Circulation and mixing processes and their effect on pollutant distribution in the western Arabian Gulf

Circulation and mixing processes and their effect on pollutant distribution in the western Arabian Gulf

Applied Ocean Research 14 (1992) 59-64 Technical Note Circulation and mixing processes and their effect on pollutant distribution in the western Arab...

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Applied Ocean Research 14 (1992) 59-64

Technical Note Circulation and mixing processes and their effect on pollutant distribution in the western Arabian Gulf V. Chandy John MacLaren Plansearch, Lavlin Inc., Suite 200, Park Lane Terraces, 5657 Spring Garden Road, Halifax, Nova Scotia, Canada B3J 3R4

(Received 24 April 1990; accepted 30 April 1991) Current measurements at four offshore locations in the western Arabian Gulf from 1986 to 1989 show periods of stagnation or low currents, and steady Shamal(northwest storms) driven currents which have significance to the mixing and transport of pollutants in this area. A major effect of Shamal on the net circulation was observed in June 1986 at the Station CM3 surface where the net drift increased from 50 km during the first two weeks to about 100km during the next two weeks. The drift direction during this period was 45 ° clockwise from Shamal wind direction. At CM4, the net drift was even reversed from southward to northward when Shamal winds changed to 'Kaus' winds (southeast storms) in August 1987. Progressive vector diagrams show that the residual current in the region controls the pollutant transport of the surface water at CM 1 generally towards the south and southwest and the bottom water towards the southeast. In contrast to CM 1, the net drift at CM2 showed smaller southeastward drift. However, the near-bottom flows at all stations were towards the southeast and southwest, except near-bottom at Station CM3, where eastward and northeastward residual advective transport is observed during the whole observation period. This unique northeastward near-bottom current has a special significance to the overall residual circulation of the Arabian Gulf. The strongest tidal current transport occurs at Station CM 1, which is near the location of an amphidromic point for M2 harmonic tidal constituent. Movement of pollutants offshore over a tidal cycle would be generally toward the southeast during flood and toward the northwest during ebb tide. Large tidal mixing is observed in the region and the associated turbulent diffusion causes pollutants to be rapidly dispersed and thereby shows almost uniform distribution in the vertical. Complex tidal mixing and transport exist at Station CM 1 where predominantly diurnal tidal elevations exist with mixed, mainly semi-diurnal tidal currents and vice versa (predominantly semi-diurnal tidal elevations exist with mixed mainly diurnal tidal currents) at CM3. An earlier study by John et al. 1 indicated that the formation of high-salinity water in the Gulf of Salwah may be one of the most important sources of the salinity-related density gradient proposed by Hunter e to drive circulation in the Arabian Gulf. However, this study shows that the net displacement of water towards the southeast was also due to the predominant northerly and northwesterly winds.

ment due to remarkable industrial expansion in the last two decades. The dredging for offshore projects causes m o v e m e n t o f sediments and m a n y associated ecological and coastal protection problems. The conflicting needs o f industry and environmental protection have to be taken into account for proper management o f marine resources. The ecology o f the coastal zone environment is strongly

INTRODUCTION Coastal areas o f the A r a b i a n G u l f region have been increasingly used for recreation and resource developApplied Ocean Research 0141-1187/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 59

