Evaluation of Check Dam's Height on Pick and Volume Flood (Case Study: Gorgandooz Watershed)

Watershed structures are among of expensive projects that are constructed to decrease erosion and sediment load and decreasing flood in watershed. Constructing any structures in flood bed can change the flood behavior. This research tries to study check dam effects on flood and water way properties in Gorgandooz watershed. For this goal, HEC-RAS model is used to simulate hydraulic in 4 Scenarios: without structure, 20 structures with 1.5, 10 structures with 3 and 8 structure with 3.75 meter for comparing of specified plan effects with each other. The results of this research show that the influence of 1.5, 3 and 3.75 meter check dam on pick flow comparing with structure was not created on the upstream of the watershed is 0, in the middle 1 and at the downstream is 2.49, 8.8 and 17.5% respectively. Also the effect of 1.5, 3 and 3.75 meter structure on decreasing of flood volume in respect of without structure is 1.16, 8.1, 13.9, respectively. The study of flow velocity shows that the construction of higher structure cause that the flow velocity decrease that shows the power of flow erosion in water way become less. Shear stress parameter that shown friction rate between flow and water way confirm the velocity results and show that shear stress decrease with increasing of structure height. The multiplying of shear stress on speed is another important parameter that is studied. This parameter shows the sediment transport power of flow. This parameter indicates the strength of sediment transport by flow that showed strength of sediment transport is less in higher structural. Not only reduce of sediment transport will cause less damage to the waterway but also sedimentation into check dam was occur soon and sediment balancing gradient is reached. As whole various Scenarios of check dam shows that the influence of structure on flood and water way become more by increasing of structure height and cause that the condition in deal with flood become well.