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Swans awaiting medical attention, they were victims of the oil discharge outside the Estonian coast in January 2006. Photo: Silver Vahtra

Right fairway may reduce coastal pollution
There was relief among the coastal inhabitants of Estonia when storms of the 2005/2006 autumn and winter season finally ended and it was clear that no major flooding or other extreme event was under way. The relief, alas, was premature: two major oil pollution accidents soiled dozens of kilometres of the northern coast.
The source of pollution in January was lost in the white darkness of winter. Two months later a cargo ship, the Runner 4, sank after colliding with another ship in a convoy with the assistance of an icebreaker six nautical miles off Vaindloo Island with over 100 tons of oil products in her tanks. Substantial pollution was detected in a few days in the vicinity of Lahemaa National Park, about 40 km from where the accident occurred. The oil spread partially under ice and partially with ice and, when visible at all, it was mixed with ice. The pollution crossed about 100 km and reached the coast of Tallinn Bay in a few weeks, and even showed up west of Tallinn. Modern means of oil pollution elimination were hardly usable in a mixture of oil and ice, and could not be applied under the firm ice cover. Oil spill response vessels also cannot work in very shallow areas. The result was many kilometers of polluted coastline in the most beautiful and vulnerable coastal area.
These events and their aftermath were reminders that many threats from the sea are man-made. An important political decision was made towards allocating approximately EEK 400 million (EUR 25.5 million) for purchasing techniques for detecting and monitoring oil pollution. One of its central components, called a lidar – a laser-based apparatus capable of detecting oil pollution and even able to specify its properties from a helicopter or airplane – has actually been constructed by Estonian scientists, and currently it is being produced by a small Estonian high-tech company, LDI.

Figure 1. Persistence of currents in surface layer.

Motions in the sea: random or organised?
The circulation pattern of the Baltic Sea has been extensively studied during the last century. No jet currents similar to the Gulf Stream or Kuroshio exist, and the system of motion at any given moment is hardly predictable. There is one exception in the Gulf of Finland – the dominating outflow to the Baltic proper in the near-surface layer in the northern part of this gulf. This is known from Rolf Witting’s time, has already been sketched in the Atlas of Finland in 1910, and has simple basic reasons – deep water inflow and surface waters entering the gulf along the Estonian coast, and discharge from large rivers such as the Neva, Narva and Kymi; the resulting flow slightly deflected by the Coriolis force and finally forming a cyclonic circulation scheme. This pattern is basically responsible for transport of everything in the sea, including oil pollution.

The pattern of surface currents usually is quite messy. The basic circulation pattern is hardly visible and frequently completely overridden by much stronger instantaneous wind and wave-induced local currents. The accident with the Runner 4 happened in ice conditions when water masses do not “feel” wind. Yet the quite intense drift carried much of the oil towards the coasts. Since the influence of ice drift on water motion is negligible, this feature should reflect some internal structure of sea currents. For an open-minded observer the question arises: why did this structure carry oil to the coasts when the general flow should go out of the gulf?

Undercover organisation and a trap on the sea surface
There is apparently an amazingly persistent subsurface “river” at depths of about 2.5–7.5 m, flowing out of the Gulf of Finland. It becomes evident in recent high-resolution numerical simulations of Finnish, Swedish and Russian scientists (Andrejev et al. 2004 a, b). Its width is about 10 km, it is clearly distinguishable from the Neva Bight up to the entrance of the gulf, and it mostly follows the bathymetry of the northern part of the gulf. The average current velocity is up to 10 cm/s, which is comparable with instantaneous wind-induced surface currents. This “river” is a specific feature of the Gulf of Finland, probably driven by joint efforts of all external factors.

The above calculations suggest that, outside the stream, water motion usually forms circular current cells called rings, which are gradually transported westwards in the northeastern part of the gulf and eastwards in its entrance area. These rings carry the water masses periodically close to the coast. The two sad “experiments” near the Estonian coast suggest that such transport of pollution, at least, qualitatively matches the estimated current patterns.

Figure 2. Persistence of currents in subsurface layer.


Although the stream exists in a statistical sense (that is, it may not be present continuously), it still presents an option for reducing the probability of coastal pollution. The trajectories of water particles and oil spills on the sea surface are affected by a number of factors: wind and wave-induced transport, surface currents, etc. Subsurface currents also affect transport: when direct wind and wave influence ceases, water viscosity with certain more refined physical processes tend to homogenise the vertical structure of currents. Since the stream is thicker than the surface layer, the latter mostly follows the former. Several fractions of oil pollution are concentrated below the water surface, thus being more strongly affected by subsurface dynamics. In particular, subsurface and surface currents under ice cover probably do not differ much and together determine oil transport patterns.

