Suche

2022, Glattbrugg, Suisse : VSA, 143 p.

Partial driftwood rack at gated ogee crest :

Scientifique ArODES

trapping rate and discharge efficiency

Michael Pfister, Loïc Bénet, Giovanni De Cesare

Journal of Hydraulic Engineering, 2022, vol. 148, no. 8, article no. 06022008

Zusammenfassung:

Driftwood belongs to natural rivers just like water and sediment do. A sound ecosystem requires driftwood, although it might jam at civil structures, altering the flow section and rise the backwater. Safety considerations suggest removing wood from rivers, whereas ecological experience asks for its presence. The situation might become critical if spillways clog during floods, so that their discharge capacity reduces. For narrow bays, full racks mounted upstream of the weir or overhanging piers trap the driftwood distant from the flow control section. The hydraulic capacity is then maintained, but the driftwood has to be removed after the event. We thus investigated herein with a physical model a novel partial rack, motivating the driftwood for uncongested appearance to partially pivot and pass, but ensuring a high discharge capacity under hypercongested appearance. The partial rack configuration was specified, together with the related trapping rate and discharge efficiency.

Performance and design of a stepped spillway aerator

Scientifique ArODES

Michael Pfister, Stéphane Terrier, Anton J. Schleiss

Water, 2022, vol. 14, no. 153

Zusammenfassung:

Stepped spillways are frequently limited to specific discharges under around 30 m2/s due to concerns about potential cavitation damages. A small air concentration can prevent such damages and the design of bottom aerators is well established for smooth chutes. The purpose of this study is to systematically investigate the performance of a deflector aerator at the beginning of stepped chutes. Six parameters (chute angle, step height, approach flow depth, approach flow Froude number, deflector angle and deflector height) are varied in a physical model. The spatial air concentration distribution downstream of the aerator, the cavity sub-pressure, water discharge and air discharges are measured. The results describe the commonly used air entrainment coefficient, the jet length, as well as the average and bottom air concentration development to design an aerator. The lowest bottom air concentration measured in all tests is higher than the air concentration recommended in literature to protect against cavitation damages. And, unlike smooth chutes, there appears to be no significant air detrainment downstream of the jet impact. One deflector aerator seems therefore sufficient to provide protection of a stepped spillway.

2021

Bottom-pressure development due to an abrupt slope reduction at stepped spillways

Scientifique ArODES

Michael Pfister, Mohammad J. Ostad Mirza Tehrani, Jorge Matos, Anton J. Schleiss

Water, 2021, vol. 14, no. 1, article no. 41

Zusammenfassung:

Fluctuating bottom-pressures on stepped chutes are relevant for the spillway design. An abrupt slope reduction causes a local alteration of the bottom-pressure development. Little information is available regarding the air–water flow properties near an abrupt slope reduction on stepped chutes, particularly on the local pressure evolution. Nevertheless, the option of providing a chute slope reduction may be of interest in spillway layout. The experiments presented herein include pressure distributions on both vertical and horizontal step faces, subsequent to an abrupt slope reduction on stepped chutes. A relatively large-scale physical model including abrupt slope reductions from 50° to 18.6° and from 50° to 30° was used, operated with skimming flow. The data indicate a substantial influence of the tested slope reductions on the bottom-pressure development. In the vicinity of the slope reduction, the mean pressure head near the edge of the horizontal step face reached 0.4 to 0.6 times the velocity head upstream of the slope reduction, for critical flow depths normalized by the step height ranging between 2.6 and 4.6.

Blockage probability modeling of large wood at reservoir spillways with piers

Scientifique ArODES

Michael Pfister, Paloma Furlan, Jorge Matos, Conceição Amado, Anton J. Schleiss

Water Resources Research, 2021, vol. 57, no. 8, article no. e2021WR029722

Zusammenfassung:

Large wood increases the morphological and hydraulic complexity of rivers, yet it may block and modify the flood discharge capacity of hydraulic structures. To assess the related risk, blockage probability estimation for hydraulic structures such as reservoir spillways is needed. This work presents unstudied parameters for blockage of large wood with a reservoir-type approach flow, where the inflow velocity has a negligible magnitude. Experiments were conducted in a channel with an ogee crested spillway equipped with piers, representing a commonly used hydraulic structure. Artificial stems were used to systematically evaluate the influence of stem length and stem draft on the blocking process. Different hydraulic conditions were evaluated by changing the water level in the reservoir. The head at the spillway crest with respect to stem draft was found to be a key parameter for blockage probability estimation at a spillway. Additionally, stem length was related to the bay width in the estimation of blockage. Larger heads tend to reduce the blocking probability of large wood, for a given stem draft, while increasing the relative stem length tends to increase the blocking probability. A logistic regression model is provided to estimate large wood blockage probability at ogee crested spillways with piers. Finally, recommendations for engineering practice are presented.

Floating woody debris :

Scientifique ArODES

blocking sensitivity of labyrinth weirs in channel and reservoir applications

Michael Pfister, Taylor Vaughn, Brian M. Crookston

Journal of Hydraulic Engineering, 2021, vol. 147, no. 11, article no. 06021016

Zusammenfassung:

Accumulation of floating woody debris at flow control structures can result in reduced discharge efficiency. Labyrinth weirs may be more likely to catch debris on the crest than other weirs due to their ability to pass large discharges with relatively small heads. Therefore, prompted from field data at Lake Brazos Dam, a laboratory-scale hydraulic model study was performed to evaluate the floating woody debris blocking sensitivity of labyrinth weirs in channel and reservoir applications, including arced labyrinth weirs. Experimental test results noted debris entrapment locations on the crest and that debris blockage probability is primarily a function of trunk diameter and upstream head. Debris accumulation tests indicated that lower heads are more sensitive to debris blockage than higher heads. Specifically, for smaller flow depths the initial or reference head (without debris) increased by up to 17% with debris, while at higher reference heads the head increase due to debris was approximately 7% or less.

Vers une diminution de la capacité des sources urbaines ? :

Scientifique ArODES

étude de la source des Pilettes - influence de l'urbanisation et du changement climatique

Michael Pfister, Nathan Bongard, Luca Rossi

Aqua & Gas, 2021, no. 6, pp. 22-31

Zusammenfassung:

Les changements climatiques et l’imperméabilisation font peser un grand nombre d’incertitudes sur l’évolution de nos ressources en eau, particulièrement sur les sources. L’analyse détaillée du comportement de la Source des Pilettes à Fribourg montre l’impact des changements déjà intervenus, les changements attendus dans le futur et donnent des pistes pour remédier à ces impacts.

Effect of 30-degree sloping smooth and stepped chute approach flow on the performance of a classical stilling basin

Scientifique ArODES

Michael Pfister, Ivan Stojnic, Jorge Matos, Anton J. Schleiss

Journal of Hydraulic Engineering, 2021, vol. 147, no. 2

Zusammenfassung:

Advances in dam construction techniques have significantly increased the number of stepped spillways implemented worldwide. Although stepped chutes provide enhanced energy dissipation along the chute, as compared to smooth chutes, an adequate energy dissipater is usually needed at their toe to govern the remaining energy. Stilling basins downstream of stepped spillways are currently designed using the approaches developed for smooth chutes. As a stepped surface alters the structure of the approaching flow, such practice is questionable. This paper reports a study on the effect of stepped chute approach flows on the performance of a classical stilling basin. Physical modeling was conducted using a large-scale facility of a smooth and stepped spillway with a 30° sloping chute. Experiments were performed under different discharges, two step heights, and variable approach flow aeration. The characteristics of the hydraulic jump were described, focusing mainly on flow depth, bottom pressure, and length. The results indicated a significant effect of stepped chute approach flows on bottom pressure and length of the hydraulic jump. Near the jump toe, pronounced fluctuating and extreme pressures were observed after stepped chutes and attributed to the higher turbulence level of the incoming flow. The normalized hydraulic jump lengths were found to be about 17% longer downstream of stepped chutes as compared to smooth chutes.

Bemessungsbeispiele von Tosbecken unterhalb von Treppenschussrinnen

Scientifique ArODES

Michael Pfister, Ivan Stojnic, Jorge Matos, Giovanni De Cesare, Anton J. Schleiss

Wasserwirtschaft, 2021, vol. 1, pp. 18-24

Zusammenfassung:

Treppenschussrinnen entstanden mit der Anwendung des Walzbetons im Talsperrenbau. Sie kommen heute auch vermehrt an den Felsfanken von Schüttdämmen zur Anwendung. Systematische Modellversuche im Rahmen eines Forschungsprojektes haben gezeigt, dass Tosbecken unterhalb von Treppenschussrinnen nicht mit den gleichen hydraulischen Ansätzen wie jene nach glatten Schussrinnen dimensioniert werden sollten. Das Ziel dieses Artikels ist es, ein angepasstes Bemessungsverfahren für Tosbecken nach Treppenschussrinnen anhand von zwei Beispielen zu verdeutlichen sowie Unterschiede zu Tosbecken nach glatten Schussrinnen aufzuzeigen.

