The specification of mixing in random walk models for dispersion in the sea

Abstract Random walk modelling has been extensively used for pollutant dispersion studies on a timescale of a few tidal cycles. The aim of this study is to assess the potential of random walk modelling for longer timescales and to assess the performance of different modelling formulations for computing the horizontal spread of material in the sea. In order to assess the validity of the longer term predictions, data from large scale dye spreading experiments off Cape Kennedy, Florida ( Pritchard et al. 1966 ) and in the North Sea (RHENO experiment, Weidemann 1973 ) have been compared with model predictions for the appropriate conditions. Results have shown a need for care in determining the concentrations from the random walk model to ensure a consistent method of predicting the dye patch concentration through time. The three case studies have shown that the model can successfully predict the dye patch dimensions and the dilution of the dye with time, and have demonstrated that the wind induced shear was the dominant factor in the spread. The model has also shown that vertical tidal shear was important for the spread, and under calm conditions would become dominant over the horizontal turbulent mixing after 8 h in high shear regimes (Cape Kennedy, April 1962 and RHENO) and after 90 h in low shear (Cape Kennedy, August 1962 experiments) conditions. In the short term (