Victor Alari

Simulation of Wave Damping Near Coast due to Offshore Wind Farms

Abstract ALARI, V. and RAUDSEPP, U., 2012. Simulation of wave damping near coast due to offshore wind farms. Two hundred wind turbines with an annual productivity of 2.3 TWh, which could produce up to 30% of the energy Estonia needs, are scheduled to be constructed on separate shallows in the NW Estonian coastal waters (the Baltic Sea), 5–20 km off shore. With numerical modeling, we have established a potential impact of the wind farms on wave heights. We concluded that the impact exists in terms of the reduction of significant wave height, but it is very marginal, not more than 1% below 10-m isobaths. This is due to a very small ratio between the turbine diameter and dominant wavelength and the favorable setup of turbines with respect to each other and the coast.

Multi-scale analysis of wave conditions and coastal changes in the north-eastern Baltic Sea

ABSTRACT Suursaar, Ü., Alari, V., Tõnisson, H., 2014. Multi-scale analysis of wave conditions and coastal changes in the northeastern Baltic Sea. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 223–228, ISSN 0749-0208. Temporal variations of shoreline changes have been analyzed and interpreted in three differently exposed Estonian coastal sections. Using coastline contours that have been recorded frequently over the last twelve years, as well as recently digitized aerial photographs, orthophotos and old topographic maps (some of them dating back to 1900), all overlaid in the Mapinfo software, areal changes over different sub-periods were calculated. To explain the shoreline changes, two different wave modelling approaches were used and mutually compared. Both the BaltAn65+ reanalysis (an ERA-40 refinement) forced SWAN model hindcast (1965–2005) and the point model runs (1966–2012), locally and independently calibrated against extensive wave measurements in these coastal study sites, confirmed specifically higher (and increasing) intensity of coastal processes in the westerly exposed study sites, and a decrease in northerly exposed sites. Some common quasi-periodic cycles with high stage approximately in 1985–1995, and probably also from 2007 can be found. However, the role of a few randomly occurring extreme winter storms (such as in 1967, 2005, 2007 and 2012) was often decisive within the sub-periods.

Transport of sediments resuspended by ferries

In this study we propose a scheme of sediments transport in Tallinn Bay. Our previous works used optical methods to estimate that fast ferries bring into motion of the order of nearly 10,000 kg of sediments per running meter of coastline per year. But the question is still open, where the sediments will be transported and which part of them will be carried out of the coastal zone. We used an Aanderaa sonde RDCP 600 to measure the wakespsila velocity and direction near the most endangered coast. The results were somewhat unexpected - the near bottom velocity (~0.1 m/s) was typical for the Tallinn Bay, but for all the measurements the direction was not along the shore but 45deg from the shore line. That means the sediments brought into motion by the fast ferries will quickly and irreversibly be transported away from the coast to the deeper (20-50 m) sea areas. Wave parameters were recorded and analyzed during the experiment also. Ferry wakes were categorized by the height as well by the period.

Comparing a 41-year model hindcast with decades of wave measurements from the Baltic Sea

Abstract We present ice-free and ice-included statistics for the Baltic Sea using a wave hindcast validated against data from 13 wave measurement sites. In the hindcast 84% of wave events with a significant wave height over 7 m occurred between November and January. The effect of the ice cover is largest in the Bay of Bothnia, where the mean significant wave height is reduced by 30% when the ice time is included in the statistics. The difference between these two statistics are less than 0.05 m below a latitude of 59.5°. The seasonal ice cover also causes measurement gaps by forcing an early recovery of the instruments. Including the time not captured by the wave buoy can affect the estimates for the significant wave height by roughly 20%. The impact below the 99th percentiles are still under 5%. The significant wave height is modelled accurately even close to the shore, but the highest peak periods are underestimated in a narrow bay. Sensitivity test show that this underestimation is most likely caused by an excessive refraction towards the shore. Reconsidering the role of the spatial resolution and the physical processes affecting the low-frequency waves is suggested as a possible solution.

