Modelling the Braer oil spill—A retrospective view

Volume 2 8 / N u m b e r 4/April 1994

Pergamon

Marine Pollution Bulletin, Vol. 28, No. 4, pp. 211-218, 1994 Elsevier Science Ltd Printed in Great Britain

Modelling the Braer Oil Spill-A Retrospective View W. R. T U R R E L L

The Scottish Office Agriculture and Fisheries Department, Marine Laboratory, Aberdeen, Scotland, UK

Following the grounding of the tanker Braer numerical model predictions of the movement of spilled oil were made available to several organizations including Scottish Natural Heritage (SNH), the Marine Pollution Control Unit (MPCU) and The Scottish Office Agriculture and Fisheries Department (SOAFD). Two numerical models from different sources were employed. Both failed at the time to predict the fate of oil as revealed by field observations. This paper describes the hydrography of the area where the Braer disaster occurred and reviews some of the numerical modelling efforts that took place during the days following the spill. It describes simple models of advection and mixing which, in conjunction with observations of oil distribution, reproduce what is now believed to be the fate of the Braer oil. Finally the paper considers the developments required to improve the modelling of similar incidents in the future.

Following the grounding of the tanker Braer on 5 January 1993 at Garths Ness at the southernmost tip of the Shetland Island (Fig. 1), 85 000 t of light crude oil was released into the sea. During the following weeks much environmental monitoring took place for which numerical modelling of the spill played an important role. Many numerical models now exist which simulate the movement and dispersion of oil and chemicals released into the environment. These models parameterize or reproduce a variety of physical and chemical processes at a number of levels of sophistication. One important feature of the hydrodynamics which may be included in models is the advection of released material by tidal and wind-driven currents (both locally wind-driven and those arising from storm surges created by the wind but some distance away from the spill site). These currents may be represented in two-dimensions (the currents are assumed to be © 1994 Crown Copyright

constant with depth) or in three-dimensions with vertical shear simulated in the models. The very simplest models assume oil remains at the surface as a slick, and is simply moved by the local wind, normally at approximately 2-3% of the wind speed, and possibly at some angle to the right in the northern hemisphere to account for the rotation of the earth. More sophisticated models now reproduce the physics of oil in water, with vertical mixing moving oil down into the water column, and the oil's own buoyancy moving it up. The vertical mixing may be caused by wind at the surface, the effect of the tide moving over the sea bed and the action of waves. Chemical processes, such as the microbiological decay of oil, and evaporation may also be included. As more physical and chemical processes are included in the model so the number of parameters that are required increases. Often these parameters, such as for example the vertical mixing coefficient, are poorly understood in reality and are often selected in order that the model reproduces known distributions of released tracer. Hence models require tuning and may be of limited use in actual prediction of a spill, especially where no observations are available. The models are perhaps best at interpolating between spatially spread and sparse observations, and may be able to extrapolate in time from one set of observations. They are also useful in examining a possible range of scenarios arising from different combinations of the model parameters, which may aid decisions in the field. The output of numerical models during the days after the Braer incident was put to a variety of uses. These included forming decisions of where and when to employ dispersants, where sensitive sites might be impacted (e.g. fish farms), the possible location of fishery exclusion zones, and where to deploy research vessels attempting to sample the spill. At the time, they failed to describe the ultimate fate of most of the spilled oil, although it is believed that retrospective modelling has now succeeded in reproducing the observed pattern of movement. A number of factors may have accounted for the earlier discrepancies and useful lessons may be learnt for future oil spills. 211

