KM-I-3: Height of storm surges
Click to enlargeSource: bevisphoto / stock.adobe.com
2023 Monitoring Report on the German Strategy for Adaptation to Climate Change
Click to enlargeSource: bevisphoto / stock.adobe.com
KM-I-3: Height of storm surges
The maximum tidal high water levels measured at six selected gauging stations in the North Sea and the Baltic Sea indicate a cyclical progress. The time series fluctuate in phases of rising and falling storm-surge water levels. The illustration based on moving 19-year mean values does not indicate any extreme individual events. The trend development differs between the gauges observed.
The line chart KM-I-3 ’Height of storm surges’ shows the moving 19-year mean of the annual maximum tidal high water in centimetres with reference to NHN for six selected gauging stations in the North See and Baltic Sea. Depending on data availability the time series illustrate the years from 1843 the earliest and 1963 the latest to 2021. The height of the maximum tidal high water measured at the gauging stations of Cuxhaven Steubenhöft (North Sea), Kiel (Baltic Sea) and Travemünde (Baltic Sea) has risen significantly during the relevant period of observation. The comparable value measured at the gauging stations of Borkum Fischerbalje (North Sea) and Sassnitz (Baltic Sea) has receded. The development at the gauging station of Wittdün (North Sea) does not indicate a significant trend. At the gauging station Cuxhaven Steubenhöft on the North Sea coast, the height of the moving mean of the annual maximum tidal high water of roughly 350 centimetres referenced to NHN exceeds the height measured at other gauging stations. The lowest value amounting to 100 centimetres (referenced to NHN) was measured at the gauging station Sassnitz.
Storm surges increase flooding risks
In coastal regions, storm surges are among the greatest natural hazards. Storm surges occur when strong onshore wind pushes large volumes of water towards the coast. Storms and hurricanes, in particular, lead to a massive build-up of wind which, depending on the tides, can lead to the development of storm surges in the North Sea. Depending on the wind intensity, the orographic nature of coastal waters and any measures taken for technical flood protection, there is a risk of serious floods in coastal lowlands. On the German coast, storm surges are not uncommon, especially in the winter half-year. On the North Sea coast of Schleswig-Holstein, weather patterns with winds from the west, and in Lower Saxony, with winds from the north are very efficient in causing storm surges, thus making them particularly hazardous.
The water level rise in the North Sea and the Baltic Sea due to climate change provides storm surges with a better starting point so that water masses can mount higher as they reach the shore. In the estuaries, the storm-surge water levels rose to even greater height owing to local anthropogenic measures. This can be seen in particular with regard to the rivers Ems, Weser and Elbe. Engineering measures such as the dyking and straightening of rivers eliminate natural flood plains.
Nowadays people refer to storm surges on the North Sea coasts when the water level rises to at least 1.50 metres above the current mean high water (MHW). Exceeding this threshold value is linked with the (fluctuating) mean sea level (cf. Indicator KM-I-2). Although that does not mean that this will necessarily lead to more frequent storm surges, it nevertheless indicates that storm surges – assuming otherwise equal meteorological framework conditions prevail – will increase in height and are thus apt to cause greater damage. Individual and particularly serious storm surges generally entail damage to coastal buildings and infrastructures. In the past, such damaging storm surges occurred repeatedly on the North Sea coast. For almost 2000 years storm surges have been documented on German coasts. One of the most devastating storm surges of the past 100 years was an event on 16th February 1962, which caused damage on the coastline of the entire German Bight, but especially in Hamburg, involving numerous fatalities. Although the storm surge of 1976 reached a greater height, there was better protection in place thanks to coastal protection having been implemented previously. The big storm surge in northern Friesland in November 1981 caused extensive damage to dunes on the North Sea islands. In December 1999 hurricane Anatol reached storm peaks up to 200 kilometres per hour and briefly caused very high increases in sea level throughout the North Sea area. In December 2013 the entire North Sea area was affected by hurricane Xaver and storm surges which were quite severe in places. The dykes on the mainland managed to withstand the onslaught of water masses, whereas on the islands in eastern and western Friesland, dunes were severely breached in many places. Latterly, in the beginning of 2022, several – partly severe and one very severe – storm surge / s (>3.5 metres above MHW) occurred on the North Sea coast. However, those storm surges have not yet been incorporated in the data base for the Indicator KM-I-3 illustrated in this report.
On the Baltic Sea, the duration, direction and intensity of the wind determine whether storm surges occur. Water levels of one meter and more above the mean sea level are regarded as storm surges here. Given that the tides hardly play a role in this sea, storm surges in the Baltic Sea can last for one or two days. To a lesser extent, the volume of water in the Baltic Sea, natural oscillations in water masses (also known as ‘seiches’) and changes in air pressure contribute to this phenomenon. The most severe storm surge in the south-western part of the Baltic Sea occurred on 13th / 14th November 1872 causing major destruction. 271 people lost their lives in this disaster.
On the German Baltic Sea coast minor storm surges occur every year. Serious storm surges are less frequent, but they did occur at the beginning of 2017 and 2019 respectively. The highest water levels were recorded in Wismar: towards the evening of 4th January 2017 with 1.83 metres, and on the afternoon of 2nd January 2019 with 1.91 metres above the mean water level. After these events had taken place, damage was recorded especially on the coasts and beaches. At the end of 2022, the low-pressure system Nadya first caused a low-water level followed instantly by a storm surge. Owing to the low-pressure system shifting, it first resulted in a strong south-west wind which made water levels recede; subsequently, the wind shifted north thus capturing also the central part of the Baltic Sea. From there the water was pushed towards the western part of the Baltic Sea while localised hurricane-type gusts from the north pushed the water towards the coast. In Flensburg, for instance, the water level dropped within roughly 16 hours from 1.59 metres to 1.49 metres above the mean water level, thus in fact rising by a total of more than 3 metres.
Changes in the intensity of storm surges caused by rising sea levels can be illustrated by the annual maximum tidal high water (HThw) measured at the North Sea gauging stations and for the annual maximum high water (HW) measured at the Baltic Sea gauging stations. For the indicator, these coefficients of water levels were calculated by the BfG by means of using a low-pass filter to determine the 19-year moving mean.
For the purpose of illustration in the indicator, meaningful individual gauging stations were selected, making sure that readings would not be influenced too much by structural changes that took place in recent years or by local circumstances. Furthermore, the objective was to ensure a regional distribution of gauges between the North Sea and the Baltic Sea, and also between the relevant coastlines. As far as the levels occurring in the North Sea are concerned, the gauging stations at Cuxhaven Steubenhöft, Borkum Fischerbalje and Wittdün on Amrum were selected. The gauging stations at Kiel, Travemünde and Sassnitz are located on the Baltic Sea coast.
The height of the storm surges measured at the selected gauging stations in the North Sea and the Baltic does not indicate a significantly rising trend. Phases of rising and receding maximum levels alternate with a periodicity of roughly 50 to 70 years. The illustration based on moving 19-year mean values does not indicate any extreme individual events. It is only when such events occur very frequently that values increase.
