A three-dimensional hydro-environmental model of Dublin Bay

This paper compares a 3-dimensional hydro-ecological model with a 2-dimensional model simulating the distribution and fate of Escherichia Coli (E.Coli) discharges from a sewage treatment plant discharging into Dublin Bay, Ireland. Before being discharged, the effluent from the sewage treatment plant is mixed with cooling water from a thermal generation plant resulting in a warm buoyant sewage plume that can be 7 − 9oC higher than the ambient water in the Bay. The mixing of the stratified plume is complicated by the tidal currents which transport the plume into and out of the estuary. These processes have a direct impact on the transport and fate of E.Coli and the model comparison demonstrates that a three-dimensional model is required to adequately represent the mixing processes in such a stratified environment. The modelling followed a two-step procedure. First, hydrodynamic simulations of water levels and flow velocities in Dublin Bay were performed using the three-dimensional model TELEMAC-3D. The resulting water level and flow velocity fields were used by the three-dimensional water quality model, SUBIEF-3D to model the transport and fate of E.Coli in the Bay. Further simulations were performed in which the wind effects on the E.Coli dispersion were included. The water quality simulation was repeated using the 2-dimensional, depth-averaged, hydrodynamic model TELEMAC-2D to compare with the three-dimensional simulations. The results showed that the three-dimensional model gives an adequate representation of the hydrodynamics and water quality in the Bay while the two-dimensional, depth-averaged, water quality model (in comparison to the three-dimensional model) delays the timing of the delivery of E.Coli to the Bay and seriously underestimats the decay rate of E.Coli and the effect of wind on the movement of the buoyant plume of pollution.