Analysis of the formation process of flushing cones at flood-control bottom outlets: A case study at the Susobana Dam

In 2017, one of the two bottom outlets equipped with a flood–control gate at the Susobana Dam in Nagano Prefecture became clogged and inoperable because of sediment and submerged wood debris. Removal of the blockage took four months, during which flood–control capability was reduced. The incident appears to have been triggered by sediment reaching the inlet level of the bottom outlet. Since many flood–control dams in Japan have bottom outlets installed at low levels to maximize flood control capacity, the risk of similar issues is likely to arise at other dams as reservoir sedimentation progresses. This study focuses on developing adaptive countermeasures to mitigate these risks and to ensure sustainable and reliable dam operation. At the Susobana Dam, sediment near the bottom outlet is eroded into a half–cone–shaped scour, causing sediment to be flushed each time the gate is operated. Although not intentionally designed, this phenomenon corresponds to the so–called “flushing cone” formed by pressure flushing. In this context, to prevent blockages, it is important to reduce the gate’s vulnerability to sediment and submerged wood debris and to avoid the gate becoming buried during sediment discharge. In this study, as a fundamental investigation toward developing countermeasures against gate clogging, we elucidate the relationship between dam operation and variations in flushing cone shapes around the bottom outlet at the Susobana Dam. Based on field observations, we analyzed how the flushing cone responds to changes in reservoir water level and discharge rate. The results indicate trends similar to those observed in previous experimental studies. Finally, we discussed potential sediment management strategies for bottom outlets.