Reservoir level rise under extreme driftwood blockage at ogee crest

Scientifique ArODES

Michael Pfister, Loïc Bénet, Giovanni De Cesare

Journal of Hydraulic Engineering, 2021, vol. 147, no.1

Zusammenfassung:

Dams are civil structures essential to modern civilization. However, they can be a threat if not properly designed and operated. A particular risk that potentially can lead to dam failure is the blocking of the spillway inlet with driftwood or debris. This study investigated, on the basis of physical modeling, this blocking as well as the related backwater rise and discharge-capacity reduction. Considerable quantities of driftwood were supplied upstream of an ogee weir with piers, and the subsequent reservoir level rise was measured. Particular focus was placed on extreme events in terms of driftwood occurrence (volume) and discharges (design value). It was found that a gated ogee blocked with driftwood performs with a reduced discharge coefficient as long as no countermeasures are taken, such as pier overhang, the removal of piers, or the installation of a rack. The performance of these countermeasures was studied, and criteria were developed to control the perturbing effect of driftwood.

2020

Hydraulic engineering of Dams

Livre ArODES

Michael Pfister, Willi Hermann Hager, Anton J. Schleiss, Robert M. Boes

2020, London, UK : CRC Press, 1080 p.

Zusammenfassung:

Hydraulic engineering of dams and their appurtenant structures counts among the essential tasks to successfully design safe water-retaining reservoirs for hydroelectric power generation, flood retention, and irrigation and water supply demands. In view of climate change, especially dams and reservoirs, among other water infrastructure, will and have to play an even more important role than in the past as part of necessary mitigation and adaptation measures to satisfy vital needs in water supply, renewable energy and food worldwide as expressed in the Sustainable Development Goals of the United Nations. This book deals with the major hydraulic aspects of dam engineering considering recent developments in research and construction, namely overflow, conveyance and dissipations structures of spillways, river diversion facilities during construction, bottom and low-level outlets as well as intake structures. Furthermore, the book covers reservoir sedimentation, impulse waves and dambreak waves, which are relevant topics in view of sustainable and safe operation of reservoirs. The book is richly illustrated with photographs, highlighting the various appurtenant structures of dams addressed in the book chapters, as well as figures and diagrams showing important relations among the governing parameters of a certain phenomenon. An extensive literature review along with an updated bibliography complete this book.

Hydraulique des canalisations :

Livre ArODES

quatrième séminaire

2020, Fribourg : HEIA-FR, xviii, 165 p.

Bemessung von klassischen Tosbecken unterhalb von Treppenschussrinnen

Professionnel ArODES

Michael Pfister, Ivan Stojnic, Jorge Matos, Giovanni De Cesare, Anton Schleiss

Wasser Energie Luft = Eau énergie air = Acqua energia aria, 2020, vol. 112, no. 3, pp. 165-170

Zusammenfassung:

Aufgrund des Fortschritts im Talsperrenbau mit Walzbeton (RCC) werden weltweit häufig Treppenschussrinnen erstellt. Nach jeder Schussrinne ist ein hydraulisches Bauwerk zur Energiedissipation vorzusehen, beispielsweise ein Tosbecken. Obwohl die Hydraulik von Treppenschussrinnen in den letzten Jahrzehnten eingehend untersucht wurde, sind aktuell nur wenige Ansätze zur Bemessung von Tosbecken nach einer Treppenschussrinne verfügbar. Die hier vorgestellten umfangreichen Modellversuche sollen diese Lücke für klassische Tosbecken mit ebener Sohle ohne Einbauten schliessen. Die Experimente zeigen, dass solche Tosbecken unterhalb von Treppenschussrinnen nicht mit den gleichen Ansätzen dimensioniert werden können, wie sie nach glatten Schussrinnen verwendet werden. Ziel dieses Artikels ist es, neue Bemessungsansätze für solche Tosbecken bereitzustellen.

Forces on buildings with openings and orientation in a steady post-tsunami free-surface flow

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Anton J. Schleiss

Coastal Engineering, 2020, vol. 161, article no. 103753

Zusammenfassung:

Steady free-surface flows around buildings occurring during flood or tsunami events can produce major damages and a quantification of the post-peak forces is essential for safety and resilience of coastal structures. The loading process is highly affected by the flow Froude number and the drag coefficient, commonly defined for highly subcritical flows, while field observations of tsunami flows reported Froude numbers close to one, visually appearing as a choked regime. The present experimental study addresses the hydrodynamic forces on emerging buildings in a subcritical choked regime, focusing on the effect of openings and orientation. Laboratory experiments indicated a substantial difference in flow depths between the up- and downstream side of the building for increasing Froude numbers. The presence of openings induced a flow through the building lowering the difference in flow depth, limiting the effect of the hydrostatic component of the loading process. The formulation of an empirical resistance coefficient CR allowed a combination of both form drag and hydrostatic forces. Whilst for impervious buildings CR was consistent with reference studies, for buildings with openings CR was directly dependent upon porosity. Contrarily to the unsteady flow conditions, results showed that the sidewalls also played a role in the loading process. Buildings with a rotated orientation resulted in slightly larger surfaces exposed to the flow and larger horizontal forces. Nevertheless, these were applied at lower cantilever arms, thus reducing the tilting moment. Altogether, this study provides experimentally-derived parameters that will support hydraulic engineers in the design of coastal structures.

Effet des bois flottants obstruant un évacuateur de crue dans des conditions extrêmes

Professionnel ArODES

Michael Pfister, Loïc Bénet, Giovanni De Cesare

Wasser, Energie, Luft, 2020, vol. 112, no. 2, pp. 77-83

Zusammenfassung:

Le transport de bois flottants peut induire des obstructions aux ouvrages hydrauliques et limiter leurs fonctionnalités et leurs capacités de décharge. Le blocage de l’entrée d’un évacuateur de crue d’un barrage par des bois flottants représente un risque particulier. La présente étude examine, sur la base de la modélisation physique, ce blocage et ses conséquences. Plus précisément, les bois flottants sont introduits dans un modèle réduit en amont d’un déversoir standard vanné, mais ouvert, et leur blocage ainsi que l’augmentation subséquente du niveau de la retenue sont étudiés. Un accent particulier est mis sur les événements extrêmes en termes d’occurrence de bois flottants, de dimensions et de débits. Sans aucune contre-mesure prise, l’efficacité du déversoir est réduite et le coefficient du déversoir s’approche d’une constante inférieure à un fonctionnement non perturbé, lorsqu’un volume de bois déterminant est bloqué. Il a été constaté qu’une barrière de bois se forme proche du déversoir. Des installations techniques pourraient être envisagées afin d’éviter que les troncs se bloquent au niveau de la crête du déversoir et perturbent l’écoulement, comme le déplacement des têtes de piliers dans le réservoir, l’enlèvement des piliers ou l’installation d’un râtelier. Les effets hydrauliques attendus, à savoir l’éloignement de la zone d’accumulation de bois flottants en amont de la section d’écoulement critique, ou le passage des troncs sembleraient se confirmer lors des expériences.

Die Entwicklung der gefalteten Wehre mit ausgewählten aktuellen Forschungsresultaten

Professionnel ArODES

BAW Mitteilungen : Feste Wehre an Bundeswasserstraßen Untersuchungen zur Machbarkeit sowie Empfehlungen zur Umsetzung, 2020, vol. 105, pp. 27-39

Hydraulic design of classical stilling basins downstream of stepped chutes

Professionnel ArODES

Michael Pfister, Ivan Stojnic, Jorge Matos, Giovanni De Cesare, Anton J. Schleiss

The International Journal of Hydropower & Dams, 2020, vol. 27, no. 5, pp. 46-54

Zusammenfassung:

New design information regarding the hydraulic behaviour of classical stilling basins downstream of stepped chutes is presented and discussed. An extensive experimental campaign revealed that classical stilling basins preceded by stepped chutes cannot be adequately designed using the principles developed for smooth chute approach flow, as is done in common engineering practice. As such, new practical design recommendations are proposed. The detailed design procedure is illustrated with examples of stilling basins downstream of smooth and stepped chutes.

Impact hydrodynamique des vagues contre les bâtiments

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Anton J. Schleiss

La Houille Blanche, 2020, no.1, pp. 34-41

Zusammenfassung:

Les tsunamis, les vagues d'impulsion et les ruptures de barrages représentent des phénomènes extrêmes qui mettent en danger la vie des personnes et la fonctionnalité des infrastructures. Des événements récents dans l'Océan Indien (2004), au Japon (2011) et en Indonésie (2018) ont montré que des mesures constructives peuvent réduire les charges induites sur les structures, et l'objectif de cette étude est d'évaluer et d'analyser le dommage potentiel des bâtiments frappés par des vagues. Au travers d'un programme expérimental étendu, cette recherche d'abord caractérise le comportement hydrodynamique des vagues sur fond sec et sur fond mouillé en termes de la célérité du front, des hauteurs d'eau et des profils de vitesse derrière le front. Ensuite, l'avantage d'avoir des bâtiments perméables pour réduire les hauteurs d'eau et les sollicitations en cas d'inondation est démontré. En outre, cette étude introduit des formules simples pour prédire la charge hydrodynamique sur un bâtiment, en tenant compte de l'effet des ouvertures au travers d'un coefficient de résistance adapté. Ces résultats fournissent des informations importantes pour le dimensionnement d'infrastructures plus sûres, qui puissent agir comme refuges verticaux en cas de dangers naturels liés à l'eau.

