Modelling of a Thermal Discharge in an Ice-covered Estuary in Finland

Abstract

Abstract Numerical modelling of ice growth, melt and transport on regional scales such as coastal seas, estuaries, rivers or lakes can provide crucial input for safe and efficient designs and installations of marine infrastructure in arctic, sub-arctic or mid-latitude regions. The modelling of ice and related complex physical processes on these regional scales is however still rather unexplored. The complexity of ice modelling on local and regional scales is best illustrated in areas where ice-covered sea water is mixed with fresh river water and a thermal discharge from for instance a refinery or power plant. Such a situation exists in the Svartbackfjarden, an estuary some 35 km to the east of Helsinki, Finland. Two minor rivers discharge into this estuary. The estuary freezes during the winter with ice thicknesses typically in the range of 20 to 50 cm. In this estuary an oil refinery takes in cooling water at a depth of about 15 m. The heated water is discharged at the surface and results in melting of the sea ice in the vicinity of the outfall. However, given the fresh water from the rivers in the early spring, and the fact that the salinity of the intake water is higher than the surface water, the discharged cooling water has been observed to flow under the relatively cold fresh layer just under the ice. Due to mixing of the plume with the surrounding water, the temperature of the fresh water layer increases, leading to melting of the ice at some distance from the plant's outfall. This paper presents a case study with Delft3D, which is a flexible integrated numerical modelling programme that enables simulation of 3-dimensional flow, sediment transport, morphology, waves, spills, water quality and ecology, in combination with a recently developed ice module. The 3-dimensional modelling with Delft3D of the thermal discharge from the oil refinery in the ice-covered Svartbackfjarden estuary is presented and compared to local observations of currents, water temperature, salinity and ice thickness. The case study will show the capability of Delft3D to cover all the relevant physical processes that determine the temporal and spatial characteristics of the ice and the thermal discharge, under influence of fresh river discharges, hydrodynamic, meteorological and atmospheric forcing. These capabilities will contribute to the further development of integrated ice modelling on regional scales, eventually benefiting the sustainability, efficiency and safety of the designs of marine structures in ice-covered waters.