The wave tank (32 m long × 2.0 m high × 0.6 m wide) at the Bedford Institute of Oceanography in Nova Scotia was used to simulate the propagation and breaking of deep water waves using a flap-type wavemaker. The water profile and velocity were measured using a wave gauge and an Acoustic Doppler Velocimeter (ADV). The wave periods of interest ranged between 1.18 and 2.08 seconds. A technique for generating breaking waves at the same location in the tank was used to obtain a spilling and a plunging breaker. We evaluated the energy dissipation rate at various depths in the tank for regular and breaking waves. Plunging breaking waves had heights of 0.25m. For the breaking experiments, the energy dissipation rate decreased from around 1.0 10−2 watts/kg a few centimeters below the surface to less than 5.0 10−4 watt/kg 20 cm deep in the water column. The regular waves had, on the average, an energy dissipation rate of 5.0 10−6 watt/kg deep in the water column. This indicates that breaking plays an important role in the dispersion of oil at sea.
In 2005, the National Research Council (NRC) published a comprehensive treatise on oil spill dispersants. Among other things, it concluded that research on dispersion effectiveness as a function of energy dissipation rate and particle size distribution was a high priority. Energy dissipation rate (turbulence and existence of breaking waves) is important to initiate and promote effective dispersion, and the particle size distribution of dispersed oil droplets affects dispersion and the ultimate fate of oil in the water column. In this paper, we discuss the use of a wave tank built on the premises of the Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada as part of collaborative research begun in 2003 by the U.S. Environmental Protection Agency (EPA) and Fisheries and Oceans Canada (DFO). This tank is able to produce breaking waves of various energy levels at precise locations in the tank. We studied the effects of 2 commercial dispersants (Corexit 9500 and SPC 1000) and a no dispersant control on two different crude oils (unweathered Alaska North Slope and weathered MESA Light) at 3 different energy dissipation rates (regular non-breaking waves, spilling breakers, and plunging breakers), amounting to 18 different treatments. We quantified the energy dissipation rates under those 3 wave conditions and measured oil dispersion in a factorial experiment involving 3 replicates of the 18 treatments over the course of the summer of 2006. Results clearly showed the importance of wave energy and the presence of a chemical dispersant on the ability to produce effective dispersion of oil into the water column. The presence of dispersants at increasing wave energies produced significantly better dispersion (p <0.05) than the no-dispersant controls. This study was conducted under batch conditions. Future work will be done under continuous flow conditions.
The studies of dispersion of oil in wave tanks
Our previous work investigated the transport of oil under regular waves at sea. This work considered irregular waves represented by a JONSWAP spectrum, which is a more realistic situation. Particle tracking was used in a Monte Carlo framework to evaluate the combined effects of wave kinematics and turbulent diffusion on the transport of oil droplets at sea. The centroids, variance and spreading coefficients of oil spills with various wave parameters were found in this study. Turbulent diffusion was assumed to be velocity-dependent, and an empirical formulation adopted from subsurface hydrology was adopted. Five hundred neutrally-buoyant oil “particles” were placed at the water surface and tracked for 1 hour. The vertical movement of the plume appeared to be comparable to the significant wave height (about one meter herein), and to decrease with depth. The increase in wind fetch caused an increase in transport and spreading of the plume. The results found in this study can be used by spill responders as a first approximation to the spread of a dispersed oil spill, or can be used as parameters as part of a more complex code used to model oil spills.
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