Marine Pollution Bulletin

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General Hydrography of the Area In order to understand how the numerical models failed to predict correctly the movement of the Braer oil spill it is necessary to examine the actual physical environment surrounding the wreck site and the processes occurring within the area. Topography The general topography of the northwest European continental shelf surrounding the Shetland Islands is shown in Fig. 1. Orkney and Shetland lie on a relatively shallow (depth < 100 m) ridge extending towards the northeast from the Scottish mainland, almost to the edge of the continental shelf (200 m contour--Fig. 1). The ridge lies across the westerly entrance to the northern North Sea. While the detailed coastal bathymetry surrounding the Braer wreck site was most influential during the initial stages of oil dispersion, it is the large scale topography which determines the general circulation of the area, and hence the ultimate fate of the spill. Water characteristics The water surrounding the Shetland Islands is predominantly Atlantic water originating from the ocean currents flowing along the shelf edge to the north and west of the islands. Although during the summer months the water column may be stratified (with the water warmer or fresher at the surface compared with that near the sea bed), at the time of the wreck the water column would have been completely well mixed from top to bottom owing to the action of storms during the preceding autumn and winter. Typical water temperat212

ures and salinities at the time were 7°C and 35.2 ppt, respectively. Tides The tidal streams between Orkney and Shetland are particularly strong, partly because the shallow ridge separating the islands intensifies the currents as they flow into and out from the North Sea. Typical maximum tidal streams in the area vary between 1 and 3 knots. Flow is directed either towards the northwest or southeast depending on the state of the tide. The currents immediately off Sumburgh Head are especially strong, and tidal races there are known as the Sumburgh Rrst. The tidal streams here flow around the tip of Shetland, in a clockwise or anti-clockwise direction, again depending on the state of the tide. The clockwise westerly flowing tide is believed to last 9 h compared to only 3 h for the easterly flowing tide, resulting in a net westerly transport in the coastal waters around Sumburgh. Residual currents The general pattern of average water flow during the winter months is shown in Fig. 1 (Turrell et al., 1992). A persistent flow of Atlantic water occurs along the edge of the continental shelf, the Slope Current. During its passage north some of this water crosses onto the shelf and flows into the North Sea around Shetland. Coastal water originating to the west of Britain flows northwards along the west coast into the area north of Scotland. Another persistent current of mixed coastal and Atlantic water, the Fair Isle Current, flows out of this area, across the ridge between Orkney and

Volume 28/Number 4/April 1994

Shetland and into the North Sea. The strength of the Fair Isle current is dependent upon the strength and direction of the wind experienced over a large area of the northwest European continental shelf. It is particularly strong for winds from the south and southwest, but reverses for winds from the north and east.

Numerical Models Two groups were involved with the numerical model predictions of the movement of oil following the grounding of the Braer; the Unit of Coastal and Estuarine Studies (UCES) and the Proudman Oceanographic Laboratory (POL) (Highfield, 1993). The predictions were readily made available to others.

Unit of Coastal and Estuarine Studies (UCES) Dr Alan Elliott of UCES used the latest version of the EUROSPILL model in order to aid the predictions of oil distribution (Highfield, 1993). This model, which was created under an EC funded project, employs a database approach with details of tidal and residual currents stored for each point in a grid covering the entire northwest European continental shelf at a resolution of 8 km (Elliott, 1991). The residual currents are climatological means for the shelf, themselves derived from a wind-driven model. Summer and winter circulation patterns are available (Elliott et al., 1992). Physical processes incorporated in the model include vertical shear of tidal and wind-driven currents and hence the model is more three-dimensional in comparison with previous versions (Elliott, 1991). Chemical processes include evaporation and decay. The model uses a random-walk particle tracking method in order to simulate oil dispersion. The EUROSPILL model was already set up for the northwest European shelf (Elliott et al., 1992), hence predictions commenced shortly after the Braer went aground. The model reproduced the early reports of oil moving northwards along the west coast of the Shetland Islands (Highfield, 1993). Proudman Oceanographic Laboratory (POL) POL constructed a model of oil dispersion based on a fine-scale numerical model of the hydrodynamics around Shetland (Highfield, 1993). This model no longer used a climatological mean circulation pattern, but used hourly predictions of the circulation. The model potentially could not only produce hindcasts of oil movement from the time the Braer went aground to a present time but, using the 36 h forecasts of wind speed and direction produced by an atmospheric model run by the Meteorological Office, could produce true 'predictions' of oil movement. The model employed the same mixing and chemical processes as the EUROSPILL model. Modelling predictions following the grounding of the Braer on 5 January commenced almost immediately. Early predictions were based upon the fine-scale Shetland model. Results showed the oil restricted to the coastal waters around Sumburgh and the west coast of Shetland (Highfield, 1993).

