Ants Erm

Device for taking samples from the bottom boundary layer of a water body

Long-term changes in the marine and lake environment are well described by their bottom sediments, particularly by the profiles of these sediments. Compact bottom sediments can be sampled by different sampling devices elaborated and manufactured in tens of variants, e.g. gravity corers, vibratory corers, grab samplers, sampling boxes connected to the supporting frame, etc. The components of floating and lighter contact layers will be pushed away using these samplers, and the sample obtained will have a highly mixed structure, which cannot be used to determine the distribution of vertical, i.e. temporal, distribution of the components. In 1975 Dipl. Ing. Martin Voll started the study of pollution with aromatic hydrocarbons of the Baltic Sea and some Estonian lakes with a unique sampler allowing taking sectioned cores. The device was continuously improved and protected with a number of patents. In 1988/1989 Voll used his device during the Pacific Ocean expedition, and water decaying bacteria were found at a depth of 6000 m at the bottom of the Pacific Ocean. In this report we are lightening an improved variant of the sampler protected with a United States patent application and being in the phase of construction today.

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.

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.

Operational observations methods during offshore sand mining — case study in Tallinn Bay, the southern Gulf of Finland

Offshore sand mining is increasing activity nowadays. Environmental impact of sand mining appears usually through the transport of suspended particulate matter (SPM) away from the actual mining area. Satellite remote sensing is efficient tool for the operational monitoring of SPM distribution during harbor dredging, but problematic in the case of sand mining, as SPM remains mainly below the water surface. We used satellite remote sensing, in-situ measurements of optical properties of seawater and combined wave, hydrodynamic and sediment transport numerical modeling for assessment of the area affected by sand mining in Tallinn Bay, the southern Gulf of Finland. Sand mining took place from October 2008 to April 2009 with short breaks that were random in time. Vertical profiles of spectral attenuation and absorption coefficients by spectrometer AC Spectra, underwater light field and albedo by radiometer Ramses-ACC-VIS were measured in situ. In satellite remote sensing MODIS images with 250 m spatial resolution were used for the qualitative estimation of the surface area that was affected by sand mining. Nested 2D hydrodynamic model and wave model SWAN with 400 m spatial resolution at mining site gave input fields of currents and bottom shear stresses to the Lagrangian type particle transport model. While in-situ measurements and satellite remote sensing give snapshot about the SPM distribution numerical modeling enables to have dynamics of the ongoing process. In-situ measurements showed that the concentrations of SPM were the highest at the mining operation. The thickness of the elevated SPM layer was about 6 m. Satellite remote sensing showed minor or no signal of elevated SPM concentrations on the water surface in comparison to surrounding area. Model result show clearly that eastward transport of SPM prevailed during the sand mining activities. The SPM covered larger area during autumn than during winter and spring. This can be attributed to the stronger winds that forced higher waves and stronger currents. Wave activity is responsible for keeping the SPM in suspension, which favors the transport of SPM away from the mining site. In the environmental point of view, the most affected area remains in the radius of 1 km from the mining site. In conclusion, the use of satellite remote sensing and in-situ measurements can be misleading when considering environmental impact assessment caused by SPM.

Optical properties of north-eastern Baltic Sea in spring and summer 2007

Underwater irradiation profiles in the north-eastern Baltic Sea near the Estonian northern and north- western coast, near Helsinki, and in the central part of Gulf of Finland were measured in spring and summer 2007. The vertical profiles of downwelling and scalar irradiance in the PAR region were measured in situ using a frame completed with two planar and a spherical PAR sensor. The measuring system allows the calculation of the mean attenuation coefficients of a water column of scalar irradiance (Ko). Optical density of the sea water was varying from 0.13 m-1 (Tallinn-Helsinki line, May) up to 0.85 m-1 (Tallinn-Helsinki line, April). The concentrations of optically active substances were highly variable; the chlorophyll a concentration varied from 0.7-21.4 mg m-3, the suspended particulate matter concentration from 1-7 mg l-1 and the concentration of dissolved organic matter from 2.1-5.8 mg l-1. The water transparency was much better in May compared to April 2007. Also, it is seen, that excepting the stations immediately near the port and ship line, the Gulf of Finland was quite clear, especially in May. Correlations between the water depth and optical density were calculated for the shallows near Hiiumaa. Water quality based on measurements conducted in summer 2007 at North-Western Estonian coastal waters indicates that the water quality was on average satisfactory; associating values for diffuse attenuation coefficient was 0.5 m-1, Secchi depth 3.7 m and chlorophyll a concentration 8 mg m-3.

Monitoring sediment transport in the coastal zone of Tallinn Bay

Continuous near bottom measurements of wave characteristics, bottom velocity and turbidity were performed using an acoustic Doppler velocimeter (Sontek ADV Ocean Hydra) integrated with turbidity meter (YSI OBS-3+) and a pressure wave gauge (PTR Group). An experimental autonomous submersible camera system (by the Center for Biorobotics) was used to monitor the motion of particles in the bottom boundary layer (BBL). The ADV measured flows consisted of wind induced currents, wave induced orbital motions and turbulence. Maximum of wind induced currents reached up to 10-15 cm/s, while the maximum near bed orbital motions peaked over 40 cm/s. From the comparison of ADV, turbidity and wave characteristics it followed that turbidity was clearly depending on the wave energy. It means only long and quite high waves generating bottom orbital velocities (calculated from the wave gauge data and/or measured using ADV) over 20 cm/s were able to resuspend bottom sediment and induce some increase in turbidity values - 5-12 NTU in stormy days. Sediment fluxes were estimated using measuring data, sediment characteristics and BBL model. It followed from the modeling results that the height of all BBL varied from 1 cm to 10 cm, and the height of the bottom-most layer (skin friction layer) varied from 0.01 cm to 1 cm. Running the model has shown also that the currents have a significant impact to the sediment resuspension only if their speed reaches to 15-20 cm/s occurring very seldom in Estonian sea areas. Absolute majority of measurements showed an error below 5% using the wave skin friction shear velocity instead of the total skin friction velocity, i.e. excluding the existence of a skin friction sublayer. Captured video clips were split into frames and pairs of images were analyzed using particle image velocimetry software by MatPIV toolbox for MATLAB. The velocity and vorticity vector fields of bottom boundary layer were visualized.