Modeling Water Exchange in the Oder River Mouth Area

Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Vol. XXXVI, No.1 Institute of Oceanography (55-67) University of Gdańsk 2007

DOI 10.2478/v10009-007-0002-9 Received: July 01, 2006 Research Paper Accepted: February 13, 2007

Modeling water exchange in the Oder River mouth area

Halina Kowalewska-Kalkowska1, Marek Kowalewski2

1Institute of Marine Sciences, University of Szczecin ul. Wąska 13, 71-415 Szczecin, Poland 2Institute of Oceanography, University of Gdańsk al. Marszałka J. Piłsudskiego 46, 81-378 Gdynia, Poland

Key words: numerical modeling, water exchange, Oder River mouth

Abstract

A three-dimensional operational hydrodynamic model of the Baltic Sea (M3D_UG) developed based on the Princeton Ocean Model (POM) was applied to model water exchange in the Oder River mouth area. Due to wind-driven back flow in the Oder mouth, a simplified operational model of river discharge was also developed based on the water budget in a stream channel. Linking the Oder discharge and Baltic Sea models into a single system allowed simulating hydrodynamic conditions in the Szczecin Lagoon and the Pomeranian Bay. Since the model adequately approximates hydrodynamic variability, it is a reliable tool for modeling water exchange in the Oder River mouth area and for assessing Oder water spread in the Baltic Sea.

1 Corresponding author: [email protected]

56 H. Kowalewska-Kalkowska, M. Kowalewski

INTRODUCTION

Situated in the southern Baltic Sea, the Oder River mouth is an area where fresh and brackish waters mix. In its downstream reaches, the Oder River discharges into the Szczecin Lagoon, which is a coastal water body of approximately 680 km2 with a mean depth of 3.8 m. It is intersected by a fairway 10-11 m deep. The Oder discharges first into the lagoon and eventually drains into the Pomeranian Bay, where depths do not exceed 20 m. The discharge proceeds via three straits: the Świna, the Dziwna, and the Peenestrom. Water exchange via the Świna accounts for three-quarters of the total exchange between the lagoon and the sea. This exchange usually proceeds as pulse-like inflows and outflows. During flooding events, however, the outflow from the lagoon into the Pomeranian Bay may become continuous (Majewski 1980, Buchholz 1991, Jasińska 1991, Mohrholz and Lass 1998). The heaviest impact on the Pomeranian Bay by Oder River discharge is observed in the straits and at the coast, while some eastward-trending asymmetry results from the domination of westerly winds in the area (Cyberski 1995). The water dynamics in the southern part of the Pomeranian Bay are governed mainly by the wind-driven Ekman current. During the prevalence of westerly winds, the coastal current transports lagoon water eastward in a narrow band along the coast of Wolin Island. During the prolonged domination of easterlies, newly discharged lagoon water spreads along the coast of Usedom Island (Lass et al. 2001). During fall and winter storm surges associated with very strong winds from the northern sector, the sea level in the Pomeranian Bay is higher than that in the Szczecin Lagoon and brackish bay water enters the lagoon raising the water level. The influx also affects the lagoon’s physical and chemical characteristics. Buchholz (1991) reported that due to the very low hydraulic slope of the Lower Oder channels, the wind-driven water back flow that forms in the Oder mouth extends as far upstream as Gozdowice. The complex hydrodynamic regime of the Oder mouth area has been described using various numerical models and has been presented in studies by Buchholz (1989), Ewertowski (1988, 2000), Jasińska (1991), Robakiewicz (1993), Mohrholz and Lass (1998), Jasińska and Robakiewicz (1999), Kałas et al. (2001), and Lass et al. (2001). The adequate approximation of hydrographic conditions in the Gulf of Gdańsk and the southern Baltic obtained using the three-dimensional hydrodynamic model of the Baltic Sea (M3D_UG) developed at the Institute of Oceanography, University of Gdańsk (Kowalewski 1997, 2002) provided the incentive for testing it in the Oder mouth area. A preliminary evaluation of the model’s performance was presented previously by Kowalewska-Kalkowska and Kowalewski (2004, 2005, 2006). The current

Modeling water exchange in the Oder River mouth area 57 paper discusses the application of the model in studies of water exchange in the Oder mouth area.

