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 four years (2016–2019) of wave buoy measurements from a coastal location outside the City of Helsinki, Gulf of Finland. The wave observations were complemented by 28 years of simulations with a parametric wave model. The sea levels and waves at this location show 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 total water levels corresponding to one hour per 100 years by 0.1–1.2 m. We tested three Archimedean copulas, of which the Gumbel copula best accounted for the mutual dependence between the two variables.

Highlights

  • 15 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, with an aim to determine its effect on the exceedance frequencies of their sum

  • Our results demonstrated that the assumption of independence of sea level and significant wave height leads to an underestimation of the total water levels corresponding to certain probabilities of exceedance

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Summary

Introduction

15 Urbanized and heavily populated coastal regions around the world face concrete consequences of sea level rise and climate change. To account for the additional effect of wind waves, Leijala et al (2018) complemented the still-water-level-based flooding risk estimates with wave run-up on a steep shore They estimated the probability distributions of both phenomena separately 5 and, assuming the two variables independent, calculated the probability distribution of their sum at two coastal locations close to Helsinki, Gulf of Finland. To account for the correlation between sea level variations and wind waves when estimating the exceedance frequencies of 15 their sum, a bivariate (two-dimensional) probability distribution is needed. We explore the correlation of sea level and wind waves near Helsinki on the northern coast of the Gulf of Finland, with an aim to determine its effect on the exceedance frequencies of their sum.

Data and methods
Wind and temperature measurements
Observed wave heights
Simulated wave heights
Ice conditions and wave statistics types
Probability distribution of the total water level
Copula-based probability distributions
Marginal distributions and extrapolation
Results
Discussion
Conclusions
Full Text
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