Investigating the Interaction of Caspian Sea Level Change and Sediment Fluctuations in Coastline changes (Case study:Tajan River shoreline)

Coastal changes are one of the major environmental issues that affect coastal areas around the world. The Caspian shores have been considered by human societies and numerous activities have taken place on the coast. Sea-level changes cause damages. Today, several methods are used to monitor coastlines throughout the world. Remote sensing data is often replaced by field studies. The aim of this study is monitoring the changes of the Caspian Sea coastline in Tajan river coast from 1955 to 2016 in order to identify relationship between coastal displacement and sea level changes and sedimentary changes in this area. In this research, coastal lines have been investigated over a long period of time when the water surface has rapid changes. In addition, the amount of sediment and water discharges have been used as an effective factor in the displacement of coastal lines.

The study area is located in the Mazandaran province and the middle part of the southern shores of the Caspian Sea, with a geographical position of 36◦48'25'' northern latitude and '53◦05'18'' eastern longitude in the western of the estuary to 36◦49'30'' northern latitude and 53◦09'50'' eastern longitude which is located in the western part of estuary. First, all aerial photographs and digital maps of 1: 25,000 from the National Mapping Organization and Google Earth images of 2013 and 2016 were prepared. Aerial photos were scanned, then carefully georeferenced by Arc GIS 10.3 software. In order to accurately estimate these images, they were matched with 1: 25,000 digital maps. Finally, the coast line was drawn by the Water Line method. In the next step, Google Earth images were received using geospatial elshayel smart software. Again, they were adapted with digital maps of 1: 25,000. Then, the coast line was drown in Arc GIS 10.3 software. The DSAS extension was used in 10.3 Arc GIS software to determine the coastline variation. For this purpose, across the coast, transects were drown perpendicular to the coastline at intervals of 100 meters and then the statistical parameters were extracted. Finally, water and sediment data from Iran Water Resources Management Company were received and the mean water and sediment discharge were calculated individually at each time interval and adapted to the position of the coastline.

The level of the Caspian Sea has been fluctuating since 1955. For the better study of the shorelines of the Caspian Sea, coastal changes were calculated based on incremental and decreasing sea level cycles. The findings of this study showed that the coastline of Tajan River has not been stable since 1955. In order to achieve the shorelines change in each sea level changes, the statistical parameters for the first period from 1955 to 1977 were examined on the coastlines in this time period. In this section, the EPR, SCE, and the NSM values were 1,765, 25.62, and19.60, respectively. All values in this period are positive and indicate the accretion. The nearest water and sediment station is Kordkhil Station where the water flow rate data is available since 1969 and the sediment discharge data is available since 1981. It does not include the first time zone, so it was not considered for the first time period. The second time period, from 1977 to 1995 includes two series of coastal lines including 1981 and 1994. In this section, the EPR was -11.90, the SCE was 155.22, and the NSM equaled -155.22. This part of the coast in this time have erosion. In the second period of time, the rate of water flux at Kordkhil station is 14.36 m 3 / s and the sediment discharge rate at Kordkhil station is 14802.79 tons per month. The third time has been around since 1995 until now which includes three series of coastlines surveys including 2006, 2013, and 2016. In this section, EPR was 5.95, SCE was 61.26, and NSM equaled 61.24. This part of the coast has a sedimentation status. In the third time period at Kordkhil station, the water flux rate is 9. m 3 / s, and the sediment discharge rate is 42406.5 tons per month.

The rise of the sea level causes beach erosion, as it causes water to deepen and finaly causes larger and stronger waves to reach the coast and the sand is transported to the sea. Also, down on the surface of the sea, causing sedimentation. This study in the Tajan River confirms the scientists' view on the role of the sea level at the location of the beaches. In the first period of time, the most changes occurred in the estuarine to the eastern or river, which indicates the effective role of sediments in the estuaries. In addition, moving sediments through longshore current along the western shores of the Caspian Sea is so important. Because these flows move from west to east and divert the sediments to the east. There is a severe erosion of the coast in the second time zone. Rising water prevents sedimentation and the highest rate of erosion in this period is in the eastern part. Totally, in this time, three factors of the sea level increase, increasing water flow, and reducing the sediment discharge can be effective. There is a reduction in the water level during the third period and the rate (EPR) in each transect is different. The highest amount of sediment is found in the estuary of the river and the east of river. The amount of water flood in this period decreased compared to the previous period and the amount of sediment has increased. High sediment supply, reduced water flow, reduced sea level, and expanding location for growing beaches has led to coastal advances towards the sea, and the longshore current moved these sediments to eastward. Finally, according to the results, it can be said that the displacement of the coastline is according to the general rules that scientists have said about the changes in the sea level and the displacement of the coastline.