PROJECT DESCRIPTION
BACKGROUND
All medium-sized reservoirs below the rock/tree line have a tendency to become silted up since the annual inflow of solid particles is high in relation to the storage capacity. In order to maintain the functional capacity of reservoirs and to avoid this problem, these types of reservoirs must be continually scavenged. The resultant high concentration of solid particles leads to severe ecological damage in downstream river systems. The damage comprises: destruction of fish stocks and benthic fauna and flora, clogging of the interstitial gravel system and depositions on the river bed. In Austria alone, this problem affects approx. 20 hydroelectric power station reservoirs. Since 1979 VKW AG had been operating the Langenegg power station with the associated Bolgenach reservoir. In 1995, the reservoir had to be emptied and due to the drain off, damages occurred in the adjacent rivers: Bolgenach, the Weissach and also the Bregenzerache. To avoid such events in the future, the owner had to develop a concept to manage the solid particles.
OBJECTIVES
The basic concept developed by the project was that sediments transported into the reservoirs during periods of high water levels would be to a large extent simultaneously removed using suitable means, and discharged at the same point in the river system as the water used to operate the power station. This would mean that the transport of solids in the whole river system would once more become a natural cycle. Three items of technical equipment would be required: a sufficiently large clearance volume in the reservoir, an adjustable dredger and a transport system for the solid particles. In addition to the transport system, several measures could be taken to reduce the solid particles.
RESULTS
To reduce the solid particles, the water inlet of the Subersach river was regulated and an additional strainer was installed. In order to ensure that the sediment deposited in the reservoir would not be removed when the reservoir was emptied for inspection purposes, an adequate clearance volume had to be provided below the deepest point of the bottom outlet sluice. This meant that the bottom outlet sluice could not be located at the deepest point of the reservoir. The clearance volume had to be of such dimensions that all the solids deposited in the reservoir could be contained in it when emptying the reservoir and for dimensioning the clearance volume it was necessary to take into account that the annual inflow of solids is subject to wide variation. In the Bolgenach reservoir the existing bottom outlet sluice was provided with a new inlet pipe located 33 metres higher up. The existing pipe at the deepest point of the reservoir was sealed off. This created a clearance volume of approximately 700 000 cubic metres. This large volume was necessary since experience had shown that although the average annual solids deposition was approximately 50 000 cubic metres, a series of years with very high extreme deposition rates could generate sediment deposition of 100 000 to 200 000 cubic metres a year. Near the bottom outlet sluice inlet a higher water inlet pipe for the water used to operate the power station was also constructed. A special machinery, which works by the principle of an air-lift, was developed and constructed for the transportation of the debris. Through the pipeline floating on the reservoir the sediments are led through the turbine inlet into adjacent rivers of the hydroelectric power plant. A safe and ecological operation of the hydroelectric power station was ensured for the future. The reservoir does not have to be scavenged any more and therefore damages to the adjacent rivers can be prevented. Throughout Europe, there are many reservoirs which are affected by problems similar to those of the Bolgenach reservoir. The solid control programme developed by the project could in principle be used for all these reservoirs, as an alternative to the scavenging process which up to now has represented the state of the art technology. In addition to reservoirs for electricity generation, other potential applications could include reservoirs for drinking water and irrigation schemes. To reduce the solid particles, the water inlet of the Subersach river was regulated and an additional strainer was installed. In order to ensure that the sediment deposited in the reservoir would not be removed when the reservoir was emptied for inspection purposes, an adequate clearance volume had to be provided below the deepest point of the bottom outlet sluice. This meant that the bottom outlet sluice could not be located at the deepest point of the reservoir. The clearance volume had to be of such dimensions that all the solids deposited in the reservoir could be contained in it when emptying the reservoir and for dimensioning the clearance volume it was necessary to take into account that the annual inflow of solids is subject to wide variation. In the Bolgenach reservoir the existing bottom outlet sluice was provided with a new inlet pipe located 33 metres higher up. The existing pipe at the deepest point of the reservoir was sealed off. This created a clearance volume of approximately 700 000 cubic metres. This large volume was necessary since experience had shown that although the average annual solids deposition was approximately 50 000 cubic metres, a series of years with very high extreme deposition rates could generate sediment deposition of 100 000 to 200 000 cubic metres a year. Near the bottom outlet sluice inlet a higher water inlet pipe for the water used to operate the power station was also constructed. A special machinery, which works by the principle of an air-lift, was developed and constructed for the transportation of the debris. Through the pipeline floating on the reservoir the sediments are led through the turbine inlet into adjacent rivers of the hydroelectric power plant. A safe and ecological operation of the hydroelectric power station was ensured for the future. The reservoir does not have to be scavenged any more and therefore damages to the adjacent rivers can be prevented. Throughout Europe, there are many reservoirs which are affected by problems similar to those of the Bolgenach reservoir. The solid control programme developed by the project could in principle be used for all these reservoirs, as an alternative to the scavenging process which up to now has represented the state of the art technology. In addition to reservoirs for electricity generation, other potential applications could include reservoirs for drinking water and irrigation schemes.
