Particle-tracking model of outfall plumes in a tidal channel

Abstract

In order to protect the environment that receives them, it is necessary to manage marine outfall discharges of wastewater effluents, which typically contain different types of contaminants even after treatment. The objective of this paper is to answer the question: ‘To what extent is the receiving environment being exposed to effluents?' The focus is on effluent spreading in the far field. A particle-tracking technique is presented, in which effluents are represented by a large number of particles and the particles' trajectories are tracked for given conditions of ambient flow and density stratification. The technique is particularly useful for examining the undesirable scenario of effluents rising to the water surface or coming into contact with the seabed. Advection, non-Fickian horizontal diffusion and Richardson number-dependent vertical diffusion are taken into account. The technique is successfully applied to the discharge of effluents into a tidal channel where the water column is density-stratified. In this application, predictions of the temporally and spatially varying effluent concentration field agree well with field data. A proper formulation of the effects of stratification on vertical mixing of the effluents in the ambient water is the key to success. The technique has shown advantages in handling large spatial gradients.