Influence on dune erosion of smaller storm surges in 2021/22
From the second half of September 2021 to April 2022, the dominance of the Western circulation was observed, with winds from the SW to NW directions. This was caused by multiple cyclones passing over the Baltic Sea from the West towards the East (Łabuz 2023b). In the autumn and winter season 2021/22, 37 storm surges were noted on the Polish sea coast, of which 30 occurred at winds from the SW to NW directions. Some of the smaller storms in the 2021/22 season were characterised by wind direction from SW to W, while larger ones occurred during winds blowing from SW to NW or NNE directions.
During the storm surges in autumn and winter 2021/22 with the sea level 0.8 m above average, the water run-up onto the coast was up to the level of 2.8 m a.s.l. At such sea level, the water covered the beaches of an average height up to 1.8 m a.s.l. (Łabuz 2023b). On the Polish coast, on those sections that are prone to erosion, the height of the beach is 1-1.5 m a.s.l. even in the summer season. These sections, where the dominance of dune erosion has been observed for years, are referred to as so-called erosion bays (Zawadzka-Kahlau 1999; Łabuz 2013). In these places, the smaller storm surges in autumn and winter of 2021 led to the erosion of low beaches and dune erosion (Łabuz 2023b). The erosion of dunes caused by smaller storm surges was 2-4 m. This depended on the beach height and the sea level during every surge (Fig. 14). As a result, the beaches had already been lowered in many locations, and the dunes had eroded even before the Marie and Nadine storms. Due to that, the erosion after smaller storms was considered in the presented analyses and their interpretation. Small storm surges from autumn and winter 2021/22 are presented in Table 1, and the course of changes in the sea level during selected surges is shown in Figure 4.
Fig. 4. Other storms - SL
Characteristics of the Marie and Nadine storm surges
On the 27-31.01.2022, two cyclones: Marie and Nadine passed over the Baltic Sea. During the passage, there was high-velocity wind blowing from the W and NW directions (Fig. 2D). This caused an increase in sea undulating and in the sea level up to over 1.2 m above average (Tab. 3).
The cyclone named Marie formed on the 25th of January 2022 above the Northern Atlantic (Fig. 1A). On the 26th of January, the cyclone passed over central Scandinavia. This resulted in the development of strong wind on the western coast of the Baltic Sea. The wind was blowing from the south-western direction to the centre of the cyclone. In the initial phase of the storm, on the 27th of January, the dominant winds on the Polish sea coast blew from the SSW-W direction with a velocity of 11-13 m/s. The wind blew from the coast to the north-east, causing a short-term decrease in the sea level (Fig. 6A). The direction and velocity of wind during both storm surges on the Polish coast of the southern Baltic Sea are presented in Figure 2D.
The decrease of sea level in the western part of the coast (Świnoujście, Kołobrzeg, see Fig. 1A) was higher than in the eastern part (Władysławowo). This resulted from the fact that the water level in the eastern part of the Baltic Sea remained high after the previous storms (Łabuz 2023b). At the same time, the cyclone Marie was moving from Estonia to Belarus (Fig. 1B). As a result, the wind direction changed to W and WNW during the night of the 28th of January (Fig. 6B). On the Polish coast, the impact of that wind led to undulation and another surge of the water. An increase in the sea level by 0.6 to 0.8 m above average was noted in harbours. The average sea level on the Polish coast was 0.78 m AMSL (Tab. 3). The highest sea level was noted in the central part of the coast, in Koszalin Bay, where the coast was exposed to the strongest wind from the WNW direction. There, the sea level reached 0.89 m AMSL (Tab. 3). As the cyclone was moving into the continent (Fig. 1C), wind velocity over the Baltic Sea decreased, and its direction changed into South. The strong wind from the sea lasted for a short time; therefore, the sea level was also decreasing rapidly.
The decreasing wind velocity towards the end of the Marie cyclone was accompanied by an almost immediate increase in wind velocity from the SW direction to 14 m/s. This signalled the start of another storm, caused by the Nadine cyclone (Fig. 1B-D). On the 28th of January, Nadine was over Iceland, while on the next day, the 29th, it had already moved to central Scandinavia (Fig. 1). Early on the 29th of January, the wind on the Polish coast was blowing from the SW direction towards the centre of the cyclone, which was located over Scandinavia (Fig. 5A). The sea level decreased to 0.2-0.4 m below average (Fig. 6A). Higher decreases were noted in the western part of the coast, where stronger winds from the land were blowing for a longer time (Fig. 6B). This was caused by the fact that the velocity of the wind blowing from land was higher and the levels remained lower after the passing of the Marie cyclone. During several subsequent hours, the sea level increased 1 m above the average (Fig. 6A), because of the change in the wind direction from SW to W, and the wind velocity up to 20 m/s (Fig. 5B). As a result of the log-term influence of the wind blowing from the W and NW direction, on the 30th of January, the sea level on the central and eastern part of the coast exceeded 1.1 m AMSL (Fig. 6), while in the western part, the maximum sea level was only up to 1 m above the average (SW - Świnoujście). The increase in the sea level was similar throughout the coast, but the maximum increase was lower in the western part because of the larger decrease between the Marie and Nadine storms (Fig. 5A, blue line). A large surge in the sea level, up to 1.2 m above average, occurred between the western and central parts of the coast (KG - Kołobrzeg and DA - Darłowo). In the eastern part, where the sea level remained high after the previous storm (Marie), high-velocity wind caused the sea level to increase even further, to 1.28 m above average (WA - Władysławowo). On the 31st of January, the Nadine cyclone moved to Latvia and further on to the south-east (Fig. 1D, 5D). The wind direction changed from NNW to NNE, and the velocity decreased. As a result, the sea level decreased to approx. 0.50 m AMSL on the 1st of February.