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V. Chandy John

influenced by the coastal currents, thermal structure, and the environment's capacity to disperse pollutants discharged into the coastal zone. An understanding of these processes is a basic prerequisite to effective conservation of the coastal zone environment. The results of this study are useful to practical problems such as siting of discharge of sewage and industrial effluents, and water intakes. Hunter z and Hughes and Hunter 3 reviewed mixing processes in the Gulf using a simple analysis of the limited oceanographic data available. Cekirge et al. 4 developed a passive pollutant transport model to estimate the transport of non-interacting pollutants in the Arabian Gulf. Some localized studies of the current pattern have been made in the coastline industrial areas of Saudi Arabia. 5 Some theoretical computer models for the Gulf have been developed, 6-9 which are capable of predicting currents as well as tidal heights. The first systematic current surveys to measure currents in the western Arabian GuW ° provided the necessary data for empirical verification of these models. The distribution of pollutants is controlled by various transport and diffusive processes which are produced by forcing mechanisms (winds, tidal elevations, and density distribution) and dispersion of pollutants by turbulence, and molecular diffusion. Tides in the Arabian Gulf enter through the strait of Hormuz and propagate as Kelvin waves] 1 Thus, tides progress in a counterclockwise pattern north along the Iranian coast and south along the Saudi Arabian Coastline. Tides in the southern Saudi Arabian Gulf coast are semi-diurnal; however, tides in the northern area are predominantly diurnal. One of the interesting weather phenomena of the study area is the Shamal which is a sub-synoptic scale wind phenomenon which is frequent and influences the local weather by seasonal northwesterly winds that occur during winter (mainly during December to February) as well as summer (mainly in June and July). 5~12 Winter storms occasionally create southeast or south winds known as 'Kaus' followed by northwesterly winds. Water depths of the western Arabian Gulf vary from 0-30 m and the sea bottom is generally sandy. This paper describes the circulation and mixing processes and transport rates of pollutants in the western Arabian Gulf.

METHODS

During the period from January 1986 to February 1989 six current meters (ENDECO Model A 174) were deployed and maintained at four locations in the western Arabian Gulf (Fig. 1) along the Saudi Coastline. Two current meters at CM1 and CM3 (at near-surface and nearbottom) and one at CM2 and CM4 (at mid-depth) were maintained on a regular basis to obtain at least one month of data for each quarter of the year. Hourly vector average currents were computed from recorded data at

three-minute intervals. The frequency distribution of the time series current data were obtained by using Statistical Analysis Package (SAS) for 30- to 45-day data blocks. These data were also analyzed for speed and direction histograms and statistical distribution of currents. Daily mean currents were used to draw progressive vector diagrams (PVDs) which show the pseudo-trajectory of particles in the sea.

RESULTS A N D DISCUSSION

The histograms of surface and bottom currents show large variations in current pattern, varying from a bimodal distribution at CM4 with predominant currents toward southeast and northwest to multi-directional currents at 30 m stations (Fig. 1). At Stations CM 1 and CM3, the tidal currents had a periodicity of 12 or 25 h and had some rotary effect (Fig. 2). However, at nearshore locations, such as, Stations CM2 and CM4, currents were translatory, since particle velocities perpendicular to the coast must vanish there. The currents at these nearshore locations flowed parallel to the local bathymetry contours. Tidal currents in the study area varied from mixed, mainly semi-diurnal at Station CMI to mixed mainly diurnal at Station CM3.13 Progressive vector diagrams (PVDs) show periods of stagnation or low currents and steady Shamal-driven currents, and also show the effect of coastal boundaries by restricting water movement near the coast. Most important, however, is the changing flow pattern as distance from the shore increases. At the 30-m depth station in the northern area (CM1) the net surface currents were most variable (southeasterly through westerly). During the period 31 August to 17 October, 1987, periods of stagnation were observed (Fig. 2) and the net drift in the southwest direction was only about 70km. The largest unidirectional southeastward and southward net surface drift of 400km was observed (flushing period) during the period 15 May to 2 July 1987. The near-bottom currents at CM1, however, showed a much smaller net drift, showing a net drift of 30 km during 3 September to 7 October 1987. However, the net drift during 10 May to 29 June 1987 was about 90 km in the southeastward direction. There is a minor difference noticed in the direction of surface and bottom currents at CM 1. Although the whole water column was affected by winds, the surface water at CM1 generally moved southeastward through westward and the bottom water moved towards the southeast. In contrast to CM1 (showing a net surface drift of up to 400 km), the net drift at CM2 showed a smaller southeastward drift of 6090 km during the whole period. At CM3 the residual current pattern was similar to that at CM1 with a near-stagnant surface drift pattern during 2 February to 21 March 1987 with a net drift of only 30 km. A net southeastward drift of up to 170 km

