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Spatiotemporal variability of wave climate in the Gulf of Riga

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Najafzadeh F. , Jankowski M. Z. , Giudici A. , Männikus R. , Suursaar Ü. , Viška M. , Soomere T.

Oceanologia

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2024

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tom Vol. 66 (1)

56--77

EN

Basic properties of wind wave climate in the Gulf of Riga, the Baltic Sea, are evaluated based on modelled wave fields, instrumentally measured and historical visually observed wave properties. Third-generation spectral wave model SWAN is applied to the entire Baltic Sea for 1990–2021 with a spatial resolution of 3 nautical miles (nmi, about 5.5 km) forced by the wind field of ERA5, to the Gulf of Riga and its entrance area with a resolution of 1 nmi (about 1.85 km), and to nearshore areas of this gulf with a resolution of 0.32 nmi (about 600 m). The calculations are performed for an idealised ice-free climate. Wave properties are represented by 36 directional and 32 frequency bins. The simulations are complemented by five sessions of instrumental measurements in the 2000s and two sets of historical visual wave observations from the island of Ruhnu and the Sõrve Peninsula for 1954–2011. Predominantly representing fetch-limited windseas, the wave climate in the gulf is milder and more intermittent than in the open Baltic Sea. The average significant wave height is mostly in the range of 0.6–0.8 m and peaks at 0.82 m inside the gulf. Typical wave periods are shorter than in the Baltic proper. The spatial pattern of wave heights, with higher wave intensity in the northern and eastern parts of the basin, follows anisotropy in wind conditions. Interannual variations are highly synchronised in different parts of the gulf. Their magnitude is less than 10% of the long-term average wave height. No long-term trend has been found in significant wave height and no distinct decadal variation exists inside the gulf.

2

Modelling nearshore hydrodynamics and circulation under the impact of high waves at the coast of Varkiza in Saronic-Athens Gulf

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Belibassakis K. A. , Karathanasi F. E.

Oceanologia

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2017

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tom No. 59 (3)

350--364

EN

A plethora of physical parameters, such as hydro-, litho- and morpho-dynamic characteristics, are essential for understanding the response of coastal systems to intense sea states in terms of sediment transport and shoreline evolution. Nowadays, numerical models are extensively applied to meet the above needs and support coastal planning and management. In the present work, a 2DH dynamic modelling system is used for simulating the hydrodynamic and meteorological/oceanographic characteristics of the Saronic Gulf, in order to examine circulation patterns and predict sediment transport phenomena under high wave conditions at the coast of Varkiza, a sandy beach in the southern Attica, Greece. Time series of wind and wave data were used as input at the open boundaries of the model domain while the model was calibrated and validated through (linear and directional) statistical measures with respect to in situ wave measurements, since there was lack of hydrodynamic data at the site of interest. The simulation period of the model was between January 3 and February 19, 2013, with consecutive high waves in-between. The good agreement of the numerical results from the wave and hydrodynamic model with in situ measurements confirmed the suitability of the model for the support of sediment transport rates at Varkiza coastal segment. Model results reveal that there is a counter-clockwise water circulation during high waves that contribute to the erosion of the examined beach, which is also confirmed by independent field measurements.

3

Assessment of wave climate and energy resources in the Baltic Sea nearshore (Lithuanian territorial water)

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Jakimavičius D. , Kriaučiūnienė J. , Šarauskienė D.

Oceanologia

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2018

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tom No. 60 (2)

207--218

EN

The main task of the present research was to analyse wave climate and evaluate energy resources in the Lithuanian territorial waters of the Baltic Sea. Wave and wind parameters were analysed according to long-term measurement site data. Distribution of wave parameters in the Baltic Sea Lithuanian nearshore was evaluated according to wave modelling results. Wave energy resources were estimated for three design years (high, median and low wave intensity). The results indicated that in the coastal area of Lithuania, waves approaching from western directions prevail with mean wave height of 0.9 m. These waves are the highest and have the greatest energy potential. The strongest winds and the highest waves are characteristic for the winter and autumn seasons. In the Baltic Sea Lithuanian nearshore, the mean wave height ranges from 0.68 to 0.98 m, while the estimated mean energy flux reaches from 0.69 to 1.90 kW m−1 during a year of different wave intensity. Distribution of energy fluxes was analysed at different isobaths in the nearshore. Moving away from the coast, both wave height and wave power flux increases significantly when water depth increases from 5 to 20 m. Values of the mentioned parameters tend to change only slightly when the sea is deeper than 20 m. In a year of median wave intensity, the mean wave energy flux changes from 1.10 kW m−1 at 10 m isobaths to 1.38 kW m−1 at 30 m isobaths. The identified differences of wave height and energy along the selected isobaths are insignificant.

