Frequency and strength oF storm surges in the oder river estuary area

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 irregularities 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 persistence 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.


introduction
The Pomeranian Bay, with its small depths and a varied shoreline, belongs to those Baltic areas that are particularly vulnerable to storm surges (Majewski et al. 1983).The main factors affecting the character of freshets include the vector wind field, the distribution of atmospheric pressure over the sea, and the filling of the Baltic Sea (Hünicke et al. 2015, Wiśniewska 1979).The most dangerous surge events of the 20th century on the southern shores of the Pomeranian Bay were associated with low-pressure systems shifting over the Baltic Sea from the Norwegian Sea towards the south-east (Sztobryn et al. 2005).The highest wa-ter level was recorded during the freshet on 9-10 February 1874, when the sea level in Świnoujście reached 696 cm, that is 196 cm above the average level therein (Richter in. 2012).The sea level changes occurring during storm surges entail an increase in the water level of the Szczecin Lagoon, and then in the river network of the lower course of the Oder River.The highest freshet on the Szczecin Lagoon was recorded on 30-31 December 1913, when the level of 637 cm was registered in Trzebież (Majewski 1980).In Szczecin, the maximum water level of 680 cm was recorded on 7 March 1850 during a storm surge, which occurred during an increased supply from the Oder River catchment (Borówka 2002).
Due to the low capacity of the straits connecting the Szczecin Lagoon with the Pomeranian Bay, changes in the water level during flood surges occur with some delay in time, depending largely on the course of sea level changes (Majewski 1980, Robakiewicz 1993, Kowalewska-Kalkowska 2012).In the river network of the lower Oder, the pace of upward tidal wave movement and freshet intensity depends largely on the atmospheric conditions, specifically wind speed and direction, and on the changes in the atmospheric pressure (Buchholz 1990, 1991, Ewertowski 2000, Kowalewska-Kalkowska and Wiśniewski 2009).The flow rate in the Oder River also influences the degree of flood wave suppression; the higher the flow, the lower the wave height reaching the Oder (Ewertowski 1992, Kowalewska-Kalkowska 2012).The aim of the 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 Sztobryn et al. (2005) from August to July of the following year.The collected material included daily water level readings in the lower reaches of the Oder River (in Gozdowice, Bielinek, Widuchowa, Gryfino, Podjuchy (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 Lagoon (in Trzebież) and off the coast of the Pomeranian 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 southern coast of the Pomeranian Bay in Świnoujście.
In the first stage of the analysis, the general characterization of freshets was developed in terms of the frequency of their occurrence in the annual cycle and over the multi-year period, their extreme values and their duration, and then the transformation up towards the lower reaches of the Oder River.In the second stage, based on hourly information on water levels in the seasons from 2010/11 to 2016/17, the impact of water inflow from the Oder River catchment and the degree of filling of the Baltic Sea on the height, range and duration of selected storm surges in the Szczecin Lagoon and in the lower reaches was examined.In order to detect the correlations between water levels read on water gauges located in the Oder River estuary region, correlation and cluster analyses were applied.In cluster analysis, the grouping procedure Fig. 1.The Oder River estuary area with the marked locations of water level stations was performed using the Ward method, whereas the measure d (x, y) = 1 -R was adopted as a measure of distance, where R is the Pearson correlation coefficient (Stanisz 2007).In order to present the variability of water levels in the Oder River estuary region, the graphs used standardized values (Z), showing the deviation of x variable from its expected value of μ, expressed in units of standard deviation σ (Balicki and Makać 2000).

