EXPERIMENTAL STUDY OF A MULTIPLE-ROW PILE BREAKWATER THEOHARRIS KOFTIS (1) , PANAYOTIS PRINOS (2) & MICHAEL AFTIAS (3) (1) Research Associate, Department of Civil Engineering, Hydraulics Laboratory, Aristotle University of Thessaloniki, AUTh, Thessaloniki, 54124, Greece. thkoftis@civil.auth.gr (2) Professor, Department of Civil Engineering, Hydraulics Laboratory, Aristotle University of Thessaloniki, AUTh, Thessaloniki, 54124, Greece. prinosp@civil.auth.gr (3) Ydronomi Consulting Engineers, Evias 32, Vlilisia, Athens, 15235, Greece. m.aftias@ydronomi.com Abstract The efficiency of a perforated breakwater that consists of rows of cylindrical piles is investigated experimentally in a wave flume. The effect of the structure's dimensions and the relative water depth on the transmission coefficient Ct is studied under regular water waves. The results show that the efficiency of the breakwater increases with the increase of the structure's width and freeboard and that the proposed structure has comparable efficiency with that of a conventional rubble mound Low Crested Structure. 1. Introduction The use of perforated breakwaters consisted of piles has been widely studied recently, as an alternative solution to conventional breakwaters, for coastal protection under mild wave conditions. The smaller construction costs and less environmental impacts compared with conventional gravity-type breakwaters as well as environmental requirements, for instance phenomena of intense shore erosion, water quality and aesthetic considerations advocate the application of such structures. Studies on such perforated breakwaters can be found in the recent literature such as a single-row pile breakwater that is consisted from a row of rectangular piles (Zhu, 2011), a multiple-row curtain wall-pile breakwaters (Ji and Suh, 2010), a partially perforated-wall caisson breakwater (Suh et al, 2006) and a perforated wall with vertical slits (Suh et al, 2011). The main goal of the present study is to evaluate the efficiency of such a perforated breakwater for wave attenuation and coastal protection of Agios Theodoros Mazotos beach, located in Cyprus in central south Cyprus. The beach is approximately 1,5 km and suffers from severe erosion; the maximum retreat that has been documented from 1973 to 2003 is of 23 m. The satellite image shown in Figure 1 gives an indication of the erosion as the present coastline can be compared to the 1973 coastline. The main idea is to propose an easy to install perforated breakwater with the following features:  is composed of industrial low cost elements. e.g. prefabricated concrete pipes  is easy and quick to install and/or remove  has less environmental impact compared with conventional rubble-mound breakwaters  partial attenuates wave energy  has small impact on the aesthetic of the beach and on the water quality