Evaluation and simulation of hydraulic performance of sediment dams, case study: Doveirej Watershed

Sediment dams are built with multi-purpose goals including flood control, erosion, sedimentation and aquifer feeding. These dams, with the aim of controlling the sediment from transferring part of it to the reservoirs of the dams in the downstream, and by causing a delay in the surface runoff, they control a part of the flood and feed the aquifer of the area. Improper design or lack of optimal implementation of these structures causes the reduction of useful life and sometimes premature destruction of these structures. Evaluating the function and technical performance of such plans with the aim of improving and managing the system is of special importance. For this reason, in this article, a number of sediment dams implemented in Abdanan City of Ilam Province were investigated and evaluated. This research was conducted in the form of hydraulic simulation and comparison of the current situation with the conditions before the implementation of the project, in a section of the waterway about seven kilometers long. It should be mentioned that six stone and mortar dams have been built during the last decade. The results of the hydraulic simulation of the flooding of waterways and constructed structures showed that, based on the discharge with a return period of 100 years, the average overflow width of the construction dams is about 25% larger than the required value and the length of the stilling basin is about 40% less than the required value. In general, in all construction dams, the length of the stilling pond is not chosen correctly, so that the short length of the stilling pond has caused the hydraulic jump to leave the pond or to form at its end. Due to the high flow speed in the area of the hydraulic jump, erosion and destruction in the end zone of the calm pond has intensified, which has been extended to the upstream side and the main structure of the sediment dam. Based on the hydraulic simulation of the flood and the relative comparison of the results in the conditions before and after the construction of the dams, it was found that the constructed dams have a positive effect on the parameters of flow speed, flow strength and flow shear stress, which are all factors that aggravate erosion in the waterway. So that for discharge with a return period of 25 years, the speed, power and shear stress parameters of the flow have decreased by 23, 29 and 27%, respectively, during the research period. Therefore, the average reduction of these parameters locally and at the construction site of the six dams was estimated as 73, 85 and 82%, respectively. In general, according to the field visits and investigations, the implementation of the plan has been effective in controlling floods and sediments, and in case of annual protection and repair, its effects will be doubled.