Operational Model for Prediction Circulation and Transport of Oil Spills in the Black Sea

KONSTANTIN A. KOROTENKO

Russian Academy of Sciences P.P. Shirshov Institute of Oceanology

36 Nakhymovsky Pr, Moscow, RUSSIA

Abstract: - An operational model for the prediction of circulation and transport of oil spill in the Black Sea is presented. The hydrodynamic block is developed on the basis of the DieCAST high-resolution low-dissipative model adapted to the Black Sea. The oil spill transport block is constructed on the basis of the random walk approach, which allows predicting motion of individual particles, the assembly of which represents the oil spill. The numerical experiments show that the model reproduces with high accuracy the principal features of the Black sea dynamics, including the genesis of the mesoscale near-shore anticyclonic eddies, and predicts the transport and dispersal of oil pollution in the Sea. Key words: - Mesoscale circulation, oil spill, operational model, the Black Sea 1. Introduction The discovery of new and development of known oil fields have stimulated the search for efficient and safe ways of oil transport. In recent years, in parallel with traditional methods of the transportation of oil and oil products with the use of tankers, projects for laying pipelines on the sea bottom have been rather intensively developed. This method is believed to make oil transportation cheaper and safer. The first and second methods, however, do not exclude accidental oil spills and contamination of vast areas of the ocean related to emergency situations. Accidents at the terminals where ships are loaded with oil are also possible. According to the data available, the annual amount of oil released into the Black Sea is about 110 000 tons. Since the Black Sea is a semiclosed basin, it is very sensitive to these kinds of accidents. Therefore, efficient and exact prediction of sea contamination and rapid reaction to mitigate its aftereffects are very important.

The spread of oil spills in the sea is a rather complex process that depends on a number of factors determined by the environmental conditions and the properties of the pollutant itself. Therefore, when one formulates the problem on the transport of oil pollution in the

sea, it is necessary to reproduce the circulation and exchange processes in the sea and adequately account for the physicochemical properties of the oil itself and the characteristics of pollutant sources. 2. Circulation of the Black Sea The hydrophysical observations made in recent years [1,9-11, 13, 19, 20], numerical modeling [7,16-18], and satellite data [4, 21] allow us to reveal the principal details of the Black Sea circulation.

Fig.1 schematically shows the details of the Black Sea circulation synthesized over all seasons. The circulation is mainly cyclonic and known as the Rim Current (RC), which becomes stronger and more stable during the autumn--winter period under the action of intensive atmospheric circulation. The main characteristic feature of this circulation is the presence of two or three cyclonic gyres in the western and eastern deep-water parts of the sea. The characteristic features of the currents in various parts, as a rule, are associated with the meandering of the RC and the formation of eddies at its periphery. Due to the interaction

Fig.1. Schematic Circulation of the Black Sea

with the characteristic features of the bottom relief on the continental slope and hydrodynamic instability, the RC is rather unstable, which manifests itself in its meandering. The intensity and period of oscillations depend on the season. A higher level of meandering of the RC is observed in the warm time of the year, from April to November. This is associated with the weakening of the atmospheric circulation and domination of calm weather over the Black Sea region in this season. In all the seasons, between the coast and RC, near-shore anticyclonic eddies (NSAEs) are observed almost over the entire periphery of the sea. However, these eddies are most intensively generated in the warm season. The most characteristic among them are the Sevastopol, the Crimean, and the Batumi antycyclinic eddies, as well as the anticyclones near the Bosporus Strait and in the region of capes Sinop and Kaliakra (Fig.1). The size of large meanders and eddies can reach 120 km off the western and southern coasts and 250 km off the northern coast [11]. Relatively small solitary NSAEs are as large as 20-60 km in diameter. Such eddies play a particular role in the ventilation of the near-shore water, which is discussed in detail in [13,19]. On the left side of the RC, its meanders may generate cyclonic eddies, which often significantly complicate the pattern of circulation in the open sea. The wide spectrum of mesoscale dynamic processes in the Black Sea is particularly difficult for modeling. Recent models (such as POM [2]), which use strongly dissipative numerical schemes (with the horizontal

