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Karavokyris said free overflow spillways are considered preferable. Their flood routing performance will have to rely on the combination of spillway crest length and available freeboard. However, in the case of dams with considerable hydropower plant components, gated spillways are generally the option of choice. Additional design considerations for flood control dams are that flood routing involves rapid fluctuation of reservoir water elevation. This is generally avoided during normal operation of the reservoir. The following precautions are important: ● For embankment dams, the upstream shoulder should be designed for these fast reservoir fluctuations, in terms of drainage and filtering.
Above: The Plastiras Dam in Greece. Esin Deniz/
Shutterstock.com
mountain catchments. Thessaly has an extensive network of levees but, Karavokyris claimed, the presence of flow control points at the gorges doesn’t allow this system to accommodate large events safely. The selected strategy now is to:
● Attenuate mountain catchment hydrographs by dams in the periphery of the plains.
● Manage runoff generated in the plains, in combination with the attenuated mountain flows in controlled flooding areas.
● Manage the large quantities of debris from the mountains with extensive catchment management measures. These involve a number of measures related to reforestation, terracing and other surface runoff sediment checks, including small check dams.
Karavokyris also spoke about the Smokovo multipurpose dam in Thessaly. Designed with high freeboard and narrow spillway, upstream of a part of the plains prone to flooding in the past, the dam was half empty at the end of the irrigation season in September and so did not spill, protecting the downstream area. Indeed it did not allow any flooding during storm Ianos or Daniel. So under the new flood management plan, priority is now being given to dam construction. Eight new planned multipurpose dams are being considered as part of the action plan for both drought and flood management.
Design considerations Going on to discuss the characteristics of flood protection
dams, Karavokyris considered whether free overflow is better than gated spillways for flood control. Undoubtedly gated spillways offer a wider choice of flood management policies, he said. They also allow real time pre-emptive operation of the spillway to increase available routing volume in the reservoirs, based on the stream of meteorological and hydrometric observations. On the other hand, free overflow spillways are passive
structures with their operation fully governed by the uncontrolled rise of the reservoir level during a flood. There can be a limited scope for pre-emptive lowering of the reservoir level through a bottom outlet if this is designed for such use. Notwithstanding the above superiority of gated spillways, their effective and safe operation relies on the availability of sufficient capacity in terms of personnel and plant for operation, maintenance and emergency intervention of necessary during an event.
In the case of primarily irrigation/water supply dams such as the multipurpose dams of Thessaly,
16 | May 2026 |
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● For the reservoir slopes of all dam types, a good investigation and addressing of potential instabilities becomes more important for flood control dams. Often stabilisation measures will include provision of slope drainage.
To conclude, Karavokyris says that in Thessaly multipurpose dams with a flood attenuation component are a cost-effective solution for the management of downstream flood risks. Their dual drought and flood protection role makes them top priority in terms of budget allocation in Greece when compared to other important interventions such as irrigation network modernisation. They should also be designed as part of a wider plan of flood management interventions. In addition, dedicated (single purpose) flood attenuation dams can be very effective and their cost would be justified, particularly upstream of urban or semi urban areas.
Floods in Poland Krzysztof Radzicki, an engineer, researcher and lecturer
at Krakow university, presented selected observations from the September 2024 Flood in Poland, when high precipitation from 12-16 September affected parts of the mountainous catchment of the Odra River. During rainfall events as extreme as this, forest retention is described as having a negligible effect on reducing flood waves – as was demonstrated by the intensive surface runoff observed across almost the entire slopes of the Klodzko Valley on 15 September. During this event, with estimates putting total losses at
three billion euros, the Pilovitz, a German masonry dam, overflowed and was damaged but is now in renovation and ‘happily survived’. While over in the Quatsko Valley, four earth dry reservoirs which were built 10-15 years ago, worked well and operated correctly. These are the: ● Szalejow Gorny (10.7Mm3 ● Krosnowice (1.9Mm3 ● Roztoki (2.7Mm3 ● Boboszow (1.4Mm3
), ), ) ). Radzicki said some of these dry reservoirs significantly
reduced local flooding in small towns situated not far downstream from the reservoirs. However, their small mountain catchments did not experience the catastrophic local rainfall that occurred in the Stronie Dam catchment. Stronie Slaskie is a 16m high earth dam with a 1.4Mm3
dry reservoir capacity, and was constructed to
German regulations over 100 years ago. After a second period of heavy rain event during this flood event in 2024, the reservoir level was higher than the dam crest. After a couple of hours, back erosion occurred in direction of the crest, a hole was created and after 30-40 minutes the dam was totally destroyed.
With hindsight, Radzicki said the pier supports located
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