4

Spatial patterns of the wave climate in the Baltic Proper and the Gulf of Finland

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Soomere T. , Räämet A.

Oceanologia

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2011

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tom No. 53 (1-TI)

335-371

EN

We make an attempt to consolidate results from a number of recent studies into spatial patterns of temporal variations in Baltic Sea wave properties. The analysis is based on historically measured and visually observed wave data, which are compared with the results of numerical hindcasts using both simple fetch-based one-point models and contemporary spectral wave models forced with different wind data sets. The focus is on the eastern regions of the Baltic Sea and the Gulf of Finland for which long-term wave data sets are available. We demonstrate that a large part of the mismatches between long-term changes to wave properties at selected sites can be explained by the rich spatial patterns in changes to the Baltic Sea wave fields that are not resolved by the existing wave observation network. The spatial scales of such patterns in the open sea vary from > 500 km for short-term interannual variations down to about 100 km for long-term changes.

5

Numerical simulations of wave climate in the Baltic Sea: a review

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Soomere T.

Oceanologia

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2023

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tom Vol. 65 (1)

117--140

EN

Efforts towards the numerical simulation of the Baltic Sea wave properties, started in the 1950s, have reached maturity by the implementation of contemporary third generation spectral wave models, such as WAM and SWAN. The purpose of this paper is to give an overview of the relevant efforts since the beginning of numerical wave simulations. The Sverdrup-Munk-Bretschneider (SMB) type models are still valuable tools for rapid estimates of some properties of wave climate in single locations. The spatial resolution of spectral wave models for the entire sea has increased from about 20 km to 1 km, and to 100–200 m in specific areas. The number of directional bins has increased from 10–15 to 24–36 and the number of spectral frequency bins from about 15 to 35–42. The models replicate all main features of the wave climate of the Baltic Sea, such as an overall mild but intermittent wave climate, the predominance of short windseas and the scarcity of long swell, east-west asymmetry, the strong impact of seasonal ice, and the specific properties of wave growth in some areas. The wave climate changes involve variations in regional wave intensity, core properties of wave-driven sediment transport and wave set-up. Reconstruction of wave properties in the nearshore, archipelago areas, and in narrow subbasins remains a challenge. These areas require finer spatial resolution and possibly advancement of wave physics to account for changes in the spectral composition of wave fields and specific features of wave growth in narrow basins. Progress in these fields is a pillar for a number of applications, from the quantification of sediment transport to proper input into management issues of the coastal zone.

6

A cost-effective method for estimating long-term effects of waves on beach erosion with application to Sitia Bay, Crete

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Karathanasi F. E. , Belibassakis K. A.

Oceanologia

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2019

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tom No. 61 (2)

276--290

EN

Considering the significant role of beaches for the sea environment and welfare of coastal communities, a variety of process-based models are applied in order to examine and understand the interaction of hydrodynamic processes with seabed material at different time scales. However, a long-term view of this interaction requires a great amount of computational time. In this work a cost-effective methodology is proposed to surpass this shortcoming and estimate bed level evolution. The technique is relied on an objective criterion to assess spectral wave time series of wave height, period and direction and identify the wave conditions that contribute to the initiation of sediment movement. After implementing the so-called Shields criterion, the full wave climate is reduced to two classes of representative wave conditions: the over-critical ones, mainly responsible for long-term erosion, and the sub-critical wave conditions. By applying a well-known process-based model, the representative wave conditions are used as input for the wave-current-sediment transport simulation and rates of bed level changes are obtained, on the basis of which the long-term effects of waves on beach erosion are estimated. Taking into account that erosion is a threatening phenomenon along the sandy beaches of Mediterranean Sea, the present method is demonstrated at a sandy coast of Sitia Bay, Crete. The bed levels derived from the proposed methodology and the full time series are compared. The results indicate reasonable agreement at the selected locations with deviations under 7%, and conformity of the tendency of seabed evolution, rendering the new methodology a useful tool.