exchange coefficients determined by the Smagorinskii formula) and approximations of the second order, cannot be used as they smooth the small-scale eddies. Therefore, to model the dynamics and transport of the admixture in the Black Sea, we used a low-dissipative model of the ocean circulation, DieCAST Ocean model developed by D. Dietrich [3]. It uses a fourth-order approximation [14]. This model was adapted for the Black Sea and udes as a hydrodynamic module in the operational model. 3. The Model Description Like most models of the transport of oil pollution, the model presented in this work is divided into three major units: (1) acquisition of input data, (2) calculation of motion trajectories and amount of oil, and (3) output of the results. The latter stage, in turn, consists of the output of the results of modeling oil slick motion and output of hydrophysical data. The procedure of predicting the oil spill behavior is divided into two parts: (1) prognostic calculations of sea circulation with the use of the hydrodynamic module and current hydrometeorological conditions and (2) modeling of an oil spot, which is based on the method of random walk. The latter, together with the calculations of circulation, allows us to predict three-dimensional motions and behavior of individual oil droplets, the assembly of which is the oil slick. Thus, the model can be conventionally divided into a transport unit and a hydrodynamic unit.

3.1 Hydrodynamic Module The displacements ( )∆ xi j k,

i j,

are defined as the

deterministic part of the motion due to the mean velocity field, V and the random

displacement, ( )ηi j k, due to fluctuations of

velocity and denotes the displacement of the k-th particle moving along the xi - axis at the j-th instant of time, Nt is the number of time steps, ∆t is the time step, Nf is the number of particles in each fraction, and the subscript f denotes a particle fraction.

The model calculation grid z-coordinate, 168x92x21) covers a region from 28E to 42E and 41N to 46.5N over the horizontal, i.e., the entire Black Sea. The resolution with respect to the longitude was chosen to be 1/12 degree. With respect to the latitude, the resolution changed so as to keep the metric ratio ∆X/∆Y equal to unity. Thus, the size of the square cells changed only with latitude from 7 to 6.4 km. The vertical step was non-uniform, with a thickening of the grid near the sea surface for better resolution of the seasonal thermocline. In the model, both horizontal and vertical turbulent diffusivities are assumed to be constant and equal 10-3m2/s, which allowed us to reproduce mesoscale NSAEs. A numerical scheme with fourth-order approximation provided the calculation stability for the 10-min time step.

The distribution of the number of particles in fractions (hydrocarbon groups) is initially assigned and distributed randomly depending on the type of oil. The total number of the particles launched in the model usually does not exceed 106; nevertheless, the behavior of the tracked particles proved to be representative of the entire spill, even though each droplet represents only a small part of the total volume of the oil. Within each fraction, each droplet is also randomly distributed to have its own half-life according to the empirical exponential laws.

As climatic data, we used mean multiyear (monthly) data on the wind stress, temperature, salinity, heat fluxes, evaporation, precipitation, riverine runoff of eleven major rivers, and exchange via the Bosporus. The data were kindly placed at our disposal by I. Staneeva [17]. The time taken to obtain the quasi-steady annual cycle of circulation is 20 years, while the major characteristics of the Black Sea circulation such as the RC, cyclonic gyres, Rossby waves, and entrained near-shore waves appear as early as four calculation years. The model can reproduce the intensification of the RC in the winter period and its weakening and meandering in the warm seasons, as follows from the field observations.

In practice, those distributions are assigned randomly by means of a random number generator giving uniform numbers chosen uniformly between 0 and 1, and then they are transformed into an exponential distribution with a weight dependent on wind speed and oil temperature. The 'long-living' fractions such as

C2, C4, C6, C7, and C8 are randomly exponentially distributed within a range corresponding to the rather slow effect of total degradation. Their half-life for total degradation is chosen to be 250 hours

As for the output data, the model presented allows us to trace the amount of evaporated oil and its concentration in the water and at the sea surface, as well as the amount of oil settled out on the bottom and discharged on beaches. More details on the model are given in [6-8].

3.2 Oil Spill Module The basic concept adopted for the

construction of this unit is similar to that used in [12], except that oil is initially divided into fractions in order to describe the evaporation process with more accuracy.

