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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.
An experimental investigation of supercritical uniform and gradually varied open channel flows is presented for a wide range of Froude numbers and flume width-to-flow depth aspect ratios. The instantaneous streamwise and vertical flow velocities were measured in a laboratory flume over the entire width using a two dimensional–laser Doppler anemometry (2D-LDA) system to determine turbulence intensities, and bed and Reynolds shear stresses. The mean velocity patterns show undulation across the flume, indicating the presence of counterrotating secondary current cells. These currents redistribute turbulence intensities and bed and Reynolds shear stresses across the flume. For aspect ratios ≤ 4−5, i.e., narrow open channel flow, the velocity-dip phenomenon is identified both in the streamwise velocity and the Reynolds shear stress distributions. For high aspect ratios, i.e., wide open channel flow, the strength of secondary currents diminish toward the flume center, resulting in a 2D flow farther away from the walls and no velocity-dip phenomenon. Froude number effects on the flow characteristics are less pronounced compared to the aspect ratio effects. At high Froude numbers, the results for narrow and wide open channel flows agree well with literature data. The log-law holds in the inner region across the entire flume width for all investigated Froude numbers and aspect ratios. The Reynolds shear stress distribution agrees well with the computed spanwise bed shear stress distribution. At the flume side walls, the bed shear stresses are 20–50 % higher than the mean values. These results are verified with an engineering example in which high sediment transport and corresponding deep abrasion patterns at the side walls were observed.
Sediment transport in high-speed flows over a fixed bed. 2: Particle impacts and abrasion prediction
(2017)
Single bed load particle impacts were experimentally investigated in supercritical open channel flow over a fixed planar bed of low relative roughness height simulating high-gradient non-alluvial mountain streams as well as hydraulic structures. Particle impact characteristics (impact velocity, impact angle, Stokes number, restitution and dynamic friction coefficients) were determined for a wide range of hydraulic parameters and particle properties. Particle impact velocity scaled with the particle velocity, and the vertical particle impact velocity increased with excess transport stage. Particle impact and rebound angles were low and decreased with transport stage. Analysis of the particle impacts with the bed revealed almost no viscous damping effects with high normal restitution coefficients exceeding unity. The normal and resultant Stokes numbers were high and above critical thresholds for viscous damping. These results are attributed to the coherent turbulent structures near the wall region, i.e. bursting motion with ejection and sweep events responsible for turbulence generation and particle transport. The tangential restitution coefficients were slightly below unity and the dynamic friction coefficients were lower than for alluvial bed data, revealing that only a small amount of horizontal energy was transferred to the bed. The abrasion prediction model formed by Sklar and Dietrich in 2004 was revised based on the new equations on vertical impact velocity and hop length covering various bed configurations. The abrasion coefficient kv was found to be vary around kv ~ 105 for hard materials (tensile strength ft > 1 MPa), one order of magnitude lower than the value assumed so far for Sklar and Dietrich's model.
Particle dynamics are investigated experimentally in supercritical high-speed open channel flow over a fixed planar bed of low relative roughness height simulating flows in high-gradient non-alluvial mountain streams and hydraulic structures. Non-dimensional equations were developed for transport mode, particle velocity, hop length and hop height accounting for a wide range of literature data encompassing sub- and supercritical flow conditions as well as planar and alluvial bed configurations. Particles were dominantly transported in saltation and particle trajectories on planar beds were rather flat and long compared with alluvial bed data due to (1) increased lift forces by spinning motion, (2) strongly downward directed secondary currents, and (3) a planar flume bed where variation in particle reflection and damping effects were minor. The analysis of particle saltation trajectories revealed that the rising and falling limbs were almost symmetrical contradicting alluvial bed data. Furthermore, no or negligible effect of particle size and shape on particle dynamics were found. Implications of experimental findings for mechanistic saltation-abrasion models are briefly discussed.
