Tarmo Kõuts

A stochastic model for the sea level in the Estonian coastal area

Abstract A stochastic model is suggested to perform the space–time optimal analysis of the sea level data recorded in 1978–1982 at 20 stations at the Estonian coast and along a part of the Latvian coast surrounding the Gulf of Riga. The original time series recorded with the time lag of 1, 6 or 12 h are divided into mean and fluctuation components. The mean field is modeled as the sum of the linear trend and annual harmonic. The mean sea level is generally higher at the stations located in the river mouth area. The estimated linear trend yielding the sea level rise of 1–3 cm/year is an approximation of the interannual variability over the selected 5-year period. The dominating annual harmonic with amplitude of 20 cm describes 40–45% of the total variability of the time series of the monthly mean sea level values. The temporal and spatial correlations of the sea level fluctuation field were estimated on the basis of the suggested stochastic model. The correlation functions were approximated by Gaussian functions yielding the temporal correlation radius (e-folding scale) of about 10 days and spatial correlation radius of 200–400 nm. According to the developed criterion, proceeding from the suggested stochastic model, at least 90% of the sea level data from the Estonian coastal area should be considered as meeting the quality requirements. There was no significant difference in the quality of data measured either continuously by mareographs or observed by reading the bench sticks. After removal of outliers, the approach was utilized to reconstruct the sea level field in the Estonian coastal area in 1978–1982 with an acceptable low reconstruction error.

Environmental monitoring of water quality in coastal sea area using remote sensing and modeling

Coastal sea of the southern Gulf of Finland is under heavy anthropogenic stress due to development of harbors and increase of ship traffic. During dredging operations, the increase of suspended particulate matter (SPM) concentration in seawater is inevitable. Water transparency decreases with an increased SPM concentration, which leads to worsening of underwater light conditions. Diminishing of light intensity that penetrates to the sea bottom has negative effect on the growth of benthic macroalgae, which is an indicator of water quality in the coastal zone. Monitoring of SPM transport and distribution along with the estimation of dredging impact on marine environment is crucial especially when sensitive and critical marine areas are close to dredging site. The monitoring system that combines satellite remote sensing and numerical modeling and is supported by measurements is presented. The modeling part consists of hydrodynamic model, particle transport model and benthic macroalgae growth model. A simple approach is followed in the formulation of the system and on the determination of required relationships that are based on the measurements. The monitoring system was applied to Pakri Bay during dredging in Paldiski North harbor, which lasted a one and half year. Comparison of SPM distributions from remote sensing images and numerical model results showed qualitatively similar patterns. Quantitative comparison allowed separating SPM concentrations due to dredging operations from background values of natural origin. Underwater light conditions were the most affected close to the harbor and in the coastal sea. Estimations of the changes of macroalgae biomass due to diminishing underwater light intensity showed minor effect, as the dredging was carried out mainly from late autumn until early spring when macroalgae growth is limited by water temperature.

Use of lightweight on-line GPS drifters for surface current and ice drift observations

A family of lightweight (less than 15 kg), long-life (up to 3–4 months) and low-cost drifters are developed in the Tallinn University of Technology and local engineering companies. Such buoys have a rugged and simple housing construction with the diameter of 11 cm and length of 1 m. The buoys are equipped with GPRS based two-way communication and they are programmable during the mission. The data are transferred in real time into one or several FTPs for the operational use in circulation and ice models for data assimilation and/or model validations. The buoys are used also for the estimation of oil drift and ice dynamics properties, e.g. ice drift, compacting, ridging, etc. Real-time (on-line) data transfer is performed via the GPRS protocol with a typical time interval of 15 minutes, but it can be programmed otherwise. The data received in FTP are ready to be used for model assimilation and in different operational applications and services like the drift of surface objects, ice and oil drift. It is possible to view the buoy data in real time via a special web solution. Two major measurement campaigns have been performed using light drifters. Firstly, the validation of High Resolution Operational Model for the Baltic Sea (HIROMB) surface currents in Estonian coastal waters in 2007 and secondly, the study of dynamic ice drift. In both cases the light on-line drifters showed good performance and stability for long-life operation. The percentage of fault positioning was higher in water, reaching up to 20%, and was mainly caused by the reflections of GPS signal from the waves, since a very small part of the buoy remains above the sea surface. In ice the fault positioning was lower, less than 10%. In the case of an ice covered sea, the buoys were from time to time pushed under ice for some period during which the positioning systems failed to transfer the signal. In 2007 during the period from August till November 17 experiments were carried out with surface drifters released in small coastal bays and the open sea of the Gulf of Finland. The longest period when a buoy was drifting lasted for 8 days. It moved across the Gulf of Finland covering a little less than 50 miles. In accordance with the drifter experiments the drifter trajectories that imitate oil spill drift were calculated from HIROMB currents and wind. The calculations were carried out with a different wind factor between 0–3%. The results of modeling showed close agreement with drifter data in some cases, but no unique wind factor suitable for the Gulf of Finland could be identified. In March-April 2010, an extensive ice drifter experiment was conducted in the eastern part of the Gulf of Finland. The development of light on-line GPS drifters continues. The platform of data logging and transfer allows adding a number of parameters to be measured by the drifter and transmitted on-line. To begin with, water temperature and salinity sensors will be added.