Operationally the model is controlled primarily by “Oil Spill Model” Dialog with a map of the Black Sea shown in Fig.2. The model is started with “Update and Run” button, which reads in bathymetry, predetermined current velocities, and model and source parameters.

In the model, displacements of each particle are estimated as given by the following expressions [6-8] ( )

);

(

)(

.

,,,

1,2,...,8fN1,2,...,k

;N1,2....,j3;1i

tVx

ff

t

kjijjikji

==

=−=

+∆=∆ η

(1)

Note that the statement that the Sevastopol anticyclonic eddy is a quasi-steady one, which has been found in many field observations, is a conventional statement. More correctly, this means that strong anticyclonic eddies are periodically formed west of Sevastopol. Then, these eddies are entrained by the RC and reach the Bulgarian coast, where they dissipate. In this respect, the Kaliakra anticyclonic eddy may be considered as a Sevastopol anticyclone coming from the northeast. It should be noted that the motion of the Sevastopol anticyclonic circulation was previously reproduced in detail from the results of satellite surveys of sea surface temperature. In [4], an analysis of the trajectory of this eddy is presented for June--August 1998. An analysis shows that the eddy found southeast of Sevastopol moves to the Kaliakra Cape over about three months. This agrees well with the model results.

Fig.2. The Model Dialog Interface

A source position is determined by the

mouse pointer location; in doing so the Lat/Long coordinates and instantaneous depth will be displayed. The user may choose whether the spill is bulk or continuous, and specifies the period of spill in the later case. Once the model is running, the user may also specify the wind and wave conditions as well as choose which fraction (evaporated, beached, deposited, etc.) to be displayed during the calculations. It should be noted that once “Update and Run” button is clicked, it runs a separate graphic window displays real-time motion of droplets. The concentration recalculated from particle density and oil fraction distributions during the spilling process are displayed at the map and in correspondent frames at the upper right corner of the main Dialog. A special Properties Dialog, which appears by clicking “Setup Hydrocarbon Groups” button allows the user to input type and properties of oil.

It is interesting that, west of the Crimea, one can sometimes observe tripole structures composed of two anticyclones and a cyclone formed in series. Such structures were registered from the data of hydrographic surveys [1,4,24]. The model is also able to reproduce such structures. As can be seen from Fig.3, in January, such a tripole structure is formed west of the Crimean Peninsula. The formation and destruction of the Batumi anticyclonic eddy (BAE) is distinctly traced from April to August (Fig.3). It is interesting that, as can be seen from the modeling, the Batumi anticyclone appears owing to the NSAE formed off the Turkish coast. Being drawn in and replenished by the RC, this near-shore anticyclonic eddy spreads in the eastern part of the sea and results in the northward deviation of the RC in this region, which can be traced up to the end of August (Fig.3). Further, in September (Fig.3), the BAE is displaced by two cyclonic eddies, which results in the formation of a tripole structure in the eastern part of the sea. The BAE then dissipates, and, in October, a strong cyclonic gyre is formed in its place, which is traced up to January (Fig.3a).

4. Results 4.1 Modeling Circulation

Figures 3f-3d show examples of calculation of the currents for four months characterizing different seasons. The results of numerical calculations show that the model reproduces the basic characteristics of the Black Sea. One can see seasonal oscillations in the quasi-steady cyclonic RC; cyclonic gyres in the deep-sea part; multiple anticyclonic meanders of the RC; and eddies located between the RC and the coast, including the Sevastopol, the Batumi, and other eddies, which are schematically presented in Fig.1.

The process of intensive formation of NSAEs is observed near the Turkish coast and northeastern coast of the Black Sea. From March to August, this process is most intensive

44

42

46

30 32 34 36 38 40

Current Velocity

N

E

January

44

42

46

30 32 34 36 38 40

Current Velocity

N

E

; April

44

42

46

30 32 34 36 38 40

Current Velocity

N

E

; September

44

42

46

30 32 34 36 38 40

Current Velocity

N

E

; August

Fig.3. Predicted surface velocity

(Figs.3). In so doing, the eddies formed become longshore, being drawn into the RC.

