The Use of Lagrangian Trajectories for Minimization of the Risk of Coastal Pollution
Bert Viikmäe, Tarmo Soomere, Nicole Delpeche
Institute of Cybernetics at Tallinn University of Technology
Akadeemia tee 21, 12618 Tallinn; bert@ioc.ee
We propose a novel way of utilising the results of circulation modelling for identification of areas of high and low risk for the marine environment. The basic idea takes into consideration statistical properties of large pools of Lagrangian trajectories computed off-line from numerically simulated current fields. As a particular application, we describe first steps made towards creating a technology for identifying such areas for fairway design in the Gulf of Finland.
The basic tool for the analysis of current-driven propagation of adverse impacts (e.g. oil pollution) is a Lagrangian trajectory model, TRACMASS (Döös K. 1995. Inter-ocean exchange of water masses. J. Geophys. Res., 100, C7, 13499–13514) with the use of three-dimensional current velocity fields calculated by the Rossby Centre global circulation model (Regional Ocean model, RCO) with a resolution of 2×2 nautical miles. Trajectories of current-driven pollution are simulated for a few weeks and the simulations for each sea point are repeated over several years.
A central question for narrow basins is how to minimize the joint probability of hitting of either of the coasts. The first order solution is the equiprobability line, the probability of propagation of pollution from which to either of the coasts is equal. This line and/or areas showing the probability of propagation of pollution to the coastal area indicate a safe fairway.
We propose two methods for numerical estimation of the location of the equiprobability line. The first method consists of the analysis of five trajectories starting from each grid cell. A count is made on if at least 50% of the trajectories travelled to either southern or northern coast. If yes, the cell is marked as being a probable source of pollution for the particular coastal section. If not, the cell is marked as a part of an undefined area, propagation of pollution from which to any of the coasts is unlikely. The separation line for probable sources of pollution to different coasts is interpreted as the estimate of the location of the equiprobability line.
Another method for specification of this line implicitly involves a smoothing process and consists of dividing the sea area into clusters of 3×3 grid cells. By tracing nine trajectories in each cluster (one from each cell) it is established whether the majority of the trajectories end up at one of the coasts or stays in the open sea area.
The equiprobability line was found to be substantially shifted northwards from the axis of the Gulf of Finland. In the western part of the gulf there is a well-defined probability gradient across the gulf, but in the eastern part there is a large area of low risk with respect to pollution affecting the coastal areas. The purpose and meaning of this area is then determined.