Acta Sci. Pol. Formatio Circumiectus 17 (3) 2018, 55–65 ENVIRONMENTAL PROCESSES www.formatiocircumiectus.actapol.net/pl/ ISSN 1644-0765 DOI: http://dx.doi.org/10.15576/ASP.FC/2018.17.3.55 ORIGINAL PAPER Accepted: FREQUENCY AND STRENGTH OF STORM SURGES IN THE ODER RIVER ESTUARY AREA Halina Kowalewska-Kalkowska Department of Earth Sciences, University of Szczecin, ul. Mickiewicza 18, Szczecin ABSTRACT The aim of the study was to estimate the frequency and strength of storm surges in the Oder River estuary area during the 1993/94–2016/17 seasons. The analyses involved water level readings recorded at gauges located in the Oder River estuary area, including the coasts of the Pomeranian Bay (Zatoka Pomorska) and the Szczecin Lagoon (Zalew Szczeciński), as well as the downstream reach of the Oder. Recognition of temporal and spatial water level variations in the region under investigation revealed significant irregulari- ties in storm surge occurrence; periods with an increased frequency of freshets followed long periods with their scant occurrence. During the year, most of the storm events were observed between November and January. In the period discussed, the most severe and extensive storm surge in the Oder River estuary area was observed in October 2009. Very high freshets were recorded in November 1995, January 2012, and January 2017. The long-lasting and extensive high water events in the downstream reach of Oder River resulted from the progressive and prolonged increase in sea level in the Pomeranian Bay. Long-term per- sistence of high water levels in the Oder River estuary area were also registered when storm surges limited the outflow of the Oder River during snow-melt events, ice jams developing on the lower Oder at the same time. Finally, severe storm events were observed under the condition of the increased water volume in the Baltic Sea. Keywords: storm surges, water level, water regime, Oder River estuary area INTRODUCTION ter level was recorded during the freshet on 9–10 Feb- ruary 1874, when the sea level in Świnoujście reached The Pomeranian Bay, with its small depths and a var- 696 cm, that is 196 cm above the average level therein ied shoreline, belongs to those Baltic areas that are (Richter in. 2012). The sea level changes occurring particularly vulnerable to storm surges (Majewski et during storm surges entail an increase in the water lev- al. 1983). The main factors affecting the character of el of the Szczecin Lagoon, and then in the river net- freshets include the vector wind field, the distribution work of the lower course of the Oder River. The high- of atmospheric pressure over the sea, and the filling est freshet on the Szczecin Lagoon was recorded on of the Baltic Sea (Hünicke et al. 2015, Wiśniewska 30–31 December 1913, when the level of 637 cm was 1979). The most dangerous surge events of the 20th registered in Trzebież (Majewski 1980). In Szczecin, century on the southern shores of the Pomeranian Bay the maximum water level of 680 cm was recorded on were associated with low-pressure systems shifting 7 March 1850 during a storm surge, which occurred over the Baltic Sea from the Norwegian Sea towards during an increased supply from the Oder River catch- the south-east (Sztobryn et al. 2005). The highest wa- ment (Borówka 2002). e-mail: © Copyright by Wydawnictwo Uniwersytetu Rolniczego w Krakowie, Kraków 2018 Kowalewska-Kalkowska, H. (2018). Frequency and strength of storm surges in the Oder River estuary area. Acta Sci. Pol., Formatio Circumiectus, 18(3), 55–65. DOI: Due to the low capacity of the straits connecting In the first stage of the analysis, the general charac- the Szczecin Lagoon with the Pomeranian Bay, chang- terization of freshets was developed in terms of the es in the water level during flood surges occur with frequency of their occurrence in the annual cycle and some delay in time, depending largely on the course over the multi-year period, their extreme values and​​ of sea level changes (Majewski 1980, Robakiewicz their duration, and then the transformation up towards 1993, Kowalewska-Kalkowska 2012). In the river the lower reaches of the Oder River. In the second network of the lower Oder, the pace of upward tidal stage, based on hourly information on water levels in wave movement and freshet intensity depends largely the seasons from 2010/11 to 2016/17, the impact of on the atmospheric conditions, specifically wind speed water inflow from the Oder River catchment and the and direction, and on the changes in the atmospher- degree of filling of the Baltic Sea on the height, range ic pressure (Buchholz 1990, 1991, Ewertowski 2000, and duration of selected storm surges in the Szczecin