2019

Hydraulique :

Professionnel ArODES

documentation technique de la norme SIA 190:2017

Michael Pfister, Jean-Louis Boillat

SIA D 0264 : SIA 190,

Zusammenfassung:

Kanalisationen bilden meist verzweigte lineare Netze, und ihr Abfluss wird von der Schwerkraft angetrieben. Um hydraulische Instabilitäten und das Zuschlagen von Leitungen zu vermeiden, werden sie im Freispiegelabfluss betrieben und mit einem ausreichenden Luftquerschnitt bemessen. Kanalisationsnetze beinhalten zahlreiche punktuelle Elemente, wie beispielsweise Krümmer oder Anschlüsse mit einer standardisierten Ausführung. Sie enthalten aber auch Spezialbauwerke wie Vereinigungen, Absturzbauwerke oder Düker, welche einer individuellen Bemessung bedürfen. Die vorliegende Dokumentation gibt einen Überblick der typischen hydraulischen Bemessungsansätze von Kanalisationen, unter Berücksichtigung der Norm SIA 190:2017. Sie konzentriert sich insbesondere auf ungünstige Bedingungen, unter denen die theoretische Abflusskapazität einer Kanalisation nicht ausgeschöpft wird, beispielsweise aufgrund des Einflusses der Turbulenz oder der eingeschlossenen Luft. Die hier beschriebenen Grundlagen beziehen sich allerdings nicht ausschliesslich auf die Anwendung der Norm SIA190:2017. Es soll auch spezifische Fachliteratur zur Behandlung von besonderen Bauwerken bzw. Fragestellungen hinzugezogen werden.

Effect of debris damming on wave-induced hydrodynamic loads against free-standing buildings with openings

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Claudia Ylla Arbol, Anton J. Schleiss

Journal of waterway, port, coastal, and ocean engineering, 2020, vol. 146, no 1

Zusammenfassung:

Tsunamis, impulse waves, and dam-break waves are rare but catastrophic events, associated with casualties and damage to infrastructures. An adequate description of these waves is vital to assure human safety and to generate resilient structures. Furthermore, a specific building geometry with openings, such as windows and doors, reduces wave-induced loads and increases the probability that a building withstands. However, waves often carry a large volume of debris, generating supplementary impact forces and creating debris dams around buildings, limiting the beneficial effects of the openings. Herein, a preliminary study on the three-dimensional (3D) effect of debris dams on postpeak wave-induced loads under unsteady flow conditions is presented based on laboratory experiments. Both wooden logs (forest) and shipping containers were tested, showing different behaviors. Shipping containers were associated with severe impact force peaks, whereas the interlocking nature of forest-type debris provoked a compact debris dam, leading to higher and longer-lasting hydrodynamic forces. The arrangement of the debris also had an influence on the resulting structural loading. All tested scenarios were analyzed in terms of horizontal force, cantilever arm, and impulse acting on the building. This study presents a methodology to support the evaluation of postpeak debris-induced loads for the design of safer resilient buildings.

Design of riverbank riprap using large, individually placed blocks

Scientifique ArODES

Michael Pfister, Mona Jafarnejad, Mário J. Franca, Anton J. Schleiss

Journal of Hydraulic Engineering, 2019, vol. 145, no. 12

Zusammenfassung:

The protection of river banks in mountain rivers by riprap requires large blocks with weights typically exceeding 1 t. Thus, the blocks have to be placed individually. Such packed riprap has a relatively small spacing between the blocks. Consequently, the interlocking forces between the blocks are high, and the resistance of this protection to erosion is increased compared with traditional dumped riprap used for bank protection in low-grade rivers. Based on 98 systematical laboratory experiments, an adapted design method was developed for the case of riprap riverbank protection using large, individually placed blocks. Three different block sizes were tested by varying the longitudinal channel slope and bank inclination. For comparison, 34 tests were performed with dumped riprap. A design relationship is presented as a function of block size relative to the flow depth and a modified block Froude number considering the mean flow velocity. The additional resistance of riprap to erosion with large, individually placed blocks compared with dumped riprap is quantified. The proposed equations are compared with existing riprap sizing equations. The suggested method proved to be reliable in the assessment of the stability of packed riprap. Furthermore, it is shown that a second layer significantly delays riprap failure.

Blockage of driftwood and resulting head increase upstream of an ogee spillway with piers

Chapter ArODES

Michael Pfister, Paloma Furlan, Jorge Matos, Anton J. Schleiss

Dans Bennett, Tony, Bibeau, Johanne, Tournier, Jean-Pierre, Sustainable and Safe Dams Around the World / Un monde de barrages durables et sécuritaires: Proceedings of the ICOLD 2019 Symposium, (ICOLD 2019), June 9-14, 2019, Ottawa, Canada / Publications du symposium CIGB 2019, juin 9-14, 2019, Ottawa, Canada (10 p.). 2019, Boca Raton, London, New York : CRC Press

Zusammenfassung:

L‘accumulation de bois flottants dans les réservoirs peut avoir des effets négatifs sur la sécurité de l‘exploitation d‘un barrage. Par conséquence, une conception adéquate des déversoirs en ce qui concerne le bois flottant est d’une importance primordiale. Ici, une étude sur le blocage du bois flottant avec une approche d‘écoulement de réservoir a été menée. Un modèle physique a été utilisé pour évaluer le blocage des tiges artificielles sur un déversoir à crête standard équipé avec des piliers. Grace a une approche systématique, l’effet des tiges bloquées sur la tête d’un réservoir a été étudié qualitativement. Des expériences avec des tiges bloquées manuellement ont été réalisées pour étudier quel effet, qu‘un blocage, peut avoir sur la tête du réservoir. Des expériences en libérant 200 tiges ont été réalisées pour étudier les formes d’encombrement et leur probabilité de provoquer une augmentation de la tête. Il a été constaté que des volumes de tiges bloqués similaires avaient des effets différents sur l‘augmentation de la tête dans le réservoir et dépendaient du fait que les tiges étaient en contact ou non avec la crête de l‘évacuateur de crue. Il a également été constaté qu‘une tête croissante tend à diminuer la probabilité de blocage, mais pas de manière linéaire.

Hydraulic design aspects for supercritical flow in vortex drop shafts

Scientifique ArODES

Michael Pfister, Gaetano Crispino, Corrado Gisonni

Urban Water Journal, 2019, vol. 16, no. 3, pp. 225-234

Zusammenfassung:

Vortex drop shafts, as special sewer manholes, operate optimally if an adequate energy dissipation is guaranteed and the integrity of the structural components is safeguarded. The results of an experimental study on a vortex drop shaft with supercritical inflow are discussed herein. The hydraulic behaviour of the spiral inlet, the vertical shaft and the dissipation chamber is described. Based on detailed flow observations, useful recommendations for designing these structures are provided. It is demonstrated that a relation adopted for supercritical bend flows provides a reliable estimation of the maximum wave height along the inlet. A procedure for predicting the rotational flow angles and the velocity distribution along vertical shafts with swirling flows is developed. Water levels and pressure measurements in the dissipation chamber are further analysed to identify maximum forces acting on the chamber invert and to derive preliminary design equations.

Statistical accuracy for estimations of large wood blockage in a reservoir environment

Scientifique ArODES

Michael Pfister, Paloma Furlan, Jorge Matos, Conceição Amado, Anton J. Schleiss

Environmental Fluid Mechanics,

Zusammenfassung:

The blockage of weirs or bridges by in-stream wood can reduce the flood discharge capacity, leading to hazardous situations. To assess the related risk, blocking probabilities quantifications are needed. However, large wood has a random behaviour and it is challenging to accurately evaluate the blockage process and the influence of different large wood or hydraulic parameters properly. Investigations of large wood processes with physical models have been performed in the past but some contradictions regarding the coherence of the results has been found. Herein, a compromise between statistical accuracy and experimental repetitions is presented. The influence of a different number of experimental repetitions on the accuracy of blocking probabilities estimations of groups of stems in an ogee crested weir equipped with piers was systematically evaluated. Statistically justified numbers of repetitions for achieving maximum standard errors of 0.20 are presented for different semi-congested LW transport regimes. The bootstrap re-sampling technique was applied to generalize the results obtained experimentally. It was found that an increasing number of stems may decrease the randomness of the blockage process. This observation allowed to decrease the number of experimental repetitions needed to achieve equal levels of statistical accuracy compared to individual stem experiments.