Applicability of SAR-based wave retrieval for wind–wave interaction analysis in the fetch-limited Baltic

ABSTRACT In this article, a method for the detection of wave field parameters from synthetic aperture radar (SAR) imagery in the fetch-limited Baltic Sea is presented. Over the Baltic Sea region, common southwest (SW) and west (W) winds induce steep waves with shorter wavelengths compared with ocean waves. Thus, with the use of previous SAR sensors (e.g. ENVISAT/ASAR), it was not possible to detect individual waves and retrieve image wave number spectra. Since the year 2007, when TerraSAR-X (TS-X) reached its orbit, high spatial resolution data is available for measuring the sea-state parameters: the individual waves up to 30 m wavelength and their refraction can be distinguished. The main objective of this work was to demonstrate the capability of detecting wave field parameter from (TS-X) imagery in the Baltic Sea. The wave field parameters obtained from the SAR imagery were compared with in situ measurements and the Simulating WAves Nearshore (SWAN) wave model. The comparison of SAR-based wave field information with buoy measurements showed high agreement in case of wave propagation direction (r = 0.95) and wavelength (r = 0.83). A significant correlation is also seen between SWAN- and SAR-derived wave propagation direction (r = 0.87) and wavelengths (r = 0.91). With the case studies, it is shown that SAR data enables one to detect land shadow effects and small-scale wave field variations in the coastal zone. It was shown that SAR data is also valuable for improving and interpreting the wave model results. In consequence of common slanting fetch cases over the Baltic Sea region, it was demonstrated that the peak wave directions differ from the mean wind directions up to 43°.

Projected Changes in Wave Conditions in the Baltic Sea by the end of 21st Century and the Corresponding Shoreline Changes

ABSTRACT Suursaar, Ü.; Tõnisson, H; Alari, V.; Raudsepp, U.; Rästas, H., and Anderson, A., 2016. Projected changes in wave conditions in the Baltic Sea by the end of 21st century and the corresponding shoreline changes. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1012 - 1016. Coconut Creek (Florida), ISSN 0749-0208. The aim of the study is to analyse possible future changes in the Baltic Sea wave conditions and to project coastal changes in six differently exposed Estonian coastal sections resulting from changing wind climates. In the open parts of the Baltic Sea, the SWAN model with 3 NM spatial resolution was used for simulation of wave fields in 1966–2100. Regional climate projection EUR-11 assuming the RCP4.5 greenhouse gas scenario was used as wind forcing. In addition, using a site-dependently calibrated fetch-based wave model, a set of semi-realistic scenario calculations was obtained by modifying the baseline wind input data in order to investigate the reaction of wave climates and coastal developments. For coastal change, past developments in the shoreline and accumulation-erosion areas were tracked using repeated GPS measurements and GIS-overlaid cartographic and photographic material. The projections showed spatially and temporally varying wave fields and a slight overall increase, which corresponds to increased south-westerly winds. Depending on exposition, the wave climates would change differently even within a single semi-enclosed sea. Using the previously established empirical relationships between wave parameters and shoreline changes, we predict that erosion will probably increase in transitional zones while accumulation increases within bays. Sea-level rise and shortening of the sea-ice duration will probably have a remarkable contribution.

Variability of Benthic Communities in Relation to Hydrodynamic Conditions in the North-Eastern Baltic Sea

ABSTRACT Herkül, K.; Torn, K.; Suursaar, Ü.; Alari, V., and Peterson, A., 2016. Variability of benthic communities in relation to hydrodynamic conditions in the north-eastern Baltic Sea. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 867–871. Coconut Creek (Florida), ISSN 0749-0208. Wave forcing is an important hydrodynamic variable that influences spatial patterns and temporal dynamics of marine benthos. In this study, we used three principally different wave models – simplified wave model (SWM), spectral wave model SWAN (simulating waves nearshore), and locally calibrated point model (LCPM) – to assess the effects of wave forcing on the distribution and temporal dynamics of macrobenthos in the north-eastern Baltic Sea. Other important environmental variables like depth, salinity, turbidity etc., were also included into analyses of benthos distribution in order to quantify the relative importance of wave forcing. Following depth, SWM was the second most important environmental predictor of spatial distribution of benthos. The importance of SWAN was the lowest among environmental variables due to its low spatial resolution (1 nautical mile) compared to SWM (25 m). Considering the temporal dynamics of benthos, wave height significantly correlated with several benthic variables, but the direction and magnitude of effects were site-specific.

