Wolfgang Cyclone Landfall in October 2023: Extreme Sea Level and Erosion on the Southern Baltic Sea Coasts

Abstract

This paper presents the hydrological and meteorological parameters of the Wolfgang storm surge on the southern Baltic Sea coast and the storm’s impact on coastal areas with highly urbanised and developed zones. The surge emerged during a rare cyclonic system that was located over Western Europe in October 2023. A high difference in air pressure between the western and eastern parts of the Baltic coast led to the high-velocity wind blowing from the eastern direction to the centre of the cyclone located over Denmark. It caused high sea levels in the western part of the Baltic Sea. On the German and Danish coasts, the inflow of water at a high wind velocity perpendicular to the coast caused a very high surge of the sea and strong undulation. In this part of the Baltic Sea, the storm caused an increase in the sea level ranging from 1.5 to 2.2 m above average. It was lower on the eastern part of the Polish coast, exceeding 0.9 m above average sea level. The erosion of the base of cliffs ranged from 2 to 7 m, depending on the sea level. The dune erosion was larger but more varied, which resulted from different heights of the beach, at a maximum of up to 18 m. The water run-up reached 5.2 m above mean sea level (AMSL). The run-up parameter is a more accurate indicator of the potential threat than the sea level height. As a result of water run-up on the coast, lowlands situated even as far as 300 m from the shore were flooded. The storm caused significant damage to the coastal infrastructure and harbours. Research was conducted based on field studies and the analysis of digital documentation from websites, with the records of water run-up and the effects of the storm. Field studies were based on measures of coast retreat. Sea levels and wind were studied based on collected data.

1. Introduction

Recent academic publications have been published concerning coast erosion due to cyclones caused by climate change and its influence on sea coasts [1,2,3,4,5,6,7,8]. Some studies have also identified places that are threatened by sea level rise (SLR) due to future climate change [9,10,11,12,13]. However, only a few works discuss the changes in local coasts that result from single, extreme storm events induced by a low air pressure system called a cyclonic system, typhoon, or hurricane depending on the geographical zone. These single phenomena often have a stronger impact on coast erosion than long-term trends related to sea level growth [2,3,4,13,14,15,16,17,18,19,20].

Studies show that the vulnerability of coastal areas to storm surges depends on the storm surge water level (SL) relative to coastal morphology [2,3,4,5,6,7,12,13,14,15,16,17,18,19,20,21,22]. Previous analyses have indicated that shoreline water run-up (SLr) is influenced by sea level and wave height, both of which are driven by wind velocity and may be key factors contributing to coastal risk during storm surges [23,24,25]. The stronger the storm, the higher the sea level and wave height, resulting in a greater water run-up. In this study, these storm surge parameters are analysed based on coastal morphology measurements, with an emphasis on run-up height, and a summary of observed inundation events that led to infrastructure damage or coastal erosion.

On the Baltic Sea coast, a semi-closed basin, the largest sea level changes that are observed in the autumn–winter period (October–March) are influenced by air pressure systems passing through from the southwest to the east or northeast over the Baltic Sea [26,27,28,29]. These cyclones are responsible for varied air pressure and strong wind action and induce sea level growth and its waves. The increase in the relative sea level (SLR) on the southern part of the Baltic Sea coast influences extreme levels related to storm surges induced by air pressure and wind force [27,28,30,31,32,33,34,35,36]. The extreme changes in the sea level that are observed on the Baltic Sea coasts and the changes in the frequency of the phenomena referred to as storm events, with sea level SL > 0.7 m AMSL [27,28,31,32,33,34,35,36], cause an increased risk of coastal erosion and flooding of low-lying areas around the Baltic Sea [37]. This increases the probability of erosion and flooding that pose a threat to infrastructure and human life [1,4,11].

Storm surges on the Baltic coast are understood as a strong wind action that produces high waves accompanied by an increase in the sea level over a short period [27,29,32,33]. The length of this phenomenon ranges from 1 to 3 days. Most of the surges on the south Baltic coast are recorded between November and February [27,30]. Storm surges in the Baltic Sea result from high-velocity wind that is caused by cyclones passing over this part of Europe [26,27,28,31,32,33,34,35,36,38,39]. The direction of the wind and the increase in sea level depend on the location of the cyclone over the land. They may increase sea levels in different parts of the Baltic Sea basin, and that is why they are important to study, due to their unpredictable nature. Deeper and longer-lasting cyclones result in the development of a longer and larger storm surge [9,10,11,22].

On the southern Baltic Sea coast, there is a medium risk of high water levels [36,39]. On the German part of the Baltic Sea coast, SW and W winds are dominant—40–50% [40], while on the Polish coast, SW to W winds account for 45–55% during the year [21]. The velocity of NW and NE storm winds exceeds 10 m/s. These winds cause significant swells and high undulation on the southern Baltic coast in the autumn and winter seasons [27,32].

In the second half of October 2023, a cyclone named Wolfgang (int. Aline) formed over the northern Atlantic (Figure 1). From 18 to 20 October, the cyclone moved in a southeasterly direction, through the British Isles to the coasts of Europe. It brought a storm on the coast of the Atlantic Ocean and also on the shores of the Baltic Sea. Similar to most such systems, the cyclone moved over the North Sea to the Baltic Sea but stopped over Western Europe, in the vicinity of the Jutland peninsula. The Wolfgang cyclone caused the development of a very strong storm on the western and eastern coasts of the Jutland peninsula. For 3 days, the wind blew with high velocity from the eastern direction to the centre of the cyclone, causing an increase in the sea level on the western coast of the Baltic Sea. A large increase in the sea level was observed throughout the western coast of the Baltic Sea in Denmark, Sweden, and Germany and partially in Poland.

It caused heavy damage to coastal infrastructure, the erosion of different types of coast, flooding, and threats to human safety. It was called in the media the storm of the century, with a maximum sea level of 2.27 m AMSL in Flensburg. The sea level on the eastern Baltic coast during the storm was significantly lower: from 0.6 to 0.9 m AMSL. Only rarely do cyclonic systems stay over the southern Baltic Sea coast for such a long period.

This phenomenon was very similar to the strongest known storm on the western Baltic Sea, on 13 November 1872, responsible for the large destruction of human infrastructure at that time [41,42,43,44]. During that event, the sea level caused by E and NE winds on the western Baltic coast exceeded 3.4 m AMSL in Travemünde [44,45]. Considering the similarity of both surges and the observed threat posed by the Wolfgang surge (cyclone) in the 21st century, a detailed investigation was conducted. Based on our own field data and analysis of collected information concerning the surge impact, several aims have been established to underline this surge’s impact on the southwestern Baltic Sea shores and coasts.

This work aimed to explain the immense destruction and shore erosion reported by the southwest Baltic states. Based on the above, this study aims to (a) analyse the development of the storm surge and its parameters, (b) analyse the relationship of beach height with coast erosion versus sea level and water run-up, (c) analyse the impact of this storm on communities along the coast, and (d) examine how sea level could be responsible for coast erosion, considering the preexisting morphological conditions of the shore–coast complex.

2. Study Area

The analysed area includes the southern Baltic Sea coast from the Gulf of Gdańsk on the east to the diverse coast with several bays on the German, Swedish, and Danish Baltic coast (Figure 2). The dominant types of shores on the southern Baltic coast are soft shores, consisting of sandbarriers with dune sands and cliffs formed in boulder clay [37,46,47,48,49,50,51,52,53,54,55,56]. Changes in sea level, vertical changes in the lithosphere, and geological structures are responsible for the formation of the coasts of the Baltic Sea basin [47,48,52,53,54,55,56]. On the other hand, the eastern Polish part of the coast is characterised by a dominance of barriers and dunes, which account for more than 76% of the total length of the Polish coast [49,50,51,53]. The dunes developed on sandbars vary in height, from low foredunes to high (up to 20 m) and eroded former inland dunes [49,50,53]. The Polish coast is characterised by frequent erosion [34]. In recent years, an increasing number of seawalls and sections with regulated beaches have been constructed. Those are divided by short moraine cliff coasts up to 30 m high. The highest moraine cliffs exist in most southern parts of the Baltic Sea; they are 60–80 m high and exist on the shores of the Wolin and Usedom islands in the Pomeranian Bay [53,55].