It should be noted that the summary pattern of the major characteristics of the Black Sea circulation presented in Fig.1 and the similar more detailed pattern presented in [8] describe only probability characteristics of the

appearance of this or another element of circulation in individual regions. Such elements can be generated in some places and transported to others, as was shown for the Sevastopol anticyclonic eddy. The results of the circulation modeling show that, in general, all near-shore anticyclonic eddies originating in certain places then move longshore. Attention should also be paid to the peculiarities of formation of the currents over the northeastern sea shelf. In the version used, we reproduce eleven major rivers, such as the Danube, the Dnieper, and the Dniester entering the northeastern sea shelf. The fields of current velocities presented in Fig.3 show that the direction of the current caused by the effect of riverine runoff significantly depends on the season. In so doing, the local circulation occurs in the zones of runoff of these rivers. The intensification of the southward transport of fresh water is mainly due to the strengthening of the northerly winds and interaction of desalinated water with the RC. 4.1 Modeling Oil Spill Transport

The operative use of any computer model for monitoring one or another phenomenon requires the development of a convenient interface that would provide the automatic input of necessary parameters of both the study agent and the environment. For the model proposed, we developed a dialogue interface (Fig.2). This interface combines the DieCAST model and a model for the oil transport in the Black Sea. The interface consists of eight major units responsible for different functions. The left unit is used for the input of the major model parameters for the prediction of pollution in emergency situations and operative data on the wind velocity and sea height. In the case of the integration of the model with a meteorological complex, these data are acquired automatically. The current velocities calculated with the use of the hydrodynamic unit constructed on the basis of the DieCAST model are also input here. In determining the accident region and the input source, a map of the specific region is used. On pressing the right button of the mouse at the site of the pollution dumping, the source at this site begins to operate and the calculation of the oil spill trajectory and oil concentration begins to

be performed in the pollution zone. The source can be both continuous and instantaneous depending on the study problem. The operator can set the required power of the source depending on the incoming emergency data. The model gives the complete pattern of distribution of the particles representing hypothetical oil spills. The accompanying information on the physicochemical transformation (destruction) of the oil product and the components of its mass balance is also available.

Fig.4 shows an example of the time trend of the oil mass balance components in an experiment with a continuous source. In this experiment, a source with a power of 130 t/h operated for 15 days. The calculations were performed over 30 days from the beginning of

the source operation. Fig.4 shows the time dependencies of the

amount of the evaporated oil and the oil dispersed in the water and at the sea surface, as well as the amount of oil settled on the bottom and discharged on the beach.

Before analyzing Fig.4, let us make some comments. There are two coordinate axes in the figure: left (basic) and right (auxiliary). On the left scale, the principal rapidly growing components of the oil balance, such as the total amount of oil and the amount of evaporated oil,

dispersed oil, and oil components in the water, are shown. The auxiliary ordinate axis represents the fractions with smaller amounts of oil: the oil discharged on the beach, settled on the bottom, and at the sea surface. The auxiliary scale with the smaller absolute amount of oil allows us to examine these fractions in detail and trace their dynamics in time.

As can be seen from Fig.4, with the spill growth, oil is discharged onto the beach and settles on the bottom. The plot shows that the contamination of the coast begins from the seventh day after the onset of the source operation. The contamination is rather serious: by the end of 30 days, about 3800 tons of oil is discharged onto the beach. About 2800 tons of oil settles on the bottom.

It is interesting to consider the behavior of

the amount of oil at the sea surface. At the beginning, when the source is in operation, in spite of evaporation, the amount of oil at the sea surface increases. Then, after the end of the oil supply, the amount of oil on the surface decreases asymptotically as evaporation, settling on the bottom, and coastal contamination continue. In spite of the relatively small current amounts of oil at the sea surface, it should be recalled that actually (and in the model as well) oil is evaporated from the sea surface; therefore, the total amount of oil at

Fig.4. Oil Mass Fate vs Time

0

500010000

1500020000

25000

3000035000

4000045000

50000

0.5 3 5.5 8 10.5 13 15.5 18 20.5 23 25.5 28

Time, days

Oil

Mas

s, to

ns

0

500

1000

1500

2000

2500

3000

3500

4000

Total EvaporatedIn water DispersedAt surface(right axis) Beached(right axis)Deposited(right axis)

Fig.5. Computed Hypothetical Oil spills at Major Ports of the Black Sea; for February( a - upper panel), for July (b - lower panel)

the surface would be determined by the actual amount of oil at the surface and the amount of oil which has been evaporated. Therefore, in total, this amount will be rather significant.