By 12 January the POL model predicted that the oil was streaming off from Sumburgh Head across the North Sea in a northwesterly direction (Highfield, 1993). On 18 January the latest predictions were used by MLA to advise the RV Michael Sars (Institute of Marine Research, Bergen) on possible survey patterns and positions to deploy several Argos satellite tracked buoys. Although sampling took place at the predicted centre of oil distribution, by 29 January, when MLA published the first Shetland Monitoring Programme Bulletin (SOAFD, 1993a), no oil had been found. In fact, when initial results from the satellite tracked buoys were released on 2 February, there were indications of a net southeasterly movement at a depth of 5 m, rather than a northeasterly drift which the POL model continued to predict up to 20 January (Highfield, 1993). Following reports of oil fouling fishing gear southeast of Fair Isle, MLA conducted a water sampling and fluorimeter survey, and by 10 February (SOAFD, 1993b) results revealed the presence of oil in the water. By 17 February preliminary results revealed extensive sediment contamination southeast of Fair Isle (SOAFD, 1993c). This was clearly contrary to the numerical model predictions.

Examination of Actual Oil Movement During the Event Wind during the event The speed and direction of the actual wind experienced at Lerwick during the grounding and subsequent dispersal of oil are shown in Fig. 2a and b. The severe weather is evident, with few periods during the following 15 days with winds of less than Beaufort force 6, while at several times winds exceeded force 8, reaching force 12 on one occasion. The direction of the wind almost exclusively lay between south and west during this period. Orkney-Shetland North Sea inflow model The mean speeds within the inflow between Orkney and Shetland were calculated using a simple empirical model of transport. During the autumn of 1987 several recording current meters were placed across the inflow along a line crossing the current to the west of Orkney. The instruments were left out for several months during which many different wind speeds and directions were encountered. The volume of water moving towards the North Sea (Q m 3 s-l) was then related to the surface wind stress (x Nm-Z), derived from the low-pass filtered wind speed (w m s-1) and direction (qb) at that time. The result was the equation: Q = 0.4 + 22.2 c o s ( ~ - 200°).x, where T

~ P a f D w2

Pa = density of air (1.29 kg m -3) CD----drag coefficient (1.3 × 10-3). This regression equation confirmed that derived by a numerical model of the wind-driven circulation of the 213

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Fig. 2 The record of (a) wind speed and (b) direction obtained from Lerwick during the incident. The upper portion of the plot shows hourly wind speed in knots. The scale along the right axis indicates Beaufort wind force. The lower plot shows the direction from which wind was blowing. (c) The estimated strength of the inflow towards the North Sea as computed using an empirical model of transport through the channel between Orkney and Shetland. The left axis shows current speed in km day -1.

northwest European Continental Shelf (Pingree & Griffiths, 1980). The maximum low-pass filtered wind speed encountered during the experiments used to derive the regression were approximately 10 m s-~, compared to an average (maximum) of 10 (16) m s-1 during the 15 days following the Braer grounding. Transport (Q) has been converted to speed (u) by 214

assuming the inflow passes through the 84 km section between Orkney and Shetland, which had a mean depth of 80 m. The regression equation, in combination with measurements of the wind during January and February 1993, has thus provided hourly estimates of the current's strength (Fig. 2c). During the first 15 days after

