SLIn Savske elektrarne Ljubljana d. o. o. (SEL), we closely monitored the forecasted precipitation and the expected increase in flows. Because of this, the Završnica reservoir was emptied to -2.4 m by August 3, 2023, at 22:00 and 23:00. During the night, there was an extremely rapid increase in flow. The maximum inflow into the lake, which we could still calculate, was 18 m3/s at 3:00. After that, the inflow continued to rise, but there is no clear data on the exact amount. We estimate that it was significantly above 20 m3/s.
At this flow, at least 72,000 m3 of water enters the lake in one hour, which means that at such a flow, the entire lake fills in less than two hours. Despite the fact that operating generator 4 used a considerable amount of water, the entire reservoir was filled by 3:00 in the morning, and the lake level rose by approximately 2.7 m. Therefore, on August 4, 2023, at 3:20, the spillway gates began to open automatically. All devices worked flawlessly, and the gate automation operated according to the planned program, maintaining an appropriate level in the accumulation basin and enabling the continuous discharge of the significantly increased flow and the huge amount of debris carried by the water. We handle debris in a similar way in other hydroelectric power plants. If the gates were to remain closed, there could be a blockage of wood and other debris, leading to the clogging of the openings of the spillways. This could cause a flood wave several times larger than the flow during that time.
At 8:00, the protocol for automatic maintenance of the reservoir level with the regulation of the spillway gates was manually interrupted. At that time, the Završnica inflow had already slightly decreased, and the amount of debris being carried into the reservoir had also decreased. After several attempts between 3:00 and 6:00, generator 4 in HE Moste could no longer be started due to blockage at the Završnica inlet with mud and wood, and it is still not in operation at this moment.
With generator 4 not working, it was necessary to safely pass the entire water inflow into the reservoir through the Završnica HE dam. The flow below the dam increased as a result, but it remained around 20 m3/s, far below the hundred-year flow, which is 70 m3/s, during the most critical time.
The dam itself is designed to safely pass flows even larger than the hundred-year flows, and it can withstand earthquakes stronger than those predicted for this area. The dam is regularly maintained and monitored, and its safety is continually upgraded. The latest safety update was done this year, and as part of this update, a warning siren was installed, primarily intended for alerting in case of dam failure.
At SEL, we are currently directing all efforts to restore normal operation of generator 4, which also requires diving work. These works are expected to be carried out in the next week. Once the generator starts operating at full capacity, there will be no more overflow of water over the dam.
Director, Mag. Uroš Koselj