The western coast of the studied south Baltic shores, which belongs to Germany, is a vast area with an irregular coastline, with numerous peninsulas, coves, and islands, of which Rügen is the largest. These are built by moraine tills and clays alternately with short sandbar sections with coastal dunes. On Rügen Island, tall chalkstone cliffs are connected to low dune banks on narrow sandbarriers [52,54]. Short sandy sections alternate with moraine cliffs 10–14 m in height. Characteristic elements are the sandbarriers that are covered with low dune banks [52,53,54]. These forms undergo constant changes as a result of erosion and changes in the sea level [34,37,46]. The exposure of the shore changes every few kilometres, which also influences the effects of undulation from various directions. An example may be the Darss Peninsula, whose western coast is usually subject to erosion, while the northern shore shows tendencies to rebuild and accumulate on the northernmost cape. Part of the coast that is shielded by islands and spits is protected from storm erosion. The western part of the Baltic Sea is dominated by low dune and cliff coasts. A more serious risk is the flooding of lowlands in the bays during increases in the sea level. Due to the threat of erosion, multiple dune sections in Usedom, Fischland, and Warnemünde are wide, constantly artificially nourished formations. Low wetland sections are protected by flood embankments [57]. The height of the embankments of the German southern coast of the Baltic Sea currently reaches 5–7 m AMSL [58].

In the northern part of the western Baltic Sea, there are numerous Danish islands with a diversified coastline and with various types of shores that consist of chalk, sandstone, clay, and gravel, as well as spit sands [37,53]. The tips of many islands are low and narrow spit sections that are covered with very low dunes. These are usually bank formations that show growth of the spit during changes in the sea level [59]. Some of the dunes on the Danish islands have been formed artificially to prevent erosion and flooding in low coastal areas [59,60]. Dunes that were expanded in this way and wide beaches are situated, among others, south of Copenhagen. On Bornholm, where rocky coasts are dominant, low dunes are present in the eastern and southern parts.

The Swedish coast in Scania consists of shores formed from old rocks but also low spits, including the characteristic peninsula of the Falsterbo Spit [53]. These coasts are exposed to the south, except for the large Sandhammaren cape, which is exposed towards the east. In the Ystad region, dunes are frequently subject to erosion [53]. Low dune coasts are usually intensively exploited for tourism [53,61], so the risk to human infrastructure caused by sea level rise may increase during storm swells on these shores.

Figure 2. Southern Baltic coast. (A)—Location on the Baltic Sea coast. (B)—A studied area affected by the Wolfgang storm in October 2023 (own map based on sources [62,63,64,65,66,67]). (C)—Selected wind roses during the surge (data-based sources: [62,67,68,69]).

Figure 2. Southern Baltic coast. (A)—Location on the Baltic Sea coast. (B)—A studied area affected by the Wolfgang storm in October 2023 (own map based on sources [62,63,64,65,66,67]). (C)—Selected wind roses during the surge (data-based sources: [62,67,68,69]).

3. Material and Methods

The materials are the changes in sea level and wind velocity during the storm caused by the Wolfgang cyclone, erosion of the coast, and destruction of infrastructure. The methods used are described below and also include the latest source of data originating from social media on the internet, recorded during the ongoing phenomenon. This study includes in situ data and was performed remotely. In this study, diversified methods and techniques of data analysis were used (Figure 3):

• Digital analysis of meteorological and hydrological data obtained from institutions;
• Field measurements of coast retreat, SL range on the coast (run-up), and destruction of infrastructure using GPS RT K, GPS positioning, and levelling of morphology in selected areas and comparison with sea level;
• Remote analysis of sea level impact on the coast, including erosion and infrastructure destruction, based on internet cameras and different internet media sources;
• Analysis of coast retreat and its relation to other variables.

Data were analysed and prepared in Microsoft Excel 2019, Quantum GIS Desktop 3.10.6, Golden Software Surfer 8 and Grapher 8, Adobe Photoshop 7.0 and CorelDRAW Graphic Suite 19. These are our own data, not generated by AI.

3.1. Hydro-Meteorological Data

For coastal gauge stations located around the southern and southwestern Baltic Sea coast (Figure 2), hourly sea level readings were analysed and compared to other variables. The meteorological and hydrological data were based on materials obtained from the Institute of Meteorology and Water Management (IMGW) [62], the Federal Office for Maritime Navigation and Hydrography (BSH) [63], the Swedish Meteorological and Hydrological Institute (SMHI) [64], the Danish Meteorological Institute (DMI) [65], and the web portal Pegelalarm from SOBOS [66]. The meteorological maps of Wolfgang’s low shift were analysed based on data from the Institute of Meteorology of the Free University of Berlin [67]. All SL height data were related to the same level used for the Baltic Sea.

Wind information was extracted from readings at the Institute of Meteorology and Water Management [62], the Institute of Meteorology of the Free University of Berlin [67], Meteorological Service in Germany [68], and the web page windy.com [69]. The wind speed and directions, as well as the changes in the sea level, were assessed. The dependencies between them were determined and analysed.

The value of the significant wave height on the open sea was taken from WAM model for the Baltic Sea Wave Hindcast service provided by Copernicus Marine Service [70]. This wave model (WAM) is used at a resolution of 2 km every 2 h.

3.2. Coast Erosion Estimate Techniques

Due to the large size of the analysed area, morphological-change computation was performed for several selected and known areas from previous field research. The analysed field research data included relief after a storm in areas of previous research, conducted at various time intervals between 2019 and 2023. The analysed material from the Polish coast, the Usedom and Rügen islands, and the Warnemünde and Travemünde areas originates from our field research that was conducted in October and November 2023. Other sections of the German coast selected for this study were measured in February 2024, excluding Denmark and the Swedish coast. The morphological changes of the coast were analysed in relation to various sea levels and the run-up of water onto the shore. Data were collected from profiles presenting shore or coast change due to surge landfall. The measure was conducted using a leveller, a GPS Hyper SR, and a common GPS.

Field measures depended on recognition of coast and shore erosion and infrastructure destruction along selected cross sections. The numerical data were obtained from in situ work by measuring thresholds of relief indicators and, less so but also still informative, through pictures or from video material, including cameras.

The main analysed variables were as follows:

• Beach (be) height (Hbe) and width (Wbe) change;
• Coast retreat for dune (fr) and cliff sections (cl) of given height (Hfr, Hcl);
• Sea level, including max. (HSL);
• Water run-up on the coast(HSLr), marked as wreck or litter material;
• Visible infrastructure damage.

The change in dunes, foredunes, or cliff toes was analysed based on profiles before and after the surge. Changes in beach width or sand volume were also analysed. The variables were computed and compared in the mentioned software. The range of flooding caused by storm landfall was searched based on field and remote data. Also, the relation between erosion and the maximum run-up was presented. The values of coastal erosion were prepared based on relief differences measured in the field and obtained remotely from secondary video images.

Wave parameters were not studied statistically due to a lack of data from shore areas. Wave height was highest on the east coast and very low on the west, where sea level was the highest, as was the observed erosion.

Additionally, both phenomena, erosion and water landfall on the coast, were observed during the surge, through video recording online during its action, and post-event on the recordings studied from internet media. The video images can provide data through rectification techniques on the known parameters of the objects observed remotely on a photo or video recording [71,72,73,74]. This simple method, based on camera photos, was used to measure coast retreat remotely in areas not visited after the surge.

3.3. Remote Data from Internet Media

In the changing world of information inflow, internet media, online recording, self-video data, and so on, it is possible to analyse environmental phenomena online, during their development or just after, right from these sources. Using other multimedia data from the web may be helpful to analyse larger areas and also during the ongoing phenomena (here, surge erosion) in situ without one’s own presence. This allows for being familiarised with a larger, broader land/coast section at once. The authors used those materials during the ongoing storm surge and afterwards.

The changes in coast position and sea level during the storm were studied based on images displayed by online cameras with a view of the coast, which are available in many towns and villages. These data were compared to the latest available satellite images and images published on Google Earth in 2023 and available maps. Apart from that, the authors used data that were available on the internet and that presented an ongoing surge and coastal changes after its end, i.e., online cameras, published videos, and photos that showed the changes after the storm, including photos from the online cameras:

(a)

Local newspapers, web pages, and websites of seaside towns and local governments containing descriptions of surge damages, photos, short videos, and information about erosion,

(b)

Coast-located video cameras with data presented on the internet during and after the surge landfall;

(c)

Different internet social networks with fresh data on ongoing surge, flooding, erosion, or infrastructure destruction, including private material on an ongoing surge.

During the storm and immediately after it, a large amount of multimedia information was published online. Based on the data, coastal retreat was analysed in selected areas. As for the places that the authors have not visited in person, private videos and photos published on social media (local media and private pages on Facebook, YouTube, TikTok, and others) were used. Materials published in local newspapers and on the official social media channels of coastal towns were also studied.

Based on all these media, the authors described the observed damage to the infrastructure on the coasts that were destroyed by storm undulation. This was an important objective for explaining the strength of the analysed phenomenon. Data were derived both from field research and remote research. Due to the large amount of information, the main areas with the highest destruction were included in selected categories (Figure 2).

