Table 7 



Masonboro Inlet Jetties 



Masonboro Inlet, North Carolina, SAW 



Date(s) Construction and Rehabilitation History 



1947- In 1947 the district built two groins on the north shore. Shortly 

 1959 thereafter, three groins were constructed on the south shore, but 

 all five proved to be ineffective in maintaining a channel through 

 the inlet. In 1950 Congress authorized a channel 14 ft deep and 

 400 ft wide across the bar at Masonboro Inlet and dual jetties ex- 

 tending to the 14-ft depth contour (Figure 10). The jetties were to 

 be constructed only if it were found impracticable to maintain the 

 channel by dredging and if a study showed the jetties economically 

 justified. The ocean entrance channel through the inlet was com- 

 pleted in 1959. 



1965- Continued shoaling in the channel and attendant maintenance dredging 

 1966 problems led to a reactivation of the project's provisional jetties 

 feature. Because of the predominant southerly littoral drift, only 

 the north jetty was completed pending future evidence of the need 

 for a south jetty. In addition, the north jetty was designed as a 

 prototype sand-weir structure to add a sand-bypassing feature to the 

 overall navigation improvements. This was the first time that the 

 sand weir bypassing feature had been incorporated into a Corps of 

 Engineers (Corps) jetty design. The overall length of the jetty was 

 3,639 ft, consisting of 1,739 ft of concrete sheet pile and 1,900 ft 

 of rubble-mound on landward and seaward sections, respectively 

 (Figure 11). The sections of sheet pile, 23.5 ft long by 3 ft wide 

 by 16 in. thick, were precast and prestressed with cables, and, once 



2 

 placed, were interconnected with 12-in. treated timber wales. Sub- 

 sequent to completion of the jetty, several sections of the timber 

 wales came loose and required rebolting or removal from the sheet 

 pile. 520 lin ft of wales were removed. It was recommended that 

 any future designs were not to incorporate timber wales. The crest 

 elevation of the shoreward 600 ft of the sheet pile varied from 

 +12 to +2 ft mlw, with the 1,100-ft weir section at a crest ele- 

 vation of +2 ft mlw. The rubble-mound portion of the north jetty 

 had design crest elevations of +6 ft mlw for 850 ft, a transition 

 from +6 to +8 ft mlw for over 100 ft, and +8 ft mlw for the seaward 

 950 ft. The design crown width was 10 ft, and the side slopes were 

 1V:1.5H and 1V:2.5H for the trunk and head sections, respectively. 

 Capstone size ranged from 7 to 12 tons. Depth-limited design wave 

 heights of 8 and 12 ft were used with Hudson's stability equation to 

 select capstone for the trunk and head sections, respectively. The 

 jetty design included a deposition basin on the leeward side of, and 

 adjacent to, the weir section. The basin would periodically be 

 dredged, with the material placed on the opposite shore as required. 



(Continued) 



(Sheet 1 of 4) 

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