Application of the CORMIX model to assess environmental impact in the coastal area: an example of the ocean disposal system for sanitary sewers in the city of Fortaleza (Ceará, Brazil)

Abstract

Cornell Mixing Zone Expert System-CORMIX software was used to simulate the behavior of effluents discharged by the underwater emissary in Fortaleza (Ceará, Brazil). This software is used to analyze and predict the discharge design in bodies of water. Application of the CORMIX model was determined to simulate the tracer discharge using local environmental parameters and underwater emissary discharge data. Data on wind, current and position in relation to the coast were used in the chosen case to obtain the discharge flow classification from the model since the hydrodynamic dispersion fields are differentiated by the predominance of different physical processes responsible for the dispersion and mixture of effluents. The simulations carried out considering the characteristics of the underwater emissary, liquid effluents and receiving medium show a very high dilution capacity. The dilution is directly related to coastal current speed ranging between 1:45 and 1:278, respectively the minimum and maximum for the mixture’s initial zone dilution, with diffuser distances between 16 and 55 meters. This gives the system the characteristic of good dilution capacity compared to other installed systems. At the limit of the simulations, 5.000 m from the diffuser, dilutions reached values between X1:251 and 1:2.688. From the simulations, it is possible to observe that given the difference in density between the effluent (freshwater) and the sea, the plume presents strong positive buoyancy. Thus, if we consider the unfavorable speeds and currents (0,25-direction and 0,04 - speed) we would have a P = 0,25 x 0,04 or P =0,01 (1%) probability of occurrence. However, for the actual situation, this probability is near zero. The results obtained from using the CORMIX model helped in the environmental monitoring of the effluent disposal area.