Proceedings on the Third International Conference on the Application of Physical Modelling to Port and Coastal Protection 1 ANALYSIS OF OVERTOPPING DISCHARGE OVER VERTICAL TIERS - HEADED WALL IN A TANDEM BREAKWATER SYSTEM CORRADO ALTOMARE (1), LEONARDO DAMIANI (2) and XAVIER GIRONELLA (3) (1) Ph.D., Water Engineering and Chemistry Department (DIAC), Technical University of Bari, via E. Orabona, Bari, 70125, ITALY. c.altomare@poliba.it (2) Professor, Water Engineering and Chemistry Department (DIAC), Technical University of Bari, via E. Orabona, Bari, 70125, ITALY. l.damiani@poliba.it (3) Professor, CIIRC-LI Group, Universitat Politecnica de Catalunya (UPC), c/ Jordi Girona, 1-3, Edif. D1, Barcelona, 08034, Spain. xavi.gironella@upc.edu The semi-empirical methods are useful tools to understand the wave overtopping phenomena, but uncertainty remains on their applicability. It’s necessary to carry out campaign surveys or laboratory tests to support them. In this paper we apply the methods proposed in the EurOtop Manual to predict the flows over a vertical breakwater, marking the need to calibrate them to take into account the peculiar geometry of the system. This was done by introducing appropriate correction factors. The results show an improved accuracy, compared with the two-dimensional physical model tests conducted at the LIM laboratory in Barcelona. Keywords: overtopping; SEMI-EMPIRICAL MODELS; tandem breakwater. 1. Introduction The overtopping is a phenomenon of wave - structure interaction that consists of waves passing over the crest of the structure in the form of continuous sheet of water, plumes or splash, depending on the seaward geometry of the structure. The overtopping occurs for a given value of wave height and depends mainly on the freeboard of the structure as well as of the sea state. It is an extremely complex and no uniform phenomenon, set by different geometric – structural and hydrodynamic factors. The importance of this phenomenon is often undervalued by the final users, because of the sporadic occurrence of the event. In terms of risk the consequences can consist of different types: danger to pedestrian and vehicles, damage for seawalls, building, infrastructure and ships, internal residual agitation. From the available experimental data it is clear that it cannot describe the phenomenon in detail. Therefore the evaluation of the mean overtopping discharge q and the total overtopped volume V is considered sufficient for the proper design of coastal structures. The analysis of the applicability of the predictive methods to vertical seawalls is described in the present paper. In detail that seawall is characterized by a peculiar configuration: in the geometry of the structure itself and because of the its coupling with a submerged breakwater. The result of 2D model tests are shown: they suggested to adjust the methods in order to take into account that rare structural layout. 2. The predicting methods The complexity in assessing the overtopping explains the huge development of multiple approaches leading to the estimate of the phenomenon: numerical models (SPH, 2006; VOF, 2007; etc), empirical methods (Van der Meer et al. 1992, 2003, 2005, Franco 2005; etc), neural networks (Medina 1999, Medina et al, 2002) and physical model experiments. In the last years different European and International projects (VOWS 2000, OPTICREST 1998-2001, CLASH 2002-2005) led to the revisions or implementation of the mentioned above methodologies. These projects prove the growing interest in a more complete and accurate assessment of the overtopping, leading to the creation of global databases of information. This would take to development of new models and methods (European Overtopping Manual 2007) supported by laboratory test results and measurements in situ, also considering the effects present in scale model tests and the complexity of the factors that affect the phenomenon in reality (wind, fluid viscosity, etc.). 2.1 The European Overtopping Manual (2007) The European Overtopping Manual is perhaps today's most advanced tool in the estimation of wave overtopping, as the formulations contained in it and the proposed tools (PC-overtopping and Neural Network) have been calibrated on data from over 10,000 test performed worldwide and collected in the database of the European CLASH project (2002-