Upstream erosion and sediment passage at piano key weirs

Scientifique ArODES

Michael Pfister, Ivan Stojnic, Mattia Noseda, Anton J. Schleiss

Journal of Hydraulic Engineering, 2019, vol. 145, no. 8, article no 04019029

Zusammenfassung:

Piano key weirs (PKWs) are a weir type characterized by an effective rating curve. Accordingly, this control structure is primarily applied at dams to increase the spillway capacity. In recent years, PKWs also have been implemented in rivers combined with low-head hydropower or to regulate waterways. For the latter application, a weir type without gates may be favorable, but the issue of the passage of sediments arises. Such sediments are either deposited in the backwater or transported to the weir during intense floods. An efficient sediment passage is necessary to avoid inundations upstream of the weir and to maintain a navigable waterway. Two options arise: (1) to flush the sediments (e.g., through a gate in the weir), or (2) to carry them over the weir crest. The second option is favorable, if upstream riverbed aggradation can be avoided, because no mechanical devices (i.e., gates) are used. This study analyzed the sediment passage over a PKW driven uniquely by the flow. Systematic physical model tests were conducted to study the upstream riverbed behavior as well as the passage of sediments over the PKW. Three PKW configurations, two sediment granulometries, and six discharges were considered. Key results refer to the modified rating curve under high riverbed levels and to the upstream scour process of sediment deposits. Finally, the sediment passage capacity was linked to the equilibrium sediment transport conditions upstream of a PKW. Pragmatically formulated, this relation indicates— at least for the tested configurations—that sediments arriving at the PKW also pass over it.

Effect of bed roughness on tsunami-like waves and induced loads on buildings

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Anton J. Schleiss

Coastal Engineering, 2019, vol. 152, article no 103508

Zusammenfassung:

Tsunami, impulse-waves and dam-break waves afflict humanity with casualties and damages. An insight into the flow mechanisms of these waves is important to provide safety and reduce reconstruction costs. This experimental study focuses on the effect of bed roughness on the main hydrodynamic properties of surges propagating on dry bed. In addition, the resulting wave impact forces on buildings with and without openings are studied. Results pointed out that dry bed surges on a rough bed had a lower front celerity and a higher flow depth, resulting into inundation depths during the impact around 20% higher as compared to the smooth bed. Furthermore, a rough bed induced a lower momentum flux during wave propagation, resulting into lower impact forces exerted on the building. The rough bed configuration also caused shorter impact durations, leading to lower impulse values transferred to the building. Results pointed out that even on rough bed, openings within the buildings linearly reduced impact forces, thus providing some helpful information for the design of safer coastal structures.

Effect of building overtopping on induced loads during extreme hydrodynamic events

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Ioan Nistor, Anton J. Schleiss

Journal of Hydraulic Research,

Zusammenfassung:

Tsunamis, impulse-waves and dam-break waves have affected humanity in recent decades and the construction of vertical shelters can provide safety to people. However, for non-critical infrastructures, typically residential houses of lower height, overtopping is accepted during such events. This study experimentally quantifies the effect of building overtopping, i.e. water flowing over the roof, on the resulting loading process. Both surges and bores were investigated and the impact against buildings with two different heights was assessed. Detailed measurements of forces and moments allowed key differences to be captured between the scenarios with and without overtopping. Results showed that overtopping induced higher downstream water depths, leading to lower horizontal forces and a reduced resistance coefficient. Furthermore, cantilever arm, moment and impulse values were constantly lower in case of overtopping. Finally, this study presents an innovative methodology to assess the main loading features of buildings subject to overtopping, supporting engineers to design safer resilient structures.

2018

Multiple inflow branches at supercritical-type vortex drop shaft

Scientifique ArODES

Michael Pfister, Gaetano Crispino, Thierry Fuchsmann, Jean-Marc Ribi, Corrado Grisoni

Journal of Hydraulic Engineering, 2018, 144, 11

Zusammenfassung:

Vortex drop shafts serve to overcome important elevation differences in drainage systems. If well designed, they are performant in terms of energy dissipation and safety. However, the standard design requires well-defined approach flow conditions to ensure the reliability of the structure. In practice, these conditions are frequently ignored due to space restrictions and the fact that several inflow branches may arrive at various elevations with supercritical and/or subcritical flows. The literature provides preliminary concepts for such situations that were not adaptable to the case discussed herein. Nevertheless, in order to apply a standard vortex drop shaft, a novel concept was developed that comprises a junction chamber to merge the inflow branches followed by a very short and steep inlet channel. Extended physical model tests have proven its feasibility. The hydraulic aspects discussed herein allow, at least partially, the adoption of the presented concept in similar situations.

Experimental study on forces exerted on buildings with openings due to extreme hydrodynamic events

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Ioan Nistor, Anton Schleiss

Coastal Engineering, 2018, 140, pp. 72-86

Zusammenfassung:

Tsunamis, landslide-generated waves, and dam failures are rare, but highly destructive phenomena, associated with extreme loading on infrastructure. Recent events showed that specific measures must be taken to guarantee safety of both people and the built environment. This experimental study investigates the forces and moments exerted on free-standing buildings that are induced by both surges and bores. The hydrodynamic impact was characterized by high splash, subsequently followed by a quasi-steady flow around the structure. For dry bed surges, the time history of the horizontal force was proportional to the momentum flux per unit width. For wet bed bores, an attenuation of the peak force due to the presence of an aerated front was observed and the introduction of a reduction coefficient was necessary to achieve a realistic force estimation. Additional force analysis in terms of peak time, wave height at maximum force and impulse also pointed out some key differences between forces exerted by dry bed surges and wet bed bores. The occurrence of the maximum tilting moment on the building coincided with the maximum horizontal force and an evaluation of the cantilever arm was possible. These findings provide engineers with practical information for the design of safer coastal structures.

Supercritical flow in junction manholes under invert- and obvert-aligned set-ups

Scientifique ArODES

Michael Pfister, Gaetano Crispino, Corrado Gisonni

Journal of Hydraulic Research, 2018

Zusammenfassung:

Junction manholes are a part of urban drainage infrastructures. They merge inlet branches into an outlet branch. Former studies focused on junction flow by considering a specific manhole layout: identical diameters were assigned to the branches and the invert was flush. Nevertheless, engineers are often involved in designing junctions under generalized geometries, with different branch diameters and, sometimes, bottom offsets at manhole inlets. For these junction arrangements, the empirical relations documented in literature are not applicable. An experimental campaign was thus conducted to determine the main flow features of junction manholes under generalized set-ups. The results include a diagram of state to be used if junctions operate under mixed flow conditions. Information regarding the main wave patterns, with detailed discussion of wave features and heights, is provided to support the hydraulic design of junction manholes under the supercritical flow regime.

Effect of a second layer on the time to failure of compressed riprap as mountain riverbank protection

Scientifique ArODES

Michael Pfister, Mona Jafarnejad Chaghooshi, Mario Franca, Anton Schleiss

Journal of Hydraulic Research, 2018

Zusammenfassung:

Recently, Jafarnejad, Franca, Pfister, and Schleiss [2017. Time-based failure analysis of compressed riverbank riprap. Journal of Hydraulic Research, 55(2), 224–235. doi:10.1080/00221686.2016.1212940] presented an experimental study on the stability of compressed riprap, which means that it is composed of individually placed blocks with a single layer as riverbank protection. The relationship between the time to failure and dimensionless bed shear stress was presented. Here, the authors complement the previous investigation with results regarding the effect of a second riprap layer. In total, 49 tests (28 tests with one and 21 tests with two riprap layers) were performed using a single block size, three longitudinal channel slopes and three different riprap bank inclinations. Under similar conditions, the second layer can significantly delay the time to failure, whereas the block erosion rate was found to increase. The second layer has a more stabilizing role when the bank angle of the riprap is approaching the angle of repose of the blocks.

Experimental repetitions and blockage of large stems at ogee crested spillways with piers

Scientifique ArODES

Michael Pfister, Paloma Furlan, Jorge Matos, Conceição Amado, Anton Schleiss

Journal of Hydraulic Research, 2018

Zusammenfassung:

Large wood is often transported by rivers into reservoirs during heavy rainfall events. When a critical section like a spillway is blocked and discharge capacity reduced, an uncontrolled increase of the reservoir water level may occur. This study aims to statistically analyse the importance of repetitions for the accuracy of experimental campaigns when studying blocking probabilities at ogee crested spillways equipped with piers. Systematic and reliable estimations based on physical models are critical for developing preventive measures against large wood blockage. Two statistical methods have been described and applied to calculate confidence intervals. A minimum number of repetitions for a maximum acceptable error is recommended for blocking probabilities. The minimum number of experimental repetitions has been statistically justified in accordance with a reasonable use of resources for experimental campaigns. In addition, a maximum acceptable level of error is proposed as a common metric of accuracy in large wood studies.

