Table 14
Jacksonville Harbor Jetties
Jacksonville, Florida, SAJ
Date
Construction and Rehabilitation History
The original design consisted of two jetties, a 9,400-ft-long north
1895
jetty,
south jetty, which were to converge
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
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
Each layer was
sunk and weighted down by placing a
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 1,000, In 1893 the south
jetty was extended 2,900 ft, to a total length of 11,300 ft, using
to 6-ton stone and 123,000 tons of
to 400-lb
15,900 tons of
stone. The south jetty was completed in 1895 at a total cost of
3,000. Although there were no in situ section geometry details
to
found for either jetty, it appears that both had been built
approximately mlw.
During this period both jetties were raised above mhw
ft),
numerous repairs were made to the jetties, the north jetty was
1928
tended seaward 2,070 ft, and the channel depth was increased to
The method of jetty construction by this time was to
-30 ft
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
crest at
(Continued)
(Sheet 1 of 4
)
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