Table 1 



Mississippi River Gulf Outlet Jetties 



Mississippi River Gulf Outlet, Louisiana 



Date(s) Construction and Rehabilitation History 



i960 The River and Harbor Act of 29 March 1956 authorized construction of 

 a 35-ft-deep by 500-ft-wide channel from New Orleans to the Gulf of 

 Mexico as an alternative and necessary route in the event of emer- 

 gency, war, or blockage of the Mississippi River. As part of the 

 navigation improvements, Jetties at the channel entrance also were 

 authorized (Figures 2 and 3). Construction of the north Jetty 

 between 2264+00 and 2420+00 was completed at this time. The design 

 section (Figure 4a) called for shell and riprap to be placed between 

 retaining dikes composed of dredged fill. The dikes were placed from 

 25 to 35 ft on either side of the planned Jetty center line and built 

 up to elevations of +1 or +3 ft mean low water (mlw) on existing 

 ground above or below -2 ft mlw, respectively. The shell material 

 was placed to a center-line elevation of +1 ft mlw and 1V:7H side 

 slopes. The 150- to 500-lb riprap was placed to an elevation of 

 +3 ft mlw, a top width of 20 ft, and 1V:3H side slopes. Due to soil 

 conditions, which consisted of predominantly fat clay with low shear 

 strength, additional capping stone was to be added at a later date, 

 thus allowing the existing soil mass to consolidate. As will be 

 seen, this method was used throughout construction of the Jetties. A 

 total of 124,580 tons of riprap and 78,300 cu yd of shell was placed 

 at a cost of $1,211,500. 



1961 Capping stone was placed on the existing north Jetty, and south Jetty 

 construction was completed between 2264+00 and 2420+00. On the north 

 jetty, between 2262+50 and 2420+00, capping stone was placed to an el- 

 evation of +5 ft mlg, a crest width of 12 ft, and 1V:1.5H side slopes 

 (Figure 4b). Stone size varied from 200 lb to 4 tons with 80 percent 

 ranging from 2 to 4 tons. The larger pieces were placed on exposed 

 surfaces. Existing Jetty elevations, prior to capping, varied from +2 

 to +3 ft mlg. A total of 59,860 tons of stone was placed at a cost of 

 $428,700. South Jetty construction was identical to I960 Phase I 

 construction of the north Jetty. The jetties are parallel and spaced 

 1,400 ft apart (Figure 3). A total of 115,190 tons of riprap and 

 90,360 cu yd of shell was placed at a cost of $846,300. 



1962 The south Jetty was capped between 2262+50 and 2420+00, with 4- to 

 6-ton stone. In addition, 500-lb to 2-ton stone was placed in a 

 2-ft- thick by 15-ft-wide layer on the channel side slope of the Jetty 

 adjacent to the capped section (Figure 4c). The cap design geometry 

 was identical to the north Jetty cap of 1961, with a +5 ft mlg crown 

 elevation, 12-ft crown width, and 1V:1.5H side slopes. Prior to 

 capping, typical elevations on the Jetty (February 1962 survey) were 

 from +1.5 to +3 ft mlg. A total of 107,500 tons of stone was placed 

 at a cost of $717,100. 



(Continued) 



11 



