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Dams In action In Slovenia there are more than 40 large dams with a


large number of smaller dams located in the Alpine regime. Klun focused on the run of river schemes, which were the main ones involved during this flood event. Slovenia’s dams are multipurpose for flood, hydropower and other uses. There is limited retention capacity and high yearly fluctuations of flows. “All of our rivers have differential characters, and we have operators used to dealing with situations of high flow variability,” Klun said. One day before event the weather agency warned that severe flooding could be expected on the Sava River – the largest Slovenian river with its origin in the country. So dam operators had to go into a flood regime in its upper reaches, lowering reservoir levels the evening before the forecasted rain on 3 August. This meant that: ● HPP Moste was 3.5m below operational level. ● HPP Mavcice was 1.4m below operational level. ● HPP Medvode was 1m below operational level. Moste dam is the largest in Slovenia at 60m high and is the most upstream structure. The highest inflow recorded into the reservoir was 123m3 Inflow to Mavcice was 106m3 and 1014m3


/sec at 4am on 4 August. /sec at 10pm on 3 August


/sec at 8am 4 August. Lun says it wasn’t


possible to keep any of this water in the reservoir because there was only 1m available. So basically, what was flowing into the reservoirs was also flowing downstream. Further down the Sava River, it flows by the capital city Ljubljana, and then goes into the lower river section where there are five run of river reservoirs which only have partial retention capacity with special operating rules during flood events. Slovenia also has dry reservoirs to help resolve flooding issues. In the southeast of the country the 10m high and 950m long Prigorica dam impounds a reservoir of 1210Mm3


.


There was a lot of debris and sediment during this flood event but in the end the system functioned as designed and the reservoirs did their job, Klun concluded.


Greece Ioannis Karavokyris, Managing director of G. Karavokyris


and Partners Consulting Engineers, gave an insight into the role of dams in flood defence in Thessaly, Central Greece. The Thessaly plain region is vulnerable to flooding, and two Medicane storm events have impacted twice in the past five years. Medicanes are tropical-like cyclones which form over the Mediterranean Sea, Karavokyris explained. They exhibit strong winds, heavy rainfall and an organised storm structure like tropical cyclones. Medicanes are generally smaller, weaker and short lived but recent ones have caused high levels of damage and loss of life. And with Medicane formation and strength strongly correlated to sea surface temperatures, climate change is expected to result in increased frequency and severity of this type of storm.


In 2020, Medicane Ianos occurred from 15-21


September. The highest rainfall occurred upstream of the plain of Thessaly. Maximum daily rainfall was 273mm in Thessaly on 18 September, with a weighted average of 299mm from 17-19 September. And Plastira hydropower station had 535mm in one day. “So this was a very severe event,” Karavokyris said.


Ianos caused considerable damage particularly in the


western part of the Thessaly plain where high intensity rainfall was concentrated. In the city of Karditsa and another 15 villages about 5000 homes and businesses


were flooded. There was also extensive damage to agricultural cultivations. Three years later, Medicane Daniel occurred from 5-12 September. “We thought Medicane Ianos was a one-off event but then along comes another which was even worse,” Karavokyris says. “It wasn’t exactly the same but did have similar mechanisms. It then moved on to Libya where it intensified and caused the failure of two dams and huge damage.” Various features increased precipitation in Sorm Daniel. Abnormally high sea surface temperatures in the Ionian and Aegean Seas led to deepening of the Daniel low and enhanced moisture uptake by the wind over the Aegean Seas. There was record daily rainfall for Greece from 757- 765mm in a day. Storm totals over three days at various stations was 1096 and 1235mm which is more than the yearly total for Greece. The flood flows produced by Daniel affected most of


the Thessaly plain with about 10,000ha remaining under water for almost a year after the flood event. There were 16 deaths in Thessaly and about 4500 people had to be airlifted or evacuated by boats in an emergency. In the City of Volos a highly damaging urban flood occurred. Major transport infrastructure was affected by inundation and the total cost of (direct) damage was estimated at two billion euros. Debris also affected the capacity of flood structure barrage dams to accommodate the floods. At the same time, Karavokyris explained that the second event coincided with the development of a new flood plan for the region. Ironically, Medicane Daniel occurred when such plans were being prepared and so was taken into consideration. Work was carried out by the National Technical University of Athens with a unified approach of the entire country. HEC HMS software was applied to estimate hydrographs, with simulation of runoff in catchments upstream of potential flood risk zones. Then HEC RAS 2D was applied for hydraulic simulation and mapping of inundation and hazard (depth and velocity). After consultation, out of a total of 31 measures covering prevention, readiness and rehabilitation, 12 were recommended for flood protection As Karavokyris explained, Thessaly has certain


hydrographic characteristics that influence the type of floods that occur, and the design of flood protection measures. Two plains are divided by hills and connected hydraulicly through a gorge, with flow from numerous


www.waterpowermagazine.com | May 2026 | 15


Above: Flooding in Poland in September 2024. Ela73/Shutterstock.com


Below: Floods in Slovenia, in the suburbs of Ljubljana. Sanja Karin Music/Shutterstock.com


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