Mobile Breakwater Study Interim Report

Abstract

The Interim Report of October, 1951, presented a survey of the surface barrier studies which had been conducted by the Hydraulic Structures Laboratory up to that date. The performance curves of a number of types of mobile breakwaters were examined, and the decision reached that the most satisfactory one, all factors considered, was the three-bulkhead structure. After investigating the effect of such parameters as freeboard height, bottom clearance, and bulkhead spacing on the overall behavior of the barrier, a scale model of a hypothetical prototype pontoon assembly was constructed which incorporated what appeared to be the most effective values of these parameters. Performance data of this so-called optimum breakwater was given in the October report. The present report continues where the previous one left off, with a more intensive consideration of certain features of barrier performance which have been but vaguely understood. Specifically, the values of the coefficient of transmission, defined as the ratio of transmitted wave height to incident wave height, were determined under various controlled wave conditions for the following bodies: (1) Fixed single bulkheads of different bottom clearances. (2) Fixed three-bulkhead barrier. (3) Floating three-bulkhead barrier with fixed baffle extending upward from the bottom. (4) Floating barrier with weighted mooring lines. (5) Floating barrier with increased virtual mass on the end bulkheads. (6) Floating barrier with hydrofoil added forward of first bulkhead. In general, these conditions imposed upon the barrier were artificial, and impractical, as far as direct application to a prototype structure is concerned. Some information was gained, however, by separating to a limited degree the various factors which influence energy transmission by the floating breakwater. Unless specified otherwise, the floating barrier referred to in this report is the optimum structure with three bulkheads spaced at 73 and 127 feet, high freeboard, and 15-foot, 5-foot and 15-foot bottom clearances. The original performance curve of this body, obtained using soft springs in the mooring lines, is the second curve of Fig.8 in the October report, and is used frequently as a convenient basis of comparison for the data obtained in the present series of tests.