Effectiveness of Pocket Wave Absorbers in Vertical-Wall, 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 structures 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 remainder 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 absorbers. BACKGROUND, PHYSICAL MODEL STUDIES: To predict design performance of pocket