With the use of the model developed, we performed numerical experiments to learn the conditions under which oil spills at the most probable sites could result in serious hazards to the coasts of the Black Sea countries. We studied the probability of oil discharge on the beaches of Bulgaria, Georgia, Russia, Romania, Turkey, and Ukraine as a result of 10-day hypothetical oil spills at nine major ports of the

Black Sea. In contrast to the case of passive tracers, wind plays a crucial role in the process of the spread of an oil spill in the near-surface sea layer. Therefore, for numerical modeling and comparison of the results, we used two months corresponding to cold and warm climatic conditions. The sites of oil discharge were chosen at the major ports of the Black Sea (see Fig.1).

The results of the modeling transport of droplets whose assembly and behavior represent oil spills are shown in Fig.5 for February and July. They show that the general direction of oil

transport is the same as that of the circulation in the near-shore water. Meanwhile, oil spills could often stay near the coast due to the dominating wind drift current opposite to the RC. As is shown in Fig.5b, in some cases, the spreading oil spills are entrained by near-shore anticyclonic eddies (Sochi and Trabzon). In the winter and autumn, strong winds intensifying the RC also contribute to the formation of elongated oil spills, which spread longshore.

Figure 5b shows a characteristic entrainment of an oil spill by the Sevastopol anticyclonic circulation. Weak near-shore flows and winds in the spring and summer seasons result in general in relatively short spills except for the region of the Sevastopol anticyclone, where the motion of the spill is determined by RC. 5. Conclusions The comprehensive model developed for the calculation of the circulation and transport of oil pollution in the Black Sea on the basis of a DieCAST low-dissipative model and the random walk method allows one to reproduce the major characteristic features of the sea hydrodynamics and to predict the sizes and motions of contaminated regions formed by possible emergency oil discharges into the sea. The model presented provides not only an operative calculation of oil concentrations in the spill, but its evaporated amount and the amounts of oil discharged onto beaches, settled on the bottom, contained in the dispersed fraction and at the surface, as well as the area of contamination of the sea and bottom. Such a model can be included in the system of operative real-time monitoring with the use of current meteorological data on the basis of a system of automatic data acquisition developed by the author, which are available via the Internet (see [15]).

It should be emphasized that the model proposed is universal and, with corresponding updates, it can be applied to any area. This model has already been efficiently used for the Caspian and Adriatic seas.

Finally, it should be noted that the structure of the currents presented in Fig.1 is rather conventional and it actually may be much more complex. In some cases, the formation of

systems of cyclonic and anticyclonic eddies can significantly change popular opinion on both the circulation in the Black Sea and matter transport. The studies undertaken in recent years (see [21]) show that such structures often lead to the intensification of the exchange between the shelf waters and the open sea, which can affect the characteristics of the oil pollution transport in this region. References: [1] Aubrey, D.G. et al., Hydroblack'91 CTD

Intercalibration Workshop, Workshop Report No. 91, UNESCO, Intergovernmental Oceanographic Commission, 1993.

[2] Blumberg, A.F. and Mellor, G.L., A Description of a Three-Dimensional Coastal Ocean Circulation Model, Three-Dimensional Coastal Ocean Models, Vol. 4, Heaps, N., Ed., Washington: AGU, 1987.

[3] Dietrich, D.E., Lin, C.A., Mestas-Nunez, A., and Ko, D.-S., A High Resolution Numerical Study of Gulf of Mexico Fronts and Eddies, Meteorol. Atmos. Phys., Vol. 64, 1997,pp. 187-201.

[4] Ginzburg, A.I., Kostianoy, A.G., Nezlin, N.P., et al., Anticyclonic Eddies in the Northwestern Black Sea, J. Mar. Syst., Vol. 32, 2002, pp. 91-106.