results and discussion characteristics of storm surges in the coastal areas of Pomeranian bay
In the seasons from 1993/94 to 2016/17, in the region off the coast of the Pomeranian Bay, there were 72 storm surges, during which the sea level in Świnoujście reached or exceeded the emergency level (H ≥ 580 cm).During 32 swells, the sea level reached at least 600 cm.The highest freshet was registered on 3-4 November 1995, when the level of 683 cm was recorded in the Port Log in Świnoujście.According to the IMGW data, the culmination of this surge in Świnoujście amounted to 669 cm (Sztobryn et al. 2005).The second highest surge was the flood of 4-6 January 2017, during which the sea level in Świnoujście reached 653 cm.During both freshets, the sea level exceeded the level of 643 cm, that is, the water level with the exceedance probability of p = 1%, estimated by Buchholz (1991) according to Pearson distribution type III.
In particular years, the freshets off the coast of the Pomeranian Bay were recorded irregularly.After long periods of time with a low frequency of freshets, the period of intensified occurrence was recorded, for instance in the 2006/07 season, when 11 surges were recorded during which the sea level in Świnoujście exceeded the emergency level.Among those surges, 7 freshets were characterized by water levels of at least 600 cm.A large number of freshets were also recorded in the 2001/02 season -during the 9 floods at that time, the emergency level was exceeded, and during 5 events, the sea level reached at least 600 cm.On the other hand, in the 2000/01 and 2005/06 seasons, the emergency level was not exceeded.The analysis of the frequency of occurrence of exceeding the emergency condition in Świnoujście demonstrated simi-lar regularities.The 2006/07 season, apart from the highest number of floods, was also characterized by the highest percentage of water levels equal or higher than 580 cm (1.6%).The high share of such levels was also recorded in the 2016/17 season (0.9%), as well as 1995/96 and 2001/02 (0.8%), and 2011/12 seasons (0.7%).As a result of correlating average annual water levels in Świnoujście with the frequency of water levels equal to or above 580 cm, a statistically significant correlation was obtained: α = 0.002 with the correlation coefficient R = 0.60, indicating that the annual water levels would explain 36% of the variation in the frequency of water levels equal to or higher than 580 cm.
In the course of the year, storm surges, during which the sea level in Świnoujście exceeded the alarm condition, were recorded from September to April (see: Figure 2).The exception was the surge at the turn of August and September 1995.Most storm surges were recorded between November and January (48 high surges).Most often, the surges appeared in January (22 freshets, of which 11 were observed at the level of at least 600 cm).In the month of January, the incidence of water levels equal to or above 580 cm reached 1.5%, and those of at least 600 cm, 0.6%.The second month in terms of the incidence of surges was November.In that month, there were 14 floods, during which the sea level reached at least 580 cm, including 7 freshets with the level of 600 cm.The frequency of the occurrence of exceeding the emergency level was 1.0%.
The duration of the levels equal to or higher than 580 cm in Świnoujście ranged from 1 hour to 47 hours.Exceeding the alarm condition for more than half a day was recorded during 28 surges, and for over a day, during 5 surges.The longest exceedance of the emergency level took place on 13-15 October 2009 (lasting for 47 hours, including states equal to or higher that 600 cm lasting for 31 hours), on 13-15 January 2012 (for 41 hours, including 32 hours of water levels reaching at least 600 cm), 4-6 January 2017 (for 40 hours, including 28 hours with levels of at least 600 cm), and 3-5 November 1995 (for 33 hours, including the level of 600 cm, registered for the duration of 17 hours).Comparison of the number of hours with water levels of at least 580 cm in the seasons from1993/94 to 2004/05 (322 hours), and from 2005/06 to 2016/17 (510 hours) showed a much higher number of such high levels in the second half of the analysed period (see: Figure 3).Similarly, the number of hours increased, at which the sea level reached at least 600 cm (76 and 188 hours in the first and second half, respectively), which indicates the increasing threat of storm floods in recent years.Similar trends off the shores of the Pomeranian Bay were reported by Sztobryn et al. (2005) and Wolski (2017).