Since the surface currents in the stream area are very nonpersistent, they usually only move water from the middle of the stream back and forth for a short distance. The simulated structure of currents suggests that rings adjacent to the stream usually only affect its periphery. Objects or pollution released on the sea surface in the middle of the stream therefore have a high chance of staying within the stream area, and/or being carried to the open Baltic proper within about two–three weeks. In other words, pollution released in the central area of the stream has a much reduced probability of hitting the coast. More specifically: if an accident occurs near the axis of the stream, the released oil will stay away from coasts for a longer time.

Different sea areas have different functions
The whole Baltic Sea has been recently declared a particularly sensitive sea area by the IMO. This is a long-expected decision, which legally confirms that this region should be handled with special care. Yet the Baltic Sea cannot be conserved like an item in a museum. It is a living environment of millions, an important source of fish and other products, and – last but not least – a major maritime transport hub.

Regulations concerning ship traffic have been introduced mostly in order to decrease the probability of accidents. A complementary key idea – regulations directed towards minimising the influence of potential disasters – is frequently used in mainland traffic. For example, transport of dangerous goods on the mainland is limited to a specific itinerary.

Analogous suggestions in marine matters may only be sensible if the “value” of different sea areas is different and keeping traffic in specific areas is beneficial in some sense.

It is not usual to divide the sea into more and less valuable parts. Only some spawning areas are under protection but they are usually located in shallow regions where large ships do not pass anyway. Yet there exists a natural division of confined seas into open sea and coastal areas. The coastal areas are generally the major life reproduction areas. If marine pollution on the open sea kills a number of creatures, coastal pollution additionally excludes the possibility of birth of new life.

This natural division is thus decisive from an oil pollution viewpoint. First, keeping the oil spill in the open sea area usually brings the least damage. Second, pollution in open waters is accessible for pollution response vessels. Both options can be achieved by keeping major ship traffic near the middle of the above-mentioned stream, as shown on the scheme on page 15. Doing so is a wise use of the intrinsic properties of the dynamics of water masses.

Figure 3. Scheme of currents in subsurface layer. Graphics: Oleg Andrejev/Kai Myrberg

Nothing is perfect
It is obvious that this solution is not a perfect one. It does not decrease the probability of oil pollution. Since the ship traffic will be concentrated in a narrow corridor, it may even increase collision risk. The location of the stream has been established by now only in numerical experiments, although its existence has been discussed earlier (Sarkkula 1991, Mikhailov and Chernyshova 1997). Its stability and other properties are known very approximately. It may not even exist during certain seasons. Reduction of damage is only possible in a statistical sense. Since the stream may change its position, for its practical usage it is necessary to create a sort of real-time forecast system supporting the ship routing practice, which is a major challenge for scientists in this area.

The stream is mostly located well northwards from the axis of the Gulf of Finland and may partially enter the territorial waters of Finland. Using that as the main fairway thus requires international co-operation and may not be an easy decision for Finnish authorities. Still this can be done, because the protection of coastal areas of any country around the Baltic Sea is a common problem for all countries. The shifting of the existing fairway may need construction of new lighthouses or seamarks, and certainly correction of nautical charts and sailing directions. Also, real benefit can only be obtained if ships proceed from harbours immediately to this stream, which mostly means sailing a longer distance.

The basic benefit of the solution is that it helps keeping the accidentally released oil in the open sea area where (i) it is easier to clear it, (ii) where it generally stays remote from the coastal areas and (iii) from where it is carried mostly out of the Gulf of Finland. Another benefit is that routine scanning of the sea surface may be performed within a narrow lane only.

//Tarmo Soomere
Professor in coastal engineering,
Senior Scientist,
Institute of Cybernetics,
Centre for nonlinear studies, Tallinn University of Technology

References:
• O. Andrejev, K. Myrberg, P. Alenius, P. A. Lundberg 2004a, Mean circulation and water exchange in the Gulf of Finland – a study based on three-dimensional modelling. Boreal Environment Research, vol. 9, 1–16.
• Andrejev, O., Myrberg, K., Lundberg, P.A. 2004b, Age and renewal time of water masses in a semi-enclosed basin – Application to the Gulf of Finland, Tellus, Series A: Dynamic Meteorology and Oceanography 56A (5) , 548–558.
• Mikhailov, A.E., Chernyshova E.S. 1997 General water circulation. In: Davidan I.N., Savchuk O.P. Baltica Project, issue 5, Part 2: 245–260. Hydrometeoizdat, St. Petersburg (in Russian).
• Sarkkula, J. 1991. Measuring and modelling water currents and quality as a part of decision-making for water pollution control. PhD thesis, University of Tartu, Estonia.