Regular and breaking waves in wave tank for dispersion effectiveness testing

A wave tank (16 m long X 2.0 m high X 0.6 m wide) was built at the Bedford Institute of Oceanography in Nova Scotia for dispersion effectiveness testing. The waves were generated using a flap-type wavemaker, and a technique for generating breaking waves at the same location in the tank was adopted. The water profile and velocity were measured using a wave gauge and an Acoustic Doppler Velocimeter (ADV). The wave periods of interest ranged between 0.75 and 1.30 seconds, and they were essentially unidirectional (i.e., cross flow was negligible). Spilling and plunging breaking waves were generated and they had heights of 0.14 and 0.17 m. Mean velocities (i.e., currents) were present in the tank, and these currents have to be accounted for in dispersant effectiveness studies in wave tanks. Currently, most studies sample downstream (along the wave propagation) from the location where the oil and dispersant are applied. The results herein indicate that such an approach would probably underestimate the amount of oil dissolved and/or dispersed deep in the water column. The energy dissipation rate per unit mass was evaluated at various depths in the tank for regular and breaking waves. For the breaking experiments, ε decreased from around 5.0 10^-2 watts/kg at 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 a value of ε= 5.0 10^-4 watt/kg. This indicates that breaking plays an important role in the dispersion of oil at sea. To our knowledge, this is the fourth study (Kitchen et al., 1997; OHMSETT, New Jersey; Page et al., 2002) that evaluates the energy dissipation rate at various depths in a wavetank.