PART
SUMMARY OF CORPS BREAKWATER AND JETTY STRUCTURES IN LMVD
3.
There are 10 projects within LMVD which contain
jetty structures
are
breakwaters. All projects are located on the Gulf
of Mexico, and of these, eight are situated near the mouth of the Mississippi
the exception of the single jetty at Grand Isle,
projects have dual jetties. There is a total length of over 153,000 lin ft of
jetty structures. All of these are rubble mound (stone and/or shell) in cross
section, with the exception of the jetties at South and Southwest Passes,
which have been supplemented with concrete caps on several occasions since
original construction. Concrete capping ha3 not been used recently and is not
expected to see future use due to its higher cost compared with stone. The
structures typically have been built on unconsolidated silts and clays which
exist throughout the area to depths of several hundred feet. Most of the
structures are located in water depths of less than 10
(The seaward ends
of the jetties at South and Southwest Passes are in water depths of 20 to
30 ft.) Construction of early jetty projects, such as those at South and
Southwest Passes, consisted of one to several layers of fascine mats, each
layer ballasted with riprap stone, with additional stone added to obtain the
desired crest elevation. (Significant portions of the Mississippi Passes
jetties also were capped with concrete during original construction.) Fascine
mats were used as late as 1939 at Southwest Pass,
at Calcasieu Pass, and
1950 at Bayou Fontanelle. Construction of the most recent projects such as
Gulf Outlet, Tiger Pass, and Baptiste Collette Bayou, has used shell materials
as a bedding and core material, which then is covered with riprap or armor
stone. These projects were constructed in two stages, the first being a wide
base of shell and riprap stone as mentioned previously, and the second a
rubble-mound section typical of jetty structures.
sidence of the Louisiana coastline (relative to existing water levels) have
created the need to periodically raise several of the structures or extend
them landward. For example, typical rates of settlement of 0.1 to 0.5
have been observed on the Southwest Pass jetties. Numerous concrete caps
*
A table of factors for converting non-SI to SI (metric) units of measure-
ment is presented on page 3.
6
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