4. Results: Storm Development and Its Impact on the Coast

4.1. Hydro-Meteorological Analysis of Surge

4.1.1. Situation over the Baltic Sea During the Wolfgang Low

Cyclone Wolfgang (int. Aline) was first noted on 18 October, at 0.00 h UTC, south of Greenland (Figure 1). At that time, the wind from the W direction was blowing over the southern Baltic Sea at a velocity of 8–9 m/s, towards the centre of the Uwe cyclone, which was over Estonia. This caused an increase in the sea level by 0.4 m on the German coast, 0.6 m on the Polish coast, and 0.8 m on the Swedish coast of the western Baltic Sea. Then, the level of the sea temporarily decreased. As Cyclone Wolfgang approached the Baltic Sea on 19 October, the wind direction changed to E (Figure 1 and Figure 4A).

Initially, the wind velocity was 8–12 m/s, and then it increased to 14 m/s at the end of the day. The water level in the gauge stations began to increase. An increase in the sea level from 40–60 cm above mean sea level (after Cyclone Uwe) was observed on the entire southern Baltic coast, starting from 3.00–5.00 UTC, 19 October. Until the end of 19 October, the sea level on the eastern Baltic coast increased only to 0.57 m above the mean sea level (GD). From Świnoujście (SW) and further to the west, the increase was higher, from 64 cm (SW) to 100 cm in Travemünde. The maximum sea level was recorded in Flensburg: 2.27 m AMSL.

On 20–21 October, Cyclone Wolfgang had two centres: over the Dutch and French coasts. The air pressure was 980 hPa (Figure 1). The wind velocity over the southern Baltic coast reached 16 m/s and up to 22 m/s in gusts (the SW station in Świnoujście). Higher wind velocities were observed in the western section of the southern Baltic coast (Figure 4B,C and Figure 5C,D). Wind blowing from the east towards the west was dominant for several consecutive days. The sea level in the harbours in the southern part of the Baltic Sea gradually increased from the east to the west. The easterly wind blowing along the coastline did not cause an increase in the sea level in Polish gauge stations (Figure 5). On the open sea coast of Poland, the sea level on the eastern section of the coast ranged from 0.57 m in Władysławowo to 0.94 m AMSL in the western section in Świnoujście and to 1.06 m AMSL in Koserow in the Pomeranian Bay. However, on the German part of the western coast, the sea level was significantly higher. In Sassnitz on Rügen Island, it was as high as 1.16 m AMSL (Figure 5A). In Mecklenburg-Vorpommern Land, the sea level was 1.5–1.6 m AMSL. It increased significantly from Warnemünde towards Kiel, from 1.5 to 2.1 m AMSL. In Schleswig-Holstein on the western German coast, the sea level was very high and reached 2–2.27 m AMSL.

The highest sea level in the gauge stations on the western section of the southern Baltic coast was noted on 20 October, between 7.00 and 11.00 pm UTC. In the central part of the coast, on the Mecklenburg-Vorpommern coast, it was observed at 11–12.00 pm UTC on 20 October. On the eastern, Polish part of the coast, the maximum level was noted later, at 3–5.00 am UTC on 20 October. This delay might have resulted from the fact that masses of water were constantly transported by the wind towards the west. The increasing level resulted from the lowered wind velocity and the gradual ending of the movement of water masses towards the west. These water masses, which were not pushed to the west, in the end increased the SL in gauge stations on the eastern coast (Figure 5A).

The sea level in the middle part of the Baltic Sea was lower on the southern coast than that noted on the northern part, in Denmark and Sweden (Figure 5B). In the central section of the coast, in the Pomeranian Bay in Świnoujście, it exceeded 0.94 m AMSL; in Sassnitz, it was 1.16 m AMSL; while in Ystad on the Swedish coast, it was 1.32 m AMSL. Further west in Wismar, the sea level reached 1.5 m AMSL, and in Køge near Copenhagen, it reached 1.8 m AMSL, and it reached 1.5 m AMSL in Skanör in Scania. A higher SL was observed on the coast, opened for wind and wave action from the east.

On the northern coast, in Denmark and Sweden, a sea level +0.6 to +0.8 m AMSL higher than that on the southern coast was noted just before the surge impact on 18 October, which resulted from the inflow of waters through the Danish straits together with wind directions associated with the Wolfgang cyclone approaching Europe from the west. At that time, the sea level on the southern German coast was only slightly above 0.4 m AMSL. The sea level on the German section of the southern Baltic coast increased rapidly on the morning of 19 October 23. This was strictly related to water inflow from the east direction due to wind and wave action (Figure 4A–C).

On 22 October, Cyclone Wolfgang moved to the north over the North Sea (Figure 1). The direction of the wind changed to SE–S, with a velocity of 5–7 m/s (Figure 4D). The sea level in most harbours decreased to 0.3–0.45 m AMSL. On 24 October, part of Cyclone Wolfgang moved to the northern parts of the North Sea, and the wind direction changed again to SE. The storm on the western Baltic coast, with a sea level exceeding 1 m above mean sea level, lasted for 15–26 h on 19–21 October 23. The sea level remained high for the longest time in harbours that are situated deep inside the coves of Schleswig-Holstein, i.e., for a maximum of 54 h in Flensburg. This was a long and high swell, and the velocity of the wind blowing from the E direction exceeded the forecast values. A high swell of water on the southern Baltic coast lasted from the 18th to 22 October 2023 (Figure 5A).

Figure 6 presents different sea levels at the same time in relation to varied shore heights in selected areas. Given similar beach heights, a higher sea level and waves caused higher run-up to the shore, which influenced the amount of losses. The higher the run-up of waves (SLr) related to sea level height (HSL), the larger the coast erosion observed. The change in coast retreat was also related to beach height (Hbe). This relation is analysed in part of the material described below.

4.1.2. Wind Direction and Velocity vs. Sea Level

During the Wolfgang storm, the sea level on the eastern section of the southern Baltic coast did not exceed 0.7 m AMSL (Figure 2 and Figure 5). The wind blew along the coastline (Figure 2C), without causing any significant swells or coastal erosion. The wave height from the WAM model on the open sea increased to 1.6–1.8 m and was from the NNE direction [70]. No further wave increases were observed due to the wind change to the E, along the coast. The wind velocity along the coast reached up to 16 m/s (US), and it was even higher than that on the Polish part of the western coast (SW). A higher sea level was noted in the Pomeranian Bay, which was exposed to NE and E winds, than inside the bay (Świnoujście). There, in the Pomeranian Bay, the significant wave height was modelled to 1 m in height. The wind velocity was nearly 15 m/s, while the sea level was 0.94 m AMSL in Świnoujście and 1.06 m AMSL in Koserow. The coasts of the Usedom island, with the measurement station in Koserow, are exposed to the wind blowing from the NE-E direction. The sea level in Koserow and in Sassnitz harbour, which is located a little further north, was higher (SL= + 1.16 m ASML).

The wind velocity and direction changed slightly from ENE to E (Figure 2C). The sea level and undulation increased significantly on the western part of the southern Baltic coast as a result of the wind change blowing with a velocity of 16–20 m/s from the E direction towards the centre of the cyclone, which was situated in the west. The sea level in the harbours exceeded 1.5 m AMSL. Like on the Polish coast, the wind blowing along the coastline did not cause a high surge. On the coast of Schleswig-Holstein, which is exposed to the E direction, the sea level was higher as a result of the swell of water flowing in from the eastern part of the Baltic Sea. The significant wave height was lower, not exceeding 0.8 m [53]. Additionally, on the western coast of the Baltic Sea, wind from the SE direction made a major contribution to the swell of water in the harbours of the eastern coasts of the Jutland peninsula (e.g., Eckernförde, with up to 45% of the SE wind having a velocity exceeding 17 m/s). The higher the wind velocity, the stronger the undulation at the same sea level, and the stronger the run-up of water on the shore. The eastern wind that was blowing along the coastline caused the waters of the Baltic Sea to move westward and to surge between islands and in coves. The wind from the SE direction pushed the water into the narrow straits and fiords that are present in Denmark and in Schleswig-Holstein. High-velocity wind from the E direction caused the strongest undulation and swell of water on parts of the coast that are exposed to the east. This phenomenon was observed on the coasts of Danish islands and the protruding capes of the German coast. The surge of water was the highest inside closed bays on the coast of Schleswig-Holstein, including inside fiords that cut deep into the land. However, the significant wave height from the WAM model did not exceed 0.5 m. This only proves that the modelled wave height on the open sea is not related to the observed water run-up on the shore with varying exposure to the main wind force. That is why wave height is a minor factor and the time of the high water level is a major factor in the coast erosion.