[5] Korotenko, K.A., Mamedov, R.M., and Mooers, C.N.K., Prediction of the Dispersal of Oil Transport in the Caspian Sea Resulting from a Continuous Release, Spill Science and Technology Bulletin, Vol. 6, No. 5/6, 2001, pp. 323-339.

[6] Korotenko, K.A., Mamedov, R.M., and Mooers, C.N.K. Prediction of the Transport and Dispersal of Oil Transport in the South Caspian Sea Resulting from Blowouts. J. Environ .Fluid Mech. Vol. 1, 2002, pp. 383-414.

[7] Korotenko, K.A., D. E. Dietrich and M. J. Bowman, Modeling Circulation and Oil Spill Transport and Dispersal in the Black Sea. Oceanology, Vol.43, No.4, 2003, pp. 411-421

[8] Korotenko, K.A, et al, Particle tracking method in the approach for prediction of oil slick transport in the sea: modeling oil pollution resulting from river input, J.

Marine Systems. Vol. 48. No.1. 2004, pp.159-170.

[9] Krivosheya, et al., Meandering of the Main Black Sea Current and Eddy Formation in the Northeastern Part of the Black Sea in the Summer of 1994, Oceanology, Vol. 38, No. 4, 1998, pp. 546--553.

[10] Krivosheya, et al., Influence of Water Circulation and Eddies on the Depth of the Upper Boundary of the Hydrogen Sulfide Zone and Ventilation of ANoxic Waters in the Black Sea, Oceanology, Vol. 40, No. 6, 2000, pp. 767-776.

[11] Oguz, T. et al., Circulation in the Surface and Intermediate Layers in the Black Sea, Deep-Sea Res., Vol. 1, No. 40, 1993, pp. 1597-1612.

[12] Proctor, R., Flather, R.A., and Elliot, A.J., Modeling Tides and Surface Drift in the Arabian Gulf: Application to the Gulf Spill, Cont. Shelf Res., Vol. 14, No. 5, 1994, pp. 531-545.

[13] Ovchinnikov, I.M. and Popov, Yu.I., Formation of the Cold Intermediate Layer in the Black Sea, Oceanology, Vol. 27, No. 5, 1987, pp. 739-746.

[14] Sanderson, B.G., Order and Resolution for

Computational Ocean Dynamics, J. Phys. Oceanogr., Vol. 28, 1998, pp.1271-1286.

[15] Spaulding, M.L., Korotenko, K.A., TRANSMAP: An Integrated, Real Time Environmental Monitoring and Forecasting System for Highways and Waterways in RI, Report for URI Transportation Center, Kingston, RI: 2001.

[16] Stanev, E.V. and Beckers, J.M., Numerical Simulations of Seasonal and Interannual Variability of the Black Sea Thermohaline Circulation, J. Mar. Sys., Vol. 22, 1999, pp. 241-267.

[17] Staneva, J.V., Dietrich, D.E., Stanev, E.V., and Bowman, M.J., Mesoscale Circulation in the Black Sea: New Results from DieCAST Model Simulations, J. Mar. Sys., Vol. 31, 2001, pp. 137-157.

[18] Sur, H.I., Ozsoy, E., Ilyin, Y.P., and Unluata, U., Boundary Current Instabilities, Upwelling, Shelf Mixing, and Eutrophication Processes in the Black Sea, Prog. Oceanogr., Vol. 23, 1994, pp. 249-302.

[19] Titov, V.B., On the Role of Eddies in the Formation of the Regime of Currents on the Shelf of the Black Sea and in the Ecology of the Coastal Zone, Oceanology, Vol. 32, No. 1, 1992, pp. 39-48.

[20] Titov, V.B., Experimental Data on the Meandfering of the Main Black Sea Current, Oceanology, Vol. 33, No. 4, 1993, pp. 521-526.

[21] Zatsepin, A.G. and Flint, M.V. Complex Studies of the Northeastern Black Sea, (Multidisciplinary Studies of the Northeastern Part of the Black Sea), Moscow: Nauka, 2002.