MODEL DESCRIPTION

The three-dimensional operational hydrodynamic model of the Baltic Sea (M3D_UG) developed at the Institute of Oceanography, University of Gdańsk is a baroclinic model that describes water circulation with due consideration of advection and diffusion processes. The model is based on the Princeton Ocean Model (POM) that is described in detail by Blumberg and Mellor (1987). Adapting the model to the Baltic Sea required certain changes in the numerical calculation algorithm, which are described in detail by Kowalewski (1997). The open boundary located between the Kattegat and Skagerrak was used to parameterize water exchange between the North and Baltic seas. A radiation boundary condition for vertically averaged flows was applied, and a monthly averaged vertical distribution of salinity and temperature gradient normal to the border and equal to zero were also assumed at the open boundary (Ołdakowski et al. 2005). The meteorological data necessary to operate the hydrodynamic model were obtained from the mesoscale weather model known as UMPL (Unified Model for Poland) developed by the Interdisciplinary Centre of Mathematical and Computational Modelling, University of Warsaw (Herman- Iżycki et al. 2002). Solar energy input was calculated based on astronomical data and meteorological conditions (Krężel 1997). Other components of the heat budget at the sea surface were taken from meteorological data and simulated sea surface temperatures (Jędrasik 1997). The model considers 153 riverine discharge events. In order to obtain adequate resolution and reliable output, two grids with different spatial spacing were applied: 5 nautical miles (NM) for the Baltic Sea and 0.5 NM for the region comprised of the Pomeranian Bay, Szczecin Lagoon, and Oder River up to the town of Police (Fig. 1). A numerical grid with sigma- transformation enabled vertical profiles at any point in the sea to be divided into 18 layers regardless of depth (Kowalewska-Kalkowska and Kowalewski 2005, Kowalewski and Ostrowski 2005). The individual layers differed in thickness from 8.3% of depth in the middle of the profile, 2.0% at the bottom, and 0.5% at the sea surface. The initial conditions for the hydrodynamic fields were adopted in accordance with their climatic distributions based on the Baltic Environmental Database at http://data.ecology.su.se/models/bed.htm. Since there is wind-driven water back flow in the Oder’s downstream reaches, a simplified operational model of river discharge was developed. This model is based on the water budget in a stream channel between two profiles for which current water level data are available; this is described in detail by

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58 H. Kowalewska-Kalkowska, M. Kowalewski

Kowalewska-Kalkowska and Kowalewski (2006). The discharge calculated can be entered into the model as the boundary condition for the next section of the river. The preceding procedure is first applied to the Gozdowic- Widuchowa section, then the discharge at Police is computed and used as the boundary condition in the hydrodynamic model of the Szczecin Lagoon. Discharge calculations are performed automatically based on water level data from gauging stations along the Lower Oder River; these are posted on the Institute of Meteorology and Water Management (IMGW) website at http://www.imgw.pl/. As far as short-term forecasts are concerned, they are derived for

Fig. 1. Modeled regions of the Baltic Police from the hydrodynamic Sea and Oder River mouth area with the model of the Baltic Sea. location of gauging stations marked. Forecasts for other stations come from calculations of multiple regression equations based on current water levels at the nearest station. Linking the Oder discharge and the Baltic Sea hydrodynamic models into a single system made it possible to simulate operationally hydrographic conditions in the Oder mouth area. Current results of the model can be viewed on the website at http://model.ocean.univ.gda.pl as maps of 60-hour forecasts of water levels, currents, water temperature, and salinity.

