In deep ice-covered lakes with temperatures below 4 °C the heat flux from the bottom sediment results in a horizontal density gradient and a consequent flow along the bottom slope. Measurements in Lake Pääjärvi, Finland, show a stable temperature field where a heat gain through the bottom and a heat loss through the ice nearly balance each other. The circulation is thermal with low velocities (less than 1.5 cm s–1). We used the 3D hydrodynamic Princeton Ocean Model as a tool to simulate the water circulation and the temperature distribution under the ice. The model forcing was based on field temperature measurements. The model simulations suggest that in midwinter the velocity field of the upper water layers is anticyclonic while that of deep layers is cyclonic. Comparison with current measurements at one site showed good agreement between the modelled and observed results. On the basis of the modelled results it is possible to better understand the distributions of some micro-organisms and the accumulation of oxygen depleted waters in the deepest part of the lake

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We consider two applications of the stochastic discrete particles method. The first one is concerned with the dispersion of a passive pollutant by a turbulent stream with a scale dependent diffusion coefficient. The second application deals with the problem of an oil spill spreading on the water surface described by transport-diffusion equation with a nonlinear diffusion coefficient. For the first problem we develop a discrete particles algorithm provided the diffusion coefficient obeys Richardson's "4/3" law and show good correspondence with the numerical and analytical results. The second problem is more involved and we develop a heuristic procedure based on the standard discrete particles random walk algorithm updating the dependence of each particle step variance on the dependent function. The obtained solution coincides well with analytical and direct one-dimensional finite-difference solutions both for instantaneous and continuous oil release.

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A model of three-dimensional forced convection and transport of condensed moisture in the lower pools of hydroprojects is developed.The existence of special spillway structures for passage of seasonal flood flows is probably one of the basic characteristic features of large-scale hydroprojects, which distinguish them from other industrial entities with operating conditions that are relatively uniform throughout the year. For a number of large-scale HPP, the pools, which are separated by a spillway dam with a height of the order of 100 m and more, are connected by surface spillways with specially designed lips/spring boards, which ensure rigorous splitting of the discharge flow; this is required to reduce scouring of the channel beyond the structure. Here, however, water dust (condensed moisture), which for unfavorable orography of the lower pool, will complicate both construction work, and also the operation of individual zones at the site of the powerhouse, forms, as a rule, in the fall-out region of the spillway jets. Figure 1 shows a photograph of the Kapanda hydroproject (Angola) on which is apparent a water-air cloud that is formed in the narrow rocky canyon in the fall-out area of the spillway jets, and which spreads throughout the lower pool of the HPP. Under certain meteorological conditions and a vigorous discharge, this cloud may penetrate into the zone of the powerhouse. Here, the intensity of the precipitation may reach 10,000 mm/day greater.The situation in question is characteristic of the incomplete construction of the hydroproject, where with two generating sets of the HPP operating (phase I), their number will be increased to four (4´130 = 520 MW) on completion of phase II in 2007, and the duration of the effect of the air cloud at the site will be sharply curtailed, and will amount to less than 30 days per year during the passage of a 100-year flood.In our study, we are apparently the first to develop and implement a spatial numerical model of the forced circulation of air masses and transport of condensed moisture in the lower pool of a large-scale hydroproject with its spillway structures operating at near-full capacity. Data derived from field observations obtained at the Kapanda HPP during the spring-flood period of 2004, are being used to calibrate and test the model. In creating the model in question, we attempted to ascertain the principal physical mechanisms determining the phenomenon under investigation, and as a result, simplify the mathematical description of the process to the maximum extent possible, while retaining relationships (possibly empirical) on specific engineering characteristic features of the entity. The study was initiated by the Center for Hydraulic Studies affiliated with the JSC "NIIÉS," and the work is being performed through the efforts of the fol-

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