TABLE 3
Fig.5. Maps A-D
Fig.6. Graph A+B
Dune erosion caused by the Marie and Nadine storms
The Marie and Nadine cyclone system consisted of two combined storm surges. The dune erosion results refer to both storms (Fig. 7-9, 10). The dune erosion after this double storm was large, and it was observed throughout the coastline (Fig. 10). In the western part of the coast, the increase in sea level did not exceed 1 m, so the erosion on the Swina Gate Sandbar, the Dziwna Sandbar, and the Sandbar of the Rega River was small (Fig. 7A, 9, 10).
The maximum retreat of the dune base reached only 0.5-2 m. On the Sandbar of the Swina Gate, only the dunes in the eastern section, exposed to undulating from the NW (km 412-416) were subject to erosion. Larger dune erosion was noted in sections situated further to the east, where the sea level was higher. In towns Kołobrzeg (KG) and Władysławowo (WA), the sea level was over 1.2 m above the average. In the region of Kołobrzeg, dune erosion reached up to 2-5 m. The differences in erosion resulted from the formation and the previous development tendencies of individual sections of the coastline (Fig. 7, 8A, 9). It was noticeable that the sections that eroded after the smaller storms in autumn of 2021 had a small share in comparison to the largest storms: Marie and Nadine. The size of dune erosion after the Marie and Nadine storms was varied, and the largest values, reaching 5-8 m, were observed on capes (Fig. 7B, 9). In places where the beach was higher than the run-up, no dune erosion was observed.
Stronger erosion occurred on sections with low beaches and sections that were usually eroded, as well as those where the beach had already been significantly lowered by smaller surges (Fig. 7). Even larger dune erosion was noted on the sandbars of the Koszalin Bay. On average, the dune bases retreated by up to 3 m, with maximum retreats of 5-7 m. Greater erosion was also observed on the eastern coast, from Rowy to Karwia (Fig. 7, 9, 10). Erosion was stronger on western sections of small capes that exist on the spits of the eastern coast. On the 36 km long Gardno and Łeba Sandbars, the erosion was varied, and it was larger on parts of the coast that are exposed to the north-western direction (Fig. 8B, 10). On the Gardno Sandbar the average retreat of dune bases reached 3.8 m, and the maximum retreat was 13 m. The coast of this sandbar is more exposed to the NW direction, from which the wind was blowing most often, and sea undulation was observed at highest sea level during the Nadine storm. On the Łeba Sandbar, the average erosion was significantly smaller. The dune retreat in specific profiles reached 0.2 to 10 these values resulted from the beach height: the erosion was weaker in locations with higher beaches. The strongest erosion was found on capes (see Fig. 8B, 10: km 186, 188, 192, 197, 204, 207, 215). Their western coasts are exposed to undulation from the NW direction.
On the 47 km long section of the Sarbsko, Kashubian, and Karwia sandbars, the average erosion ranged from 3 to 4.5 m (Fig. 9, 10). Strong erosion was observed in the eastern part of the Sarbsko Sandbar. There, on an over 1 km long section, the dune retreat on the cape reached up to 5-9 m (km 173-174.5). Moreover, greater erosion was observed at the location of all capes on the Kashubian and Karwia sandbars (km 163, 168, 155, 150, and 144). On the western sides of the capes, during the storm with winds from the NW direction, the whole waterfront consisting of dunes was sheared. On capes, usually on 100-200 m long sections, the beaches are significantly narrowed and lowered to a height of 0.5 m, and the base of dunes is subject to stronger erosion. The lowering of beaches in these locations was caused by the previous, smaller storm surges.
On the Hel Spit, the dune erosion was large; largest in places where coast exposure changes (Fig. 7C, 10). Although the coast of the spit is generally oriented to the north-east, i.e. opposite to NW surges, the dune erosion was strong. On the Hel Spit, during storms from the NW direction, the resultant current and undulation were oriented along the coast of the peninsula, which caused damages to sections with narrower beaches. In locations where capes exist, the coastline turns slightly to the south, and the beaches are narrow and low. These places undergo constant erosion from the NW direction. On the other hand, from the E/SE direction, the beaches remain wider and higher, and erosion has not been observed. As a result, in the southern part of the Hel Spit, dune erosion is found alternately with sections of stable coast.
