depth was increased to 

 -30 ft mlw. The method of jetty construction by this time was to 

 place the stone directly on the natural bottom with the smaller 

 stone placed at the bottom and the larger stone placed above mlw. 

 The size of the largest armor stone used increased during this time 

 from a typical size of 4 to 7 tons. Figure 23b shows a cross sec- 

 tion of the north jetty taken shortly after work was completed 

 during 1923. This design section consisted of a 10-ft-wide crest at 



(Continued) 



(Sheet 1 of 4) 

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