| In order to prevent sedimentation in the reservoir and ensure the continuityof sediment transport at the river, the Koshibu dam sediment bypass tunnel wasplanned and completed in September 2016. From the viewpoint of structure, environmentand sediment budget, monitoring plan was examined by the monitoringcommittee and several trial operations have been carried out.Since the purposes of the Koshibu dam is including flood control, irrigationand power generation, the sediment bypass tunnel is planned to operate duringthe flood control. In order to adapt to the flood control rule, two crests placed onthe two orifices is designed for the inlet structure. The target bypassing sedimentsare gravel having a grain size of about 100 mm or less, sand and silt (clay). Asa countermeasure against abrasion damage caused by the sediment transport,around the gate with a relatively low flow velocity was protected with "rubber steel"and the downstream section with high flow velocity was protected with steel lining.In the monitoring plan, the objectives of monitoring at each viewpoint werearranged by selecting suitable methods. In sediment budget monitoring, as thefirst case of the sediment bypass tunnel in Japan, sediment observation deviceswere installed on the bottom of the tunnel outlet part.After completion of the bypass facility, several trial discharge operations werecarried out. Even though the trial discharge was a small scale compared with theplanned maximum bypass discharge and comparatively short time, as a result ofmonitoring, it was confirmed that there was no problem in gate operation, and nosignificant abrasion damage and environmental changes in the downstream river.Moreover, it was able to estimate the sediment budget and sediment transportcharacteristics during bypass operations. Regarding abrasion damages, long-termmonitoring is needed since the flow rate and operation time in the trial dischargewas limited. |
