Effectiveness of Pocket Wave
Absorbers in Vertical-Wall,
Coastal Entrance Structures
by Edward F. Thompson, Robert R. Bottin, Jr.,
and James P. Selegean
Coastal and Hydraulics Engineering Technical Note (CHETN) provides
preliminary information on the effectiveness of pocket wave absorbers (relative to wave conditions)
in vertical steel sheet-pile coastal entrance structures.
OVERVIEW: The U.S. Army Corps of Engineers is responsible for dozens of harbor entrances in
the Great Lakes constructed with parallel jetties. These jetties, many in operation for more than
100 years, were typically constructed of rock-filled timber cribs. Over time, the wood cribbing has
experienced significant deterioration, thus causing the jetty to be rather porous. Many of these struc-
tures have been rehabilitated. The typical rehabilitation approach has been to drive steel sheet pile
around the existing structure and place a concrete cap on top, thereby encasing the original structure.
After completion of the rehabilitation projects, the wave climate between the jetties appears to
increase significantly causing navigational difficulties and damage to moored vessels within the
harbor. This is apparently due to the fact that the timber crib jetties were rough, porous structures,
especially in their deteriorated state, and were much more effective at dampening wave energy than
the rehabilitated, sheet-pile encased jetties. The steel sheet-pile structures, being considerably more
reflective than the deteriorating timber structures, are largely responsible for the increasingly
energetic wave climate. To mitigate for the more energetic wave climate, the Corps has removed
short sections of steel sheet piling at selected harbors and replaced them with pocket wave absorbers.
A pocket wave absorber is created when a section of the sheet-pile wall is recessed from the remain-
der of the jetty and stone is placed in the area to provide a rough, porous sloping surface that is
intended to dissipate wave energy. The crest of the stone is usually offset from the steel sheet-pile
wall, thus creating a pocket. The typical length of a pocket is 61 to 91 m (200 to 300 ft). An example
of a pocket wave absorber is shown in Figure 1. The U.S. Army Engineer District, Detroit, has
installed 10 pocket wave absorbers in six Federal harbors. In some instances the pockets are located
at the landward ends of the jetties, while others are situated more lakeward. The wave absorbers
have been installed as a single pocket, and in pairs, on opposite sides of the channel. Little or no
design guidance was available for predicting the effectiveness of the many variations of wave
BACKGROUND, PHYSICAL MODEL STUDIES: To predict design performance of pocket
wave absorbers, physical model experiments were conducted by the University of Michigan,
Department of Civil and Environmental Engineering (Wright and Carpenter 1999; Carpenter 2001).
A generic model, representative of typical dimensions for various rehabilitated harbor jetties, was
constructed to a scale of 1:50. The model layout consisted of two parallel jetties 1.2 m (4 ft) apart
and 9.4 m (31 ft) long with a water depth of 0.09 m (0.32 ft) (corresponding to prototype dimensions
of 61 m (200 ft) in width and 1,550 ft in length with a water depth of 457.2 m (16 ft)). Design
parameters such as pocket length, slope of stone, and stone size were varied.