Monitoring and modelling of coastal currents and wastewater discharge: a case study

The coastal areas neighbouring wastewater outfalls are particularly sensitive and vulnerable, therefore they should be continuously monitored. The present paper examines the results of a monitoring survey carried out in July 2001 offshore the Bari town, in the Southern Adriatic Sea (South Italy), close to the outfall of its wastewater treatment plant, named Bari East. Measurements of horizontal and vertical velocity components were carried out with a Vessel Mounted Acoustic Doppler Profiler. Also salinity and temperature were assessed at the same time and locations by means of a CTD probe. The investigation confirms the pivotal role played by currents magnitude and direction, wind, tide and stratification in the process of diffusion and dispersion of passive tracers (such as temperature and salinity). As a second step, the MIKE 3FM, a 3D numerical model by the Danish Hydraulic Institute (DHI) is tested to reproduce the hydrodynamic current pattern and the diffusion of the plume in the target area. The model was implemented with initial and boundary conditions relative to the survey day and the assessed measurements were used to calibrate it, by tuning some parameters, such as the wind drag coefficient, the bottom roughness and a turbulence closure model coefficient. A satisfactory agreement was found between field measurements and model results, showing that in the target area the modelled hydrodynamics was prevalently influenced by the wind drag coefficient and less affected by bottom roughness and turbulence. The present approach confirms that, once calibrated and validated, a numerical model could be a powerful instrument to support both planning and management of coastal activities. In fact, it could allow to predict the possible dispersion of a polluting tracer when a scenario is established, thus providing some useful maps of spreading.