PERFORMANCE OF THE MODEL

Model performance was tested using the observed and calculated data sets for water level, currents, water temperature, and salinity in the southern part of the Pomeranian Bay, the Oder Bank, and the Szczecin Lagoon. Sets of data collected routinely (observed data) for 2002-2004 were obtained from the Szczecin-Świnoujście Harbor Master’s Office and from the websites of IMGM

Modeling water exchange in the Oder River mouth area 59

(Poland) and Bundesamt für Seeschifffahrt und Hydrographie (Germany). First, standard statistical parameters were calculated for a given set of observations and numerical simulations. Then, regression analysis and Student’s t tests for paired averages as well as the F test for two variances were run to evaluate the fit between empirical and predicted data series at a significance level of α=0.05. With respect to water level series, the correlation between the observed and predicted data sets was very high for both the Pomeranian Bay coastal gauging stations (Koserow and Świnoujście) and those of the lagoon (Trzebież and Ueckermünde). Although the best fit was seen in the 0-h forecast (correlation coefficients above 0.90), correlation coefficients higher than 0.81 for all the predictions were indicative of high statistical significance (Fig. 2). The highest correlation between the above variables was noted at the Trzebież gauging station where correlation coefficients ranged from 0.89 (a 60-h forecast) to 0.93 (a 0-h forecast). The correlation coefficients for the Pomeranian Bay stations

Fig. 2. Correlation coefficients between observed and predicted (0- to 60-h forecasts) water levels at individual stations. were somewhat lower. It should be mentioned that the fit between simulations and observations was comparable to that obtained for the readings from tide- gauges located along the southern Baltic as presented by Kowalewski (1998, 2002) and Jędrasik (2005), among others, as well as those obtained from the HIROMB model (Kałas et al. 2001). The best fit between average values was obtained for Ueckermünde where mean water levels were underestimated by 0.5 cm (0-h forecast) to 1.9 cm (60-h forecast). At other stations, the model overestimates averaged values by a few centimeters. The differences between averages may also result from the various leveling nets that are used in different countries as mentioned by Kałas et al. (2001). The modeled extreme values alternated with the observed ones, however, distributions of the relevant modeled and observed water levels assumed identical shapes at all the stations. The coefficients of variation ranged from 0.03 to 0.05 and indicated that higher

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60 H. Kowalewska-Kalkowska, M. Kowalewski differentiation is typical of data from the Pomeranian Bay stations in comparison to those for the lagoon. As already stated (Kowalewska-Kalkowska and Kowalewski 2005), the numerical data are only relative, which is to say that, due to the imperfection of the coastal conditions applied to the open boundary in the Skagerrak and Kattegat, it is difficult to render them comparable to the average sea level and to calculate the absolute sea levels, for example with reference to NN Amsterdam 1955. Hence, when the modeled and observed values are compared a certain constant is added to render the initial forecast and the observed values identical. A very good fit was also obtained for water temperature data. The correlation coefficients between the empirical and the numerical data sets, for both for the Pomeranian Bay and the lagoon stations, exceeded 0.98 and revealed high statistical significance. Moreover, the modeled mean values were underestimated by less than 1°C in relation to those calculated for the observed data. Higher variability was observed at coastal stations. The results obtained were similar to those for other areas of the southern Baltic and described by Kowalewski (1997, 2002) and Jędrasik (2005), among others. It was only in rare cases that modeled and observed temperatures differed by a few degrees; these differences could have been related to the meteorological forecast of the UMPL weather model that included the climatic sea temperature. Observed and calculated salinity values showed a poorer fit; however, the relevant correlation coefficients were statistically significant. For Oder Bank salinity at a depth of 3 m, the correlation coefficients ranged from 0.53 (60-h forecast) to 0.61 (0-h forecast). The model overestimated salinity values by an average of 1.0 PSU. Extreme values were overestimated as well. The model accurately reflected the seasonal salinity variations, although, in some cases, Pomeranian Bay salinity was overestimated by more than 1 PSU. The comparison of the present results with those obtained for the Gulf of Gdańsk (Kowalewski 1997, Jędrasik 2005) showed the fit between the calculated and measured distributions in this study to be somewhat poorer. The differences between observed and calculated salinity values could have been brought about by the initial conditions of hydrodynamic fields and the bathymetric map of the region being modeled.

