The risk of the whole cascade reservoirs of the earth rockfill dams under extreme operating con-dition is rely on the security of the control cascade reservoir. Such earth rockfill dam with extra height isclassified into ClassⅠand ClassⅡ dam based on the existing Chinese dam classification systems recently,but which one should design the unusual discharge facilities to reduce the risk level of the dams duringthe extreme operating condition is blank. Based on the theoretical framework of single DB-IWHR model...