Experimental study of tsunami-like waves generated with a vertical release technique on dry and wet beds

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Ioan Nistor, Anton Schleiss

Journal of Waterway, Port, Coastal, and Ocean Engineering, 2018, 144, 4

Zusammenfassung:

Tsunamis, impulse waves, and dam failures are disasters that challenge humanity, often leading to massive casualties and extreme economic losses. The highly unsteady flow conditions generated by such events are often in the form of turbulent bores. The purpose of this study was to investigate and validate a new generation system for bores propagating over dry and wet bed conditions. There are multiple techniques to generate such waves experimentally, and the study focused on the generation of tsunami-like inundation conditions through the vertical release of a water volume. A detailed methodology to characterize the generated waves hydraulically, in terms of their wave heights and flow velocities, is presented, and good agreement with the classical dam-break case for both dry bed surges and wet bed bores was demonstrated. Because of the importance of estimating the impact forces induced by such waves, particular attention was given to the wavefront celerity and the velocity profiles measured behind the wavefront; these were found in agreement with Prandtl’s power law for open channel flows, and in-depth measurements allowed for the definition of an expression to estimate flow deceleration behind the wavefront. Along with considerations of the Froude number and momentum, this paper provides relevant information to assist engineers in designing safer infrastructures in areas prone to such extreme loading.

Experimental study on the hydrodynamic impact of tsunami-like waves against impervious free-standing buildings

Scientifique ArODES

Michael Pfister, Davide Wüthrich, Ioan Nistor, Anton Schleiss

Coastal Engineering Journal, 2018, 60, 2, pp. 180-199

Zusammenfassung:

2017

Bodenöffnungen :

Professionnel ArODES

Ergänzende Betrachtungen

Michael Pfister, Jean-Marc Ribi

Aqua & Gas, 2017, no. 11, pp. 46-51

Michael Pfister, David Dorthe

Zusammenfassung:

Die Zuverlässigkeit von Bodenöffnungen in Siedlungsentwässerungssystemen hängt stark von den Zuflussbedingungen ab. Physikalische Modellversuche zum Einfluss der Rohrneigung und einer exzentrischen Anströmung verfeinern bestehende Bemessungsregeln. Die Bedeutung einer störungsfreien und ausreichend langen Zuflussstrecke wird unterstrichen.

Piano Key Weir as overflow on sedimentation basin of wastewater treatment plant

Chapter ArODES

Dans Erpicum, Sébastien, Ho Ta Khanh, Michel, Laugier, Frédéric, Pfister, Michael, Labyrinth and Piano Key Weirs III – PKW 2017 ; Proceedings of the 3rd International Workshop on Labyrinth and Piano Key Weirs (PKW 2017), 22-24 February 2017, Qui Nhon, Vietnam (pp. 175-184). 2017, London : CRC Press

Zusammenfassung:

Numerical simulations documented in literature and experiences with driftwood blockage suggested that the flow field upstream of Piano Key Weirs (PKWs) includes an active zone reaching far down near of the weir. In other words, a pronounced vertical upwards-oriented flow active along the weir front presumably occurs, in contrast to a standard overfall. An extended flow field is related to relatively low velocities with a reduced particle transport capacity. This capacity is of interest for sedimentation basins (settling tanks) of wastewater treatment plants. If the particle re-suspension capacity is reduced under similar unit discharges, then either the basin volume can be lessened or the efficiency of the basin augments for the same volume. The paper presents preliminary numerical simulations of the flow velocity distribution in sedimentation basins near both, a sharp-crested weir and a PKW as overfall. Further, some preliminary physical model tests are presented, giving indications about the re-suspension potential of particles, as a function of their vicinity to the weir crest and the unit discharge.

Comparative analyses of phase-detective intrusive probes in high-velocity air–water flows

Scientifique ArODES

Michael Pfister, Stefan Felder

International Journal of Multiphase Flow, 2017, vol. 90, pp. 88-101

Zusammenfassung:

A comparative analysis of a wide range of air–water flow properties was conducted for two types of phase-detection intrusive probes including fiber-optical and conductivity probes. Experiments were conducted on a stepped spillway model for a skimming flow discharge q = 0.478 m2/s and for Re = 4.7 105 in a flow region just downstream of the inception point of free-surface aeration and in the fully developed flow region. The comparison of a large number of key air–water flow properties showed a very close agreement for the two sensor types including void fraction, interfacial velocity and equivalent clear water flow depth enabling a direct comparison of past and future data collected with either phase-detection probe type. Minor differences were observed in terms of chord sizes, clustered properties and interparticle arrival times linked with the slightly smaller sensor size of the fiber-optical probe. The in-line positioning of the leading and trailing tips of the fiber-optical probe affected the trailing tip properties resulting in elevated turbulence intensities. An optimum dual-tip phase-detection probe design should consist of small probe tips positioned side-by-side.

Probabilistic failure analysis of riprap as riverbank protection under flood uncertainties

Scientifique ArODES

Michael Pfister, Mona Jafarnejad, Eugen Brühwiler, Anton J. Schleiss

Stochastic Environmental Research and Risk Assessment, 2017, vol. 31, pp. 1839-1851

Zusammenfassung:

Existing riverbank riprap could face the risk of failure if the flood regime changes in future. Additionally, changed sediment transport in rivers, as a possible result of climate change, impacts the failure risk of flood protection measures. Evaluation of this potential failure is the primary issue of riprap stability and safety assessment. The consequences of the bank failure are probably uncontrolled erosion and flooding with disastrous consequences in residential areas or damage to infrastructures. Thus, a probabilistic analysis of riprap failure considering different mechanisms due to the flood and sediment transport uncertainties is required to assess embankment stability. In this article, the concept of a probabilistic assessment model based on Monte Carlo simulation method, moment analysis methods, and Rosenblueth point estimation method are presented to define the failure risk of riprap as the river bank protection. The probability of failure in different modes, namely direct block erosion, toe scouring and overtopping, has been defined by taking into account the river bed level variation based on bedload transport described with a probabilistic function of the peak discharge. The result of three models comparison revealed a good agreement (the average deviation of less than 2%) in estimation of riprap failure probability. This model is a strategical tool to search the critical river reaches and helps to evaluate the risk maps. So that, the model could cover the engineering aspect of environmental stability in the rivers with riprap as the bank protections.

2016

Effect of an abrupt slope change on air entrainment and flow depths at stepped spillways

Scientifique ArODES

Michael Pfister, Mohammad J. Ostad Mirza, Jorge Matos, Anton J. Schleiss

Journal of Hydraulic Research, 2017, vol. 55, no. 3, pp. 362-375

Zusammenfassung:

Several stepped spillways have been built in recent decades, mostly integrated in the downstream faces of roller compacted concrete dams. Among them, only a few have changing bottom slopes. Comprehensive information on the effect of a slope change on the flow features is lacking. This paper reports a systematic study of air entrainment and flow bulking in skimming flow along stepped spillways in the vicinity of abrupt slope changes. Physical modelling was conducted using a large facility with abrupt pseudo-bottom slope changes from 50° to 30° and from 50° to 18.6°. Air–water flow measurements were conducted at several flow cross-sections upstream and downstream of the slope change. The results indicate a significant influence of the slope change on the air pattern and flow bulking. Four main flow sub-regions were identified to describe the typical air–water flow patterns. The normalized length of the influence reach induced by the slope change was found to depend mainly on the critical flow depth, regardless of the slope change and step height.

Studies of two-phase flow at a chute aerator with experiments and CFD modelling

Scientifique ArODES

Michael Pfister, Penghua Teng, James Yang

Modelling and Simulation in Engineering, 2016, article no. 4729128

Zusammenfassung:

The chute aerator of a spillway is a structure in such a sense that air is, in the intense emulsification, entrained into the high-velocity water flow. Correctly predicting the air entrainment and two-phase flow pattern at the aerator would contribute to reliable spillway operation. Based on experimental data, 2D numerical simulations are preformed to predict streamwise air concentrations in the aerated flow, in which a two-fluid model is used. Depending on the air bubble size, relatively good agreement is seen with the experiments in the air cavity zone. The simulations give rise to higher air concentration downstream of the cavity, which is presumably due to underestimation of the interfacial forces in the two-fluid model.

Time-based failure analysis of compressed riverbank riprap

Scientifique ArODES

Michael Pfister, Mona Jafarnejad, Mario J. Franca, Anton J. Schleiss

Journal of Hydraulic Research, 2017, vol. 55, no. 2, pp. 224-235

Zusammenfassung:

Methods to design riprap-lined channels usually refer to dumped material. Large blocks placed individually by machinery are used when more stability is required. They offer additional resistance against flow erosion since space between blocks is minimized and interlocking increased. The behaviour of this protection has rarely been studied. An experimental investigation was carried out on the stability of compressed riprap as riverbank protection. Riprap was reproduced by uniform crushed limestones with three block sizes. Tests were conducted for three channel slopes under supercritical flow conditions and for constant bank slope. A time-based analysis allowed establishing relations among time to failure, friction velocity, and dimensionless bed shear stress. The results of 45 tests confirm that the rate of block erosion is significantly reduced with increase in the riprap diameter. The time to failure of the riprap protection depends strongly on the longitudinal slope and on the block sizes. An empirical prediction to estimate the riprap time to failure is shown.

