1 Introduction Uvod The basics of the hydrodynamics of pollution discharge in the sea are: advection convective dispersion, and turbulent diffusion [1]. The above processes are characterized by mixing of internal energy and mass of dissolved or suspended matter, and the one-dimensional approach cannot be applied to represent reality [2]. The analysis of pollution discharge from submarine outfalls requires a combination of physical, chemical and biological processes. This results in efforts to develop numerical models that include all three processes [3]. Modelling of pollution discharge in marine aquatories cannot be based on constant parameters, but requires simulations in representative time periods, including temporally variable parameters. Such parameters include aspects of the pollution outflow, sea temperature, salinity, as well as changes of light intensity and movement of sea currents [4, 5]. In the analysis of pollution introduced into the sea through the diffuser part of the submarine outfall, the influence of factors in the wider spatial domain may also change. Therefore, the space is usually divided into near- field and far-field zones [4]. The near-field zone covers the space near the diffuser, characterized by upward flow dominated by the influence of the discharge rate and difference of densities of pollution and sea water. Further, a buoyant lifting plume is formed, that is influenced by density difference and sea stream velocity [6] and typically reaches the horizontal barrier (pycnocline layer), at which the plume enters the far-field zone and propagates in the 255 D Vouk, . G. Lončar, D. Bekić ISSN 1330-3651 UDC/UDK 628.312.5:628.241.2]:519.6(497.5 Poreč) NUMERICAL MODELLING OF EFFLUENT TRANSPORT IN COASTAL ZONES Dražen Vouk, Goran Lončar, Damir Bekić Effluent transport in the aquatic environment can be described by using very complex mathematical formulations that include the most relevant processes and their unsteadiness and non-uniformity. This paper gives some evidence of the capability and importance of numerical modelling implementation in the analysis of effluent transport, following the discharge from sewerage submarine outfalls. Numerical models Cormix, Mike 21fm, Mike 3fm and ROMS are described in brief and applied to a particular e in the numerical model spatial domain: north, Island south, Petalon i Koversada. Two scenarios of effluent treatment and discharge are modelled that represent the present and the future-planned state according to the development concept project. Positive aspects from the planned activities are obtained through comparison between the faecal coliforms concentration dynamics. xample in the vicinity of the city of Poreč. Five separate sewerage systems and associated submarine outfalls were encompassed with Lanterna, Červar-Porat, Poreč Sv. Nikola, Poreč Keywords: Cormix, Mike 21fm, Mike 3fm, ROMS, numerical modelling, submarine outfall, pollution Original scientific paper Pronos oblaka zagađenja u morskim akvatorijima može se opisati izuzetno složenim matematičkim operacijama, koje uključuju veliki broj utjecajnih čimbenika te njihovu nejednolikost i nestacionarnost. U radu su prikazane mogućnosti i važnost korištenja različitih matematičkih modela pomoću kojih se na učinkovit i relativno brz način može dobiti realna slika pronosa zagađenja iz podmorskih ispusta. Opisani su sofisticirani matematički modeli Cormix, Mike 21fm, Mike 3fm i ROMS, uz prikaz njihove primjene na konkretnom primjeru. Odabrano je područje Poreštine uz ispitivanje utjecaja podmorskih ispusta sustava javne odvodnje Lanterna, Červar-Porat, Poreč sjever, Otok Sv. Nikola, Poreč jug, Petalon i Koversada, na promjene stanja u morskom akvatoriju šireg područja. Razmatrana su dva slučaja, od kojih se prvi odnosi na sadašnji stupanj izgrađenosti sustava odvodnje, a drugi uključuje sve planske promjene prema usvojenoj koncepciji razvoja. Kao osnovni pokazatelj kakvoće vode u efluentu usvojena je koncentracija fekalnih koliforma. Dodatno je razmatran i utjecaj biološke razgradnje organske tvari. Ključne riječi: Cormix, Mike 21fm, Mike 3fm, ROMS, numeričko modeliranje, podmorski ispust, zagađenje Izvorni znanstveni članak Numeričko modeliranje pronosa zagađenja u morskom akvatoriju Numeričko modeliranje pronosa zagađenja u morskom akvatoriju Technical Gazette 17, (2010), 3 255-262 direction of prevailing ocean currents [7]. Concentrations in the pollution plume at the end of the near-field zone, defined by some adequate model specialized for the analysis in the near-field zone, may be used for initiating of pollution field in the numeric 2D or 3D models, which are further used in the analysis of discharge in the far-field zone. This approach avoids very dense calculation network in 2D or 3D numerical models in the near-field zone. This paper presents an example of integrated numerical modelling of pollution discharge in several spatial and temporal levels through establishing of the model system continuously linked to most important properties of hydraulic characteristics of outflow from submarine outfalls and dynamics of the surrounding ocean. The analysis was concerned with the numerical models used in the numerical analyses were Cormix, Mike 21fm, Mike 3fm and ROMS. Two cases were analyzed – the present status of development when each system has a submarine a to neighbouring larger systems. The results of the analysis indicate potential reduction of pollution concentration in the zone meant for sports and recreation (300 m from shoreline) provided the planned works are implemented. The basic index of water quality in relation to pollution is the concentration of faecal coliforms. The numerical analyses also take into account biological decomposition. interaction of several submarine outfalls of public sewerage in the area of Poreč – sub-systems Lanterna, Červar-Porat, Poreč North, Island Sv. Nikola, Poreč South, Petalon and Koversada. The outfall, and the planned future development which includes connecting of the minor systems Červar-Porat, Island Sv. Nikola and Koversad