engleski

Tailwater drawdown at the Dubrava plant due to downstream river erosion

The paper describes the tailwater drawdown effect and its impact on the plant operation, as well as available technical solutions for keeping this effect under control. The tailwater drawdown trend recorded at the Dubrava HPP since its putting into operation (1989-2007) is presented. This is a known phenomenon in river courses running through gravel and sand alluvial deposits, which also may be triggered by construction of water structures in the river courses. Generally, the water level drawdown has been recorded at the Drava river for over 100 years, and it is due to comprehensive regulation works and gravel mining in the 19th and 20th century, i.e. even before the hydroelectric power plants had been built, which is documented in the paper. The paper presents the analysis results on the water level correlation to the Drava flow downstream from the Mura mouth into the Drava (i.e. downstream from the Dubrava HPP) which shows a considerable drawdown trend for low and mean water, as presented herein. The Dubrava HPP design had foreseen the tailwater drawdown, however after almost twenty years of the plant operation the predicted drawdown has been significantly exceeded already. The tailwater level dropped by approximately 1.5 m, so an average hydroelectric power plant head nowadays is 18.5 m instead of the design head of 17.0 m. This might also be explained by the fact that the plans for construction of downstream power plants in the Drava Hydropower System have been abandoned, which put the Dubrava HPP into a position of the farthest downstream Drava power plant. Since this phenomenon has a considerable impact on the plant operation, technical measures have been taken towards its resolving, in addition to monitoring and analyses. Due to cavitation limitations, it has been decided that the turbines be adjusted to the tailwater drawdown by replacing the existing runners with the new ones. The project is currently in preparation and its implementation is planned for the period 2009-2011. Additionally, the possibility is considered of the tailwater regulation by the power plant regulation structures, which would be a permanent solution to the problem. Until such project is carried out, i.e. a sill or a system of sills and barrages built, the Dubrava HPP operation mode must be adjusted to the limitations imposed by the tailwater drawdown.

Tailwater drawdown; dubrava plant; river erosion

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