Tomasz Dabrowski

Influence of seasonal circulation on flushing of the Irish Sea.

We applied a three-dimensional general ocean and coastal circulation model to the Irish Sea in order to determine water renewal time scales in the region. The model was forced with meteorological data for 1995, a year with relatively warm summer and when extensive hydrographic surveys were conducted in the Irish Sea. We investigated intra-annual variability in the rates of net flow through the Irish Sea and carried out several flushing simulations based on conservative tracer transport. The results indicate that the net northward flow of 2.50 km(3)/d is seasonally highly variable and under certain conditions is reversed to southward. The variability in obtained residence times is high; baroclinic effects are significant. Obtained results point at the importance of spatial and temporal consideration for transport of pollutants in the shelf seas. Implications for management are numerous and involve activities such as transport, fishing, use of resources, nature conservation, monitoring, tourism and recreation.

Determination of flushing characteristics of the Irish Sea: A spatial approach

The Authors devised a novel generic approach to assessing the flushing of the Irish Sea through the determination of spatially distributed residence times and the development of flushing homogeneity curves. Results indicate that flushing of the Irish Sea is both spatially and temporally highly variable. Average residence times of the material introduced in winter may be up to 28% higher than the material introduced in summer, and the aerial flushing deviation index may reach up to 470 day. The spatial approach to flushing is an extremely useful complement to classical flushing analysis considering significant implications for management of water quality.

Use of numerical models for determination of best sampling locations for monitoring of large lakes.

The implementation of the European Union Water Framework Directive (WFD) that requires all surface waters to achieve at least good status by year 2015 presents a challenge to local authorities responsible for the monitoring programmes. Obtaining a single representative value for each water quality parameter may be extremely difficult to achieve in the case of large lakes, which may be characterised by significant spatial and temporal variation of water quality parameters. The aim of this paper is to present a methodology, which can support the decision making as to the best locations for water quality sampling when developing monitoring systems of large lakes. The methodology is based on three-dimensional hydrodynamic and a conservative tracer transport numerical modelling and utilises the concept of flushing and residence time. The 3D numerical model of Lough Corrib located in the west of Ireland was created and used to calculate its residence time and flushing pathways. Based on the results it was possible to identify the areas in the lake that are characterised by the local residence time values close to the lakes average as well as by the values significantly lower and higher. Three water classes have been introduced and the authors recommend that sampling takes place in all three classes at various frequencies. Spatial distribution of the flushing pathways allows identifying the areas in the lake that best represent the lakes average conditions both in the long term and seasonally, where monitoring can be carried out at the required frequency; worst affected areas can also be identified. The methodology can be used to improve robustness of the monitoring programme and may also contribute towards the reduction of both sampling and analytical costs, both at the design and operational stages.

Assessment of Tc-99 monitoring within the western Irish Sea using a numerical model.

Water circulation patterns and associated material transport within a highly dynamic system such as the Irish Sea are complex phenomena. Although Tc-99 monitoring programme undertaken by the Radiological Protection Institute of Ireland provides a good insight to the material distribution on the east coast of Ireland, transport patterns within the Irish Sea have not been fully explored. In this study a validated transport model was used to hindcast transport of Tc-99 discharged from the Sellafield plant to determine extents of Tc-99 migration within the Irish Sea and reassess transit times to east coast of Ireland. Transit times are also estimated within a context of changes in meteorological conditions and fluctuations in discharges. Additionally, seasonal and inter-annual circulation patterns were examined. Relationships between discharge times and timing of far field concentrations are highly variable and are dependent on sea dynamics controlling the accumulation and removal of Tc-99 mass. Transport towards the Irish east coast, and consequently transit times, vary intra- and inter-annually, and depend on the prevailing hydrodynamic conditions resulting from meteorological conditions. The transit times from Sellafield to Balbriggan fall within the wide range of 30-240 days; with summer releases resulting in the shortest transit times. The model also indicated a strong relationship between summer concentration peaks on the east coast of Ireland and the strength of the Western Irish Gyre. Sudden increases of Tc-99 concentrations at Balbriggan coincide with peak of sea surface temperatures when the gyre is strongest and when advection is fastest. The adequacy of the current radionuclide monitoring programme within the western Irish Sea is evaluated, and recommendations are made for the development of a more optimised monitoring programme.

Effects of complex hydrodynamic processes on the horizontal and vertical distribution of Tc-99 in the Irish Sea

The increased discharge of Tc-99 from the Sellafield plant following the commissioning of the Enhance Actinide Removal Plant in 1994 was reflected in higher Tc-99 activity concentrations over much of the Irish Sea. The presence of this radionuclide in the marine environment is of concern not only because of its long half life but also high bio-concentration factor in commercially valuable species, such Norway lobster (Nephrops norvegicus) and common lobster (Homarus gammarus). Accurate predictions of the transport, and spatial and temporal distributions of Tc-99 in the Irish Sea have important environmental and commercial implications. In this study, transport of the Tc-99 material was simulated in order to develop an increased understanding of long-term horizontal and vertical distributions. In particular, impact of seasonal hydrodynamic features such as the summer stratification on the surface-to-bottom Tc-99 ratio was of interest. Also, material retention mechanisms within the western Irish Sea were explored and flushing rates under various release conditions and meteorological forcing were estimated. The results show that highest vertical gradients are observed between June and July in the deepest regions of the North Channel and the western Irish Sea where radionuclide-rich saline-poor water overlays radionuclide-poor saline-rich Atlantic water masses. Strong correlation between top-to-bottom ratio of Tc-99 and strength of stratification was found. Flushing studies demonstrate that as the stratification intensifies, residence times within the western Irish Sea increase. In stratified waters of the gyre Tc-99 material is flushed out from the upper layer much quicker than from the bottom zone. The research also shows that in the gyre the biologically active upper layers above the thermocline are likely to contain higher concentrations than the near-bed region. Long-term horizontal and vertical distributions as determined in this study provide a basis for assessment of a potential biota exposure to Tc-99.

Modelling hydrodynamics of Irish Sea

Publisher Summary Two- and three-dimensional models are applied to simulate hydrodynamic circulation patterns in the Irish Sea. There are two main features in the region, including a relatively deep and narrow North Channel and a wider and shallower St. Georges Channel. The main force governing circulation in the region is the semidiurnal tide, while summer solar heating is also important. The model domain is divided into 65,000 squares, each having 2 km sides to perform hydrodynamic calculations in three dimensions; a 1 km rectangular grid is also used for hydrodynamic calculations in two dimensions. Extensive simulations for calibration of the hydrodynamic model are carried out for different types of boundary conditions and diversities of roughness coefficients. Models are calibrated accordingly to available data that include current velocities at 14 locations and tidal elevations at 15 locations along the Irish and British coasts. Comparison between individual-model runs are made and results are related to field data indicating that models represent circulation pattern reasonably well.