| Summary: | The Gorgan Bay has situated in the South east of the Caspian Sea and water entersthe Bay from the Sea .it covers an area about 450 squar kilometers that is connected to the sea only by the passage of Ashouradeh in the north east of the bay . The water balance is almost constant and is not tidal. In this study, a model made for the bay on the basis of its water quality and quantity. Its reactions evaluated under different qualified scenarios and its surrounding water quality survey during next 29 years. Mike 21 software was used for 2 dimensional modeling . At first hydrodynamic modeling of the bay was done .one of the important factor was water depth which calculated by hydrography method of the bay. Then, a model made for the salinity of the bay. Since, salinity is a stable parameter, a sub-program TM ( Mike 21) which has the ability to make a model from the process of advection – dispersion was used. The amount of Manning (0.025) and Dispersion (50) coefficient calculated simultaneously with calibration of salinity model (D:square meter / second). Results from data verification of salinity model indicated about 3 percent mean error. The pollution dispersion type in the bay evaluated at six points of the bay using the method of dot constant source of pollution. Results showed that around 99.9 percent of pollution decreases under the effect of dilution process till the 100 m distance. Then modeling of dissolved oxygen , Ammonia,nitrite and nitrate was done by using of subprogram ECOLAB at the Mike software To make these models, first the sensitivity analysis of these parameters was done. Important parameters chose for model calibration and then data verified and also the model calibrated. The result of data verification test showed the mean error of oxygen , ammonia , nitrite and nitrate were respectively about 3, 4, 6.3 and 37 percent. Then modeling made according to four different scenarios which defined on the basis of pollutions originated from aquaculture in the bay (scenario 1: aquaculture at the first ...
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