ERDC/CHL CHETN-III-66
June 2003
Periodic Inspections of Concrete-Armored
Coastal Structures
by Robert R. Bottin, Jr.
PURPOSE: This Coastal and Hydraulic Engineering Technical Note (CHETN) provides
information on the long-term structural performance of selected concrete-armored navigation
structures to their environment. Inspections of coastal structures at Ofu Harbor, American Samoa;
Nawiliwili, Laupahoehoe, and Kahului Harbors, HI; and Manasquan Inlet, NJ, are discussed herein.
The response of stone-armored coastal structures to their environment was presented in ERDC/CHL
CHETN-III-65.
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. 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 movement) 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 stereo-pair 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 inspection data can be analyzed to determine if structure changes are
occurring that indicate possible failure modes and the need to monitor the structure(s) more closely.
Underwater stone and toe armor below the water surface are not evaluated.
Normally, base conditions are established and documented in the initial effort, and the site is re-
inspected 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 the Ofu, Nawiliwili, Laupahoehoe, and Kahului breakwaters and the Manasquan Inlet jetties.
These 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 selected, targeted armor units were analyzed, as
well as comparisons of armor unit centroid data and/or rotation angles. Orthophotos of the structures
were also developed. In addition, databases of broken armor units above water were established.
Results of the inspections are summarized herein. Other concrete-armored coastal structures where
base level conditions have been established include the Crescent City Harbor, CA, breakwater,
Humboldt Bay, CA, jetties, and Cleveland Harbor, OH, east breakwater.
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