Abstract
Abstract. Both sea level variations and wind-generated waves affect coastal flooding risks. The correlation of these two phenomena complicates the estimates of their joint effect on the exceedance levels for the continuous water mass. In the northern Baltic Sea the seasonal occurrence of sea ice further influences the situation. We analysed this correlation with 28 years (1992–2019) of sea level data, and 4 years (2016–2019) of wave buoy measurements from a coastal location outside the City of Helsinki, Gulf of Finland in the Baltic Sea. The wave observations were complemented by 28 years of simulations with a parametric wave model. The sea levels and significant wave heights at this location show the strongest positive correlation (τ=0.5) for southwesterly winds, while for northeasterly winds the correlation is negative (−0.3). The results were qualitatively similar when only the open water period was considered, or when the ice season was included either with zero wave heights or hypothetical no-ice wave heights. We calculated the observed probability distribution of the sum of the sea level and the highest individual wave crest from the simultaneous time series. Compared to this, a probability distribution of the sum calculated by assuming that the two variables are independent underestimates the exceedance frequencies of high total water levels. We tested nine different copulas for their ability to account for the mutual dependence between the two variables.
Highlights
Urbanized and heavily populated coastal regions around the world face concrete consequences of sea level rise and climate change
We explore the correlation of sea level and wind waves near Helsinki on the northern coast of the Gulf of Finland (GoF), with the aim of determining its effect on the exceedance frequencies of their sum
The sea level variations and significant wave heights show a positive correlation in general (τ = 0.20)
Summary
Urbanized and heavily populated coastal regions around the world face concrete consequences of sea level rise and climate change. To ensure safe and effective coastal protection and city planning in the future, accurate and location-targeted estimates of coastal flood probabilities are in demand. The ultimate height to which the sea water rises in a coastal flood is determined by both the sea level variations – so-called still water level – as well as wind-generated waves on top of that. When estimating coastal flooding risks, these both need to be considered. The flooding risks related to still water level have been extensively studied using long, high-quality tide gauge records, which in many locations date back to the 19th century (e.g. Johansson et al, 2001; Eelsalu et al, 2014; Soomere et al, 2015; Kulikov and Medvedev, 2017). These data have enabled detailed analyses of sea level extremes and probability distributions useful for practical applications in coastal planning (e.g. Kahma et al, 2014)
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