Two-dimensional modeling of the effect of rivers on the salinity of Gorgan Bay

Gorgan Bay is the largest bay of Iran in the waters of the southeastern shores of the Caspian Sea. In this research, the effect of rivers on the salinity of Gorgan Bay in a period of one year has been investigated using the Mike 21 model on unstructured meshes and under two scenarios of the presence of rivers and the absence of rivers. The results of the research showed that the salinity level in the bay has an increasing trend from east to west in most seasons of the year and the average salinity was 16.4 psu in 2011. The salinity values in Gorgan Bay are closely related to the annual rise and fall of the water level in the Caspian Sea. Based on this, during the hot season of the year and the increase in the water level, the amount of salinity in Gorgan Bay decreases, and vice versa, the amount of salinity increases in the winter season along with the decrease in the water level. The salinity level in the western part of the bay with the maximum salinity of 25.6 psu was modeled more than the other areas due to the increase in water exchange time. From the beginning of autumn to the end of winter, due to the decrease of the water level and the movement of the water masses of the bay from the west to the east, the salinity values in the deep areas of the bay during this period compared to other seasons with a maximum of 16.5 psu increases. The entry of water from the sea into the bay and the evaporation process has the greatest effect on the salinity changes of the bay. The effect of rivers on the salinity changes of the southern coast of the bay has been low. Among the 13 rivers entering the bay, only the Qarasu River has had a larger area of influence (up to a distance of 500 meters from the estuary) compared to other rivers due to its higher water flow. In general, the input of rivers had the least effect on the salinity changes of Gorgan Bay, and the contribution of rivers to the salinity changes of Gorgan Bay was less than two hundred percent in 2011.