Simulating wave-surge interaction in a non-tidal bay during cyclone Gudrun in January 2005

Cyclone Gudrun (Erwin) crossed the Baltic Sea in 8-9 January 2005. The maximum sustained wind speed measured over the Baltic Proper reached 28 m/s (gusts 37.5 m/s) and prior the high surge (2.75 m in Pärnu city), wind blew mainly from the sector SW-WSW. The hydrodynamic consequences, coastal damages and wave conditions in Baltic Proper and Gulf of Finland resulting from windstorm Gudrun have been analyzed previously. Lacking was the knowledge of wave-surge interaction and the role of wave induced setup. The aim of this paper is to study the effects of surge upon surface wave field dynamics and to reconstruct the possible wave induced set-up at a natural beach by means of numerical modeling. Modeling system consisting of a spectral wave model SWAN and non-hydrostatic depth-averaged free surface flow model SWASH was implemented. Spectral model was implemented to describe wave conditions in the Baltic Sea during the passage of cyclone and for providing boundary data to SWASH model, which in turn is used to calculate setup and inundation. Modeling relies profoundly on the quality of modeled wind fields -hence the accuracy of downscaled ERA40 wind fields during cyclone Gudrun is analyzed. We conclude an overall good level of agreement between modeled winds and observations and suggest using it in further modeling studies. Significant wave height in Pärnu bay increases up to 1 m by taking into account the additional deepening of water due to surge. The transformation of waves over the swash zone results in wave induced setup of 0.51 m and additional inundation of 130 m.

Near bottom velocity and turbidity measurements in coastal waters of NW Estonia

Dependence of near bottom currents and turbidity on wind and wave parameters is analyzed. Measurements campaigns with an acoustic Doppler velocymeter (ADV Field/Hydra, SonTek/YSI) were curried out in two bays in north western Estonia — the first one on Naissaar Shallow in Tallinn Bay (22.12.2009–12.01.2010, water depth 9 m, 37 cm from the bottom) and the second one in Keibu Bay (03.06.2010–26.06.2010, water depth 7m, 27 cm). Near bottom velocities were recorded with frequencies 2 Hz (currents) and 0.2 Hz (wave induced orbital motion). Additionally the water turbidity at the same level as flow measurements was performed using an integrated turbidity meter OBS 3+ (YSI). Wave parameters were recorded using a pressure wave gauge (PTR Group, Tallinn). The ADV measured flows consist of wind induced currents, wave induced orbital motions and turbulence. Maximum of wind induced currents reached meanly 10–15 cm/s at both measurement locations, while the maximum near bed orbital motions peaked over 40 cm/s. Measurements showed that the near bottom velocities in Keibu Bay were in correlation with wind speed, but turbidity values showed a significant increase only in some special weather conditions. From the comparison of ADV, turbidity meter and wave gauge characteristics it followed that turbidity was clearly depending on the wave energy. It means only quite long and high waves inducing bottom orbital velocities (calculated from the wave gauge data) over 20 cm/s were able to resuspend bottom sediments.

Estimation of wave field parameters from TerraSAR-X imagery in the Baltic Sea

Current study is focused on detection of wave field parameters from SAR imagery in the Baltic Sea. Study is carried out over the Baltic Sea region where common SW and W winds induce steep waves with shorter wavelengths compared to ocean waves. As TerraSAR-X data has high spatial resolution (0.75-1.5m per pixel) compared to previous SAR sensors (e.g. ENVISAT/ASAR), it enables to detect the two-dimensional wave spectrum even in the Baltic Sea. Main objective of this work was to demonstrate the capability of detecting wave field parameter from TerraSAR-X imagery in the Baltic Sea. The wave field parameters obtained from SAR imagery was compared with in situ measurements and SWAN (Simulating Waves Nearshore) wave model. The comparison showed significant correlation between SAR derived and buoy measured wave propagation direction (r = 0.89, rmsd = 45°, bias = 8.5°) and wavelengths (r = 0.71, rmsd = 14.8m, bias 0.4m). Comparison of SAR based wave field information with SWAN wave model outputs showed also good agreement in case of wave propagation direction (r = 0.72, rmsd = 35°, bias = 14.8°) and wavelengths (r = 0.85, rmsd = 12.1m, bias = 2.4m). Peak period was also calculated from SAR based 2D wavenumber spectrum and compared with SWAN results. Case studies showed that SAR data enables to detect land shadow effects and small scale wave field variations in the coastal zones.