4.1.3. Sea Level vs. Run-Up Height

Long-term observations on the southern Baltic coast have shown that each increase in sea level is related to a proportionate height of water run-up (SLr) on the coast [75]. The height of run-up is determined by the maximum reach of water on the coast [23,24,25]. It is the sum of the sea level and the height of waves that results from the velocity of the wind. The stronger the storm, which results from a large difference in air pressure, the higher the waves, the sea level, and the observed run-up. During this surge, the wave direction was along the southern Baltic Sea and opposite to its western part, including Denmark and the German coast. The analysis of wave climate during the surge did not prove its importance in coastal erosion. The highest waves of up to 1.5–1.8 m were modelled [70] for the east coast. The only cross-shore waves were modelled for the Danish island coast, which is exposed to the E direction. From NNE to ENE, most waves were up to 1.2 m high. On the west part of the German coast, exposed to the E waves, they were only 0.3–0.5 m [70]. There, the sea level and its undulation were the highest. The wave climate had no impact on coast erosion.

The factor that influences the extent of erosion is the sea level and wave height, expressed as the maximum run-up of water on the coast [24]. The height of run-up is an important indicator of the potential value of dune retreat during the storm. It is not directly related to the sea level but is a result of the height of waves that are generated by the wind during the storm.

On the Polish coast during storm Wolfgang, the wind blowing almost constantly along the coast did not cause a significant increase in the sea level or run-up. The sea level was +0.6 to +0.9 m AMSL. The run-up on the shore was marked by debris or small embryo dune cuts just up to 1.5 m AMSL (Figure 7A), even with a beach height lower than 1 m AMSL. The erosion of the dune toe was observed only in places with a beach elevation (Hbe) lower than the run-up height (HSlr). The sea level was higher in the western part of the Baltic Sea. The higher the SL, the higher the marked run-up ranges (SLr). In Warnemünde, with SL = +1.5 m AMSL, the run-up reached 3.0 m AMSL. On the coast near Flensburg, which is exposed to the east, the SL + 2 m AMSL was comparable to the run-up range, even 5 m AMSL. Nevertheless, there, the run-up height was more diverse concerning the exposure angle to the wind–wave direction. At a similar sea level, the run-up on the German coast was different, due to a different exposure to the storm. The run-up was higher on parts of the coast that are exposed to the east or northeast, sections that were strictly exposed to the wind–wave direction. This resulted in the erosion of beaches as well as cliff and dune coasts, as in Kolpinsee or Prora (Figure 7B,C). On sections with the beach higher than the run-up, there was no erosion (Figure 7D).

The height of the run-up (SLr) determines the height of the beach (coast) where the wetlands, dunes (or cliffs), or waterfront will be eroded. At sea levels up to 1 m AMSL (SW), the run-up reached 1.5 m AMSL. At sea levels up to 1.5 m AMSL (WR), the run-up reached 3–3.5 m AMSL. On the western part of the coast, the run-up reached up to 5 m AMSL. Along the southern Baltic coast, the run-up was related to the sea level, and the highest reached 5.2 m AMSL in Flensburg (Figure 8).

4.2. Scale of Coast Erosion and Threats

4.2.1. Surge Destruction Effects

As the sea level was low, no damage to the seaside tourist infrastructure in harbours and havens was observed on the Polish coast. In the Pomeranian Bay, in the vicinity of Świnoujście, the storm disconnected two buoys that mark the fairway to the harbour from the sea bottom. The buoys were transported to a beach on Usedom island, near Zinnowitz. In Świnoujście, the ferry traffic of ferries to Sweden was stopped. Many ferries were also stopped in Denmark and Sweden. Summarised data from the described coastal areas are in

Table 1. Highlighted southern Baltic coast areas with different erosion and threats.

Location	Sea Level, (m AMSL)	Run-Up, (m AMSL)	Coast Erosion (Dune Cliff Retreat, m)	Infrastructure, Protection Measures, Damage, and Flooding
East Polish coast (Gdańsk-Łeba)	0.57–0.60	0.7–1.0	No erosion, only where beach < SLr	No damage
Middle Polish coast (Ustka-Kołobrzeg)	0.74–0.80	0.9–1.2	No erosion, only where beach < SLr	No damage
Pomeranian Bay, Polish part	0.68–0.94	1.3–1.6	Minor dune retreat, ≤1.5 m	No damage, disruption of ferry traffic
Pomeranian Bay, German part	0.94–1.06	1.6–2.7	Minor dune retreat, 2–4 m	No significant damage, artificial dune erosion
Rügen Island	1.06–1.13	2.5–2.9	Dune erosion 2–18 m, cliffs damaged 1–2 m	Destroyed promenade in Sassnitz, flooding of lowlands, seawalls flooded
East and middle German coast (Zingst–Wismar)	1.4–2.2	3.4–3.7	Severe dune erosion 5–18 m, cliff retreat 2–7 m	Damage to infrastructure, seawalls overtopped, flooding of lowlands
Fehmarn Island and Mecklenburg Bay	1.5–2.2	4.0–4.8	Heavy dune erosion 4–18 m, cliff retreat 1–9 m	Damage to infrastructure, roads, and marinas, flooding of lowlands
West German coast (Kiel-Eckernförde)	2.15	4.6–5.0	Heavy dune and cliff erosion 3–20 m, low land overtopping	Damage to infrastructure and marinas, no access to roads, flooding of lowlands, disruption of ferry traffic
West German coast (Flensburg-Kappeln)	2.27	4.9–5.4	Dune erosion in bays, erosion of cliffs	Damage to buildings, harbour piers, and transport infrastructure, flooding of lowlands, overtopping of seawalls and dykes
Denmark (Jutland, Fyn, Sealand, Bornholm)	1.67–2.20	3.4–4.5	Dune erosion (3–7 m), flooding of harbours, overtopping of low land	Damage to infrastructure and harbours, flooding, overtopping of seawalls and dykes, disruption of ferry traffic
Sweden (Scania)	1.30–1.50	3.5–4.0	Dune erosion (2–3.5 m), flooding of harbours, overtopping of low land	Flooding and damage to infrastructure areas, disruption of ferry traffic

. In Figure 2, main known impacts on the infrastructure are marked. The number of observed threats is larger in areas with a higher sea level (Figure 2, Table 1).

On the eastern section of the Polish coast, with a beach up to 1 m high, dune retreat was very low. It was observed, for example, near Łeba (LE), at the sea level SL = 0.68 m AMSL (4.00 am UTC, 21 October). Cliff erosion was observed on the western coast of Poland, in sections where the beach was lower than the run-up (Gąski, Łukęcin, and Wolin Island). On the Świna Gate Sandbar in Świnoujście, only the erosion of embryo dunes up to 1.6 m high was observed at the sea level of 0.94 m AMSL (SW). The value of the retreat of the dune base on the Polish coast did not exceed 1.5 m.

On the German side of the coast of the Pomeranian Bay, from Ahlbeck to Heringsdorf, dune erosion was not observed. The eroded sections were cliffs with a low beach north of Heringsdorf and in the vicinity of the seawall in Koserow. Immediately after the storm, sandy sediment landslides were observed that covered the base of the cliff formed from clay. In those places, the height of the beach was lower than the run-up that emerged at the sea level of 1.06 m AMSL in Koserow (7.00 pm UTC, 20.10). A 2–4 m retreat of the base of artificial dunes was observed in Kolpinsee and north of the seawall in Koserow. On the dune section from Zinnowitz to Karlshagen, the water reached the base of the dune. There, the beaches were higher than the run-up of 2.5–2.7 m at a sea level of 1.06 m AMSL. A slight retreat of low dunes was measured in the northern part of Usedom island, in Karlshagen. In the towns and villages on Usedom island, no damage to infrastructure was observed. In the Greifswald-Stralsund section, flooding of the lowlands was observed.

On Rügen Island, the storm caused dunes to be eroded and posed a threat to the seafront promenades in Binz and Glowe. The seawalls in Sassnitz and Fischerdorf were flooded. The promenade behind the seawall in Sassnitz, which is situated up to 2.5 m above sea level, was destroyed. In the town of Binz, the water reached above the seawall, up to 2.5 m AMSL. On the dune section in Prora, the erosion of the dune was 2–5 m. The dunes situated on the spit in the town of Glowe also eroded, with a base retreat of 2–3 m. The chalk cliffs on the island were damaged, although the extent of the erosion was not determined due to the absence of reference points.

On the western Baltic coast, west of Rügen Island, the erosion and infrastructural damage were very large. The number of places with damaged tourist infrastructure and sea havens increased towards the west (from Warnemünde to the west). This was where the sea level exceeded 1.5 to 2.2 m above mean sea level.