Precise Hydrodynamic Leveling by Using Pressure Gauges

This study investigates the applicability of hydrodynamic leveling by means of contemporary pressure gauges for achieving geodetic accuracy in height determination. The main problems associated with pressure gauges are rigorous connection to a national leveling network and data processing, for example, determination of time-dependent drift and data filtering principles. The equipment and methodology were tested in a test area in the Baltic Sea. It can be concluded that the year-long sea level series may provide ±2.0 cm accuracy for hydrodynamic leveling within the water stretches up to 65 km. This is confirmed by alternative height determination methods and additional field experiments.

Observations of near-bottom currents in the Gulf of Riga, Baltic Sea

Abstract: Near-bottom currents and temperature were measured at 2 and 4 m above bottom (m.a.b.) in the Gulf of Riga, a semi-enclosed sub basin of the Baltic Sea. Measurements were taken during three periods of different vertical stratification and weather conditions. Water depth was 54 meters. The studied area comprised the outer region of the bottom boundary layer, as judged by the observed Ekman veering of the current vector.¶Local wind was the primary driving force of near-bottom currents in the deepest area of the Gulf of Riga. Imposed wind stress induced surface-layer flow to the right, while compensating return flow developed in the bottom boundary layer. The response of current field was dominated by low-frequency variability with a clear indication of topographic steering and minor variability due to near-inertial frequency processes. The near-inertial frequency processes were generated also by interaction of low-frequency waves with the topography. The low-frequency and near-inertial frequency processes were of equal value in producing vertical shear of horizontal velocity in the near-bottom layer. The quasi-steady flow in autumn was forced by strong net inflow to the Gulf of Riga through the Irbe Strait. Lateral advection of water is important for the deep-water renewal that is due to dominating low-frequency currents.

Sub-regional observing and forecast system for the NE Baltic: Needs and first results

Following the operational observing system development under BOOS and application of local stand-alone coastal circulation models, acquisition of HIROMB forecasts started regularly in 2005. The forecasts, working on-line with the operational observations (automatic coastal stations and recently, also FerryBox), proved very useful in numerous cases, especially during storm surges and broadcasting relevant warnings well in advance of critical floodings. For the intermediate and downstream services that require resolving of detailed patterns of mesoscale and coastal circulation, installation and application of 0.5-mile resolution HIROMB started in 2007. The system is integrated with the Baltic-wide HIROMB and is driven by sub-regional HIRLAM weather data with enhanced resolution, forecasted and delivered by the Estonian Meteorological and Hydrological Institute.

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.

BOOS/HIROMB-based marine forecasts in Estonia: Problems, experiences and challenges

In January 2005, a severe storm hit the Baltic Sea region. Estonian towns Pärnu and Haapsalu were flooded with the storm surge waters. Considerable damages and economic loss occurred. Due to the BOOS cooperation, numerical forecasts became available and they were convincingly much more accurate than the forecasts made by the traditional methods. Advantages of modern operational oceanography were clearly demonstrated and Estonian Center of Environmental Investments launched a project to establish HIROMB-based marine forecasts in Estonia. In winter 2006, two severe oil pollution events occurred in the Gulf of Finland. Hind- and forecasts of oil drift, done with the HIROMB/SeaTrackWeb were of great importance in the practical management of those critical situations. The presentation gives an overview of problems, experiences and challenges on the implementation of BOOS/HIROMB-based marine forecasts in Estonia. We consider also validation of forecasts and the need to apply local VHR forecast models that are linked to the Baltic-wide operational system.

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.