Environmental Factors Affecting the Movement and Dispersal of Oil Spills With Particular Reference to the Gulf

Lynagh, N. (Noble Denton and Assocs. Ltd.)

OnePetro 

Abstract Currents, tidal streams and winds all influence the movement and dispersal of oil spills. Examination of the surface circulation and wind climate of the Gulf highlights the coasts most likely to be at risk of pollution by spills from the major offshore production areas. Methods used in forecasting movement of spills are discussed. Introduction Wherever oil is produced from offshore oilfields, there is always a threat of oil spills which may, in turn, cause serious pollution on adjacent beaches. In a relatively small landlocked sea such as the Gulf, the risk of pollution on beaches is particularly high. Oil floats on the surface of the sea and so movement of an oil spill is directly related to the movement of the surface water. A certain amount of diffusion or spreading also takes place. The net surface water movement comprises the vector sum of the following components : Background current Tidal stream Wind-generated current Surge current The resultant surface flow over a period of a few days can be very complex, especially if none of the constituents is particularly dominant over all the others. This is often the case in the Gulf. BACKGROUND CURRENT IN THE GULF Within the confines of the Gulf, there is very little true background current. There is some evidence to suggest that in most months of 'the year there is a net flow westwards at the surface through the Strait of Hormuz into the Gulf. This means that anything which is floating on the surface in the Gulf is unlikely to flow out through the Strait of Hormuz. TIDES AND TIDAL STREAMS IN THE GULF The tidal regime in the Gulf is particularly complex. The two main constituents of the semi-diurnal tide, M2 and S2, each have two amphidromic points in the Gulf, one near Jazirat Arzanah in the southern Gulf and one between Ras al Khafji and Bushehr in the north. The two main constituents of the diurnal tide, K1 and O1, have an amphidromic point north of the Qatar peninsula. The net result of this complex regime is that in some parts of the Gulf the tides are predominantly diurnal while in others they are predominantly semi-diurnal. High water at one location can occur at the same time as low water less than 100 miles away. Tidal Streams In general the tidal stream sets towards the WNW on the flood and in the reverse direction on the ebb. A branch of the flood sets south past both sides of Bahrain returning northwards on the ebb. Figs. 1 and 2 show the general flow. Due to the complex nature of the tidal regime, there is no general relationship between the time of turn of the tidal stream and the height of the tide. At one location the stream nay turn near high and low water while at another it may turn at mid-tide. Tidal streams run at up to about 0.8/s at springs though there is considerable local fluctuation. The streams are normally semi-diurnal but there is often considerable inequality between the rates of the two streams setting in a particular direction in a 24-hour period. The magnitude of this inequality is variable depending on the declination of the moon. P. 95

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