Modeling erosion and sedimentation changes of Sojasrood River before and after construction of Glaber Dam by GCD method

Humans have turned to dam construction to meet water needs, develop agricultural and industrial activities, and control floods since ancient times. These problems are evident both upstream of dams and downstream. Downstream problems of dams and rivers include changes in erosion and sedimentation processes in the river bed, redirection, and depth. The other part is the problems with environmental issues. The goal of this study was to analyze and investigate the effect of Glaber Dam before and after its construction on erosion and sedimentation downstream of the Sojasrood River. For this purpose, after extracting time series Dems before and after the construction of Glaber Dam from Aster satellite imagery, these Dems entered the gcd extension in ArcGIS software. The gcd model revealed the morphological changes of the riverbed before and after dam construction and the Rate of changes in volume and area of erosion and sediment along the riverbed was determined.

Sojasrood basin is located in the northwest of Iran, south of Zanjan province, between 36 degrees and 7 minutes to 36 degrees and 30 minutes north, longitudes 48 degrees and 16 minutes to 48 degrees and 50 minutes east. According to the studies, it is about 2494 square kilometers, of which 1298 square kilometers are alluvial deposits and the rest are elevations. In this catchment area, the plains are in the form of major hills and a rugged valley that is separated by high elevations or low-lying Polyoplostocene deposits. The Sojasrood River originates from the east and from the Mazidabad plain and flows into the Qezel Ozan River with the east-west trend after passing through the city of Sojas in the Downstream of the village of YengiKand
To conduct this research, images of L1A and L1B series aster, Google Earth Engine system, R programming language, topographic maps of 50,000 armed forces geographic organizations, digital elevation model ALOS, GPSgarmine device, ARCGIS software, GCD software and extension, ENVY 5.3 software, and field visit were used. The research methodology is applied in terms of purpose and in terms of the nature of the survey and analytical. After preparing the required historical Dems, the model was implemented in ArcGIS and gcd extension by three methods of Minimum Level of Detection, Propagated Errors, and Probabilistic Thresholding

By implementing the minimum detection level method with thresholds between 0.10 m to 0.80 m in GCD extension in the period before dam construction (2003-2007) based on the maps and output diagrams of the Minimum Level of Detection method, it was found that the sedimentation process in the riverbed dominates the erosion process. According to the output of the method, the Minimum Level of Detection in the period after dam operation, the sedimentation process is intensified and erosion is minimized. The output of the riverbed geomorphological changes model in the Propagated Errors method shows that between 2003 and 2007, which is the stage before dam construction, the sedimentation rate in the riverbed, like the Minimum Level of Detection model, is almost twice the erosion level which is completely in line with the output of the runoff chart generated in the google earth engine system. This trend has decreased exponentially after the construction and operation of Glaber Dam, i.e. between 2013 and 2019. Due to the storage of water behind the dam and the reduction of runoff, the erosion rate has decreased exponentially from about 220 thousand to 78 thousand cubic meters. In the method of determining the Probabilistic Thresholding, thresholds were analyzed based on statistical models and by determining the different confidence intervals of volumetric changes of erosion and sediment to obtain a more accurate estimate of the geomorphological changes of the riverbed. The Probabilistic thresholding model in the second time series, ie 2013 to 2019, shows that the volume changes related to the sedimentation process in the riverbed are more than 2.5 times the changes related to erosion. The slope of the changes in the low confidence interval is very high compared to the 95 % confidence interval. Similar to the Minimum Level of Detection model for the second time series in this model, the total difference in net volume is more than the volume changes of erosion surfaces. Therefore, it can be easily understood that by constructing Glaber Dam, while reducing river runoff downstream of the dam, the share of erosion in the bed has decreased and the sedimentation rate has increased. Also, in the time series 2013 to 2019 with 95 % confidence level, erosion levels reached 19808 m3, and sedimentation levels reached 53277 m3 in the whole substrate.

The output of the graphs and the three methods of the GCD model show that, firstly, in the whole period studied (2003-2009), the sedimentation process prevails over the erosion rate. Second, the deposition process in the time series 2019-2013 increased compared to the time series 2007-2003 and on the contrary, the erosion rate was decreasing. Therefore, considering that the amount of rainfall is not statistically significant between the two periods and there are no extensive land use changes around the river downstream, and also the amount of runoff in the second time series has decreased compared to the first time series, It can be concluded that Glabar Dam has a direct effect on reducing runoff and river dynamics downstream and has caused a general reduction in erosion and sedimentation rates and deepening of the flow bed. The results of this study can be fruitful in identifying the different effects of geomorphology, environmental impacts of dam construction on rivers and help geomorphologists to better manage the environment.