Matjaž Četina

Modelling of circulation and dispersion of radioactive pollutants in the Japan Sea

Abstract A large amount of radioactive waste was deposited in the Japan Sea, at a depth of about 3 000 m by the former Soviet Union. Research was carried out to determine to what extent the surface waters could be contaminated by possible leakage from the dumped containers. A three-dimensional, non-linear, baroclinic model was used to determine the circulation and pollutant dispersion. The computations were carried out in the diagnostic mode, taking into account data on winter and summer temperature and salinity distribution. Thermohaline forcing due to strong temperatures and salinity gradients is the main forcing factor influencing the bottom circulation. Wind forcing and the inflow/outflow surface currents were also taken into account. The simulated velocity fields show relatively good agreement with observed surface currents and with some measurements of bottom currents. The simulated hydrodynamic field is in visibly closer agreement with the observed surface circulation when topographic stress is taken into account. After the release of radionuclides at the sea bottom, the first very small contamination would reach the surface layers after 3 years. The maximum concentrations of about 10–3 Bq·m–3 would be attained after 30 years. But everywhere the predicted radionuclide concentrations would be about two orders of magnitude smaller than the background values, caused by global fallout from nuclear weapons tests. Therefore, it will be impossible to determine the effect of leakage of wastes from the dumping sites over the present background levels.

Numerical and physical model study of a vertical slot fishway

Abstract This paper presents the results of an experimental and numerical study of a vertical slot fishway (VSF). A 2-D depth-averaged shallow water numerical model PCFLOW2D coupled with three different turbulent models (constant eddy viscosity, Smagorinsky and k - ε) was used. A detailed analysis of numerical parameters needed for a correct simulation of the phenomenon was carried out. Besides the velocity field, attention was paid to important hydraulic parameters such as maximum velocity in the slot region and energy dissipation rate ε in order to evaluate the performance of VSF. A scaled physical hydraulic model was built to ensure reliable experimental data for the validation of the numerical model. Simulations of variant configurations of VSF showed that even small changes in geometry can produce more fishfriendly flow characteristics in pools. The present study indicates that the PCFLOW2D program is an appropriate tool to meet the main demands of the VSF design.

Different ways of defining wall shear in smoothed particle hydrodynamics simulations of a dam-break wave

ABSTRACT The paper describes two different ways of defining the terrain roughness in smoothed particle hydrodynamics (SPH) simulations performed with the Tis Isat model, developed at the University of Ljubljana. The model introduces into the SPH method a non-discrete boundary condition with friction. Two basic definitions of terrain roughness are used: (a) as a hydraulically smooth wall, where roughness was controlled by the wall–particle eddy viscosity coefficient; and (b) as a hydraulically rough terrain by elevating the mesh-nodes. The undertaken SPH simulations relate to a dam break at the upper storage reservoir of the pumped-storage hydro power plant Kolarjev vrh in Slovenia. For the first time, such study was performed on a real topography. Water depths at the gauges along the valley were compared with measurements on a physical model and to results obtained using a finite volume (FV) model. The comparison showed satisfactory agreement with the measurements, which are comparable with the FV model simulations.

Modelling of toxic mercury in coastal seas: case study: Yatsushiro Sea, Japan

After the well-known mercury contamination of the Minamata Bay around 1956, measurements of mercury content in the bottom sediment of the neighboring Yatsushiro Sea have shown, that some mercury has been transported from the Minamata Bay into the Yatsushiro Sea. To understand the phenomenon, and to help at the environmental protection, simulations of current velocities in the Yatsushiro Sea have been done. They are further on used for calculation of mass balance in the Minamata Bay, where the potential sources of Hg and their relative magnitudes are shown. By combining measurements with hydrodynamic and Hg cycling modelling, meaningful predictions for future behavior of Hg in both Minamata Bay and Yatsushiro Sea are possible.