Auel, Christian
Entlandung des Stausees Solis mit Hilfe eines Geschiebeumleitstollens – hydraulische Modellversuche
(2009)
Das Reservoir der Talsperre Solis in Graubünden wurde 1986 vom Elektrizitätswerk der Stadt Zürich (ewz) in Betrieb genommen. Bei Hochwasserereignissen werden grosse Sedimentmengen in den Stausee transportiert. Über 25 % des ursprünglichen Reservoirvolumens sind bereits verlandet. Aus diesem Grund plant ewz den Bau eines Geschiebeumleitstollens, der das bei Hochwasser ankommende Sediment um die Talsperre herum in Richtung Unterwasser leitet. Die Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie (VAW) der ETH Zürich untersucht und optimiert in einem hydraulischen Modell den projektierten Umleitstollen. Bei Hochwasser wird der Stausee auf den minimalen Betriebswasserspiegel abgesenkt, der Verlandungskörper liegt grösstenteils frei, das Sediment erodiert. Mit Hilfe einer Leitkonstruktion wird die Strömung in Richtung Einlaufbauwerk des Stollens gelenkt und der Abfluss sowie das Geschiebe durch diesen geleitet. Bei Abflüssen, die die Ausbaukapazität des Stollens (HQ5) überschreiten, wird die überschüssige Strömung in den vorderen Bereich des Sees in Richtung Grundablass abgeführt. Das Geschiebe wird dennoch nahezu vollständig in den Stollen eingeleitet. Bei Hochwasser mit Schwemmholzaufkommen wird mit Hilfe einer Tauchwand sowie einer Teilablleitung des Hochwasserabflusses in den vorderen Seebereich verhindert, dass Schwemmholz in den Umleitstollen
eingezogen wird.
Sediment bypass tunnels are an effective countermeasure against reservoir sedimenta-tion. They are operated at supercritical sediment-laden open channel flow conditions. The major drawback of these tunnels, besides high construction costs, is the severe invert abrasion caused by these flows provoking high annual maintenance costs. The project goal was to analyze the fundamental physical processes and to develop design criteria to decrease these negative effects. A laboratory study was performed in a scaled hydraulic model flume. The project was divided in three main test phases giving new insights into the dynamics of turbulence structures and particle motions, resulting bed abrasion and their interactions in a supercritical open channel flow, respectively. In phase A the mean and turbulent flow characteristics were investigated. In phase B single sediment particle motion was analyzed. In phase C the invert abrasion development in time and space was examined.
Phase A revealed that secondary current cells affect the turbulent flow pattern leading to high bed shear stress at the wall vicinity. In phase B it was found, that particles were dominantly transported in saltation. Relationships between the saltation probability, and particle hop lengths and heights to the flow Shields parameter were found. The specific impact energy was determined by the impact velocity, number of impacts and the amount of particles transported in time. In phase C the results show that bed abrasion progresses with time both in the lateral and vertical direction. Two lateral incision chan-nels developed along the flume side walls at narrow flow conditions occurring at low flume-width to flow-depth aspect ratios b/h < 4-5, whereas randomly distributed pot-holes were found at wide channels where b/h > 4-5. The observed abrasion patterns match well with the spanwise bed shear stress distributions found in phase A. Further-more it was found that the abraded mass linearly increases with the transported sedi-ment mass allowing for a linear fit. Further results showed that abrasion increased with flow intensity and sediment transport rate, with highest values for the mean particle diameter category, whereas abrasion decreased with increasing material strength.
Finally, a new formulation was developed based on Sklar’s saltation abrasion model. A new abrasion coefficient CA is introduced correlating the impact energy and material properties with the gravimetric abrasion rate.
This paper deals with an experimental investigation of the mean and turbulence characteristics of supercritical quasi-uniform and gradually varied open-channel flows over a transitional rough bed. These conditions are typical for sediment bypass tunnels. The results show that the log-law holds well in the inner region across the channel. The roughness shifts the velocity profiles downward by an amount of ΔU+. The velocity-dip phenomenon and strong secondary currents exist in the channel for narrow open-channel flow. These currents cause the Reynolds shear stress distributions to deviate from the linear distribution and an undulation on the transversal distribution of the bed shear stress, which matches with the bed abrasion pattern. The streamwise turbulence intensity profiles deviate from the semi-empirical universal function whereas the vertical turbulence intensity profiles fit well with it only at the centerline of the channel. A strong wall effect exists on the turbulence intensities in the outer region.