Investigation of Flow Pattern in Gorgan Gulf Considering Changes in Water Level of the Caspian Sea and Using Numerical Model

Introduction
Wetlands and gulfs are peculiar ecosystems comprising of both land and water habitats. This, gives rise to a rich biological diversity which seldom occur elsewhere on the planet. Unhappily, must of our wetlands countrywide have undergone massive negative changes during the recent years losing much of their surface and depth. These changes have been the result of multiple causes such as climate changes, decreased rainfall in line with increased evaporation, human activities such as dam construction. Our coastal wetlands have been affected by sea water level changes. Gorgan Gulf is the sole gulf in Iran that has been located in southern part of the Caspean Sea. It has been registered as biosphere reserve in Ramsar convention. The only permanent way that connects Gorgan Gulf to Caspian water is Ashoursdeh-Bandar Torkaman; a way with approximate width of 2500 meters and depth of at most 3.3 meters when the Caspian water level rises in northeast parts of the Gulf. Caspian water level may vary as a result of one or a combination of multiple factors such as: climate change, tectonic processes and human activities. The effect of each one is not the same as the other factors. Needless to say, that any alteration in Caspian level shall exert direct effect on Anzali wetland and river systems resulting in manipulation of morphology and animal life and even economy of the coastal areas. Furthermore, since wetlands are usually affected by seas and rivers, any change in sea and river dynamics shall definitely affect . In this study, different layers of Gorgan Gulf current pattern for one year with inclusion of Caspian level changes using Mike 21 has been simulated and analyzed.

Materials and Methods
Mike 21 hydrodynamic module was used for the purpose of studying Caspian current pattern. This module is the most basic model of Mike 21, and other modules of this model are dependent on the outcome of this module. This module is able to simulate water level changes and currents in two dimensions; therefore, it has the capability to study full details of a current in different spots. This model is helpful in studying the water level and current patterns of seas, rivers, gulfs and coastal areas which are affected by wind and tides. This module is also used to simulate combinational effects of such phenomena. It can model inconstant currents with inclusion of bed changes, mass changes and tide changes.

Discussion and Results
This model is able to solve the three-dimensional incompressible Navier-Stokes equations with the inclusion of Boussinesq approximation and hydrostatic pressure, using unstructured mesh. Unstructured meshes are better than rectangular structured meshes for the purpose of covering complicated borders such as coastal lines and islands borders. Horizontal unstructured mesh is a collection of 20 layers, perpendicular in direction, for sigma and z level system which uses 10 sigma layers for the distance between surface level up to -40 m, and 10 sigma layers from -40 to sea bed and 10 layers with fixed thickness (z level) 4-150. We use finite volume as our numerical methodology to solve equations.
This study covers the whole Caspian Sea. We use unstructured mesh to simulate currents. Mesh dimensions vary from 0.25 degrees in northern part to 0.01 degree in some spots of southern part. To layer the model vertically, sigma and z level system was applied. In the implemented current model, time step was fixed between 0.01 to 60 seconds.
Results demonstrate that Caspian currents are often counter-clockwise, which had previously been reported by some researchers as well. Western current is north-south and southern current (parallel to coast) is east-west, and eastern current is south-north. Current rate starts to fade in bottom layers, and current direction tends towards right due to Ekman transport. It can be seen in all seasons of the year. Furthermore, a current when is parallel to coast may be affected much more from topography. Sometimes currents may be deviated towards deep water as a result of being affected by bed. This can be observed in bottom layer more than elsewhere. Results indicated that overall Caspian current in western border more intensified in autumns, while weaker in eastern border. However, in winters, big storms take place and overall current becomes intensified in southern part of the Caspian Sea. In summers, which weather is often calm, currents are weak in central part of Caspian Sea, yet intensified in eastern border. It is evident that in Gorgan Gulf and surrounding area, where coast has low slop, wind pattern and topography of currents is weak and current rate begins to slow down in central and western parts. In southeast part of Caspian Sea, current rate and current direction reach to the least levels possible in various layers as a result of low depth and weak rate of currents. In this part, current layering cannot be observed.

Effect of Water Level on Surface Area of Gorgan Gulf
To study the effect of water level changes to Gorgan Gulf morphology, we study the effect of past year decrease of water level on Gorgan Gulf. We calculated Gorgan Gulf surface area for various years along with water levels. We found that water level has a nearly 15 cm of decrease; therefore we assumed a trend of 5 cm decrease in water level for Gorgan Gulf for the future. We extracted Caspian water level from the Anzali station records available.
Given the low depth of Gorgan Gulf as well as rather low slope in most parts of the Gulf, it can be anticipated that if water level continues to decrease, most part of gulf area and particularly gulf inlet (Chapgholi channel and Ashouradeh channel which connect Gorgan Gulf to Caspian Sea) shall undergo remarkable morphological alterations.

Conclusions
Generally, it can be suggested that the current has had a similar pattern during the first three months (Gregorian calendar) which occurs when current rate has the utmost rate in western spots of middle Caspian, yet the current has less intensity in western part. In these three seasons, current rate is very low and near to zero in various layers surrounding Gorgan Gulf. In June, July and August, it is observed that current starts to dominate in a direction parallel to the coast of Gorgan Gulf. This domination is evident in cold months as well. However, its intensity starts to fade when warm months approach. It is noteworthy this occurs in July as a result of both the coastal-parallel current and the effect of north-east current in eastern border.
Results indicate that the current in eastern border as well as the current risen parallel to southern coast have immense effect on current pattern of Gorgan Gulf. When the direction of the eastern border current is towards south-north, the intensity of the current inside the gulf starts to decrease, but when this direction is towards north-south, the current starts to dominate the gulf evidently. Also, when the current that is risen parallel to southern coast has noticeable intensity, part of this current penetrates to the gulf causing the gulf water to circulate. The coincidence of the eastern border current in north-south direction and the current risen parallel to southern cost exerts the highest effect on water circulation in Gorgan Gulf giving rise to the highest current rate in the gulf.