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Title: Erosion control of scour during construction. Report 5, Experimental measurements of refraction and diffraction downcoast of an oblique breakwater
Authors: Coastal Engineering Research Center (U.S.)
Hales, Lyndell Z.
Keywords: Scour
Hydraulic structures
Construction projects
Water waves
Shore protection
Publisher: Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical report (U.S. Army Engineer Waterways Experiment Station) ; HL-80-3 rept. 5.
Description: Technical Report
Abstract: When major stone structures are erected in the coastal zone, they alter currents that are in dynamic equilibrium with the existing conditions. Waves breaking on the structures may cause bottom material to be suspended and transported from the region. The result is scour, or erosion, that usually develops along the toe of the structure. To minimize potential cost increases due to scour during construction, it is necessary to quantify the probability and ultimate extent of potential scour during the scheduled construction period. Objectives of this research program are to develop techniques to minimize and control scour during nearshore construction, and to predict the probable magnitude of scour that may result as a function of currents and wave climate. One phase of the research program is development of numerical techniques (incorporating both refraction and diffraction effects near the structure) for computing wave-induced velocities, tidal currents, and wave heights in the vicinity of structures, and applying these results to determine sediment transport of the bottom material at the particular site. The present state of nearshore current and wave theories has reached the point where detailed experimental investigations are required for the verification of analytical developments and numerical models. To provide a foundation for further advancements, a simple beach profile consisting of straight, uniform contours parallel with the shoreline was experimentally studied by Hales (1980). A shore-connected, vertical, thin, impermeable barrier (breakwater) was installed perpendicular to the shoreline to simulate prototype jetties and breakwaters commonly occurring along many coasts. The purpose of the present study is to extend the previous work of Hales (1980) by installing a shore-connected, vertical, thin, impermeable breakwater at a 60-deg angle to the shoreline to simulate a larger range of prototype jetties and breakwaters in existence at the present time. Experimental measurements of refraction and diffraction downcoast of this oblique structure were made to obtain quantitative knowledge of this phenomenon in the lee of the jetty or shore-connected breakwater. These data were then compared with the uniformly valid asymptotic theory of Liu, Lozano, and Pantazaras (1979) for the same arrangement. The numerical model for determining wave heights downcoast of a straight breakwater at an angle to the shoreline under combined refraction and diffraction, based on the uniformly valid asymptotic theory, was obtained by contract with Dr. Philip Liu, Cornell University. Because the uniformly valid asymptotic theory is developed from the small amplitude wave assumptions, the effect of varying the incident wave height on the wave-height amplification factor, H/Hₒ, was investigated. The theory and experimental data were found to compare favorably for all wave heights tested in the shadow zone, but diverged with increase in incident wave height farther downcoast out of the shadow zone. While the uniformly valid asymptotic theory is far superior to diffraction theory alone under these conditions, additional numerical work should incorporate nonlinear wave theories for completeness.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Report

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