This abstract has also been accepted to ICS 2011 conference.
Authors: Bert Viikmäe, Tarmo Soomere, Nicole Delpeche-Ellmann
Oil transportation as one of the largest threats to the Baltic Sea has drastically increased over the last 15 years. The drift of oil spills is influenced by wind stress, waves, and currents. The properties of transport by wind and waves are relatively well known, but the prediction of current-induced transport is more challenging. The existence of quasi-persistent patterns of currents in various parts of the Baltic Sea and the presence of rapid pathways of the current-driven transport opens a new way towards a technology that uses the marine dynamics for the reduction of environmental risks stemming from shipping and offshore and coastal engineering activities. The key benefit of this particular technology is an increase in the time during which an adverse impact (for example an oil spill) reaches the coastal zone. The idea is to identify areas (of reduced risk), which are statistically safer to travel to in terms of the probability of the transport of accidental pollution to the vulnerable areas. The coastal areas usually have the largest ecological value, thus in this study we consider the nearshore as a generic example of a valuable area. While the probability of coastal pollution for most of the open ocean coasts can be reduced by shifting ship routes to a larger distance from the coast, the problem for narrow bays, like the Gulf of Finland, is how to minimize the probability of hitting either of the opposite coasts.
The first order solution to this problem is the equiprobability line, the probability of propagation of pollution from which to either of the coasts is equal. The safe fairway would either follow the equiprobability line or cross an area of reduced risk. Owing to extreme complexity and high variability of the instantaneous patterns of current fields, we use a large number of single simulations in order to estimate the pathways of current-induced drift patterns. The propagation of pollution is calculated with the use of the Lagrangian trajectory model TRACMASS (Döös, 1995; de Vries and Döös, 2001) that uses pre-computed Eulerian velocities calculated by the Rossby Centre global circulation model (Regional Ocean model, RCO, Meier, 2001) with a horizontal resolution of 2×2 nautical miles and 41 vertical levels. The trajectories of pollution (particle) propagation are calculated based on a linear interpolation of the velocity field in each point of grid cells. The position of the trajectories is updated every six hours. Trajectories of particles are simulated for a few weeks and saved for further analysis. Simulations with the same initial positions of particles are restarted from another time instant and the process is repeated over a chosen time period 1987-1992.
Two methods are used for numerical estimation of the spatial distribution of the probability of hitting the opposite coasts. Firstly, four particles (Ni = 4, 1 ≤ i ≤ N) are placed in each grid cell. If three or all four particles reach the nearshore of a particular coast, the cell is assumed the value of c = ± 1 depending on which coast was hit. If no more than two tracers reached a coast within the time period, the cell is assumed the value c = 0. Secondly, we used another method, involving a certain local smoothing, by dividing the sea area into clusters of 3×3 cells and placing one particle in each cell. By tracing nine trajectories in each cluster it is established whether or not the majority of the trajectories end up at one of the coasts. The basic idea is the same as above; only the values of (Ni = 9, 1 ≤ i ≤ N) and the initial positions of the tracer with respect to the centres of the grid cells are different. Both methods produced qualitatively similar probability maps for coastal hits that show substantial seasonal and also certain inter-annual variability. A highly interesting feature of the resulting distributions is that some open sea regions contain a clear probability gradient while some other regions of basically the same size exhibit extensive areas with very low (and essentially constant) probability of hitting either of the coasts. In the former areas it is possible to clearly define the equiprobability line whereas the latter areas can be identified as areas of reduced risk. The distance between different estimates for the location of the equiprobability line serves as an implicit measure of uncertainty related with this sort of solution. We demonstrate the location of the equiprobability line and areas of reduced risk for the Gulf of Finland and the northern Baltic Proper. An alternative method for identification of the equiprobability line and the areas of reduced risk consists in constructing spatial maps of time necessary for the current-induced transport of adverse impacts to the coastal zone. We also provide the analysis of most frequently hit coastal areas from pollution sources occurring along the equiprobability line.
The presented results confirm that it is possible to considerably reduce the probability coastal pollution by adverse impacts released from ships by means of optimising the fairways. The relatively small difference in the location of the optimum fairways obtained by different methods indicates a reasonable level of uncertainty connected with this type of solution. A highly interesting side result is the discovery of substantially different regions in the underlying spatial distributions of the probability of coastal hits. This feature probably reflects certain intrinsic difference in the dynamics of sea currents and the corresponding pollution transport between different sea areas.
Bibliography:
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Soomere, T. and Quak, E, On the potential of reducing coastal pollution by a proper choice of the fairway, J. Coast. Res., Special Issue 50, 678-682 2007
Soomere, T., Viikmäe, B., Delpeche, N., Myrberg, K., Towards identification of areas of reduced risk in the Gulf of Finland, Proceedings of the Estonian Academy of Sciences, 59 (2), 156–165 2010