Hydro-Environmental Modeling of Sewage and Riverine Discharges into a Coastal Area: Comparison of Depth-averaged and Three-Dimensional Models

This study applies and compares two hydrodynamic and water quality models; a depth-averaged (TELEMAC-2D) and a three-dimensional model (TELEMAC-3D) on their performance in simulating the transport and fate of Escherichia coli (a main microbial bathing water quality indicator) in the coastal waters of Bray, Ireland subjected to sewage discharges and freshwater inflows from the River Dargle. The models first calibrated and validated against hydrodynamic and water quality data, were used to simulate Escherichia coli distribution patterns based on mean spring and mean neap tides for dry and wet weather scenarios. The hydrodynamic calibration yielded a good match between both models (TELEMAC-2D and TELEMAC-3D) and measured velocities. The E. coli model calibrations showed that TELEMAC-2D resulted in a lower value for decay rate (higher T90 value) than TELEMAC-3D in order to match the measured E. coli concentrations. E. coli surface distributions at the time of HW resulted in TELEMAC-2D plumes that were lesser in extent and concentrations than those of TELEMAC-3D due to the fact that depth-averaged hydrodynamics underestimate the surface water velocity resulting in lower concentrations of E. coli at the water surface compared to TELEMAC-3D. The wet weather scenarios of both TELEMAC-2D and TELEMAC-3D exhibited high E. coli concentrations at the water surface that exceed the “Sufficient” limit of the Bathing Water Directive, the latter finding highlights the need for including Ultra Violet disinfection in the treatment process at Shanganagh Sewage Treatment Works