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Title: Design of quarry-stone cover layers for rubble-mound breakwaters : hydraulic laboratory investigation
Authors: U.S. Army Engineer Waterways Experiment Station.
Keywords: Breakwaters
Rubble-mound breakwaters
Wave action
Wave runup
Hydraulic structures
Coastal structures
Cover layers
Stone armor
Issue Date: Jul-1958
Publisher: U.S. Army Engineer Waterways Experiment Station.
Engineer Research and Development Center (U.S.)
Series/Report no.: Research report (U.S. Army Engineer Waterways Experiment Station) ; no. 2-2.
Description: Research Report
Summary: The need for fundamental data for use in designing rubble-mound breakwaters led to a laboratory investigation to develop design criteria; the first phase of this investigation, which dealt with the design of quarry-stone cover layers, is described in this report. For the laboratory tests, small-scale rubble-mound breakwater sections are hand-constructed in a concrete flume 119 ft long, 5 ft wide, and 4 ft deep, and subjected to mechanically generated waves. The limit of stability of the armor units forming the protective cover layer is determined as a function of the wave dimensions, slope of exposed breakwater face, weight and specific weight of the armor units, and specific weight of the water in which the test sections are situated. To date pertinent relations between these variables have been determined for the condition of incipient instability, using armor units simulating quarry stones of irregular shape with rounded edges and a fairly smooth surface. Tests have also been conducted in which damage to the quarry-stone-type cover layer was determined as a function of wave height; in these tests waves with heights greater than those corresponding to incipient instability were used. A general stability equation was first derived, based on a rational analysis of the forces exerted on armor units when waves impinge on rubble-mound breakwaters, and used to guide the experimental program and correlate the test data. Test results indicate that the assumptions upon which analysis of the phenomenon was based are essentially correct. Using the experimental data to determine the unknown functions in the derived equation, a new formula was obtained for the weight of armor units required to insure their stability when used as the cover layer of rubble-mound breakwaters. The new formula is dimensionally homogeneous and contains only one experimental coefficient. The test data obtained to date indicate that the experimental coefficient varies primarily with the shape of armor unit and the amount of damage to the cover layer. For the quarry-stone-type armor unit, and for the no-damage and no-overtopping criteria, the experimental coefficient is constant. In conjunction with the stability tests, wave run-up data were obtained for each rubble-mound test section and wave condition. It was found that the height to which waves will advance up the slope of breakwaters of the type tested is a function primarily of the breakwater slope and the steepness of the waves. Wave run-up was found to increase when the breakwater slope was steepened. A decrease in the wave steepness also caused an increase in wave run-up. Measurements to determine the average thickness and porosity of cover layers composed of quarry-stone-type armor units were also obtained in this phase of the investigation. The new breakwater stability formula, and the experimental data obtained to date, provide the essential information for an improved method of designing rubble-mound breakwaters with cover layers composed of quarry stones placed pell-mell. It is believed that tests now in progress, in which similar experimental data are being obtained for other special shapes of armor units, will increase considerably the accuracy of rubble-mound breakwater design techniques.
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