2022/08/18 by Jensen, Jürgen, Mai, Habib, Beckmann, Simon
#Baltic Sea #FOS: Natural sciences #Küsteningenieurwesen #Meeresregionen #Travemünde #detrending #storm surge
paper · doi:10.18171/1.092102
The protection of human life against flood events is an important task of public interest, especially in times of climate change. For this purpose, the robust design of flood protection structures (e. g. dikes and dams), as well as flood and disaster management are of great importance. Therefore, high-quality hydrological time series (e. g. water levels, discharges) as well as information about water levels of historical storm surges are needed, e. g. to derive design values and assigned probabilities of occurrence using extreme value statistical approaches. Existing methods for dimensioning coastal protection structures are based on the evaluation of systematic recordings (e. g. water levels, discharges), which are usually only available on the German Baltic Sea coast for the last decades. Floods and storm surges are natural events that have always posed a threat to life on the coast and may also cause great damage in the future due to the more intensive use of coastal space and the consequences of climate change. In the past centuries, these extreme events have repeatedly caused catastrophic floods in inland as well as coastal areas and, as a consequence, many fatalities and enormous damage in Germany. The devastating Baltic Sea storm surge of November 12/13, 1872, in which 277 people lost their lives, led to the highest water levels ever recorded on the German Baltic Sea. In this context, the question arises whether the Baltic Sea storm surge of 1872 is representative for coastal protection or has a singular and consequently not design-relevant character. Therefore, the aim of this study is the compilation and evaluation of historical storm surge water levels over the past 1,000 years at the German Baltic Sea coast with special regard to the gauge Travemünde. Taking into account the mean sea level rise (MSLR) and the different gauge datums or different height reference systems, a homogeneous and comparable time series of storm surge water levels is provided for the Travemünde gauge, related to the current mean sea level (MSL) in 2020, as a basis for the design of coastal protection structures. For the last 1,000 years, the development over time of the MSL („Best Estimate”) was reconstructed; over this period, the MSL has risen by about 70 cm by the year 2020. At Travemünde gauge, based on 19-year averages, MSL reached NHN ±0 cm at 1960/61; based on a linear MSLR, the intercept with NHN ±0 cm is about 1972/73. In 2020, MSL reaches a height of about NHN+8.5 cm. In the course of climate change, a significantly higher MSLR can be assumed in the future. The storm surge of 1872 also reached the highest water levels in the context of the historical storm surges of the last 1,000 years at Travemünde gauge and almost at the entire German Baltic Sea coast (at Travemünde gauge at NHN+3.14 m in the fairway and +3.29 m at the nearby Vogtei). However, by detrending the MSL changes, the often-presented singularity of this storm surge cannot be confirmed. The maximum water levels of the storm surges of 1320 and 1625 may have been only 1 to 5 decimeters lower in relation to MSL2020 than the corresponding water levels of the storm surge of 1872. Until today, the water levels of all storm surges after 1872 have been more than 1 m lower than those of the storm surge of 1872. The devastating storm surge of 1872 should continue to be used as a reference for coastal protection on the Baltic Sea coast, regardless of the „appropriate” safety (return period) for which coastal protection on the Baltic Sea is designed. A clear recommendation which maximum peak water level (NHN+3.14 m or +3.29 m) at the storm surge of 1872 should be used for statistical analyses of the time series for the Travemünde gauge cannot be given. To avoid a possible underestimation, the higher value should be used in statistical analysis. Flood and disaster management should definitely assume water levels similar to the storm surge of 1872, plus MSLR.