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Title: Design of cover layers for rubble-mound breakwaters subjected to nonbreaking waves; hydraulic laboratory investigation
Authors: Jackson, R. A. (Robert Atkins), 1908-
Keywords: Breakwaters
Rubble-mound breakwaters
Water waves experiments
Hydraulic structures
Coastal structures
Stone armor
Cover layers
Issue Date: Jun-1968
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. 2-11.
Description: Technical Memorandum
Abstract: 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 smooth quarrystone cover layers on breakwater trunks in depths of water sufficient to prevent the breaking of waves due to depth limitation, was described in a previous report by Hudson. The present report is concerned with the design of rubble-mound breakwaters constructed of rough and smooth quarrystones, quadripods, tetrapods, hexapods, tribars, modified cubes, and truncated tetrahedrons. For the tests described herein, the layered type of construction was used for both breakwater trunks and breakwater heads. The small-scal e rubble-mound breakwater sections were hand constructed in concrete flumes 119 ft long, 5 and 12 .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 was determined as a function of the shape of the armor unit, method of placing the armor unit, thickness of the armor unit cover layer, 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 were situated. For all tests conducted, the water depth was sufficient to prevent the waves from breaking due to depth limitation. The pertinent relations between the above-mentioned variables were determined for the condition of incipient instability. Tests were also conducted in which damage to the cover layers was determined as a function of wave height. In these tests wave heights greater than those corresponding to incipient instability were used. The general stability equation, based on a rational analysis of the forces exerted on armor units when waves impinge on rubble-mound breakwaters and the results of small-scale stability tests, was used to guide the experimental program and correlate the test data for the present study. Test results indicate that the assumptions upon which analysis of the phenomena was based are essentially correct. The experimental data were used to determine the unknown functions in the derived equation, which is dimensionally homogeneous and contains one experimental coefficient. The test data obtained to date indicate that the experimental coefficient varies primarily with shape of armor unit, thickness of armor layer, method of armor unit placement, type of breakwater element (head or trunk section), and the amount of damage that can be allowed to occur to the cover layer. For a given type of breakwater element, armor unit, and method of placement, and for the no-damage and no-overtopping criteria, the experimental coefficient is practically constant. The experimental coefficient for breakwater head sections is less than the corresponding coefficient for breakwater trunk sections. Wave runup data were obtained for each rubble-mound test section and wave condition; wave rundown data were obtained for some of the rubblemound test sections and wave conditions. It was found that the elevation to which waves will advance up the slope of rubble breakwaters of the types tested and the elevation to which waves withdraw down the slope of the breakwater are primarily a function of the breakwater slope and the steepness of the waves . When the breakwater slope was steepened, wave runup and wave rundown increased; when the wave steepness decreased, wave runup and wave rundown increased. Measurements were made to determine the average thickness and percentage of voids for all types of armor units used in this investigation. NOTE: This file is large. Allow your browser several minutes to download the file.
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