Stability Analysis of Riprap Layer at Downstream of Stilling Basin Based on Reliability Analysis

Determining the exact size of a stable riprap to control scouring at downstream of stilling basin is a challenge in hydraulic engineering due to various uncertainties that can lead a project become unsafe or uneconomical. Accordingly, in the present study, using the reliability analysis method, the stability of the riprap layer at downstream of the Kan diversion dam constructed at Northwest of Tehran city as a case study was studied. Then, the probabilistic properties of the parameters affecting the riprap design were determined. The results showed that the failure probability of the used riprap is about 0.2% for the 100-year flood, while it is more than 10% for the 500-year flood. Next, the relationship between reliability index and safety factor in deterministic design method calculated from the average value of effective parameters was investigated. The results showed that this relationship is linear and the safety factor corresponding to the reliability index of 3 is required to be 2.1. Finally, the sensitivity analysis of the parameters affecting the stability of the riprap layer showed that the parameters of flow discharge, turbulence intensity and relative stone density have the greatest effect on the change of the reliability index.