Abrasion in a concrete-lined sediment bypass tunnel is estimated using a Japanese state-of-the-art prediction model and validated by measured invert abrasion data at Asahi Reservoir, Japan. The model is described in detail, certain shortcomings are disclosed, and a revised version is proposed. The model consists of a kinetic energy term accounting for the impact by saltating particles, and a friction work term accounting for the grinding stress. It is found that the latter term yields concrete abrasion values being consistently a multiple compared to its kinetic term contradicting other research. Based on that, and a possible particle impact angle inconsistency, it is proposed to omit the friction work term. It is shown that the calculated abrasion is overestimated by 138% on average compared with that measured, if both terms are accounted for. However, promising results are obtained with only 30% overestimation by neglecting the friction work term.
Four dams in Japan and Switzerland with Sediment Bypass Tunnels (SBT) as a measure against reservoir sedimentation were monitored to analyse the effects of sediment supply on the downstream environment based on up- to downstream differences in geomorphological and biological characteristics. SBT operation times ranged from 93 years at Pfaffensprung and 17 at Asahi to only three years at Solis and no operation at Koshibu. Sediment grain size distribution was monitored, and microhabitats and invertebrates were analysed in terms of richness and composition. Results showed that grain sizes were coarser down- than upstream at dams with newly established SBTs, while they were similar or finer for dams with long SBT operation. Analysis of biotic data revealed that microhabitat and invertebrate richness was low directly below the dam but increased further downstream the longer the SBT operation. Sedentary species dominated at locations where bed conditions were stable, e.g. directly downstream of the dam at Koshibu. Recovery of downstream environment with increasing SBT operation time was disclosed by the Bray–Curtis similarity index, which evaluated an overlap between up- and downstream reaches for both microhabitat composition and invertebrate communities. With increasing operation time, both indices increased, revealing the positive effects of long-term SBT operation.
Nordrhein-Westfalen braucht eine gut aufgestellte Wasserforschung,
um den heutigen und künftigen Herausforderungen im
Land sowie auch national und international gewachsen zu sein.
Eine erste Erfassung der Wasserforschung in Nordrhein-Westfalen
im Sommer 2017 vermittelte Fakten zu Bestand, Defiziten
und Entwicklungspotenzialen. Mehr als 750 Wissenschaftler
forschen in Nordrhein-Westfalen in über neunzig Forschungsgruppen
verschiedener Disziplinen zum Thema Wasser. Die
Schwerpunkte liegen primär in den Ingenieurwissenschaften,
aber auch in den Natur-, Geo-, Sozial-, Wirtschafts- und Rechtswissenschaften.
Universitäten, Fachhochschulen, An-Institute
sowie außeruniversitäre Forschungsinstitutionen tragen zur
breit aufgestellten Wasserforschung bei, die unter anderem in
sechs Forschungszentren und zahlreichen Kooperationsprojekten
untereinander vernetzt ist. Der guten nationalen Vernetzung der
Akteure stehen befriedigende internationale und stark ausbaufähige
europäische Kontakte gegenüber. Die internationale
Sichtbarkeit und Attraktivität der Wasserforschung in Nordrhein-
Westfalen wird ihren Spitzenleistungen in einzelnen Bereichen
nicht gerecht. Ein vertiefter Praxistransfer von Forschungsergebnissen
offeriert der Wasserwirtschaft innovative, praxistaugliche
Lösungen und zudem der Umweltwirtschaft interessantere
Marktchancen. Besonders günstige Randbedingungen
sind in den innovationsfähigen Kommunen und Wasserverbänden
des großen, dicht besiedelten und an Landschaftstypen reichen
Bundeslandes Nordrhein-Westfalen zu sehen. Im nächsten
Schritt wird zu analysieren sein, wie die Potenziale der Wasserforschung
in Nordrhein-Westfalen effizienter genutzt und ausgebaut
werden können, um zur zukunftssicheren und nachhaltigen
Ausrichtung der Wasserwirtschaft in Nordrhein-Westfalen,
zu wissenschaftlicher Exzellenz und umweltwirtschaftlichem Erfolg
des Landes beizutragen.
Wärmeentnahme aus Abwasser
(1984)
Sauber aus dem Reaktor
(2004)
Quo vadis Baubetrieb?
(2001)
Entwicklung eines Bewertungssystems für ökonomisches und ökologisches Bauen und gesundes Wohnen
(2003)
BASIC-Programme
(1985)
ATV-Arbeitsblatt A 127
(1000)
Standsicherheit von Linern
(2000)