The oil spill model OILTRANS and its application to the Celtic Sea ...

1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465weather conditions during a spill event. After Fingas (2011), the order of importance of theweathering processes encoded in the model are: evaporation, emulsification and natural dispersion.Implementation of the weathering process algorithms chosen for the OILTRANS model are detailedin subsequent sections.2.2. OILTRANS oil fate governing equationsThe processes encoded in OILTRANS to describe the physical and chemical weathering processesof spilled oil include: advection of the slick due to currents and wind drift, diffusion of the oilparticles due to random motions, the mechanical spreading of the slick under gravity and viscousforces, the evaporation from the slick of the lighter components of the spilled oil, the entrainment ofwater into the oil slick forming emulsions, and the dispersion of the smaller droplets of oil into thewater column through the action of breaking waves. These processes are considered by the authorsto be the dominant mechanisms through which oil is removed from the marine environment. Theprocesses of dissolution, biodegradation, sedimentation and photo-oxidation have not been encodedin the current version of OILTRANS as it is considered that they represent the removal of only avery small fraction of spilled oil during the first three days of a spill, the period of the operationaloil spill forecast system. In a novel departure from other oil spill modelling systems, OILTRANSallows for user to select a number of different algorithms for the processes of mechanical spreading,evaporation and emulsification. Details of the algorithms for the current implementation ofOILTRANS are presented below, with the alternate encoded formulations referenced briefly.2.2.1. Mechanical spreadingOil will spread on the surface of a water body even without external forces such as tidal currents orwind stresses. The spreading of the oil on calm waters is due to the force of gravity and theinterfacial tension between oil and water, with the oil viscosity and inertia retarding the spreadingforces (Fay, 1969). The most widely used formulations for determining the rate of spread of oil onthe water‟s surface are the equations proposed by Fay (1971), or versions thereof. Fay dividedspreading into three phases. The first phase known as the gravity-inertial phase generally lasts forless than an hour, except for the largest of spills and is not modelled by OILTRANS. As the timefor the first gravity-inertial phase is so short, OILTRANS begins the oil weathering processes oncethe end time, t o , of this first phase has been reached, which was determined by Fay (1971) to be: