ERDC/CHL CHETN-III-65
March 2003
length of the breakwater was within 0.3048 m (1 ft) of its design elevation, or between el +36 and
el +37; and approximately 66 percent of the structure was between el +35 and el +37, or within
0.6096 m (2 ft) of its design elevation. About 29 percent of the structure length was below el +35.
Most of the low area (that below el +35) was concentrated between stas 13+70 and 15+10. For the
lower portion of the structure, the elevation of about 50 percent of the length of the breakwater was
within 0.3048 m (1 ft) of its design elevation, or between el +29 and el +30; and approximately
89 percent of the structure was within 0.6096 m (2 ft) of its design elevation, or between el +28 and
el +30. Only 2 percent of the structure length was below el +28. The initial broken armor unit
survey of the St. Paul Harbor breakwater extension occurred during July 1993. The survey revealed
73 broken or cracked armor stones above the waterline. Of the 73 stones, 7 stones were located on
the crest, 31 on the seaward slope, and 35 on the harbor side.
The May 1996 photogrammetric survey of the breakwater revealed very slight change in breakwater
elevation relative to the 1994 survey. Contours showing the difference in elevations, indicated
essentially no change along the crown of the structure. In the vicinity of sta 9+50, a change up to
0.9144 m (3 ft) occurred along the waterline on the sea side of the structure. In this area, however,
emergency repairs were made in 1995. Other changes (between 0.3048 and 0.9144 m (1 and 3 ft))
generally occurred on the harbor side of the breakwater. The data indicated essentially no settlement
of the structure between 1994 and 1996. Examination of cross section data revealed similar sections
for both 1994 and 1996. A broken armor unit survey conducted during June 1996 revealed
230 broken/cracked armor stones on the main breakwater (versus 73 in July 1993). Of the
230 stones, 54 were located on the crest, 105 on the seaward slope, and 71 on the harbor-side slope.
Broken stones, generally, were evenly distributed along the length of the structure. The survey
indicated that 49 percent of the broken stones were located on the shoreward half of the breakwater
extension, and 51 percent on the outer half.
Examination of breakwater topography for the June 2000 photogrammetric survey indicated that
essentially no change had occurred in the crest elevation of the breakwater since 1996. Maps
developed depicting changes in contours between 1996 and 2000 revealed some areas in the
breakwater where voids or subsidence (on the order of 1.5 m (5 ft)) had occurred. Decreases in
breakwater elevation were noted along areas on both the seaward and shoreward slopes. Difference
contour maps also revealed voids in several areas where single armor stones had been displaced. In
addition, breakwater cross sections developed revealed low areas in the structure at some locations.
The overall shape and elevation of the breakwater appeared similar, but voids were noted in the 2000
survey that had not been present previously. A broken armor unit survey conducted during July
2000 revealed 221 broken or cracked armor stones above the waterline. Twenty-four new broken
stones were noted since the June 1996 inspection, and 33 broken stones documented during the
previous inspection could not be found. They could have been moved away by wave and/or ice
action. Just fragments of most these stones remained. Of the 221 broken stones identified in the
2000 inspection, 56 were located along the breakwater crest, 96 on the sea-side slope, and 69 on the
harbor-side slope. In general, broken stones were evenly distributed along the length of the structure
with 54 percent located on the shoreward half of the breakwater extension and 46 percent on the
outer half. An obvious void observed during the broken armor unit survey was in a stretch along the
waterline on the sea-side slope between stas 8+80 and 9+70 where core stone was exposed in one
area. The rate of breakage appeared to have declined at St. Paul breakwater. Only 24 new broken
armor stones were observed in the 4-year period between 1996 and 2000 as opposed to 157 broken
stones that occurred during the original 3-year monitoring period between 1993 and 1996.
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