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the grounding typical speeds were 10 km day -1, although this reduced after the seventeenth day to more variable flows. Movement of oil The severe weather conditions at Shetland prevented early observations on the dispersion of oil. Daily observations on surface sheens made from the air were reported by MPCU. Water sampling to the west of Shetland by MLA began on 13 January, and was extended offshore on 18 January. These data were subsequently released in a series of weekly Monitoring Bulletins, the first being released on 29 January. These bulletins show that initially, the highest concentrations of hydrocarbons in water were found in inshore waters, particularly in the vicinity of the wreck. In Quendale Bay a level of 4295 ppb was measured on 16 January, this being more than 2000 times the background level. High concentrations were also measured in samples along the western margin of the southern peninsula. Further offshore to the west, lower levels of hydrocarbons (30-50 times background) were found in discrete water samples collected in January. More extensive measurements made using a towed fluorimeter confirmed this picture. Little oil appears to have reached the inshore or offshore waters to the east of Shetlan d. In January levels were, at most, five times background. There was evidence of oil to the south of Shetland, although only one very high value was measured at 20 m depth. More generally, levels in surface waters in this area ranged up to seven times background in January. Following reports of oiled fishing gear from fishermen working" to the southeast of Fair Isle, water analyses and towed fluorimeter measurements were made over an extensive area south of Shetland in early February. The concentrations of hydrocarbons in water sampled from various depths at that time, were, with one exception (six times background at 25 m), within the background range or slightly above. Sediment hydrocarbon levels obtained by the monitoring programme were elevated above background out to 20 km West of the wreck. Particularly high levels were found in the sounds and voes (fjords) to the west of the Shetland peninsula with a mean level of 2000 ppm, some 100 times the expected background value. Higher concentrations of > 4000 ppm were found in a discrete area covering some 80 km 2 situated 10 km west of Burra Isle. Samples from the east of the peninsula revealed lower concentrations with a mean around two or three times background. Sediment sampling also discovered another large (4000 km 2) area of elevated hydrocarbon levels some 50 km southeast of Fair Isle. Maximum concentrations in this area were 3500 ppm and were confirmed by fingerprinting as being of Braer origin. It is now evident that the complex tidal streams and residual currents surrounding Sumburgh Head made prediction of the movement of spilled oil most difficult. The initial westerly movement was presumably caused by the predominant westerly flowing tide but was aided

by brief periods of southeasterly winds (0800-1300, 10 January 1993; 0000-0400, 15 January 1993; 16002100, 16 January 1993). Subsequent movement of oil from the wreck site is likely to have been determined by how deep the oil was found. Surface oil behaves quite differently from oil entrained deeper into the water column. The depth at which oil is found is itself dependent upon the size of droplets which the oil forms in the water, and the degree of mixing that is occurring in the sea under the action of tide, wind and waves. Oil in the sea generally forms droplets of between 10 and 500 ~tm diameter (Elliott, 1991). For the Norwegian Gulfaks crude oil, under the high wind and wave energy conditions which the oil was subjected to on first release into the environment, it is postulated that the diameters of oil droplets in the sea may have been at the small end of the range, that is, 10-100 ~tm. Such droplets, under the conditions present at the time of the release, rapidly mix down through the water.

Vertical mixing model A simple model of oil droplet mixing has been used to estimate typical times required for oil to come in contact with the sea bed for different total depths. Two forces are assumed to act upon the oil; the oil's own buoyancy results in an upward directed drift of the droplets, while the droplets also move up and down randomly owing to the mixing action of the wind, waves and tide. The speed with which the oil drifts up is dependent upon the density of the oil (po---0.882 g cm -3 for Gulfaks crude) compared to that of seawater (pw--1.027 g c m - 3 in this case) and the diameter of the droplets (d cm). The actual speed (UB cm s-l), for droplets having diameters less than a critical value (de) is given by Stokes Law (Elliott, 1986): U B~

gd2(1 -- po/Pw) 18v

where g = acceleration due to gravity (981 cm s-e) v =viscosity of seawater (1.64 x 10 -2 cm 2 s-l). The critical diameter (de) for Gulfaks oil being approximately 1200 ~tm. The distance each droplet moves owing to mixing is determined by a diffusion coefficient (Kz) which is generally derived from observations. For the purposes of this study values of 50 and 100 cm 2 s-1 were used. In the North Sea 50 cm 2 s -1 has been measured in reasonably rough conditions. No measurements exist under the extreme conditions experienced during this event. The model was run over a range of water depths, and with a range of droplet sizes. The model simulates vertical diffusion using a random-walk technique. The distance moved (dz) by an oil droplet each time step (dt) is given by: dz = UBdt + Rnd.(2Kzdt ) ~/2,

