Simulating the effects of dredging on the pollutant particles originating from a wastewater treatment facility in a tidal river

A 2-D hydrodynamic finite element model with a Lagrangian particle module is used to investigate the effects of dredging on the horizontal dilution of pollutant particles originating from a Wastewater Treatment Facility (WWTF) in a tidal river. The model is driven by the semi-diurnal (M₂) tidal component and includes the effect of flooding and drying of mud flats. The particle tracking method consists of tidal advection plus a horizontal random walk model of subgrid scale turbulent processes. Our approach is to perform continuous pollutant particle releases from the outfall, simulating three different scenarios: a base-case representing the present conditions and two different dredged channel / outfall location configurations. Lagrangian particle concentrations are simulated on finite elements and dilution improvement ratios are presented for both scenarios. Results show that although no particles leave the river in a single M2 cycle, flushing takes place in longer time scales. Simulated dilution maps show that relocation of the WWTF outfall into the dredged main channel is required for increased dilution performance. The addition of a pool at the head of the river also improves dilution by adding to the tidal volume. Case oriented short-term investigations of coastal hydrodynamic problems suitable for Lagrangian particle methods should be encouraged to improve our knowledge of estuarine processes and how we model them while providing solutions to the management community in time and budget constrained decision making.