Effect of circulation and mixing processes on pollutants in the Gulf

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Fig. 1. Histograms of currents. was noticed during 9 February to 1 April 1986. However, the near bottom flow during the whole period was towards east and northeast (Fig. 3). The unique northeastward near-bottom current at Station CM3 was opposite to the

surface and the near-bottom flow observed at the remaining stations could be due to a flow of high salinity water from the Gulf of Salwah.~ Since this water mass could not be traced further north it is suspected that it moves

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V. Chandy John

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Fig. 2. Progressive vector diagrams at Stations CMI and CM2. towards the Strait of Hormuz due to the effect of Coriolis force deflecting it to the right. The directions of net drift at CM4 were similar to that at CM2, flowing toward south and southeast. However, the rates at CM4 were much stronger, reaching up to 170 km during 10 January to 13 February 1989. Hunter 2 proposed that the pressure gradients arising from evaporation-induced variations are the dominant forcing function of the net Arabian Gulf circulation. The inflow of low-salinity surface water at the Straits of Hormuz is balanced by the outflow of deep saline water. A subsequent study by John et al. t indicated that the formation of high-salinity water in the Gulf of Salwah may be one of the most important sources of the salinity-related density gradient proposed by H u n t e r 2 to drive circulation in the Arabian Gulf. However, this study shows that the net displacement of water towards the southeast was also

due to the predominant northerly and northwesterly winds (Fig. 2). The effect of Shamal on the net circulation was observed, especially during 28 May to 26 June 1986 at the CM3 surface station, where the net drift during the first two weeks was only 50 km. However, a strong Shamal wind during the next two weeks increased the net drift to about 100 km. The drift direction during this period was 45 ° to the clockwise from Shamal wind direction. At CM4, the net drift was even reversed from southward to northward when the Shamal winds changed to Kaus winds in August 1987 (Fig. 3). Generally, the net current closely followed wind directions at this station.

Transport rates of pollutants The periods of stagnation or low currents and steady Shamal-driven currents in the area discussed above have

Effect of circulation and mixing processes on pollutants in the Gulf

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Fig. 3. Progressive vector diagrams at Stations CM3 and CM4. significance to the mixing and transport of pollutants in this area. The worst periods for waste disposal are the stagnant or very weak current periods during which the discharged effluents form stagnant pools with very little transport and mixing and there are chances of onshore transport of this effluent. Periods of strong Shamaldriven currents parallel to the shore are more favorable for pollutant sources located offshore because of better effluent transport and dispersal. Large tidal mixing was observed in the region and the associated turbulent diffusion caused pollutants to be of almost uniform vertical distribution. Major tidal current constituents which cause such mixing consist of M2, $2, O1, and K1.13

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Complex mixing and transport exist at Station CM1 where predominantly diurnal tidal elevations exist with mixed, mainly semi-diurnal tidal currents and vice versa at CM3. The strongest tidal current transport occurred at Station CM1, which is near the location of an amphidromic point for an M2 harmonic tidal constituent. Movement of pollutants offshore over a tidal cycle would be generally toward the southeast during flood and toward the northwest during ebb tide. The net movement of water discussed above govern the movement of pollutants in the western Arabian Gulf. PVDs show that Shamal dominates net pollutant transport during high wind periods.

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V. Chandy John

CONCLUSIONS The results of this study show periods of stagnation or low currents and steady Shamal-driven currents in the area which have significance to the mixing and transport of pollutants in this area. The strongest tidal current transport occurred at Station CM1, which is near the location of an amphidromic point for an M2 harmonic tidal constituent. Movement of pollutants offshore over a tidal cycle would be generally toward the southeast during flood and toward the northwest during ebb tide. Progressive vector diagrams show that the residual current in the region controlling the pollutant transport is generally towards south and southeast except nearbottom at Station CM3, where northeastward residual advective transport is observed. This unique northeastward near-bottom current at Station CM3 has a special significance to the overall residual circulation of the Arabian G u l f and therefore to the transport and dispersal of effluents discharged into this area. Large tidal mixing is observed in the region and the associated turbulent diffusion causes pollutants to be rapidly dispersed and thereby show almost uniform distribution in the vertical. Major tidal current constituents which cause such mixing consist of M2, $2, O1, and K I . Complex mixing and transport exist at Station C M I where predominantly diurnal tidal elevations exist with mixed, mainly semi-diurnal tidal currents and vice versa at CM 3. The major effect of Shamal on the net circulation was observed in June 1986 at the CM3 surface where the net drift was increased from 5 0 k m to about 100km. The drift direction during this period was 45 ° to the clockwise from Shamal wind direction. At CM4, the net drift was even reversed from southward to northward when Shamal winds changed to Kaus winds in August 1987. Generally wind directions closely followed the net current at this station. An earlier study by John et al. ~ indicated that the formation of high-salinity water in the Gulf of Salwah may be one of the most important sources of the salinity-related density gradient proposed by Hunter 2 to drive circulation in the Arabian Gulf. However, this study shows that the net displacement of water towards the southeast was also due to dominance of flood over ebb currents in addition to the predominant northerly and northwesterly winds.