Mobile riverbed scour downstream of a piano key weir

Scientifique ArODES

Michael Pfister, Stefan Jüstrich, Anton J. Schleiss

Journal of Hydraulic Engineering, 2016, vol. 142, no. 11, article no. 04016043-1

Zusammenfassung:

In parallel to their most frequent application as an auxiliary dam spillway, piano key weirs (PKWs) are also installed on low-head barrages in combination with run-off-the-river power plants. Their efficient rating curve, advantageous behavior under driftwood blockage, and the absence of mechanical elements are optimal for the hydraulic conditions associated with these run-off-the-river projects. However, in the absence of technical protection measures, scouring of the riverbed can occur at the downstream foundation. The unhindered scour formation and corresponding ridge generation caused by a PKW are investigated in this paper. Physical model tests were conducted under various conditions, providing a general estimation of the equilibrium scour and ridge dimensions. The maximum scour-hole depth dominates the shape of the hole and of the ridge. It was further shown that the maximum scour slope corresponds to the angle of repose of the sediments on the riverbed. Finally, a comparison of the present PKW scour data with predictions provided in the literature revealed that PKW-generated scour might be considered as jet-induced scour.

Approaches to reduce friction losses in headrace waterways of hydropower plants

Scientifique ArODES

Michael Pfister, Helena I. S. Nogueira, Anton J. Schleiss

Journal of Hydraulic Engineering, 2016, vol. 142, no. 5, article no. 02516001

Zusammenfassung:

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

2015

Das Klaviertastenwehr (PKW) als effizientes Einlaufbauwerk zur Erhöhung der Abflusskapazität bestehender Hochwasserentlastungsanlagen

Professionnel ArODES

Korrespondenz Wasserwirtschaft, 2015, vol. 8, no. 11, pp. 673-679

Zusammenfassung:

Freie Überfallwehre sind hydraulisch effizient und zuverlässig im Betrieb. Die Überfallkapazität hängt dabei direkt von der hydraulisch aktiven Abwicklungslänge des Wehrs ab. Gefaltete Wehrtypen wie das Klaviertastenwehr (PKW) stellen eine über proportionale Abwicklungslänge zur Veifügung und sind daher besonders wirkungsvoll, insbesondere für relativ geringe Energiehöhen. Die kompakte Bauweise des PKW erlaubt zudem dessen wirtschaftliche und schnelle Erstellung auf Kronen von Schwergewichtsstaumauern. Daher bietet sich das PKW an, wenn bei bestehenden Hochwasserentlastungsanlagen die Abflusskapazität zu erhöhen ist, oder sofern eine Hilfsentlastung unter beengten Platzverhältnissen anzuordnen ist. Die hydraulische Funktionsfähigkeit der PKW wurde mittels mehreren Modellstudien getestet und optimiert. Weiter sind inzwischen mehrere PKW gebaut worden und haben ihre Zuverlässigkeit unter Hochwasser bestätigt.

Debris-blocking sensitivity of piano key weirs under reservoir-type approach flow

Scientifique ArODES

Michael Pfister, Blake Tullis, Anton J. Schleiss

Journal of Hydraulic Engineering, 2015, vol. 141, no. 10, article no. 07015013

Zusammenfassung:

The collection of floating woody debris at flow control structures, such as spillways and weirs, can potentially result in reduced discharge efficiency (higher upstream head for a given weir discharge). Compared to less hydraulically-efficient control structures, piano key weirs have higher discharge efficiency (lower upstream heads for a given discharge), which may increase the likelihood of woody debris collection. A systematic laboratory study was conducted to evaluate the interaction between various piano key weir geometries and woody debris types and sizes. The results of individual (noncumulative) debris tests indicated that floating debris blockage probability is highly influenced by trunk diameter and upstream head. The effects of debris accumulation on the upstream head varied with the value of the debris-free reference upstream head condition. At lower upstream reference head values, the cumulative debris tests indicated a relative increase of the debris-associated upstream head of approximately 70%; higher upstream reference head values produced upstream head increases limited to approximately 20%.

Hydraulic behaviour of junction manholes under supercritical flow conditions

Scientifique ArODES

Michael Pfister, Corrado Gisonni

Journal of Hydraulic Research, 2015, vol. 53, no. 2, pp. 286-289

Wave-reducing stern flap on ship convoys to protect riverbanks

Scientifique ArODES

Michael Pfister, Robin Amacher, Théodora Cohen Liechti, Giovanni De Cesare, Anton J. Schleiss

Naval Engineers Journal, 2015, no. 127-1

Zusammenfassung:

Inland navigable waterways are significant in cargo transport and leisure activities. In parallel, these channels and rivers often suffer from the ship traffic, as the generated waves may damage waterway banks, along with their riparian fauna. As a consequence, speed limits have commonly been introduced. In some cases, adaptations on the vessel might be more appropriate as an alternative. This paper describes a flap that is mounted at the stern of a barge and is operated by a pusher tug. The barge is used to transport around 170 tons of waste per course from the City of Geneva (Switzerland) to an incineration plant. The optimum shape of the flap was derived from numerical and physical modeling, and its effect tested in situ. The latter indicated that, on site, the wave energy at 20 m distance to the convoy was reduced by half with the use of the flap.

Wave reducing stern flap on ship convoy to protect river banks

Scientifique

Robin Amacher, Theodora Cohen Liechti, Michael Pfister, Giovanni De Cesare, Anton Schleiss

Naval Engineers Journal, 2015 , vol. 127, no 1, pp. 95-102

2014

Trajectories and air flow features of ski jump-generated jets

Scientifique ArODES

Michael Pfister, Will H. Hager, Robert, M. Boes

Journal of Hydraulic Research, 2014, vol. 52, no. 3, pp. 336-346

Bruno Spahni, Fabienne Favre Boivin, Michael Pfister

Zusammenfassung:

Ski jumps are frequently applied as spillways of high dams. The resulting jet impact location on the plunge pool surface is often distant from the dam toe so that the latter is protected from scouring. Furthermore, the jet disintegrates and disperses prior to its impact, thereby reducing the specific energy addition to the plunge pool. The present research addresses four aspects, based on three physical modelling campaigns: (1) geometry of upper and lower jet trajectories; (2) virtual jet take-off angles for the trajectory computations; (3) average and minimum cross-sectional air concentrations along the jet; and (4) general jet air concentration profiles. It is shown that the trajectory parabola may also be applied for negative jet take-off angles, and that these are smaller than the bucket angle. As for the air concentration distribution along the jet, tests indicate that the latter depends exclusively on the relative jet black-water core length.

History and significance of the Morton number in hydraulic engineering

Scientifique

Michael Pfister, Willi H. Hager

Journal of Hydraulic Engineering, 2014 , vol. 140, no 5

Head losses in junction manholes for free surface flows in circular conduits

Scientifique

Michael Pfister, Corrado Gisonni

Journal of Hydraulic Engineering, 2014 , vol. 140, no 9

Two-phase air-water flows: Scale effects in physical modeling

Scientifique

Michael Pfister, Hubert Chanson

Journal of Hydrodynamics, 2014 , vol. 26, no 2, pp. 291-298

2013

Debris blocking sensitivity of Piano Key weirs under reservoir type approach flow

Scientifique

Michael Pfister, Damiano Capobianco, Blake Tullis, Anton Schleiss

Journal of Hydraulic Engineering, 2013 , vol. 139, no 11, pp. 1134-1141

2012

Hydraulic design of A-type Piano Key Weirs

Scientifique

Marcello Leite Ribeiro, Michael Pfister, Anton Schleiss, Jean-Louis Boillat

Journal of Hydraulic Research, 2012 , vol. 50, no 4, pp. 400-408

2022

Application du PKW comme déversoir sur un décanteur primaire d'une station d'épuration

Conference ArODES

Actes du Cinquième Séminaire Hydraulique des canalisations VSA, 6 septembre 2022, Fribourg, Suisse

Fabienne Favre Boivin, Michael Pfister, Santiago Sandoval

Zusammenfassung:

