Simulation of River Morphological Changes under Voshmgir Dam Removal Condition- Gorganrud River

Dam removal is one of the methods of restoring the river ecosystem. In the present study, the removal of the Voshmgir Dam on the Gorganrud River, Iran, was considered, because of the high volume of sediment deposition in the reservoir and the loss of its useful life. Three scenarios were selected for the removal of the dam: 1- complete removal; 2- stepped removal of the spillway; and 3- removal with stable sediments. Numerical modeling for river morphological changes was performed using HEC-RAS model for unsteady flows with sediment transport. The erosion and sedimentation processes were simulated in six reaches (from the Voshmgir Dam to the Caspian Sea), with total length of 128 km. The results indicated that the first scenario causes the highest rate of both erosion and sedimentation in the river reaches. In the second scenario, the intensity of river-bed changes is considerably reduced, and the highest deposition and erosion occurs in the first and the fifth reaches, respectively. The simulation results from the third scenario were uncertain. The second scenario (stepped removal of the spillway) is recommended due to the less impacts on the river-bed changes, and gradual processes of river-ecosystem rehabilitation