On the Fischland peninsula, the dunes exposed to the north were eroded (Zingst-Prerow). In the area of Zingst, the calculated retreat of the dune base was 2–4.5 m. On the western side of the Darss Peninsula, the dune coasts with narrow and low beaches were undercut. As a result, old beech trees that had grown on the edge of the dune fell over. The damage to the coast continued from the Darss Cape to the town of Ahrenshoop, where a clay cliff and an artificial dune embankment were eroded. In the town of Wustrow, the artificially nourished beach with an artificial dune was higher than the run-up, and no dune erosion was observed.

The wide beach in Warnemünde was flooded to 1/3 of its width (observations from cameras above sea level, 1.21 m AMSL, 20 October). In the town of Nienhagen, a clay and silt cliff was eroded. The water reached up to 3 m AMSL. High waves attacked the lower part of the cliff slope. The cliff retreat could range from 0.5 to 1 m. In Heiligendamm, the water reached above the seawall; therefore, the sediment between the rocks and the promenade was washed out (Figure 9). The run-up reached up to 4.2 m AMSL. In Kuhlungsborn, water flowed over the low waterfronts up to a height of 4 m AMSL.

Starting from Warnemünde towards the west, all low and medium-height cliffs formed from clay and silt and exposed to the northeast were eroded (e.g., Nienhagen, Kuhlungsborn, Boltenhagen, Brodten-Niendorf, Dahme, and Fehmarn Island). It was caused by beaches or platforms (Hbe) under cliffs (cl) being lower than the sea level. The retreat of the cliff base at the locations that were analysed in detail was estimated at 2–5 m. On the cliff shore in Boltenhagen, which is the northernmost point in Mecklenburg Bay, the cliff edge retreat reached up to 7 m at several locations. During the run-up of waves up to the height of 4 m AMSL, the bottom half of the cliffs remained constantly underwater. With such a high run-up that persisted for many hours, the cliffs retreated at a high rate.

Even greater erosion was observed on sections with low dunes, ranging from 5 to 18 m of retreat depending on the height of the beach (Hbe). On the low coast with dunes that were exposed to the NE direction on Fehmarn Island, in the towns of Heiligenhafen, Hohwacht, Schönberg, and Schönhagen, water flowed over to the seaside promenade. It destroyed the tourist infrastructure and caused dune erosion. Most reports on flooding and damage were found and analysed from the Mecklenburg Bay, where the sea level exceeded 1.8 m AMSL and the run-up was over 4 m AMSL. In many places, the water flowed over the seawalls, which led to damage to low waterfronts (e.g., Hohenfelde, Fehmarn). The seawalls were also damaged (Fehmarn, Heiligenhafen, and Schleimünde).

In some towns, an interesting phenomenon was observed on the low coast. As a result of the coast retreat, water carried the sediment 50–60 m onto the land. This was a sandy sediment, but it also contained gravel and pebbles. In the southern part of Fehmarn, erosion of the low moraine cliffs reached 4–6 m. The high water carried large amounts of coarse-grained sediment washed out from the beach to the land (as in Damp). Accommodation facilities (campsites) were damaged, as well as leisure infrastructure and access roads. The height of the waterfront at that point does not exceed 4 m AMSL. The reach of the run-up was up to 4.2 m. The seaside roads and tourist promenades on the eastern and northern coasts of Fehmarn Island were partly damaged (in Marienleuchte and Fehmarn). The dunes on the spit in Fehmarn, covered with infrastructure, were also eroded. The water from the bay in the town of Fehmarn flooded the promenade to a height of 3.6 m AMSL. Large erosion and water run-up on the coast were also observed on the southern shore, east of Fehmarn.

The sea level in the port of Wismar, located in a cove that cuts deep into the land to the south, exceeded 1.58 m ASML. The waterfronts of the town were flooded by water up to a height of approximately 3.7 m AMSL. In Travemünde, the high water of 1.75 m AMSL (7.00 pm UTC, 20 October) caused the washing out of the beach and overflowing of waves above the 4.2 m high seawall. In the Scharbeutz resort, situated inside Mecklenburg Bay, the dunes were narrowed by 4–11 m on a 9 km long section. The beach infrastructure and part of the tourist infrastructure on the nearby promenade were destroyed. In the northern part of the bay, the cliffs were eroded. The erosion varied in the section; erosion niches cutting deep into the land (2–5 m deep) emerged. In Dahme, 10–20 cm thick sandy sediment originating from beach erosion covered the promenade and the base of the tourist infrastructure on the waterfront. Buildings and roads were damaged. According to the Coast Protection Services documents (the Schleswig-Holstein State Water Act), the coast in Dahme is secure, and no limitations in using the land behind anti-storm embankments are foreseen [75]. In many areas along this coastal section, anti-storm embankments up to 5 metres above mean sea level (AMSL) failed to protect the low-lying waterfronts from storm-induced flooding. The most severe damage caused by storm Wolfgang was observed on the southern and eastern coasts of Schleswig-Holstein in the western part of the Baltic Sea. This was where the run-up was the highest and reached 4.5 to 5.2 m (Figure 8). That is why so many photos and videos presenting major damage to the seaside infrastructure in this region were published. In small marinas, tens of yachts and sailboats were damaged, sunk, or thrown on the low harbour waterfronts. In all locations where the waterfront was less than 3 m AMSL high, the water damaged seaside boulevards, bicycle lanes, roads, trees, and infrastructure.

On the western coast of the Eckernförde fiord, in the town of Karlsminde, water broke through 4.5 m high dunes and burst 100–200 m inside the land. The campsites situated there were flooded up to a height of 1.3 m. In Kellenhusen, south of Dahme, on the low, 4–5 m high coast, the water created a storm landfall over 200–230 m from the shore. The promenade was covered with sediment from the eroded beach. The leisure and recreation infrastructure was damaged, along with the catering facilities. In Holm, which is situated near Schönberg, water damaged the dunes and then burst into the sea-facing slope of the anti-storm embankment to a height of 7 m AMSL. In an approximately 8 km long section, the dunes formed on the slope of the embankment were completely washed away, and the resulting sediment and plants formed a hummock dyke on the slope of the storm embankment at a height of 4–4.6 m AMSL (the Brasilien and Kalifornien tourist villages).

Large erosion of low dunes and cliffs was also observed on the coasts of the Jutland peninsula exposed to the east, where the sea level exceeded 2 m AMSL. Even cliffs protected by seawalls and waterfront thresholds were eroded, for example, in Kiel-Schilksee. The water that was overflowing significantly higher than the crown of the banks caused flooding of the beach up to a height of 4.0 m. The high rise led to erosion and landslides of the cliff with development on top.

The 3 km long beach with low dunes inside the cove between Eckernförde and Wilhelmsthal was flooded to the height of the seaside promenade, approximately 4.0 m AMSL. The erosion of the dune ranged from 6 to 12 m and up to 18 m at the maximum. In some places, the water flowed through the promenade and the Spa Park. The maximum sea level there was 2.15 m ASML on the evening of 20 October 23.

In Damp, located north of Eckernförde, on the low (up to 4 m) cliff coast with a gravel beach, the storm flooded the area of nearby campsites above the concrete seawall. The infrastructure was destroyed, and the ground was covered with a thick layer of gravel and pebbles that were transported into the land by the inflowing water (Damp, Schuby Strand). This coast is exposed to the eastern direction, i.e., the one from which the highest waves were coming. The run-up at the sea level of 2.15 m AMSL (Eckernförde) reached 4.8 m AMSL (Figure 8). In other places with a low coast covered with gravel and pebbles, a similar phenomenon was observed: coarse-grained sediment was thrown over the low-lying coast. At the same time, the erosion of developed waterfronts caused the collapse of roads and promenades (Hohenfelde-Malmsteg, Hohwacht, Schönberg, and other towns).

In all estuaries and river basins from Wismar to Flensburg, the coasts of the dune and cliffs were eroded, and the promenades, jetties, and harbour infrastructure were flooded. The observations reveal that the sea level on the harbour piers reached 0.6–1.2 m, which caused transport paralysis and made it difficult to take emergency rescue actions. All tourist beaches on the coasts of these narrow bays were reduced by 50% (Wismar, Travemünde, Lübeck, Kiel, Eckernförde, and Flensburg).

In Flensburg, the highest sea level during storm Wolfgang was noted: 2.27 m ASML (11.00 pm UTC, 20 October). The roads and houses in the harbour area were flooded. The water level was even higher than during the previous storms Axel in 4–6 January 2017 and Zeetje in 1–3 January 2019, when the streets of the city had also been flooded (SL = 0.94 m Axel, SL= 1.04 m Zeetje, see

Table 2. Comparison of destructive storm surges (cm AMSL) in the recent 10 years on the southern Baltic Sea coast to some of the largest in 1872 and 1914. Town, gauge station locations in Figure 2.