Les déversoirs à touche de piano (PKW) ont des propriétés hydrauliques qui peuvent être utilisées sur les décanteurs primaires des stations d’épuration des eaux usées (STEP). L'efficacité de leur courbe de tarage et leur influence sur l’efficacité de décantation sont particulièrement considérées dans cette étude. Les deux propriétés ne pouvant être déterminées de manière concluante à l'aide de simulations numériques ou de modèles réduits, des expériences in situ ont été réalisées sur la STEP de Vétroz-Conthey (VS). Ses conditions sont idéales dans la mesure où elle dispose notamment de deux bassins de décantation primaires identiques, équipés dans le cadre de cette étude respectivement d'un déversoir à paroi mince et d'un PKW. Quatre campagnes de mesures, d'une durée d'environ 14 jours chacune, ont été menées. Les débits sont déterminés par mesure de hauteur d’eau. Les concentrations en matières en suspension sont mesurées par turbidimétrie, en continu. Les charges massiques et les efficacités de traitement sont ensuite calculées en utilisant les débits et les concentrations de MES calculées. Le débit et la charge en MES ont été déterminés pour chaque bassin à l'entrée et à la sortie. En ce qui concerne les flux d'eau, la courbe de tarage du déversoir à paroi mince et du PKW installés au bout des deux bassins de décantation sont déduites. Le premier correspond au comportement connu, ce qui souligne la validité du concept de mesure, et le second confirme l'efficacité hydraulique du PKW attendue selon la littérature. Plus précisément, le déversement d'un PKW correspond environ 5.5 fois à celui d’un déversoir à paroi mince (PM) pour des charges basses de l'ordre de quelques millimètres, et environ 2.4 fois pour une charge d'un décimètre. Le dispositif expérimental de mesure des MES permet de retracer fidèlement le fonctionnement cyclique d’une station d’épuration. Du fait de la géométrie de la STEP, et malgré des mesures compensatoires prises pendant les campagnes, la voie munie d’un PKW a reçu une charge de MES supérieure à celle munie d’une paroi mince (différence de 9 et 24% pour les campagnes 3 et 4 respectivement). En ce qui concerne les débits, ils sont également légèrement supérieurs (2% pour les campagnes 3 et 4 respectivement) du côté du bassin équipé d’un PKW. L’efficacité de traitement calculée sur chaque campagne montre peu de différence (non significative) entre les deux déversoirs : en moyenne sur les deux campagnes, le PKW a une efficacité de 55% et le PM de 51%. Cependant, les deux décanteurs déversent des eaux dont les concentrations en MES sont similaires. Il semble donc que les différences observées proviennent de la charge en MES inférieure reçu par le PM. Une analyse des paramètres d’influence du fonctionnement des déversoirs (débit, concentrations en MES à l’entrée, charges de MES à l’entrée) ne montre pas de relation claire entre efficacité de traitement et charge de MES, débit, ou concentrations en MES à l’entrée. Le déversoir à touche de piano (PKW) peut être considéré et recommandé comme déversoir aval d'un bassin de décantation primaire. Son utilisation est particulièrement utile lors de l'adaptation des stations d'épuration existantes pour augmenter leurs capacités de traitement en offrant notamment un grand volume utile pour une même grandeur de bassin et en offrant une courbe de tarage plus favorable.

Comparaison des modèles hydrologiques afin d’estimer l'infiltration profonde dans les milieux urbains à l’échelle de la parcelle

Conference ArODES

Actes du Cinquième Séminaire Hydraulique des canalisations VSA, 6 septembre 2022, Fribourg, Suisse

Zusammenfassung:

Cette étude a pour objectif de comparer la performance de simulation des volumes d’eau d’infiltration profonde dans un bassin versant urbain (Source de Pilettes, Fribourg, Suisse) entre un modèle simplifié (méthode rationnelle) MR et l’outil SWMM. Les simulations sont également confrontées à des valeurs de débit mesurées au cours d’une période de 40 ans. Les écarts entre les valeurs d’infiltration annuelle simulées et observées sont d’environ 3.2 fois plus grands pour MR par rapport à SWMM. Néanmoins, tous les deux modèles présentent une estimation satisfaisante des volumes d’infiltration cumulés sur une période de 40 ans (écart de 1% pour MR et de 2% pour SWMM). La performance du modèle SWMM montre l’importance de la variabilité journalière de la pluie et l’évapotranspiration afin d’expliquer les dynamiques des eaux d’infiltration profonde à l’échelle annuelle. Les deux modèles reproduisent une décroissance des volumes d’infiltration profonde au cours des décennies, ce qui correspond aux données d’infiltration mesurées. Les hypothèses de modélisation établies suggèrent que cette décroissance de l’infiltration s’explique plutôt par la météorologie, c.à.d. par l’augmentation/réduction de l’évapotranspiration et/ou des volumes de pluie, au lieu de changements des caractéristiques physiques liés e.g. à la réduction de la couverture végétale.

Écoulements torrentiels dans les regards coudés

Conference ArODES

Michael Pfister, David Dorthe, Gaetano Crispino, Corrado Gisonni

Actes du Cinquième Séminaire Hydraulique des canalisations VSA, 6 septembre 2022, Fribourg, Suisse

Zusammenfassung:

Les regards coudés sont des éléments fréquents des réseaux d’assainissement urbain. La déviation de l'écoulement torrentiel génère des ondes de choc à l'intérieur du regard, pouvant frapper l'extrémité du regard ou générer un ressaut hydraulique. Le régime d'écoulement à surface libre peut alors s’interrompre brusquement et la mise en charge se propager sur le réseau et générer des débordements. Il est donc utile de connaître le débit maximal pouvant s’écouler en toute sécurité à travers un regard coudé en fonction de sa géométrie. Cet article présente une étude basée sur des simulations numériques effectuées pour évaluer le fonctionnement hydraulique de regards coudés en régime torrentiel avec différentes caractéristiques géométriques (angle de déviation, rayon de courbure, longueur d'extension droite en aval). Le modèle numérique a été préalablement validé avec des données provenant de tests sur modèles physiques documentés dans la littérature. Les simulations numériques ont permis de montrer que la capacité hydraulique d’un regard coudé était améliorée pour des rayons de courbure accrus et avec un regard prolongé en aval, alors que l'effet de l'angle de déviation est moins significatif. Une formule empirique a pu être établie pour évaluer la capacité d’évacuation du regard coudé en régime torrentiel, en fonction des paramètres géométriques déterminants, ainsi que du taux de remplissage de la conduite amont.

Effect of blocked driftwood on the hydraulic performance of a gated standard weir

Conference ArODES

Michael Pfister, Loïc Bénet, Giovanni De Cesare

Proceedings of the 39th IAHR World Congress, 19-254 June 2022, Granada, Spain

Zusammenfassung:

Driftwood belongs to riverine ecosystems and is stored and transported in every natural stream. Hydraulic structures built in streams typically alter the flow characteristics and might consequently get in conflict with driftwood. The latter can get trapped at limited cross-sections and then hinders the water to pass. The upstream water level subsequently raises, since the flow needs more energy to pass the obstacle. This might lead to inundations of upstream zones or to overtopping of dams. Both are not acceptable. The herein presented study addresses this issue, focusing on a standard weir with piers (to hold gates or flaps). Such a configuration is frequently used to regulate the flow on dam spillways. We have conducted systematic model tests supplying large driftwood volumes, varying the discharge and the bay width. The reduced discharge coefficient under driftwood impact was derived, allowing to determine the related reservoir level rise. Furthermore, three technical installations, denoted as countermeasures, were tested to avoid the observed discharge capacity limitation of a jammed weir. These measures included (i) overhanging piers (protruding into the reservoir), (ii) driftwood racks installed upstream of the weir, as well as (iii) the removal of the piers generating “wide” bays. The tests indicated that, under the herein tested conditions, all measures were highly efficient. The discharge coefficient remained typically at almost the free weir flow capacity (>90%), even under a high driftwood occurrence.

2020

Les modèles réduits des ouvrages particuliers dans les réseaux de canalisations

Conference ArODES

Michael Pfister, Jean-Marc Ribi

Actes du quatrième séminaire "Hydraulique des canalisations", 8 septembre 2020, Fribourg, Suisse

Zusammenfassung:

Le présent article définit le cadre d’utilisation de la modélisation physique dans le projet d’ouvrages du réseau de canalisations. Après l’énoncé des avantages et inconvénients des modèles physiques, on y trouve des explications sur le périmètre de la modélisation, les lois de similitude, les effets d'échelle, ceux du modèle. Un aperçu est donné sur les instruments de mesures les plus utilisés. Des estimations de coûts et de durée de construction et d’exploitation d’un modèle y sont proposées. Enfin, les exigences à mentionner au cahier des charges d'une étude sur modèle physique y sont spécifiées.

Sponge cities and the concept of green infrastructure :

Conference ArODES

an introduction

Géraldine Bullinger, Fabienne Favre Boivin, Michael Pfister, Sabine Chamoun

Actes du quatrième séminaire "Hydraulique des canalisations", 8 septembre 2020, Fribourg, Suisse

Zusammenfassung:

This article aims to introduce the concept of sponge cities and its multiple benefits, as frequently discussed in professional literature. Green infrastructure (GI) used to create a sponge city with a natural water storage is summarized. Specific concepts, applicability and technical characteristics are documented herein, all also applicable in the Swiss context.

Bottom pressure characteristics in a stilling basin downstream of a stepped spillway for two different chute slopes

Conference ArODES

Michael Pfister, Ivan Stojnic, Jorge Matos, Anton J. Schleiss

Proceedings of the 8th IAHR International Symposium on Hydraulic Structures ISHS2020, 12-15 May 2020, Santiago, Chile

Zusammenfassung:

In the last decades numerous stepped spillways were built, mostly on the downstream face of roller compacted concrete (RCC) dams. Stilling basins are often used as an energy dissipater. Although the hydraulics of stepped chutes was extensively investigated in the last decades, only fragmentary information is available on the hydraulic characteristics of stilling basins preceded by stepped chutes. Therefore, an experimental campaign was performed on a large-scale physical model of a stepped chute with adjustable slope terminating in a plain stilling basin, to study the effect of stepped chute approach flows on the hydraulic features in the basin. Experiments were conducted for two chute slopes and various discharges. Results on the basin flow features as tailwater depths, dynamic bottom pressures as well as roller and jump length characteristics are presented and discussed, focusing on the effect of stepped chute slope on the basin bottom pressure characteristics. The results show that increasing the chute slope from 30º to 50º pronounces the bottom mean, fluctuating and extreme pressures up to approximately one tailwater depth downstream of the jump toe. For 50° sloping stepped chutes, the extreme pressure coefficients reached up to about 3 times the values reported in literature for smooth chutes.

