ERDC/CHL CHETN-III-65
March 2003
Periodic Inspections of Coastal
Structures with Stone Armor
by Robert R. Bottin, Jr.
information on the long-term structural performance of selected stone-armored navigation structures
to their environment. Coastal structures at St. Paul Harbor, AK, and Burns Harbor, IN, are discussed
herein. The response of concrete-armored coastal structures to their environment will be presented
in a subsequent CHETN.
OVERVIEW: In the "Periodic Inspections" work unit of the Monitoring Completed Navigation
Projects (MCNP) Program, selected coastal navigation structures are periodically monitored to gain
an understanding of their long-term structural response. Periodic data sets are evaluated to improve
knowledge in design, construction, and maintenance of both existing and proposed coastal
navigation projects, and will help avoid repeating past designs that have failed and/or resulted in
high maintenance costs. Relatively low-cost remote sensing tools and techniques, with limited
ground truthing surveys, are the primary inspection tools used in the periodic monitoring efforts.
Most periodic inspections consist of capturing above-water conditions of the structures at periodic
intervals using high-resolution aerial photography. Structural changes (primary armor unit move-
ment) are quantified through photogrammetric techniques. When a coastal structure is photographed
at low tide, an accurate permanent record of all visible armor units is obtained. Through the use of
stereoscopic, photogrammetric instruments in conjunction with stereopair photographs, details of
structure geometry can be defined at a point in time. By direct comparison of photographs taken at
different times, as well as the photogrammetric data resolved from each set of photographs,
geometric changes (i.e., armor unit movement and/or breakage) on the structure can be defined as a
function of time. Thus, periodic inspections of the structures capture permanent data that can be
compared and analyzed to determine if structure changes are occurring that indicate possible failure
modes and the need to monitor the structure(s) more closely. Normally, base conditions are
established and documented in the initial effort, and the site is reinspected periodically to obtain
long-term structural performance data. Also, periodic monitoring usually includes detailed broken
armor unit surveys. Base level conditions were initially established for both the St. Paul Harbor,
AK, and Burns Harbor, IN, breakwaters. Both structures also have been revisited through the
"Periodic Inspections" work unit of the MCNP Program. Monitoring included limited ground
surveys for control and a photogrammetric survey of the above-water armor layers. Precise positions
of armor units were analyzed, and contour maps of the breakwaters as well as cross sections, point
plot maps, and orthophotos of the structures were developed. In addition, a database of broken
armor stones was established. Results of the inspections are summarized herein.
ST. PAUL HARBOR BREAKWATER , AK: St. Paul Harbor is located in a cove on the southern
tip of St. Paul Island. The island is part of the Pribilof chain in the eastern Bering Sea. Construction
of the current harbor was completed in 1989. It is protected by a 548.6-m- (1,800-ft-) long main
breakwater and a 295.7-m- (970-ft-) long detached structure. The project was monitored under the
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