PART II: SUMMARY OF CORPS BREAKWATER AND JETTY STRUCTURES IN LMVD 



3. There are 10 projects within LMVD which contain 19 Jetty structures 

 (Figure 1). There are no breakwaters. All projects are located on the Gulf 

 of Mexico, and of these, eight are situated near the mouth of the Mississippi 

 River (Figure 2). With the exception of the single Jetty at Grand Isle, all 

 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 has 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 ft*. (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, 1942 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. 



4. Settlement of structures due to soil consolidation and general sub- 

 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 ft/yr 

 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. 



