Port Mansfield Jetties

 

Port Mansfield, Texas

 

Date(s)            Construction and Rehabilitation History

 

1957-1961       Local interests dredged a channel across Padre Island and constructed two tetrapod jetties at the entrance to the Gulf of Mexico.  The north and south jetties were 1,600 and 800 ft long, respectively, and spaced 1,000 ft apart. The tetrapods weighed 5, 8, and 16 tons each and were placed directly on the existing bottom to a cross section of +5 ft mit, a 10-ft top width, and 1V:1H side slopes. Within a year these Jetties were no longer providing adequate channel protection.  The tetrapods had subsided, the shore ends were in water 2 tOC4 ft deep, scour channels existed along sections of both jetties, and extensive shoaling had occurred in the entrance channel.  By 1961 the inlet had closed and the tetrapods were nearly submerged.

 

1962                Two rubble-mound jetties were constructed under Corps management and Federal funding as part of navigational improvements between Port Mansfield and the Gulf of Mexico. The north and south jetties (2,300 and 2,270 ft long, respectively) were constructed on the south sides of the existing tetrapod jetties.  The parallel jetties, spaced 1,000 ft apart terminated at approximately the -9 ft mit contour.  The jetties were staggered with the south jetty 490 ft east Of the north jetty. The design geometry consisted of a +8 ft crown elevation, 16-ft crown width, and 1V:3H sideslopes.  The cross section was built on a 3-ft- thick bedding layer of 0.5-in. to 200-lb stone.  The bedding layer was to extend beyond the ends of the cover stone layer, typically 2 ft at the landward ends to 15 ft at the seaward ends.  The core was built up using 200- to 1,000-lb stone along landward sections (each several hundred feet long) and 200- to 4,000-lb stone along the remaining seaward sections.  Jetty permeability was decreased by supplementing a midsection of the core with 0.5- to 5-in. filler stone.  The geometry of this combined core stone extended below the full crown width at 1V:1H side slopes.  The cubically shaped granite cover stone increased in size from 1 to 10 tons, between landward and seaward ends of each jetty, respectively.  The seaward 100 ft of each jetty had an additional layer of 6- to 10-ton cover stone placed on the side slopes.  Stone sizes were selected using Hudson's slope stability formula and depth-limited wave heights of up to 12 ft at the seaward ends (+6 ft mit surge level).  The foundation for the jetties consisted of various sand and clay layers, including a 10- t 15-ft-thick layer of soft clay.  Soil tests indicated that the clay layer could consolidate up to 0.75 ft (seaward end), and total structure settlement on the order of 3 ft was expected (consolidation of the foundation and Jetty materials).  Total cost was $2,736,000 with 307,030 tons of stone placed and 10,000 tons of blanket stone stockpiled.

 

1963-1964       Landward sections on the channel sides of both jetties had scoured and lost blanket stone, allowing the outermost cover stones to shift, resulting in exposure of interior core and blanket stone.  These sections were located where the bedding apron was 2 ft wide.  The seaward side of the north jetty had a 10-ft-wide bedding apron and the south jetty, which had a 2-ft apron, was accumulating sediments along its seaward side.  The exposed section varied in width from 6 to 10 ft on the south jetty and 2 to 4 ft on the north jetty.

 

1964-1965       Repairs were made to the jetties using 2- to 4-ton cover stone and resetting existing cover stone.  Repairs were made also to sections of the channel revetment using 0.5-in. to 200-lb blanket stone and 2- to 4-ton cover stone.  A total of 10,420 and 5,900 tons of cover and blanket stone, respectively, was placed (about 3,800 tons of cover stone used on the jetties) at a total cost of $191,000.

 

1967-1968       The shore ends of the jetties and adjacent shore protection received minor damages due to the passage of Hurricane Beulah in 1967.  Repairs were made in 1968 using 780 tons of stone and resetting existing cover stone at a total cost of $18,900.

 

1980                The jetties were inspected and found to be in good condition, with the exception of some damage at their heads.  The landward end of the south jetty was flanked by a channel maintained by tidal flows.