ERDC/CHL CHETN-III-65
March 2003
To minimize further breakwater damage and reduce overtopping of the main breakwater, the
construction of three submerged reef breakwaters seaward of the structure was completed by the
U.S. Army Engineer District, Alaska, during the 2001 construction season. In addition, the void area
between stas 8+80 and 9+70 was repaired. The 2000 periodic inspection of the breakwater extension
not only determined changes in the armor unit field since previous studies, but established new base
conditions since construction of the reef breakwaters. Subsequent inspections will evaluate the
effectiveness and analyze the performance of the improved project.
BURNS HARBOR BREAKWATER, IN: Burns Harbor is located on the southern shoreline of
Lake Michigan. It is protected by north and west breakwaters that have an aggregate length of
1,780 m (5,840 ft). Construction of the breakwaters was completed in 1968. The project was
monitored under the MCNP Program during the period 1985 through 1989 (McGehee et al. 1997);
however, no quantifiable data relative to armor-stone positions were obtained during this effort.
Base conditions were established in 1994/1995 for the 1,414.3-m- (4,640-ft-) long north breakwater
under the "Periodic Inspections" work unit (Bottin and Matthews 1996). The structure was revisited
in 1999 (Bottin and Tibbetts 2000).
The north breakwater was constructed with a multilayered design and random placement of the
armor-stone cover layer. The design crest elevation of the breakwater was el +14. 1 Armor stones
consisted of rectangular-cut Indiana Bedford limestone blocks ranging from 9,071.8 to 13,607.8 kg
(10 to 15 tons) on the trunk and 13,607.8 to 18,143.7 kg (15 to 20 tons) on the head. Side slopes
were 1V:1.5H. After completion of construction, extensive breakwater damage occurred, and
maintenance of the crest elevation and structure cross section required the addition of large amounts
of stone.
The initial photogrammetric survey of November 1994 revealed low areas along much of the
breakwater. Breakwater topography indicated that, cumulatively, about 344.4 m (1,130 ft), or
24 percent, of the total breakwater length was below its design crest el +14. Most of the low areas
ranged from el +12 to el +14. Only about 65.5 m (215 ft) cumulative length of the breakwater
(4.6 percent) was below el + 12. Topography and cross-section data also revealed that the design
width of el +17 was not maintained in many areas. In addition, on some portions of the structure, the
slope of the harbor side of the breakwater was much steeper than the original 1V:1.5H design. The
initial broken armor unit survey of the Burns Harbor north breakwater was conducted during July
1995. The survey revealed a total of 165 broken or cracked armor stones above the waterline. Of
the 165 stones, 26 (16 percent) were located on the breakwater crest, 95 (57 percent) on the lake-side
slope, and 44 (27 percent) on the harbor-side slope. No broken armor stones were observed around
the head. Broken stones occurred along the entire breakwater, however in general, higher
concentrations were found along the easternmost portion of the structure. About 50 percent of the
broken units were located on the eastern one-third of the breakwater.
Examination of breakwater topography for the July 1999 photogrammetric survey indicated
continued loss of structure elevation. Cumulatively, about 646.2 m (2,120 ft), or 46 percent, of the
total length of the breakwater was below its design el +14 (versus 24 percent in the previous survey
of 1994). About 152.4 m (500 ft) cumulative length of the breakwater (11 percent) was below
1
Elevations (el) cited herein for Burns Harbor are in feet referenced to low water datum (lwd) (to convert feet to meters, multiply
number of feet by 0.3048).
4
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