APPLICATION OF THE MODEL TO A CASE STUDY OF WATER EXCHANGE IN THE ODER ESTUARY

The good fit between simulations and observations in the Oder mouth area provided the incentive for testing the applicability of the model for describing coastal processes such as fresh and saline water mixing and Oder River water spread in the Pomeranian Bay. Temporal variations of hydrodynamic conditions

Modeling water exchange in the Oder River mouth area 61 in the region, as approximated by the model, can be visualized for a case involving a long-term storm surge on March 6-16 2002. The storm surge resulted from the passage of a series of low-pressure systems over the Baltic Sea. Initially, a decrease in water level at the Pomeranian Bay coastal stations was observed on March 6 (484 cm in Świnoujście at 19:00). The agreement between the empirical minimum and the forecasts for March 6 in the timing and extent of the drop was good; however, the model slightly underestimated the drop (Fig. 3). Subsequently, the water level in Świnoujście was observed to increase rapidly up to 579 cm on March 7 at 15:00 as a result of the low- pressure center (999 hPa) shift over the Pomeranian Bay. The March 6 and 7 forecasts predicted the timing of the maximum highly accurately, although they underestimated it slightly. During the next two days, the ensuing drop in the sea level was accurately forecast by the model. Over the following days, temporal variations of sea level were also correctly reflected by the model. Some overestimates were produced only on March 10 from the March 9 and 10 predictions. Beginning on March 13, the sea level began to rise as a result of the advection of a new depression from the Atlantic Ocean over the southern Baltic; the maximum of 590 cm was reached on March 14 at 11:00. The forecasts from March 13 and 14 of this maximum for Świnoujście correctly approximated its timing and extent. The ensuing drop in the sea level over the following days was accurately mimicked by the model.

Fig.3. Observed and predicted water level changes in Świnoujście during the storm surge of March 6-16 2002.

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62 H. Kowalewska-Kalkowska, M. Kowalewski

During the storm surges discussed, the Szczecin Lagoon stations showed much weaker water level fluctuations that were delayed in time. At Trzebież, a drop in water level was recorded on March 6 at 23:00. The timing and extent of the minimum were very accurately reflected by the model (Fig. 4). Subsequently, the water level began to rise to reach a maximum of 565 cm on March 8 at 03:00 (with a delay of 12 hours with respect to the sea level maximum). The March 7 and 8 forecasts overestimated this maximum slightly. Over the following days, a drop in the water level was first recorded, but then it began to rise slowly on March 9. These water level fluctuations were reflected well by the model; however, the March 9 model results were overestimated. The accuracy of the maximum water level modeled at Trzebież was also good (580 cm on March 14 at 23:00, with a 12-hour delay with respect to the sea level maximum). During the next few days, the water level drop was accurately simulated by the model. Figure 5 illustrates the three-dimensional operational numerical model simulation of water level changes in the Pomeranian Bay and the Szczecin Lagoon during the March storm surge described.

Fig. 4. Observed and predicted water level changes in Trzebież during the storm surge of March 6-16 2002.

Other changes in the physical variables in the Pomeranian Bay and the Szczecin Lagoon were also observed during the March 2002 storm surge. Due to the prevalence of westerly winds in early March, the lagoon water spread eastward along the coast of Wolin Island. Although salinity simulations accurately reflected the extent of the eastward spread of lagoon water, the model usually overestimated the true (measured) values in the bay by about 1.0- 2.0 PSU (Fig. 6). On March 8, the reversed slope of the water free surface

Modeling water exchange in the Oder River mouth area 63

Fig. 5. Spatial distribution of water level during the March 2002 storm surge, simulated with the M3D_UG model (in m, relative to mean water level).

