Directional variation of return periods of water level extremes in Moonsund and in the Gulf of Riga, Baltic Sea

Abstract

We evaluate the regime of water level extremes (both maxima and minima) in the northern part of the Gulf of Riga and in Moonsund, a sub-basin of the Baltic Sea between the Baltic proper and Gulf of Riga. The analysis is performed using water level data from Moonsund and adjacent coastal areas by comparison of single extremes and estimates of return periods of water levels using an ensemble of projections. The water level maxima are less prominent in Moonsund than in the Gulf of Riga or the Baltic proper shores of the West Estonian Archipelago whereas the minima follow those in the eastern Baltic proper. Different from the Gulf of Riga, the background water level in Moonsund mostly follows the one in the Baltic proper. Shallow-water effects play minor role in the formation of high water levels on the shores of Moonsund. High water levels predominantly occur during south-western winds from a narrow direction range while very low water levels are associated with eastern winds from a much wider range of directions. This inference has a direct use as an input for estimates of worst-case scenarios of coastal erosion as well as for the design of coastal engineering structures.