| 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 rise at other dams as reservoir sedimentation progress in the future. This study focuses on developing adaptive countermeasures to mitigate these risks and to ensure sustainable and reliable dam operation. At Susobana Dam, sediment approaching the bottom outlet erodes 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.
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, with the flushing cone enlarging as the water level decreases and the discharge increases. In addition, we examined methods for controlling the flushing cone shape using a physical model and numerical simulations. Finally, we discussed potential sediment management strategies for bottom outlets.
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