Year	1872	1914	2017	2019	2020	2022	2023	2024	2025
Day	11–13	8–9	3–6	1–3	14–15	29–31	19–21	2–4	9–13
Month	Nov	Jan	Jan	Jan	Oct	Jan	Oct	Feb	Jan
Name of surge	-	-	Axel	Zeetje	Gisela	Nadine	Wolfgang	Annielle	Charly
Wind azimuth	E-ENE	NE	NW-NE	W-NNE	NE	W-NNE	ENE-E	E-NNE	NNE-N
Flensburg	327	nd	94	104	116	150	227	128	106
Travemünde	330	195	101	123	139	123	181	125	127
Warnemünde	270	nd	99	128	126	131	150	123	115
Sasnitz	200	nd	97	101	99	105	113	83	103
Świnoujście	140	196	142	133	110	99	94	94	121
Kołobrzeg	150	195	150	137	71	120	76	72	120
Ustka	200	nd	145	121	46	102	74	62	117
Władysławowo	no gauge	no gauge	136	135	54	128	57	57	111
Gdańsk	105	156	118	128	49	121	60	54	108

Source: Own data summary based on literature and gauge stations.

). On the Holnis peninsula, which closes the entry to the Flensburg harbour, water was overflowing the low, 5 m high waterfront. Seawalls, tourist facilities, and roads were damaged.

Not far from Flensburg, on the Danish coast in Kegnaes, water washed out part of the flood-protection dyke. The waterfront with the dyke retreated by 4–5 m, which caused extensive damage to an important transport road on the crown. On the Jutland peninsula, flooding of roads and harbour piers was observed. Due to the storm, the traffic of many ferries between the Danish islands was interrupted. The flight departures from Copenhagen Airport were also halted. These actions were justified. It should be noted that in January 2019, during the Zeetje storm, strong wind on the bridge between Denmark and Sweden caused the death of several people on a passenger train, when the tarpaulin and cargo on the passing freight train were torn off and thrown onto the other train. In addition to the sea, the wind is an important factor that causes damage.

On the coasts of the Danish islands, all harbours situated on the eastern coast that were exposed to storm undulation were flooded. Harbour piers and infrastructure were flooded and damaged, including in the towns of Køge, Rødvig, Faxe Ladeplads, Hesnaes, Gedser, and Rødby. Dozens of yachts were sunk and damaged. The water flew over the harbour jetties and seawalls above 3 m AMSL to the harbours and waterfronts.

The chalk and silt cliffs on the Danish coast were eroded. On many sections of the Danish coast, the dunes are artificially maintained [43] due to the low amount of available sandy sediment (e.g., in Copenhagen-Brøndby). This is why water could easily flow over the low, 3–4 m high dunes to the area behind them. These formations were also eroded. The retreat of dunes in the cove south of Copenhagen, in Greve, reached 4–7 m, and 4–5 m high embankments were flooded and broken, and storm cones emerged on the promenade behind the dunes (Figure 6D). In Strøby Egede, water flowed over the seawall to a height of 3.4 m AMSL. In Boto Bay (Falster Island), storm cones emerged on the crown of the dune to a height of 3.5–4.1 m AMSL. In Køge, the land was protected by a seawall and a jetty, and the water flowed over them up to 3–3.5 m AMSL at the sea level of 1.8 m AMSL (Figure 8). In Faxe Ladeplads, the water above the seawall reached 4.0 m AMSL. The dunes in the area of Faxe Ladeplads and from Hesnaes to Geder and Rødby were eroded. On these islands, at a sea level of 1.67 m AMSL, low dunes were destroyed. The erosion of the dune base was 3–6 m.

The erosion of low dunes was observed on the eastern and southern coast of Bornholm Island, in the area of Snogebæk and on the Dueodde cape. The waves caused by the strong wind were up to 5 metres high there. The dune retreat reached 3–5 m.

On the Swedish coast, in Scania, all harbour piers were flooded. Dunes were eroded on the Sandhammaren cape, in the area of Ystad, and on the western coast of the Falsterbo Spit (near Skanör). In Ystad, the run-up reached 3.5 m AMSL at the sea level of 1.3 m AMSL (Figure 8), which caused low dunes to retreat by approx. 2–3.5 m. In Ystad, the maximum sea level was noted on 21 October at 2.00 am UTC. A similar situation occurred on the Falsterbo Spit, where the sea level was higher and exceeded 1.5 m AMSL. The beaches in southern Sweden are narrow and low, and the dunes are 3–5 m high [53]. They are prone to damage at such a high sea level. On 22 October, after the maximum surge, the Marko Polo ferry ran aground on a sandbank on the fairway to the Karlshamn harbour. The sandbank may have emerged as a result of the transport of the sediment by a sea current generated by the wind and undulation during the storm discussed (position marked in Figure 2).

4.2.2. Coast Exposure to the Direction of Undulation by Sea Run-Up (SLr)

Research conducted on the southern Baltic coast confirmed that the exposure of the coast to undulation during storms influences the height of run-up [76]. The change in the orientation of the coast and its exposure modify the water level in the given section of the coast [40]. The analyses of materials that show the erosion of coasts with a different orientation during storm Wolfgang confirm this regularity.

Sections of the coast that were exposed to wind with an undulation from the E direction were flooded to a greater height. This was in sections where the largest dune and cliff retreats were observed: from 4 m on the Danish islands to 8 m on the German coast, to a maximum of 12 m deep inside the coves on the western Baltic coast. Furthermore, the coastal sections that were oriented perpendicularly to the direction of undulation were the most eroded. This applied to both coast types, more-resistant moraine cliffs and spit dunes. On the coasts of Mecklenburg-Vorpommern Land, with the northern exposure of the shores, the erosion was weaker. Small erosion was also noted on the Polish coast, where the wind blowing from the E direction caused a movement of undulation and currents along the shore.

It is noticeable that coasts that are exposed to the direction of the storm waves are eroded more strongly. The coast exposed directly to strong wind and waves from the east (e.g., Prora) was eroded because the water swell was higher than the height of the beaches. Also, 4–5 m high storm embankments and seawalls did not protect from the water run-up on the sections of the western part of the southern Baltic coast that were exposed to the wind from E. There, at the sea level exceeding 2 m AMSL, the run-up reached 5.0–5.2 m AMSL. On the western German coast of the Baltic Sea, storm embankments are up to 5 m and, at the maximum, 7 m high [57,58]. Their height did not protect the land from such a high increase and inflow of water. In some places, the water flowed over these embankments and even caused their erosion, flooding the lowlands that were occupied by tourist facilities (usually campsites). Figure 9 presents examples of three shores with protective bands of similar height and with varying sea level and run-up during the Wolfgang surge. The 4 m high band did not protect land against water inflow with SL = +1.65 m AMSL (Figure 9A). In the presented areas, the threat was not related to the beach or protection band height but only to the SL height and projected run-up range on shore (Figure 9B).

4.2.3. Coast Erosion vs. the Beach Height (Hbe) and Sea Level (HSL)

Dune or cliff erosion is understood as the retreat of the toe or edges of the coastal landforms and the reduction of their volume. As the sea level increases, the height of the water run-up on the shore increases. As a result of higher sea level than usually predicted, coastal land erosion increases, causing an increase in the threats. The probability of coasts erosion and threat with higher beaches also becomes larger (Hbe). The factor that has the strongest influence on coast erosion is the parameter/variable of the storm duration (tHSL), with the sea level (HSL) and run-up (HSLr) higher than the beach (Hbe) [76]. On the other hand, coast erosion depends on the beach height (Hbe) in relation to the run-up height (HSLr). Therefore, the higher the beach, the lower the probability of coastal erosion. Beaches that are lower than the water run-up do not protect dunes and cliffs from erosion [76]. Coasts that are lower than the run-up (SLr) are flooded by the sea [23,24,25], which has frequently been noted on the Schleswig-Holstein coast. This section noted a very high sea level and run-up, up to 4.5–5.2 m AMSL, during storm Wolfgang. Figure 10 presents the measured erosion of the toe of the dunes in selected areas concerning the change in sea level (SL) and run-up (SLr).

