Study On The Three-Dimensional Oil Spill Numerical Model And Its Application In The Coastal Area

Accidental oil spills have the potential to cause serious impact on the marine environment so that considerable amounts of work have been directed towards developing spill models predicting the trajectory and fate of spilled oil in order to understand and quantify the spill processes. In this paper, the present situation of marine oil pollution and damage are briefly introduced, and the comprensive review of coastal hydrodynamics and oil spill numerical modeling is given. On the theory basis, the three-dimension oil spill numerical model is proposed.The behavior of oil at sea is determined by the environmental conditions involving sea winds, waves and water currents. Accurate environment information is essential for the reliable prediction of the transport and fate of oil pollution, so forecasting these dynamic factors accurately is the base of the oil spill model. A high-resolution circulation model describing the complex hydrodynamics of coastal waters is required to provide appropriate flow fields. For this purpose, the sigma-coordinate primitive equation Princeton Ocean Model (POM) coupled with a third-generation wave model for coastal regions SWAN (Simulating WAves Nearshore) has been employed. Expressions for depth-dependent radiation stresses derived on the basis of linear wave theory by Mellor are introduced. The radiation stresses at different water depths are discussed, and the result demonstrates that the wave radiation streeses concentrated near the free surface. By adding the depth-dependent radiation stresses to the POM threee dimensional flow model, the wave set-up and undertow are studied. A comparison of the numerical results with the experimental data shows good conformity. It shows that the three dimensional flow model with the depth-dependent radiation stresses can be applid for the coastal hydrodynamic problems.The main mechanisms which govern the fate of an oil slick are advection, diffusion, mechanical spreading, vertical dispersion, evaporation, emulsification. dissolution and the interactions of these processes, which can also curtly recapitulate two modules- transport and weathering. Recent studies have shown that the trajectories of drifters on the ocean surface have a fractal structure that is far from being described using ordinary Brownian motion. Thus, in modeling the diffusion process, a discrete method has been employed for the generation of fractional Brownian motion (fBm) to illustrate superdiffusive transport. The numerical experiment demonstrates that when the Husr value equals 0.75, the simulated results are more consistent with the observations.After proving the equivalence bewteeen a particle tracking model and a concentration model, a hybrid particle tracking/Eulerian-Lagrangian approach for the simulation of spilled oil in coastal areas. Oil discharge from the source is modeled by the release of particles. When the oil slick thickness or the oil concentration reaches a critical value, particles are mapped on slick thicknesses or node concentrations, and the calculations proceed in the Eulerian-Lagrangian mode. To obtain a three-dimensional oil model, a mathematical description of the vertical movement of an oil droplet in the ocean is proposed based on the Langeven equation. The results manifest that the buoyant effect and the vertical turbulent variations are very important mechanisms for vertical movement of oil in the water column. The transport model is used to join other weathering modules for the prediction of the the horizontal movement of surface oil slick, the vertical distribution of oil particles, the concentration in the water column and the mass balance of spilled oil. An accidental "Arteaga" oil spill near Dalian coastal waters is simulated to validate the developed model. Compared with the satellite images of oil slicks on the surface, the numerical results indicate that the model has a reasonable accuracy.