Abstract

This paper outlines the experimental study on the wave attenuation characteristic of multiple plates with wide spacing guided by Bragg resonance condition. One array of multiple plates was fixed horizontally on still free surface, the other penetrated vertically through free surface. The spacing was kept equal for all model tests. A series experiments was conducted with various immerged depth, plate numbers and wave steepness. The free surface elevation was measured by wave gauges. The variation of the transmission coefficient K versus dimensionless parameter 2S L were presented. The results show that there are lower transmission coefficients while the wavelength is approximately twice the plate spacing.

Introduction

The breakwater plays an important role in wave obstruction and structure protection for seaboard, port, quay and some marine engineering projects. In recent years, free surface breakwater, also known as an open type breakwater, including permeable breakwater and floating breakwater, has aroused great interest of coastal and ocean engineering reseachers. Its main structure locates near free surface where the wave energy is concentrated, and the effect of water circulation and coastal erosion is relatively low it also could be applicable for deep water field and soft geological seabed. Free surface breakwater is quite effective when the environment wave period does not exceed 5 seconds (Teh, 2013). But it is rather difficult for free surface breakwater to attenuate a 6~12s long period wave which generally appears in the operation sea environment. Therefore, the purpose of this paper is to find an effective way to obstruct such a long period wave to protect costal and ocean engineering structures.

Bragg reflection resonance phenomenon is firstly known as to analyze the crystallographic structure with x-ray (Mei, 2005). In costal engineering, Heathershaw (1982) first presented that Bragg resonance occurred in the interaction of surface waves with submerged bars by experimental researches using small amplitude monochromatic waves. The incident waves are reflected significantly while the incident wavelength is twice the bedform wavelengths. Mei (1985) investigated the resonant reflection of surface water waves by periodic sandbars applying linearized shallow-water theory. After that, many scholars(Zeng (2013), Zhang (2012), Hsu (2007)) researched the interaction of submerged breakwaters with water wave based on Bragg resanance by numerical or experimental method, and discussed effects of different parameters, such as shape, number, height, breath and spacing of the submerged breakwater, on the condition and intensity of resonant reflection.

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