Erosion also occurred on the eastern section of the southern Baltic coast. There, only dune coasts with beaches up to 1 m high were eroded; that is, their height was lower than the run-up during storm Wolfgang, with an HSL = 0.6 m AMSL (Figure 10). On Usedom island, the beach up to 1.6 m high was flooded and did not protect the cliff base from erosion at HSL = 1.06 m AMSL. However, the 2.5 m high beach in the northern part of the island stopped dune erosion. The high, artificially formed beach in Wustrow protected the artificial dune from being eroded. On the short spits of the Danish islands and the southern Baltic coast in Germany, low and narrow beaches failed to protect the low dunes from strong erosion.

The moraine cliff coast erosion was also related to the erosive platform or beach height versus sea level. Other erosion of moraine cliffs was observed at a similar beach height, 1.0 to 1.5 m above mean sea level, due to a different sea level (Figure 11). On the Polish coast, at the sea level of 0.76 m in Kołobrzeg, the run-up covered the beach to the base of the cliff, at the height of 1.5 to 1.7 m AMSL. This run-up caused only small erosion of the cliff bases, including those that were not protected by embryo dunes (e.g., Gąski, Łukęcin). The high cliff on Wolin Island was also undercut only on parts of the sandy landslips on beaches up to 1.5 m high. Similar processes were observed on the high cliffs of Usedom island. There, at the sea level of 1.06 m in Koserow, water undercut the sandy landslips on the slopes of cliffs 1–2 km to the north of Heringsdorf and in the Kolpinsee-Koserow region, where the beaches are up to 1.8 m AMSL high. No large damage was observed, and the cliff edge did not retreat.

The moraine cliff coast (cl) erosion was also related to the erosive platform or beach height (Hbe) versus sea level (HSL). Other erosion of moraine cliffs was observed at a similar beach height, 1.0 to 1.5 m above mean sea level, due to a different sea level (Figure 11). On the Polish coast, at the sea level of 0.76 m in Kołobrzeg, the run-up covered the beach to the base of the cliff, at the height of 1.5 to 1.7 m AMSL. This run-up caused only small erosion of the cliff bases, including those that were not protected by embryo dunes (e.g., Gąski, Łukęcin). The high cliff on Wolin Island was also undercut only on parts with the sandy landslips on beaches up to 1.5 m high. Similar processes were observed on the high cliffs of Usedom Island(Figure 11A). There, at the sea level of 1.06 m in Koserow, water undercut the sandy landslips on the slopes of cliffs 1–2 km to the north of Heringsdorf and in the Kolpinsee-Koserow region, where the beaches are up to 1.8 m AMSL high. No large damage was observed, and the cliff edge did not retreat.

On the west coast, at a sea level exceeding 1.5 m, low beaches situated below 1.5 or even 2 m high cliffs were unable to absorb the energy of the waves. In Nienhagen, situated in an area with a sea level of 1.5 m AMSL (Warnemünde), the cliff edge erosion reached 2 m, while the run-up reached almost 3.6 m of the cliff height. At the sea level from 1.6 to 1.8 m AMSL, the cliff edge in Boltenhagen retreated by 7 m. During the run-up of waves up to the height of 4 m AMSL, the bottom half of the cliff remained constantly underwater (Figure 11). The cliff sections of the eastern coast of the Jutland peninsula retreated by 2–5 m at the sea level of 2.2 m AMSL. However, the observation data are incomplete, so it is possible that in certain locations, the erosion was even higher. Low cliffs in the Mecklenburg Bay, which consist mainly of coarse-grained sediment and silt and, thus, are resistant to erosion, retreated by 4–6 m (e.g., on Fehmarn Island and in Damp and Pottloch). Chalk or limestone cliffs that exist on Danish islands were not analysed.

In the dune sections of the coast, the differences in coastal erosion were connected to the differences in beach height (Hbe) compared to run-up height (HSLr). This relation is presented in Figure 12. The higher the sea level, the larger the dune erosion measured. The volume of erosion from the dune depended on the height of the beach and the elevation of the run-up. On usually eroded sections with low beaches, erosion occurred even at a lower sea level. For example, on the eastern coast in Poland, the sea level was 0.57–0.68 m AMSL, and the dunes were eroded in places where the beaches were lower than 1 m. At the sea level from 0.94 m to 1.16 m AMSL, from the Pomeranian Bay to Rügen Island, dune erosion occurred where the beaches were lower than 2–2.5 m AMSL high. Only a small erosion of embryo dunes was noted on the sandy coast east of Świnoujście, where the beach elevation was higher than the run-up (Figure 7A and Figure 10). From Świnoujście through Ahlbeck to Bansin, no dune erosion was observed. The situation was similar in Zinnowitz. On this section of the coast, the beach height was higher than the run-up. Dune erosion was related to sea level, run-up, and beach height (Figure 8).

On the accumulative shores, the beaches are usually 2.5–3.5 m above mean sea level. In these places, only very high run-up at a high sea level caused the erosion of dune bases (in natural and artificial dunes). In conclusion, in places where the run-up was lower than the height of the upper beach, dune erosion did not occur. This was true, for example, in Wustrow, in a section with artificially nourished dunes and a beach with run-up of approximately 3.7 m AMSL (Figure 7D).

On the coasts of Mecklenburg-Vorpommern, Schleswig-Holstein, and the Danish islands (Falster, Zealand), the height of the beaches on short spit-and-dune sections does not exceed 1.5–2.5 m. Due to that, dune erosion occurred on all these sections. Its volume depended on the height of the run-up. On the high, artificially formed beaches in Copenhagen, dune erosion was not observed. In Greve, with a 2.5 m high beach, the retreat of low dunes reached 4–7 m at the sea level of 1.84 m AMSL. The water flowed over the dunes at a height of more than 4 m. In Faxe Ladeplads, the erosion of the dune reached up to 7 m at a sea level of 1.69 m AMSL. In Gedser on Falster Island, it reached 6 m.

On the dune coasts inside the Mecklenburg Bay, the dune retreat was larger on the coasts exposed to undulation from E (5 to 15 m from Travemünde to Fehmarn) and smaller on coasts exposed to the west (from Heilgendamm to Wismar, 3–6 m). A larger retreat of the dune was observed on the protruding coasts in Schleswig-Holstein at a sea level exceeding 2 m AMSL. The retreat of low dunes reached 6–18 m (Schönberg, Damp). Large erosion of the dune was also observed inside the coves and fiords. In Travemünde, the low dune retreat ranged from 6 to 18 m, and in Scharbeutz, from 5 to 11 m. Dunes that were lower than the run-up were washed out completely (HSLr > Hfe).

The statement is that the higher the sea level (SL), related to the run-up (SLr), the higher the dune toe erosion. Figure 12 presents an increase in the retreat of the dune toe in relation to different heights of the sea level during storm Wolfgang.

5. Discussion

The increase in the relative sea level (RSL) of the Baltic Sea in the southern part of the coast [33,34,36] influences the extreme levels during storm surges [28,34,35,75]. This phenomenon is presented in the latest research from other coastal areas [6,7,8,10,11,12,14]. Others are underlining increasing changes in cyclonic activity that lead to more frequent storm surge development [6,7,29,35,36].

Both factors, sea level growth and the intensity change of cyclones, are increasing the probability of erosion and flooding that pose a threat to infrastructure and to human life [1,7,8,11]. The extreme changes in sea level in the Baltic Sea and changes in the frequency of phenomena classified as storms (SL = 0.7 m AMSL [27,28,32,36,76] lead to an increasing threat of coastal erosion and flooding of low areas [37]. Additionally, coasts that are exposed to the direction of the storm waves are eroded more strongly [15,17,21,22,76].

In October 2023, an extremely large storm emerged on the southern coast of the western Baltic Sea as a result of the inflow of water from the extremely rare easterly direction from the open Baltic Sea. As a result of the very-high-velocity wind blowing from the east to the centre of the cyclone that was situated over the Netherlands, the water masses were transported in the westward direction. On the eastern coast of the Baltic Sea, the wind blew along the coast, and no damage was observed to the land, except for short sections of the shore where the beaches were lower than the run-up (Hbe < HSLr). The strongest erosion was noted on all areas of the western Baltic coast. The extent of the erosion of land consisting of clay, silt, and sand increased towards the west from the Usedom and Rügen islands. Tourist and leisure infrastructure was destroyed on the coastal land, suspected to be safe, due to strong storm surges. This is proved by the height of the used coastal protection structures on this part of the Baltic Sea coast that were constructed up to 4 m AMSL [57,58]. The water damaged the infrastructure on the beaches and waterfronts above the eroded low coasts, up to a height of 5.2 m AMSL. The water flowed over seawalls, flood embankments, and jetties. It caused the sinking of yachts and boats in harbours and marinas. In Danish and German digital media, the storm was referred to as the “high water of the century”.

