Explanation Regarding the Operation of Hydroelectric Power Plants in the Medvode Area During the Floods on August 4, 2023

The mission of Savske elektrarne Ljubljana d. o. o. (SEL) is the safe and reliable production of electrical energy from water. We are aware of this responsibility and act accordingly. Therefore, we continuously monitor forecasted precipitation and the anticipated increase in water flows. Hydrological forecasts from August 3, 2023, predicted a significant increase in flows in the Sava River basin on Friday during the day, while later afternoon forecasts anticipated a night-time increase in flows. During the night of August 3 to August 4, 2023, there was indeed a rapid increase in flows in the upper course of the Sava River. The increase in the Sava Dolinka basin was smaller, but more water came from Sava Bohinjka, Radovna, Završnica, Tržiška Bistrica, Kokra, and Sora even reached the highest recorded flow values to date. 

High waters in the Medvode area in previous years 

From 1990 to 2012, the Mavčiče and Medvode hydroelectric power plants faced flows higher than 900 m3/s at least six times. Some notable examples include the flood on November 1, 1990, with a flow of 900 m3/s, and on December 6, 1992, with a flow of 1000 m3/s. On November 7, 2000, the highest flow of 1060 m3/s was recorded, and on September 19, 2007 (during the Železniki event), the flow in Mavčiče was 930 m3/s. Lastly, a very high flow of 930 m3/s was recorded on November 5, 2012 (during the Drava floods).

In the listed examples, the Sava flows reached the following maximum values in m3/s:

Date HPP Mavčiče and Medvode VP Sora Suha  VP Sora Medvode  VP Sava Medno
1. 11. 1990  900  687  732  1351
6. 12. 1992  1000  303  336  1097
7. 11. 2000  1060  356  422  1194
19. 9. 2007  930  431  424  1157
19. 9. 2010  730  467  519  1177
5. 11. 2012  1014  426  423  1319

Pre-emptying of reservoirs and river flows on August 4, 2023

Due to the forecasted higher flows, on Thursday, August 3, 2023, at 22:00, the reservoirs were appropriately emptied as follows: Moste -353 cm, Mavčiče -145 cm, and Medvode -100 cm. HE Vrhovo started a slow denivelation and had -15 cm at that time. Flows were at 33 m3/s in Moste, 106 m3/s in Mavčiče, and 277 m3/s in Vrhovo at 22:00, then began to rise on the upper hydroelectric plants. The maximum flow of 126 m3/s was reached in Moste at 4:00, then decreased to 65 m3/s by 8:00 and remained around 60 m3/s for most of the day. In Mavčiče, the maximum flow was already 1014 m3/s at 8:00. Between 3:00 and 6:00, it rose from 218 to 836 m3/s, indicating an increase of 200 m3/s/h.

At 6:30, VP Suha on the Sora measured a level of 575 cm and a flow of 687 m3/s. At 6:50, the water level reached 600 cm, and after that, the station did not provide data. VP Medvode on the Sora showed a level of 534 cm and a flow of 675 m3/s at 6:40. It then reported the last data of 697 cm at 9:20, and after that, it stopped transmitting data. It resumed at 13:40, showing a level of 545 cm and a flow of 700 m3/s. The highest measured level was at least 1.5 m higher than the level at which the flow was calculated. Therefore, we estimate that the maximum flow of the Sora was at least 900 m3/s, possibly even 1000 m3/s. The Sora thus significantly exceeded the previously recorded highest flow on November 1, 1990, which was 687 m3/s in Suha and 732 m3/s in Medvode. The consequences of this flood are visible throughout the Sora basin. At the Medno gauge station, the Sava recorded a flow above 1700 m3/s between 9:30 and 12:00, with a continuous flow above 1800 m3/s from 9:50 to 10:50. The maximum of 1929 m3/s was recorded at 10:30. From these data, we infer that the flows through the Medvode hydroelectric power plant and the inflow of the Sora at the confluence were almost equal. This year, a coincidence occurred, as the peaks of the Sava and Sora occurred practically simultaneously, which did not happen in previous high-water events, where peaks were usually staggered by several hours.