characteristics of storm surges in the szczecin lagoon and downstream oder river system
The occurrence of storm surges in the Szczecin Lagoon and in the river network of the lower Oder River is associated with their occurrence off the coast of the Pomeranian Bay.In the seasons between 1993/94 and 2016/17, the highest water level in Trzebież, amounting to 625 cm, was recorded during a storm off the coast of the Szczecin Lagoon on 15 October 2009.It was 9 cm higher than the water level with a probability of exceeding p = 1 % (616 cm, according to Buchholz (1991)).The next highest in terms of high surges were the floods noted: in mid-January 2012 (on 15 January, the water level in Trzebież reached 610 cm); at the beginning of November 1995 (on 4 November, the level of 608 cm was reached), and at the beginning of January 2017 (on 5 January, the level of 604 cm was recorded).The highest frequency of surges during which the emergency level was exceeded in Trzebież (H ≥ 560 cm) was observed during the seasons of: 2006/07 (4.4% of all readings), 2001/02 (4.0% of all observations), and 2011/12 (3.1% of all recorded data).Similarly as off the coast of the Pomeranian Bay, in the 2000/01 and 2005/06 seasons no exceedance of the emergency level was recorded.
In the course of the year, from October to March, over 1.5% of all observations in Trzebież indicated that the emergency level was exceeded.As in Świnoujście, the emergency level was most often exceeded in January (5.1% of the readings).Over 2% of the data indicated that the emergency level was exceeded also in November (2.4%) and in March (2.2%).Due to the The prolonged, multi-day exceedance of the emergency level on the Szczecin Lagoon was usually accompanied by situations where several storm surges occurred one after the other off the coast of the Pomeranian Bay.Such a situation took place in January and February 2007, when the 9 storm surges, occurring one after the other, caused a multi-day upheaval on the Szczecin Lagoon (in total, the emergency level exceeded in Trzebież was recorded for 300 hours).Long-term persistence of very high water levels in the lagoon was also observed in January 2005, in October 2009, and in January 2012.
On the Western Oder River in Szczecin, near the Długi Most, and on the Eastern Oder River in Podjuchy and Gryfino, the largest surge, just like in the Szczecin Lagoon, was the one registered in October 2009.On 15 October, the level of 631 cm was registered in Szczecin at the Most Długi, while 644 cm was recorded in Podjuchy, and 638 cm in Gryfino.During the culmination of the flood in Szczecin near the Most Długi, the water level was exceeded with the 5% probability of exceedance, while in Gryfino the observed maximum state reached approximately the water level with a 10% probability of exceedance (620 cm and 640 cm respectively, according to Buchholz (1991)).The next highest surges on the lower Oder River were the freshets in November 1995, as well as in January 2012 and January 2017.On the water gauges in Szczecin (near Długi Bridge and in Podjuchy) the exceeded emergency level during storm surges was recorded only 9 times.It lasted for up to two days.In Gryfino, the exceedance was recorded 21 times during freshets, but sometimes very high water levels on this gauge resulted from the freshets in the Oder River, as during a storm surge in December 2010 when the water level in Gryfino reached 641 cm due to the formation of ice phenomena.
In Widuchowa, the influence of storm surges was manifested in the form of an increase in the level of water in the Oder River, sometimes matching the relative increase of the sea accumulation.Exceeding the emergency level most often resulted from the overlap of the high water level in the river, which was associated with increased supply from the Oder catchment and/or the emergence of ice phenomena (winter-spring freshet of the Oder River in 2002, and at the turn of 2010/11).
During the high storm surges, lasting several days each, the increase in the water level on the Szczecin Lagoon and on the lower Oder River up to Widuchowa remained within the range of several dozen centimetres, reaching 90 cm, including during freshets at the turn of August and September 1995, in November 1995, and in October 2009.In Bielinek, the increase in water level in the Oder River did not usually exceed about a dozen centimetres.The exceptions were the floods at the turn of August and September 1995, in October 2009, and in October 2016 (increase in the Oder River level by over 40 cm).In Gozdowice, the impact of storm surges on changes in water levels during the analysed situations was negligible.
The highest storm surges in the period covered by the analysis included a freshet recorded at the beginning of January 2017, related to the movement of the deep and extensive low-pressure area called Axel over the Baltic Sea, with the water level in the Pomeranian Bay being higher than usual at this time of year (average water level in Świnoujście in December 2016 amounted to 514 cm).Off the coast of the Pomeranian Bay in Świnoujście, on 4 January, the sea level reached 653 cm, that is 73 cm above the emergency level.The surge in the sea caused a 60-70 cm increase in the water level on the Szczecin Lagoon and in the Oder River network up to Widuchowa (see: Figure 4).In Bielinek, the water level has risen by 12 cm (see: Table 1).The analysis of the obtained dendrogram of grouping the water level sequences in the Oder River estuary area on 3-7 January 2017 showed a distant influence of the sea impacts, reaching Bielinek (see: Figure 5).The levels of water from Trzebież to Widuchowa formed a cluster with a very high degree of mutual correlation, which was then closely related to the water levels in Bielinek (at linkage distance of 0.4).The water levels from Trzebież to Widuchowa were then related to the water levels in Świnoujście (at linkage distance of 0.9).

