Extreme weather requires dredging measures at the Klammsee reservoir in Kaprun

In February 2024, VERBUND emptied the two high-mountain reservoirs of Mooserboden and Wasserfallboden of the Kaprun power plant group to inspect the dam facilities in accordance with official requirements. Since then, the base drains have been open and the reservoirs are in free-flow mode. 

Draining of the reservoirs normally takes place in the winter months at a time of low natural inflow. The situation in winter 2023/2024 told another story: either the weather was marked by very heavy snowfall or rain or it was unusually warm. The temperatures in February and March 2024 deviated from the long-term average by +6 °C and +3.3 °C respectively. These conditions resulted in huge volumes of melt water and the ground around and inside the emptied reservoirs was not frozen as in past decades. This is currently stirring up larger amounts of mobilisation of sediment, which is finding its way into the Klammsee. 

To avoid sediment accumulating around the base drain in the Klammsee, VERBUND has to carry out separate dredging measures to safeguard the public interest. To this end, excavations have been ongoing since 16 February 2024 to maintain a deep channel to ensure that the Kapruner Ache flows into the Klammsee in the most controlled way possible within the confines of the embankment. The removed material is temporarily stored on the bank and then transported to officially approved sites. So far, a total of 32,000 m³ of sediment have been excavated from the Klammsee working around the clock on every day of the week. 

Despite the intensive effort by the Kaprun power plant group, the Klammsee is currently almost entirely silted up and additional measures are now having to be taken. In close coordination with the authorities, a slow and controlled lowering of the Klammsee has been selected as the most suitable measure.

From the afternoon of Wednesday, 24 April, 38,000 m³ of remaining water will be slowly drained under supervision over the next few days. The additional discharge this produces will hardly be visible in the Kapruner Ache. The low flow rate will stir up very little sediment. In addition, the discharge from the Klammsee will be diluted with pure water from streams of the power plant main stage to keep the load as low as possible. The excavators can then remove up to 4,000 m³ of sediment a day from the empty Klammsee over the next 3 to 4 weeks.

In consultation with the responsible authority and the affected landowner, an agricultural meadow was found in the immediate vicinity to which the sediment can be transported. This reduces the number of lorry trips. Refilling of the Mooserboden will commence on 26 April 2024; the Wasserfallboden and Klammsee will follow in mid-May. 

VERBUND asks the residents of Kaprun for their understanding and support while this necessary work is ongoing.