Physical modelling of mixing processes from ballast water discharge in ports

The predominant factor in biological invasion in port areas is the discharge of ballast water from commercial cargo ships which is a global threat to marine ecosystems. We report Rhodamine-WT tracer dye experiments carried out in a contolled environment to simulate ballast water propagation in order to better understand the diffusion and retention characteristics of ballast water discharges in ports at various wave climates. A Cyclops C7 Turner Rhodamine sensor was used to measure dye concentrations in proximity of the discharge point. Several wave heights and wave periods showed different concentration rates. In particular, the wave frequency accelerated the mixing of dye in the wave basin whilst the wave height had less impact on the dilution. However, further testing is imperative to gain a more complete understanding of all the factors affecting diffusion processes. Our findings suggest that this experimental setup is an efficient way to determine mixing and diffusion coefficients to better analyse Ballast Water Management protocols. Future experiments should include the use of multiple sensors and sufficient rest in between experimental runs. With a targeted parameter set of various sea states diffusion coefficients of discharge jets as well as molecular diffusion rates can be determined.