storm surges at the elevated level oF the baltic sea
The occurrence of storm surges off the coast of the Pomeranian Bay one after the other, usually recorded in periods of increased cyclone activity and at an elevated level of the Baltic Sea, causes a prolonged rise in the water level in the Szczecin Lagoon, and in the river network of the lower Oder (Kowalewska-Kalkowska 2012).The elevated overall level of the Baltic Sea affected the height and duration of storm surges in the 2011/12 season.Already in December 2011, the average sea level in Świnoujście was higher than the average (519 cm).
At that time, a high storm surge was registered, during which, on 17 December, the sea level in Świnoujście reached the level of 623 cm.In January 2012, at a significantly higher than average mean sea level (539 cm), two events were recorded during which the sea level in Świnoujście exceeded the level of 600 cm.The first freshet was recorded on 4-7 January.In Świnoujście, as a result of an increase in the sea level by 165 cm on 6 January, the level of 612 cm was recorded.The level of emergency remained exceeded for 21 hours.The second summit, preceded by two storm surges, during which the pre-emergency (warning) state was exceeded in Świnoujście, was recorded in mid-January.As a result of the increase in   the sea level by 155 cm on 14 January, in Świnoujście, a level of 642 cm was recorded, i.e. approximately the level of water with exceedance probability of p = 1% (according to Buchholz (1991)).The emergency level remained exceeded for 41 hours.
The occurrence of several storm surges one after the other in the period from 4 to 26 January 2012 off the coast of the Pomeranian Bay, at the simultaneous elevated level of the Baltic Sea, caused the inflow of water from the Pomeranian Bay to the Szczecin Lagoon, and significant inhibition of free outflow of inland waters to the sea.As a result, the level of water in the Szczecin Lagoon and in the river network of the lower Oder River increased considerably, which led to a multi-day persistence of a very high water level in this area (see: Figure 6).In total, on the Zalew Szczeciński, the emergency level remained exceeded for 204 hours.In Trzebież, the highest water level was recorded on 15 January at 610 cm, which means that the water level was exceeded with exceedance probability of p = 5% (602 cm, after Buchholz (1991)).On the lower Oder River, the maximum water levels were recorded on 14 January.The water level in the Oder River in Szczecin reached the level of 618 cm, in Podjuchy, 630 cm, and in Gryfino, 624 cm.Exceedance of the emergency level lasted between 40 and 53 hours.In Widuchowa, the pre-emergency level was exceeded.Water level increases on the Szczecin Lagoon and in the river network up to Widuchowa ranged between 77-90 cm during the flood on 4-7 January, and 67-80 cm during the flood on 12-18 January.The reach of both freshets extended to Bielinek (13 and 25 cm increase in the level of water in the Oder River).
The analysis of the correlation between water conditions read on 4-26 January 2012 showed a significant role of variability of the sea level in the Pomeranian Bay in shaping the variability of the water level of the Oder River estuary area (see: Table 2).The water lev-   els in Świnoujście were most strongly correlated with water levels in the Szczecin Lagoon, and within the Szczecin Water Junction ((Szczeciński Węzeł Wodny) at R ≈ 0.7.Slightly weaker correlation was obtained between water levels in Świnoujście and water levels in Gryfino (R = 0.65) and Widuchowa (R = 0.54).The influence of sea level on the level of water in the lower reaches of the Oder River decreased gradually along with the distance from the sea.While the variability of water levels in Świnoujście could be explained by 50% of the variability of water levels at the Szczecin Lagoon and within the Szczecin Water Junction, in this way it was possible to explain only 29% of the variability of the water levels in Widuchowa.The sea impact on the water levels in Bielinek and Gozdowice was negligible.The analysis of the dendrogram of grouping of the sequences of water levels recorded at the water gauges in the Oder River estuary on 4 -26 January 2012 confirmed the important role of the sea impacts in shaping the variability of water levels in the Oder River's lower network up till Widuchowa (see: Figure 7).The strongest correlations were obtained between water levels from Trzebież to Gryfino, whereas water levels in Widuchowa were least correlated.The water conditions from Trzebież to Widuchowa were then related to the water conditions in Świnoujście (at linkage distance of 0.5).The construction of the dendrogram indicated the lack of significant correlations between the water levels in Świnoujście and those in Bielinek and Gozdowice, which formed a separate surge with a high degree of mutual correlation.