The storm, named Wolfgang, may be compared to the largest known sea surge in the western part of the Baltic Sea on 11–13 November 1872. At that time, the cyclone was also located over Western Europe [42,43,44,77,78,79]. Based on the analysis of historical documents, the erosion and destruction were determined to be large [42,44,77,78,79], and the effects of the storm were analysed multiple times [41,42,43,44,45,77,78,79]. The erosion and large damage occurred on the coasts of Schleswig-Holstein [77] and the Danish islands [41]. During the storm in 1872, the maximum sea level in Travemünde was 3.4 m AMSL at the wind direction E-ENE [40,41,42,43], while in October 2023, the sea level in Travemünde was 1.84 m AMSL at the wind from the E direction. In the western part of the Baltic, exposed to wind–wave action from the east, the highest sea level was marked in Flensburg in November 1872 and 2023. These were the highest SLs noticed on most western parts of the Baltic Sea coasts (Figure 8, Table 1 and Table 2).

Table 2 presents the sea levels for the southern Baltic Sea coast in the last 10 years, including the largest from 1872 and 1914. The higher SL on the western part of the southern Baltic Sea was always impacted by winds from the E or NE. The rise in the sea level each time was related to the coast’s exposure to the wind–wave action. The highest was observed in gauge stations located perpendicular to the wind and wave run-up (Table 2). These surges, which developed during the wind from the NW to NE direction, were the highest on the eastern part of the south Baltic coast [75,76,80,81,82,83,84,85,86]. High sea levels were often observed on the German Baltic coast during storms from the E or NE direction [34,39]. However, high storm surges on the western Baltic coasts when the cyclone is situated on the southwestern Baltic coast occur rarely. That is why there was a large destruction of coastal infrastructure, because the probability of such strong water inflow was neglected as not being possible. This can be proven by the maximum height of protection measures [59], where part of them were overtopped by the storm or broken, causing flooding. Even including other known large surges from the latest years from the eastern part of the southern Baltic coasts, the destruction of infrastructure during the Wolfgang surge was on the largest scale (Table 2).

The cyclone’s appearance and movement caused storms to make landfall on coastal areas with less-predictable values [5,6,7,8,11,26,34,75,76,84,86]. This is explained by climate change, which caused the rare phenomenon of larger surges than expected [1,2,5,7,9,10,12,14,21,37,82,83,84,85,86]. Very high storm surges on the German southern Baltic coast were noted in 1872, 1904, and 1914, with a sea level of 2 m AMSL and with the wind blowing from E to NE [34,40,41,42,43,44,77,78,79,80,81]. High storm surges on the Polish coast occurred at the same time [27,82,84], e.g., during two storms in December 1913 and January 1914, with a sea level from 1.5 to 1.9 m AMSL. During those storms, the sea level on the Polish coast exceeded 1.4–1.6 m AMSL. The double storm of 31 December 1913 from the W-NW direction and 9 January 1914 from the NE-NNE direction led to a large erosion of the whole southern Baltic coast. Numerous seaside buildings were destroyed, from Gdansk (SL = 1.56 m) through Ustka, Kołobrzeg, and Warnemünde to Kiel (SL = 1.82 m).

In the 20th century, very high sea levels were noted during storms that developed from the NW to NE directions, including in January 1983 (SL = 1.43 m), December 1988 (SL= 1.47 m), January 1993 (SL= 1.41 m), and November 1995 (SL= 1.61 m), when the highest level was observed in the eastern part of southern Baltic coast [27,28,35].

In the 21st century, high water surges were also observed in the western part of the southern Baltic Sea, during long storms caused by the cyclones Pia in November 2004, Wimar in October 2009, Andrea in January 2012, Axel in January 2017, Zeetje in January 2019, double Marie&Nadine in January 2022 [35,76,83,84,85,86] and Charly in January 2025 (Table 2). During those storms, strong erosion occurred on the shores of the eastern Baltic coast in Poland [83,84,85,86] and on the central part of the German coast, while lower erosion was noted on the western Baltic coast. The wind and undulation during those surges came from the NW to NNE direction at various stages of their development (Table 2).

Each instance of elevated sea level also increases the water run-up reaching the shore [24,25]. Along the southern Baltic Sea coast, the risk of flooding and erosion statistically increases with rising sea levels [32,51]. This was observed during the Wolfgang surge (Figure 13). The magnitude of the run-up on the open southern Baltic coast was found to be greater than that observed in bays along the German coastline during the analysed storm surge from the east. It may be related to significantly higher waves on the open coast than in straits or bays.

Run-up serves as an indicator of the potential inland extent of seawater intrusion into low-lying coastal areas.

During storm Wolfgang, the most severe erosion occurred on the western part of the southern Baltic coast, coinciding with a very high sea level. This was the result of low atmospheric pressure and the influx of water masses from the east, which caused a higher-than-usual water run-up inland. As the sea level increased, so too did the run-up—understood here as the sum of sea level and wave height (HSLr = HSL + HWa). The relationship between run-up height and coastal elevation is a key indicator in identifying zones susceptible to erosion [23,24,25]. The extent of land erosion and the overtopping of waterfronts are primarily influenced by the sea level and the associated water run-up along the shoreline [24,25,76]. A large scale of erosion is usually related to a higher sea level, higher waves, and a higher shore or coast height [2,3,5,6,11,13,14,15,16,17,18,19,20,21,22,37,76,83,84,85,86]. During the observed surge, land erosion intensified westward in conjunction with rising sea levels (Figure 2, Figure 5, Figure 8, Figure 10 and Figure 11). The run-up was observed to be 1 to 1.5 m higher than the sea level itself (Figure 13), and these values should be taken into account in future coastal management and planning. The run-up is a more adequate parameter than sea level or wave height itself [23,24,25].

On the eastern coast, where sea levels reached up to 1 metre AMSL, erosion was observed only in isolated locations, primarily where beach elevations were lower than the maximum sea level. At sea levels of approximately 1.5 m AMSL, erosion occurred across all sections of cliff and dune coastlines where beaches are typically narrow and low. The volume of erosion correlated with both the sea level height and the duration of elevated water levels. When the sea level exceeded 2 m, the run-up surpassed 4 metres. Consequently, coastal land and waterfront areas below this threshold sustained significant damage. In regions where the sea level exceeded 2.0 m, all coastal types experienced erosion and flooding to elevations above 4.8–5.2 m. At such high run-up values, dune ridges and flood embankments were overtopped or breached, resulting in the inundation of low-lying areas.

These observations suggest that flood embankments lower than 5 m on low-lying coasts [57] are insufficient to withstand storm surges from the E–NE direction. Comparable conclusions have been drawn in other studies [24,34].

6. Conclusions

As a result of the development of a large storm surge caused by the Wolfgang cyclone (in October 2023), erosion of dunes, cliffs, and low shores was observed in the western part of the southern Baltic coast. This was caused by the wind and undulation from the east direction, which occurs extremely rarely in this World region.

In the southern part of the Baltic Sea, storm surges have different levels during the passage of the same cyclone, due to the exposure of the coast to undulation. The position of the cyclone leads to the development of undulation and wind that blows from the given direction to the centre of the cyclone. This is why, during storm Wolfgang, different levels of erosion were noted at a similar sea level and run-up on the shores that were exposed to the east, north, or west. During storm Wolfgang, significant coast erosion was observed on sections of the coast that are exposed to the east. This resulted from the inflow of masses of water from the eastern part of the sea. An important local morphological condition is the elevation of the beach above the maximum run-up. During the discussed storm, no land erosion was observed in those sections where the beaches were higher than the run-up of water.

In the eastern part of the southern Baltic coast, the storm did not cause significant damage, while on the western coast, erosion and infrastructure damage were greater than during the last large storm swells in the 21st century: in January 2017 and 2019. On the other hand, these storms caused major damage on the southern coast of the eastern Baltic Sea. This shows that large storms cause significant damage, depending on the position of the cyclone with respect to the whole Baltic Sea, the duration of the storm, and the direction of undulation from NW to NE.

The volume of erosion during the Wolfgang storm depended on the following factors:

• The position of the given area in relation to the cyclone and the inflow of air masses that caused a surge in the sea level and undulation height;
• The orientation of the coast to the main direction of the wind and undulation;
• The sea level and the maximum run-up resulting from the pressure and the direction and the velocity of the wind;
• The fetch length and wind and the wave height on the open shore;
• Morphological conditions such as beach height, and, partly, its width, and the resistance of sediment to erosion (cohesive rocks or silt, clay, gravel, peat, or sand).

To summarise, this work highlights a real threat to shores and build-up areas where even long-term protection failed due to a extreme rare phenomenon. This should be a warning for ongoing tourist investments on coasts, even those so far rarely affected by larger storms and high surges.