Investigation of the River Bed Changes along the Downstream of Karkheh Reservoir Using Mathematical Model GSTARS3

Construction of dams in rivers is cause of sediment accumulation behind the dams and therefore clear water flowing downstream. Flowing of water with high capacity of sediment and low in sediment load will affect downstream morphological parameters of the river. In the present study, morphological changes of Karkh river in the segment located downstream of the Karkheh reservoir is analyzed with the aids of a mathematical model. The length of the river reach was about 200 km located between Paaipol station and Hamidieh station. GSTARS3 model is developed version of the stream tube model for simulating the alluvial rivers presented by the river hydraulics and sedimentation branch of USBR. The hydraulic data of the river between 1998 and 2009 (12 years period) and measured cross sections in 2005 are used. Calibration of the model showed that the Ackers-White and Laursen-Madden are suitable sediment transport equation for this river. By using these equations for a priod of 10 years, the changes of the sediment and morphological parameters such as the variations of the cross sectional and longitudinal profiles, bed slope of the river were analyzed and discussed. In this period the greatest change in slope at the interval of the UpperKarkhehBasin has been observed.