ACKNOWLEDGEMENTS The author wishes to acknowledge the support o f Saudi Aramco and the Research Institute of the King Fahd University o f Petroleum and Minerals for this work under K F U P M / R I Project N u m b e r 24079. The author

also thanks the Saudi Arabian Ministry of Petroleum and Mineral Resources for their authorization to publish this paper. Special thanks are due to the staff members who contributed to acquisition of data, Dr John Hunter for helpful comments, and Mr Ezzat Abdelkader and Mr A b o b a k r Abozed for their help in the analysis of data.

REFERENCES 1. John, V.C., Coles, S.L. & Abobakr, I.A., Seasonal cycles of temperature, salinity and water masses of the Western Arabian Gulf. Oceanologica Acta, 13(3) (1990) 273-81. 2. Hunter, J.R., A review of the residual circulation and mixing process in the KAP region, with reference to applicable modelling techniques. In Oceanographic Modelling of the Kuwait Action Plan Region, ed. M.I. E1-Sabh. UNESCO Reports in Marine Sciences, 28 (1984) 37-45. 3. Hughes, P. & Hunter, J. A proposal for a physical oceanography program and numerical modelling of the KAP region. Project for KAP 2/2, UNESCO, Paris, 1979. 4. Cekirge, H.M., AI-Rabeh, A.H. & Gunay, N., Passive pollutant transport in the Arabian Gulf, in press. 5. Williams, R., 1979 Meteorologic and Oceanographic' Data. Book for the Eastern Provinee Region of Saudi Arabia. Aramco Technical Servies Division, Dhahran, Saudi Arabia. 6. Von Trepka, 1968 Investigations of the tides in the Persian Gulf by means of a hydrodynamic-numerical model. Proc. Symposium on Mathematical-Hydrodynamical Investigations of the Physical Processes in the Sea, lnstitut fur Meereskunde der Universitat Hamburg, Hamburg, 1968, pp. 59-63. 7. Evan-Roberts, D.J., Tides in the Persian Gulf. In The Consulting Engineer, June 1979. 8. Lardner, R.W., Cekirge, H.M. & Gunay, N. Numerical solution of the two-dimensional tidal equations using method of characteristics. Comp. and Maths. with Appls., 12A(10) (1986) 1065-80. 9. Elahi, K., Tidal charts of the Arabian Sea North of 20°N. In Oceanographic Modelling of the Kuwait Action Plan Region, ed. M.I. EI-Sabh. UNESCO Reports in Marine Sciences, 28 (1983) 68. 10. KFUPM/RI, Aramco Sustaining research project environmental studies. Third annual report 1986/87. Volume II: Oceanography, Prepared for the Arabian Oil Company by the Water Resourcs and Environment Division, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, 1988. 11. Le Provost, C., Models for tides in the KAP region. In Oceanographic Modelling of the Kuwait Action Plan Region. ed. M.I. E1-Sabh. UNESCO Reports in Marine Sciences, 28 (1983) 37--45. 12. Perrone, T.J., Winter Shamal in the Persian Gulf, Naval Environmental Prediction Research Facility. Technical Report I.R.-79-06, Monterey, California, 1981. 13. John, V. C., Harmonic Tidal Current Constituents of the Western Arabian Gulf from Moored Current Measurements. Coastal Engineering, in press.