where Rnd is either --1 or 1, selected by the computer's 215

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It is estimated that 10 ~tm oil droplets may reach a depth of 40 m between 24 and 48 h after release. Droplets of 100 txm reach the same depth possibly between 5 and 10 days. Under extreme conditions 10 (100) ~tm droplets will reach a depth of 100 m within 5 (15) days.

random number generator. For these model runs dt was selected as 1 min. Owing to the random nature of the model, for each combination it was run 10 times yielding the mean time taken for particles to arrive at the selected depth. The results are summarized in Fig. 3.

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It is evident that the Braer oil did not form a thick surface film or slick but became mixed with the water. While surface oil may be directly driven by the wind, oil mixed into the water must move with the prevailing currents. For oil entering the North Sea inflow running between Orkney and Shetland (see Fig. 1) this means a general movement towards the southeast. During the grounding and subsequent release it may be expected that these currents were flowing at approximately 10 km day-1. For total depths of 100 m, the oil droplets reached the sea bed between 5 and 15 days after the initial release, and so may have travelled between 50 and 150 km from the wreck site. This corresponds with the observed sediment contamination (Fig. 4).

ultimate size of areas of impacted sediment and to provide early warning to potential coastal impact sites (e.g. in this case potentially Norway according to some model runs). For this purpose the EUROSPILL approach, employing climatological circulation details held in a database, may also play an important role. It has the advantage that it can be run quickly to provide a range of future scenarios, enabling administrators to be briefed on the possible implications of an incident.

Recommendations for future research It is now clear that the oil released from the Braer did not behave in the manner expected from observations on earlier oil spills. In retrospect, now that the pattern of distribution of the oil is known, its behaviour

Discussion The movement of water, and oil in water, under the action of tides and wind may be simulated using computer models. Several such simulations were attempted during the Braer spill, but did not successfully predict the movement of oil to the southeast. It is easy to be wise after such an event, however. At the time a number of difficulties made modelling especially difficult. Major changes to existing models were necessary. Moreover, a number of other factors affected the accuracy of the predictions. The environmental conditions experienced after the release were extreme. Computer models rely on parameters to simulate actual processes that occur in nature, such as wind and wave mixing. These parameters are derived from measurements at sea, which are difficult to perform under severe weather conditions. Hence the parameters available to the models may not have truly reflected conditions off Shetland at the time of release. The hydrographic conditions surrounding Sumburgh Head are extremely complex. As can be seen from Fig. 1, it makes a great difference to the eventual fate of the oil if it is initially mixed into the North Sea inflow occurring between Orkney and Shetland, or whether it escapes this current and moves east into the northern North Sea. Hence the ultimate drift of oil may have been determined within a very short time of its release within the small hydrodynamically complex area around Sumburgh Head. The models may not have been able to simulate conditions on a fine scale within such a small area sufficiently accurately. I n general two time scales may be imagined of impGrtance to statutory bodies dealing with the aftermath of an oil spill. Short-term predictions of oil distribution, incorporating real-time meteorological data, are required for such purposes as dispersant applications, survey vessel deployment and the protection of sensitive sites (e.g. booming of fish farms or coastal industrial plants). Models of the type used by POL are best suited for these type of predictions, particularly if they are tuned to initial observations of oil distribution. Long-term predictions are required for other purposes. For example, the estimation of the effect of the spill on a fishery, the early determination of the size and duration of a fishery exclusion zone, estimation of the

