Estimating riprap size to prevent scouring in downstream of stilling basins(Case study: Namrood Dam)

Local scouring immediately downstream of stilling basin is unavoidable when a local velocity exceeds incipient motion. This phenomenon causes a difference in the level of basin concrete bed and natural river bed that is turn affects the rate of scouring. In this study hydraulic model of Namrood dam was used the experiments. Results of this study showed that the stable stone size increases with a reduction of tail water, promotion of critical depth and the level of degradation. Having discharge and tail water depth, the developed relationships in this study could be used to estimate the size of stable stone for any level of degradation