On the Vistula Spit, stronger dune erosion was observed in the eastern section, on coasts exposed to the NW direction (located to the east of the new canal and Krynica Morska). The central and western parts are protected by the Hel Peninsula. This usually shields the southern coast of the Gdansk Bay from stronger erosion. The maximum erosion on the Vistula Spit was lower than in the central part of the coast and reached up to 4 m. In the eastern part, the mean dune retreat was 3.5 m, while in the central part, it was up to 2.5 m.
During the Nadine storm, the mean erosion of dunes reached 3-4.5 m, and in sections where the direction of the coastline changes, up to 8-14 m. The strongest erosion was noted in sections that were exposed to the dominant wind and undulation from the W to NW direction: the Koszalin Bay, Ustka Bay, eastern parts of the Vistula Spit and the Swina Gate Spit (Fig. 10).
Fig. 10. Map A+B
Sea level vs run-up height
The analyses revealed that each increase in the sea level is linked to a proportionate height of run-up of water on the shore. The height of run-up is determined by the maximum reach of water on the shore (Nielsen and Hanslow 1991; Stockdon et al. 2006; Łabuz 2022). It is the sum of the sea level and the height of waves that results from wind velocity. The stronger the storm, the higher the sea level and the observed run-up on the shore. Based on that, one may determine the locations where the strongest erosion may occur or has occurred. The relation between the changes in the sea level and the increase in run-up height is shown in Figure 11. It presents the average values from long-term measurements (2001-22), max. surges in 21st century and indicates the run-up that was measured during surges in the 2021/22 season (including Nadine). The height of run-up determines the height of the coast where the dune (or cliff) waterfront will be eroded.
At sea level up to 0.6 m above average, the run-up of water during the storms in 2021 ranged from 0.9 to 1.1 m a.s.l. This height is like that from long-term measurements. During smaller surges, with the sea level reaching 0.6-0.7 m above average, beaches up to 1.2 m high were eroded. The run-up resulted in lowering the beaches or to the forming of micro-cliffs.
Fig. 11 SL vs SLR
Beach height vs dune erosion
The erosion of dunes in the given section depended on the height of the beach (Łabuz 2022; 2023a). This refers to low, average surges and to the high Nadine storm (Fig. 11, 12A). Dune erosion occurred in places where the beaches were significantly lower than the run-up of water on the shore. On sections with beaches of the height of 3-3.5 m a.s.l., erosion was not observed. It was not observed, either, on the western coast of Poland, where the sea level was lower: HSL = 1 m AMSL.
During average storm surges, with sea level up to 0.8 m AMSL, dunes were eroded on sections with beaches of the height up to 1.6 m (Fig. 11, 12A, the Gerhild storm). These are coasts that are prone to erosion, where the beach is narrow and low above sea level. Such beaches can be found on western coasts of numerous capes. On such sections, the beaches are constantly narrow and low, whether before or after the storms, and the dune has a high, cliff-like cutdown. During larger storm surges with HSL > 1 m ASML (the Ida storm), dunes were eroded on sections with beaches up to 2.2 m high. At the sea level HSL > 1.2 m ASML, dune erosion covered the longest fragments of the coast, including the accumulative coast sections, where the beaches are the highest (Fig. 11, 12A, Nadine storm).
In general, as a rule, the higher the sea level is, the stronger the erosion of dunes. The likelihood of dune (cliff) erosion during storm phenomena increases on coastal sections with low beaches. They are usually eroded even during smaller storm surges (Fig. 12B). Therefore, the erosion along the coastline is highly differentiated, as the height of beaches varies even in adjacent sections. An example might be the large difference in beach height and dune erosion that results from maximum run-up, as presented for the Gardno Spit (km 213-215, Fig. 14).
On sections with lower beaches, up to 1.5 m a.s.l., dune erosion may occur even during surges that are categorised as medium storms, HSL = 0.8 m AMSL. This is the average height of beaches on the Polish sea coast (Zawadzka-Kahlau 2012; Łabuz 2013). A coast with a beach up to approx. 2 m a.s.l. high protects the dunes from medium surges, of max. HSL = 1 m AMSL. Furthermore, accumulated sections with beaches higher than 2.5 m a.s.l. are destroyed only by the high storm surges HSL > 1.2 m AMSL. In these locations, the dunes did not retreat, which is shown, among others, in Fig. 8. The mean size of erosion depending on beach height, equal to the run-up, is presented in Table 4. These data refer to the storms from the 2021/22 season. Due to the fact that the height of most of the beaches ranges from 1.5 and 2.5 m, most coasts are eroded by a run-up of 2.8-3.4 m AMSL.
Fig. 12 A-B
TABLE 4