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can be simulated. The incident may therefore improve our understanding of the processes that influence the movement of oil. Hence, prospects for modelling future oil spills hopefully will improve. From the experiences gained during the days following the grounding of the Braer a number of recommendations may be made concerning future developments of oil spill tracking models. These arise from extensive discussions between scientists at the MLA involved in the oil spill follow-up operations, members of the Ecological Steering Group for the Oil Spill in Shetland (ESGOSS) and scientists at POL and UCES. 1. High resolution model grids should be developed for particular areas of the UK coastal waters which are at potential risk from oil spills (e.g. Minch, Orkneys, Clyde, Forth, Irish Sea, English Channel). These should be meshed with the low-resolution shelf-wide models (both dynamically and for the cross-transfer of particles). These models could be held and regularly maintained by an appropriate agency for rapid response in the event of future spills. This would avoid potential errors introduced by on-line developments rushed through during an incident. 2. Consideration should be given to the range of data that could most readily be collected shortly after an incident with which to tune short-term high-resolution numerical models (e.g. satellite tracked buoys, fluorimeter surveys). It is important to consider how this data is disseminated to the various groups involved and used for the preparation of response plans. Feedback of observational data to the modellers is important. 3. A model of the transport of oil in the atmosphere should be developed and integrated with a hydrodynamic model. Its preparation may require some research into atmospheric transport mechanisms and ejection/deposition mechanisms. The availability of such a model would avoid the confusion that arose through the observation of surface sheen over wide areas following the Braer release. The model would also be of use when considering the impact on lee shorelines and coastal land areas. 4. Research should be carried out into the uptake of oil by bed-load and suspended sediments and the mechan-

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isms involved in the transport and deposition of oil associated with suspended particles. A better understanding of such processes should lead to their eventual inclusion in numerical models. 5. Better understanding is required of the processes affecting the behaviour of oil at shorelines exposed to severe weather conditions. 6. Research should be carried out on the effects of wind and waves on vertical mixing under extreme environmental conditions. Novel tracer methods or other observational methods may be required. The use of wave models should also be investigated. 7. Research should be carried out on droplet size distributions of typical oil types that are transported in UK waters under a variety of environmental conditions. This could be done by deciding on a likely extreme droplet size range and making model simulations for different conditions. The effect of dispersants on droplet sizes should also be considered. Dr Alan Kelly (SOAFD) supplied Fig. 4. The author would like to acknowledge many helpful discussions with Dr Alan Elliott, Dr Roger Proctor, Dr John Davies and Professor A. D. Hawkins.

Elliott, A. J. (1986). Shear diffusion and the spread of oil in the surface layers of the North Sea. Dt. Hydrogr. Z. 39, 113-137. Elliott, A. J. (1991). EUROSPILL: Oceanographic processes and NW European shelf databases. Mar. Pollut. Bull. 22,548-553. Elliott, A. J., Dale, A. C. & Proctor, R. (1992). Modelling the movement of pollutants in the UK shelf seas. Mar. Pollut. Bull. 24, 614-619. Highfield, R. (1993). Spilt oil gathers beneath the waves. Daily Telegraph, 25 January 1993, p. 14. Pingree, R. D. & Griffiths, D. K. (1980). Currents driven by a steady uniform wind stress on the shelf seas around the British Isles. Oceanologica Acta 3(2), 227-236. POL (1993). Proudman Oceanographic Laboratory Information Faxes. Dated 14 January 1993 to 3 February 1993. SOAFD (1993a). Shetland Monitoring Programme, Bulletin No. 1, 29 January 1993. SOAFD (1993b). Shetland Monitoring Programme, Bulletin No. 3, 10 February 1993. SOAFD (1993c). Shetland Monitoring Programme, Bulletin No. 4, 17 February 1993. Turrell, W. R., Henderson, E. W. & Slesser, G. (1990). Residual transport within the Fair Isle Current observed during the Autumn Circulation Experiment (ACE). Cont. ShelfRes. 10(6), 521-543. Turrell, W. R., Henderson, E. W., Slesser, G., Payne, R. & Adams, R. D. (1992). Seasonal changes in the circulation of the northern North Sea. Cont. ShelfRes. 12(2/3), 257-286.