Kowalewska-Kalkowska and Wiśniewski 2009). The Lagoon and in the lower reaches was examined. In flow rate in the Oder River also influences the degree order to detect the correlations between water levels of flood wave suppression; the higher the flow, the read on water gauges located in the Oder River es- lower the wave height reaching the Oder (Ewertowski tuary region, correlation and cluster analyses were 1992, Kowalewska-Kalkowska 2012). The aim of the applied. In cluster analysis, the grouping procedure research presented in this work was to estimate the frequency and intensity of storm surges in the Oder River’s estuary region in the seasons between 1993/94 and 2016/17. MATERIAL AND METHODS The analysis of storm surges was carried out based on water level readings from water gauges located in the Oder River estuary region (see: Figure 1) in the seasons from 1993/94 to 2016/17, designated after Sz- tobryn et al. (2005) from August to July of the follow- ing year. The collected material included daily water level readings in the lower reaches of the Oder River (in Gozdowice, Bielinek, Widuchowa, Gryfino, Pod- juchy (Szczecin Podjuchy) and Szczecin (Szczecin Most Długi), obtained from the IMGW-PIB website (https://dane.imgw.pl) and the RZGW in Szczecin. Information about water levels in the Szczecin La- goon (in Trzebież) and off the coast of the Pomera- nian Bay (in Świnoujście) was provided courtesy of the Maritime Authority in Szczecin In the Port Logs in Świnoujście, hydrological data were recorded every hour, while in Szczecin and Trzebież, it was recorded every four hours. Since May 2010, it has been possible to obtain hourly information on water levels from the entire Oder River estuary region from the IMGW-PIB website (http://monitor.pogodynka.pl). Storm surges were analysed, during which the emergency state (H ≥ 580 cm) was exceeded at the Fig. 1. The Oder River estuary area with the marked loca- southern coast of the Pomeranian Bay in Świnoujście. tions of water level stations 56 www.formatiocircumiectus.actapol.net/pl/ Kowalewska-Kalkowska, H. (2018). Frequency and strength of storm surges in the Oder River estuary area. Acta Sci. Pol., Formatio Circumiectus, 18(3), 55–65. DOI: was performed using the Ward method, whereas the lar regularities. The 2006/07 season, apart from the measure d (x, y) = 1 – R was adopted as a measure of highest number of floods, was also characterized by distance, where R is the Pearson correlation coeffi- the highest percentage of water levels equal or higher cient (Stanisz 2007). In order to present the variability than 580 cm (1.6%). The high share of such levels of water levels in the Oder River estuary region, the was also recorded in the 2016/17 season (0.9%), as graphs used standardized values (​​Z), showing the de- well as 1995/96 and 2001/02 (0.8%), and 2011/12 viation of x variable from its expected value of μ, ex- seasons (0.7%). As a result of correlating average an- pressed in units of standard deviation σ (Balicki and nual water levels in Świnoujście with the frequency Makać 2000). of water levels equal to or above 580 cm, a statisti- cally significant correlation was obtained: α = 0.002 RESULTS AND DISCUSSION with the correlation coefficient R = 0.60, indicating that the annual water levels would explain 36% of the Characteristics of storm surges in the coastal variation in the frequency of water levels equal to or areas of Pomeranian Bay higher than 580 cm. In the seasons from 1993/94 to 2016/17, in the re- In the course of the year, storm surges, during gion off the coast of the Pomeranian Bay, there which the sea level in Świnoujście exceeded the were 72 storm surges, during which the sea level in alarm condition, were recorded from September to Świnoujście reached or exceeded the emergency level April (see: Figure 2). The exception was the surge at (H ≥ 580 cm). During 32 swells, the sea level reached the turn of August and September 1995. Most storm at least 600 cm. The highest freshet was registered on surges were recorded between November and January 3-4 November 1995, when the level of 683 cm was (48 high surges). Most often, the surges appeared in recorded in the Port Log in Świnoujście. According January (22 freshets, of which 11 were observed at the to the IMGW data, the culmination of this surge in level of at least 600 cm). In the month of January, the Świnoujście amounted to 669 cm (Sztobryn et al. incidence of water levels equal to or above 580 cm 2005). The second highest surge was the flood of 4–6 reached 1.5%, and those of at least 600 cm, 0.6%. January 2017, during which the sea level in Świnou- The second month in terms of the incidence of surges jście reached 653 cm.