storm surges With the increased Water suPPly From the oder river catchment
The Oder River is characterized by a snow-and-rain supply regime, with a clear predominance of the snow supply (Mikulski 1963).Occurrence of increased storm surges during the increased supply from the Oder River catchment hinders the outflow of river waters into the sea, and it causes the surge wave to overlap with the high level of water on the lower stretch of the Oder.This situation results in long-term persistence of a very high water level in the Oder River estuary, and in exceptional cases, it may lead to flooding of low-lying areas around the Szczecin Lagoon and the Dąbie Lake, and areas adjacent to the Eastern and Western Oder (Buchholz 1990, Ewertowski 2000, Kowalewska-Kalkowska 2012).A particular threat of flooding arises when ice phenomena are additionally observed.The latter take on different forms of icing in particular sections of the lower Oder (Buchholz 1990).One of the most dangerous events of this type was the Oder River surge at the turn of 2010/2011 (Figure 8).It was caused by ice jams, and later by melt-water runoff from the Oder River catchment (Assessment ... 2010(Assessment ... , 2011)).The maximum levels recorded in Widuchowa and Gryfino during the freshet (767 cm on 23 December 2010 and 657 cm on 9 January 2011, respectively) were higher than during the Oder River flood season in the summer of 1997.They exceeded the water levels of 5% exceedance probability (749 cm in Widuchowa, and 653 cm in Gryfino, according to Buchholz (1991)).The appearance of 5 surges at the turn of 2010/2011 additionally resulted in an increase in the level of water in the Szczecin Lagoon and on the lower Oder River (up to half a meter) for their duration.The highest storm surge was recorded on 11-13 February 2011, when the sea level reached 632 cm in Świnoujście.Correlation analysis carried out between water levels in the period from 21 November 2010 to 26 February 2011 showed a significantly smaller role of variability of the sea level in the Pomeranian Bay in shaping the variability of the water in the Oder River estuary area (see: Table 3).This impact was limited to the Szczecin  Lagoon and the Szczecin Water Junction.Water levels in Świnoujście were most strongly correlated with water levels in the Szczecin Lagoon (R = 0.76).With the increasing distance from the sea, the correlation between the water levels in Świnoujście and the water levels in the lower reaches of the Oder River declined significantly.Whereas in Szczecin, the correlation coefficient was 0.67 at the Most Długi, and 0.50 in Podjuchy, in Gryfino its value decreased down to 0.095.Although the variability of water levels in Świnoujście could explain 58% of the variability of water levels in Trzebież, it would explain only 25-44% of the varia-bility at water gauges in Szczecin.The impact of the sea on the water levels in Gryfino was negligible.
Analysis of the constructed dendrogram of grouping the water level series of the Oder River estuary area in the period from 21 November 2010 to 26 February 2011 confirmed the limited impact of coastal influences in shaping the variability of water levels in the lower Oder River system (see: Figure 9).The strongest correlations were obtained between the water states recorded in Trzebież and within the Szczecin Water Junction.They formed a close concentration, which was related to the water conditions in Świnoujście (at   linkage distance of 0.5).On the other hand, the water levels in Widuchowa and Gryfino also formed a close concentration, which was related to another concentration in which the water conditions in Gozdowice and Bielinek were located (at linkage distance of 0.6).
The construction of the dendrogram indicated the lack of significant correlations between the water levels in Świnoujście and the water levels upstream of the Oder River from Gryfino; the impact of the sea was limited to the Szczecin Water Junction.