Fig. 6. Spatial distribution of salinity (PSU) during the March 2002 storm surge, simulated with the M3D_UG operational model.

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64 H. Kowalewska-Kalkowska, M. Kowalewski resulted in the intrusion of brackish water from the Pomeranian Bay into the Szczecin Lagoon. Salinity simulations from March 8 clearly reflected the inflow by showing the presence of saline water in the northern part of the lagoon. During the next few days, the water level drop in the bay and the prolonged prevalence of the westerlies resulted in the discharge of lagoon water into the bay and an eastward spread of this water along Wolin Island. However, the storm-driven change of wind direction to the northwest-northeast forced a sea water intrusion into the Szczecin Lagoon again, which was well approximated by the model in the simulations of March 14. Finally, the change of wind direction to the southeast on March 14 and the ensuing sea level decrease at the Pomeranian Bay coast over the following days resulted in the discharge of lagoon water into the bay and the spread of this water northeastward off the bay’s coast. On the other hand, the water temperature simulations reflected a typical spring pattern with continuous water warming and the increasing impact of warmer fresh Oder River water that first entered the Szczecin Lagoon and was then discharged to the Pomeranian Bay via the Świna, Dziwna, and Peenestrom (Fig. 7.). The March 14 water temperature simulations also showed an intrusion of colder brackish waters into the lagoon. The simulated water temperatures in the Pomeranian Bay and Szczecin Lagoon during the storm were an average of approximately 0.6°C lower than the measured values.

Fig. 7. Spatial distribution of water temperature (°C) during the March 2002 storm surge, simulated with the M3D_UG model.

Modeling water exchange in the Oder River mouth area 65

CONCLUSIONS

The three-dimensional hydrodynamic model of the Baltic Sea (M3D_UG) has been generating time series of water levels, currents, water temperature, and salinity for the southern Baltic Sea and the Gulf of Gdańsk since 1999. Linking the Oder discharge and the Baltic Sea models into a single system made it also possible to operationally simulate hydrographic conditions for the Pomeranian Bay and the Szczecin Lagoon. Current results of the model are posted daily on the University of Gdańsk website at http://model.ocean.univ.gda.pl as maps of 60-hour forecasts of water level, currents, water temperature, and salinity for the Pomeranian Bay and the Szczecin Lagoon. The evaluation of the model’s performance in the Oder mouth area showed a good fit between modeled and observed distributions of water levels, temperatures, and salinities during a three-year period (2002-2004). The modeled water levels and temperatures produced a very good fit, while a somewhat poorer fit was obtained for salinities. The model reflects correctly the hydrographic conditions and seasonal variability of the variables analyzed and also generates relatively good flow simulations. The adequate approximation of hydrographic conditions in the Pomeranian Bay and Szczecin Lagoon generated by the model means that it is a reliable tool for studying coastal processes such as the mixing of fresh and saline waters in the Oder mouth area, the spread of Oder water in the coastal zone of the Pomeranian Bay, and storm surges. A better fit between simulations and observations can be achieved by including instantaneous salinity and temperature. Using a more accurate bathymetric map of the Pomeranian Bay coastal zone and applying data provided by satellite thermal images and measurement buoys may further improve the prognostic reliability of the model.

ACKNOWLEDGEMENT

We would like to thank Dr. Teresa Radziejewska of the Institute of Marine Science, University of Szczecin for assistance in preparing the English version of the manuscript. The work was supported by the Ministry of Science and Higher Education under the research project Impact of storm surges at the southern Baltic coast on hydrodynamic conditions in the Oder mouth area (grant no. N306 028 31/1644). The information contained in this paper was presented during a scientific conference “Water exchange in estuarine waters in the Southern Baltic - hydrology and ecology” organized by the Institute of Oceanography, University of Gdansk on April 21, 2006. The conference was dedicated to Professor Jerzy

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66 H. Kowalewska-Kalkowska, M. Kowalewski

Cyberski in celebration of his scientific career which has spanned forty-five years.

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