Abstract:The Wenchuan earthquake on May 12

2008 has induced many secondary disasters

such as collapses

landslides

dammed lakes

and a great area of loose soils and rocks. The debris flows of larger scales may occur at the stimulus of rainstorms that would cause events of ordinary scales before the quake. Based on the analysis of historic debris flows following the earthquakes and the debris flows in 2008

the optimal opportunity of controlling the potential debris flows was considered. The results indicated that the control measures should be taken three to five years later in the central quake hit areas. In other affected areas the engineering structures can be extensively made. It is suggested that the projects on the debris flows mitigation works in the most severe areas should be postponed up to three to five years.