Both reasons (the highest recorded flow of the Sora and the simultaneity of the peak with the Sava) caused extensive floods in the Medvode area.

Ensuring safety

During high-water conditions, the primary task of our company is to ensure the safety of people, prevent environmental damage, and protect property using all available means. In HE Moste, due to the considerable free volume, we were able to retain the rising Sava Dolinka for a long enough time to successfully evacuate people from the Šobec camp, as the water there exceeded 200 m3/s due to Radovna, Završnica, and smaller inflows. Then, the Dolinka stabilized at around 250 m3/s.

In the Završnica valley below the Završnica dam, the flow reached approximately 20 m3/s in the morning, leading the Municipal Civil Protection Headquarters to order the evacuation of residents from around 10 houses. Generator 4 in HE Moste could not be started at 4:00 due to blockage at the Završnica inlet with mud and wood.

In HE Mavčiče, the flow was already 500 m3/s at 4:00. By 5:00, the flow reached 700 m3/s, prompting our control center to inform the Information and Management Center of SEL according to the notification protocol. The flow of HE Medvode must follow the flow of HE Mavčiče, as their reservoirs are relatively small. HE Mavčiče has a useful volume of approximately 1.7 million m3, and HE Medvode has 1.2 million m3. The useful volume is the difference between the normal highest and lowest operating levels. With very large flows of around 1000 m3/s, 3.6 million m3 of water can flow in one hour, making it impossible to “store” this water. Instead, we aim to safely pass it on as it comes. As the inflow into the first power plant increases, the outflow from it and the next power plant must also increase. Flows that are much larger than the turbines can handle are transferred through spillways or gates, which have a much greater capacity for water passage. In HE Mavčiče and Medvode, the gates, as well as all other installed hydro-mechanical equipment and the dam, are designed for much larger flows than have ever occurred. The equipment is well-maintained and operated by trained personnel.

Our teams dealt with enormous amounts of debris during the last event, which we safely passed through the spillways without causing any blockages. We managed flows in such a way that the outflow from HE Medvode approximately matched the inflow to HE Mavčiče. The flow in Mavčiče was roughly equal to the sum of the flows from the Okroglo gauge station on the Sava and the Kokra River. The former had a flow of 650 m3/s at 7:00 and remained above 700 m3/s throughout the morning. The peak of 743 m3/s occurred at 12:00. The gauge station on the Kokra in Kranj had a flow above 260 m3/s between 5:30 and 10:30, with the peak from 6:30 to 7:00 when the water level was 428 cm, and the flow was above 300 m3/s. The total inflow in Mavčiče was above 900 m3/s during the morning, with a peak above 1000 m3/s. The flow through HE Medvode varied between 900 and 950 m3/s between 7:00 and 14:00.

Due to the overflow of debris, it was necessary to maneuver the gates multiple times. This involved alternately closing one and increasing the opening of the adjacent gate to maintain the total flow. Short-term changes in downstream flows were inevitable during these maneuvers, minimal deviations from the average value over a longer interval. By continuously passing the debris, we helped maintain a stable flow through HE Medvode, which was crucial because the Sora, which exceeded all previously recorded flows in Medvode, greatly worsened flood conditions.

In summary, both power plants, HE Mavčiče and HE Medvode, with the help of trained personnel, safely managed high flows of the Sava without worsening downstream conditions. Moreover, by preventing the formation of blockages with debris, which could have caused a flood of greater proportions, they helped maintain stable conditions in this area.

At no time was there any fear that the safety of individuals would be additionally jeopardized due to our facilities and their operation. We informed concerned emergency services and individuals who managed to contact our staff with these data in real-time. Unfortunately, we could not respond to all callers immediately. This prepared report will, we hope, contribute to clarifying certain circumstances. The interested public has the opportunity to review our operations, as SEL, as a member of the GEN Group, operates transparently and sustainably.

Director, Msc. Uroš Koselj


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