Changes in Hydrographic Conditions Associated with 2005–2007 Storm Surges in the Odra Mouth – a Numerical Study H. Kowalewska-Kalkowska 1 and M. Kowalewski 2 1 Institute of Marine Sciences, University of Szczecin Wąska 13 Street, 71-415 Szczecin, Poland 2 Institute of Oceanography, University of Gdańsk Piłsudskiego 46 Avenue, 81-378 Gdynia, Poland This work was supported by the Ministry of Science and Higher Education under the research project “Impact of storm surges along the southern Baltic coast on hydrodynamic conditions in the Odra river mouth area” (grant No. N306 028 31/1644). Abstract-A pre-operational hydrodynamic model of the Baltic Sea (M3D_UG), developed at the Institute of Oceanography, University of Gdańsk was used to describe changes in hydrodynamic conditions in the Odra mouth (the southern Baltic Sea) during storm surges that occurred in the years 2005–2007. The model was based on the coastal ocean circulation model known as the Princeton Ocean Model (POM). Because of wind-driven water backflow in the Odra mouth, a simplified operational model of river discharge, based on water budget in a stream channel, was developed. Linking these two models into a single system made it possible to forecast water levels as well as water physical features in the Szczecin Lagoon and the Pomeranian Bay. Evaluation of the model’s performance for two simulations that were carried out showed a good fit between the observed and computed data. The model correctly reflected events involving high-amplitude and rapid water level fluctuations as well as fast changes of physical properties of the water; it also generated relatively good flow simulations. I. INTRODUCTION The Odra mouth area (a complex structure encompassing the lower Odra branches, the Szczecin Lagoon, and the inshore Pomeranian Bay in the southern Baltic Sea off the Lagoon’s discharge), is a site of fast changes of physical properties of the water and rapid water level fluctuations. The Odra River opens first into the Szczecin Lagoon, a coastal semi-enclosed water body of about 680 km 2 surface area and 3.8 m in mean depth, intersected by the 10–11 m deep fairway (Fig.1). Then it drains into the shallow Pomeranian Bay via three narrow straits: the Świna, the Dziwna, and the Peenestrom. In the Bay, an area not deeper than 20 m, the spread of the lagoon’s waters depends mostly on wind conditions [1], [2]. In the Szczecin Lagoon and in the lower Odra channels water levels are strongly affected by sea level fluctuations. Particularly during autumn and winter storm surges associated with strong winds from northern sector, when the sea level in the Bay is higher than that in the Lagoon, the Bay’s brackish water enters the Lagoon and raises the water level both there and in the Lower Odra channels. As a result of a very low gradient of the Lower Odra River channels, the wind-driven water back-up, forming in the Odra mouth, reaches as high upriver as to Gozdowice (160 km south from the sea), as shown in [3]. The influx also affects the Lagoon’s physical and chemical characteristics. Storm surges at the Baltic coasts occur as a result of the passage of a deep and intensive low-pressure system over the Baltic Sea. The sea level fluctuations in question result from the combined effects of persistent wind over a shallow body of water and changes in atmospheric pressure on the sea surface. They produce flooding events of coastal areas, impact shore and beach stability, result in coastal erosion, negatively affect port operations and navigation and impinge on the coastal zone infrastructure. A complex nature of meteorological effects renders forecasting sea level fluctuations very difficult, particularly in the coastal zone where influences of additional factors should be taken into account as well. Hence over the recent years, sea level fluctuations along the Baltic coast have been in the focus of numerous studies. The 2D hydrodynamic model was used for describing extreme sea levels in the Gulf of Riga and the Väinameri Sea [4], [5], [6]. In the Gulf of Finland extreme sea levels were studied with the CARDINAL numerical model [7]. The High Resolution Operational Model for the Baltic Sea (HIROMB) was developed at the Bundesamt für Seeschifffahrt und Hydrographie (BSH) in Hamburg (Germany) and subsequently extended in cooperation with the Swedish Meteorological and Hydrological Institute (SMHI) in Norrköping (Sweden) [8], [9]. It is now run operationally by the SMHI (http://www.smhi.se/oceanografi/oce_info_data/models/hiromb.htm). The Maritime Institute in Gdańsk (Poland) extended the model onto the Polish zone of the Baltic [10]. At present, the Maritime Branch of the Institute of 978-1-4244-2268-5/08/$25.00 ©2008 IEEE