conclusions
In the seasons from 1993/94 to 2016/17, storm surges off the coast of the Pomeranian Bay were characterized by a high irregularity of occurrence.Most of the registered 72 storm surges were observed between the months of November and January.The highest water level of 683 cm was registered in November 1995.In the Zalew Szczeciński (Szczecin Lagoon), and in the river network of the lower Oder River course, the largest surges were recorded in October 2009.The water level increases in the Oder River estuary region were 80-90 cm up to Widuchowa, and the water level in the Szczecin Lagoon reached 625 cm in Trzebież.The floods observed in November 1995, in January 2012, and in January 2017 also accounted for very high freshets.
In the Oder River estuary region, high and longterm storm surges towards the far reaches of the Oder River occurred whenever high and several-day-long storm surges were observed off the coast of the Pomeranian Bay, with gradual elevation of the sea level.Such situations were observed at the turn of August and September 1995, in October 2009, and in January 2017.The range of this type of floods reached as far as Bielinek.
Intensification of freshets in the Oder River estuary region was favoured by the occurrence of storm surges during increased supply from the Oder River catchment and/or the emergence of ice phenomena (the winter-spring freshet of the Oder in 2002, and at the turn of 2010/11).Long-lasting periods with very high water levels in the Szczecin Lagoon and on the lower Oder River were also noted during the elevated water levels of the Baltic Sea (in January and February 2007, and in January 2012).

Fig. 2 .
Fig. 2 .The number of surges and seasonal frequency of sea levels reaching or exceeding the state of emergency at Świnoujście in the 1993/94-2016/17 seasons

Fig. 3 .
Fig. 3.The number of hours with sea level reaching or exceeding 580 cm and 600 cm at Świnoujście in the 1993/94-2016/17 seasons

Fig. 4 .
Fig. 4 .Water level changes in the Oder River estuary area between 3 and 7 January 2017 (water level readings were converted to standardized values (Z)).

Fig. 5 .
Fig. 5. Dendrogram grouping the water levels in the Oder River estuary area between 3 and 7 January 2017.

Fig. 6 .
Fig. 6 .Water level changes in the Oder River estuary area between 4 and 26 January 2012 (water level readings were converted to standardized values (Z)).

Fig. 7 .
Fig. 7. Dendrogram grouping the water levels in the Oder River estuary area between 4 and 26 January 2012.

Fig. 8 .
Fig. 8. Water level changes in the Oder River estuary area between 21 November 2010 and 26 February 2011 (water level readings were converted to standardized values (Z)). reFerences

Fig. 9 .
Fig. 9. Dendrogram grouping the water levels in the Oder River estuary area between 21 November 2010 and 26 February 2011

table 1 .
Maximum water levels and water level changes in the Oder River estuary area on 3-7 January 2017Source: item 1 -data from the Maritime Authority (Urząd Morski) in Szczecin, items 2-8 -data from the IMGW-BIP

Table 2 .
Triangular matrix of the correlation of water levels in the Oder River estuary area between 4 and 26 January 2012 (bold font denotes statistically significant correlation coefficients at the level of α = 0.05).

table 3 .
Triangular matrix of the correlation of water levels in the Oder River estuary area between 21 November 2010 and 26 February 2011 (bold font denotes statistically significant correlation coefficients at α = 0.05 level).