2019

Hydraulic jump downstream of a stepped chute :

Conference ArODES

an experimental study

Michael Pfister, Ivan Stojnic, Jorge Matos, Anton J. Schleiss

Proceedings of the 38th IAHR World Congress, 1-6 september 2019, Panama City, Panama

Zusammenfassung:

The number of stepped spillways significantly increased in the past decades. Among other reasons, this is due to their energy dissipation potential leading to a reduction of the residual energy at the toe of the spillway. To date, only fragmentary designed guidelines are available for the stilling basin design in conjunction with stepped chutes. Therefore, tests were carried out on a large-scale physical model of a 30° stepped chute in combination with a classical stilling basin, to study the effect of stepped chute inflow on the hydraulic jump features. Detailed air-water flowmeasurements of the inflow conditions were conducted, as well as of the free surface and dynamic bottom pressures along the hydraulic jump. Specifically results on the free surface, bottom pressure and length characteristics are presented and discussed herein.

Experimental and numerical study on wave-impact on buildings

Conference ArODES

Michael Pfister, Davide Wüthrich, Daisuke Nishiura, Shun Nomura, Mikito Furuichi

Proceedings of the 38th IAHR World Congress, 1-6 september 2019, Panama City, Panama

Zusammenfassung:

Unsteady flows such as tsunamis, impulse waves and dam-break waves can lead to damages and human losses. Hence, specific research to limit casualties and reconstruction costs is needed. The complexity of the phenomena involved suggests that a hybrid experimental-numerical approach should be used to gain a more comprehensive understanding of the process. This paper presents an explorative study on the comparison of SPH numerical simulations using a highly effective parallel computing technique with large scale experimental data for dry bed surges and wet bed bores impacting free-standing buildings with and without openings. These preliminary results showed a relatively good agreement between the two approaches in the estimation of water depths around the buildings, which represents a key parameter for the design of vertical shelters. Nevertheless, some differences observed during the impact phase may be attributed to the incapability of the SPH numerical simulations to fully capture the turbulent nature of the process and its air-entrainment. These validation tests with detailed experimental data are a promising approach to improve the numerical models toward the development of reliable numerical tools for a safer design of resilient structures.

Which phase-detection intrusive probe to use in high-velocity air-water flows?

Conference ArODES

Michael Pfister, Stefan Felder, Benjamin Hohermuth, Robert M. Boes

Proceedings of the 38th IAHR World Congress, 1-6 september 2019, Panama City, Panama

Zusammenfassung:

Air entrainment is a common feature of high-velocity free-surface flows. Air-water flow properties inside the flows are typically measured with phase-detection intrusive probes comprising fibre-optical and conductivity probes. Despite their common use, the most-suited phase-detection probe system for high-velocity flows is unknown. Novel comparative analyses of phase-detection probes were conducted in high-velocity air-water flows on a stepped spillway and in a tunnel chute. Experiments were performed with a side-by-side double-tip conductivity probe and several double-tip fibre-optical probes with different inline positioning of the probe tips as well as a fibre-optical side-by-side probe. Experiments were conducted for a range of flow conditions and flow velocities up to 15 m/s using the same acquisition system and post-processing software.While void fraction and interfacial velocities were in close agreement for the side-by-side probes in the tunnel chute and for the two probes used in the stepped spillway configuration, the comparison indicated that the probe tip geometry, especially the orientation of the probe tip to the main flow, can significantly affect the results. Several air-water flow properties showed differences including the bubble count rate, chord times and cluster properties linked with different tip sizes. The results indicate that the side-by-side phase-detection probe tip design has advantages in high-velocity air-water flows because the inline positioning affects the trailing tip data and the calculation of the interfacial velocity. The experiments highlight the need for a sturdy probe design for measurements in high-velocity flows, favoring the conductivity probe for such flow conditions and potential future employment in prototype-scale air-water flows.

Hydraulics of a physical model of supercritical vortex drop shaft

Conference ArODES

Michael Pfister, Gaetano Crispino, Jean-Marc Ribi, Corrado Gisonni

Proceedings of the 38th IAHR World Congress, 1-6 september 2019, Panama City, Panama

Zusammenfassung:

Vortex drop shafts are special drop structures typically installed in urban drainage systems or rarely as spillways for hydropower plants. The knowledge about the hydraulic behavior of vortex drop shafts was improved over the years but some gaps still have to be filled, with particular concern to the hydraulic aspects of the intake structure and of the vertical shaft. In this paper, an experimental campaign on a reduced-scaled physical model of vortex drop shaft serving an urban sewer system is described. Free-surface observations were used to characterize the hydraulics of the spiral inlet at the head of the shaft. A computational procedure based on the momentum approach is applied to derive velocity and flow rotation angle profiles over the vertical shaft. Such hydraulic features are fundamental into the design of a vortex drop shaft. If they are known with a sufficient reliability, then the energy dissipation efficiency of the shaft can be easily derived.

Numerical approach in the study of tsunami-like waves and comparison with experimental data

Conference ArODES

Michael Pfister, Daisuke Nishiura, Davide Wüthrich, Mikito Furuichi, Shun Nomura, Giovanni De Cesare

Proceedings of the 29th International Ocean and Polar Engineering Conference, 16-21 June 2019, Honolulu, Hawaii, USA

Zusammenfassung:

A comprehensive understanding of physical phenomena based on a hybrid experimental-numerical approach supports a safer design of critical infrastructures. However, for a large-scale tsunami-like waves, the reliability of numerical models was insufficiently validated. Herein, we validate our numerical model of tsunami-like waves by comparing simulation data (obtained using a smoothed particle hydrodynamics method based on a highly efficient parallel computing technique) with large-scale experimental data for both, dry bed surges and wet bed bores. The presented preliminary results are believed to aid the development of experimentally validated numerical tools for a better understanding of tsunami-like flows.

2018

Spillway blockage caused by large wood in reservoirs

Conference ArODES

Michael Pfister, Paloma Furlan, Jorge Matos, Anton Schleiss

Proceedings of the E3S Web of Conferences 40, 02037 (2018), River Flow 2018 - Ninth International Conference on Fluvial Hydraulics, Lyon-Villeurbanne, France, 5-8 September 2018

Zusammenfassung:

Large wood (LW) elements are often transported by rivers into reservoirs during heavy rainfall events. Large wood has important environmental attributes that benefit the diversification of riverine ecosystems. There are several studies dealing with the transport and behaviour of LW inside streams. However, during flood events, LW tends to create jams or blockages at diverse hydraulic structures inside streams, creating significant problems such as discharge limitations and increased water levels. Even though knowledge on the effect of LW at bridges in rivers with relatively high flow velocities starts to be available, the latter is hardly applicable for reservoir approach flow conditions. Understanding LW blockage processes at a reservoir spillway is essential regarding the safety evaluation of a dam and the surrounding areas. The geomorphologic benefits of wood for stream restoration depends also on our present ability to manage jams and the risk they imply for civil structures when blocked. Therefore, series of systematic laboratory experiments were conducted to analyse blockage of floating stems at an ogee crested spillway equipped with piers. Different LW characteristics were represented in a physical model with cylindrical stems. Results associate the size of stem groups to blockage probabilities and the effect blockages can have on the discharge capacity of a spillway.

Wave impact on oriented impervious buildings

Conference ArODES

Michael Pfister, Claudia Ylla Arbos, Davide Wüthrich, Anton Schleiss

Proceedings of 5th IAHR EUROPE CONGRESS – New challenges in hydraulic research and engineering, 12-14 June, Trento, Italy

Zusammenfassung:

Recent studies showed that a specific design can reduce the impact of water waves on structures, thus limiting the damages. Herein the impact of waves on buildings is addressed, pointing out the influence of orientation on the hydrodynamic process.

Influence of density of large stems on the blocking probability at spillways

Conference ArODES

Michael Pfister, Paloma Furlan, Jorge Matos, Anton Schleiss

Proceedings of the 7th International Symposium on Hydraulic Structures, Aachen, Germany, 15-18 May 2018

Zusammenfassung:

Dam safety is strongly linked to the probability of occurrence of large floods. Floods can transport large wood (LW) into reservoirs and towards water release structures as spillways. Due to blocking and clogging, LW may significantly influence the discharge capacity of spillways and thus result in dangerous rise of the water level in the reservoir. For a better assessment of the related risk, the behaviour of LW in contact with hydraulic structures has to be known. Thus the understanding of LW blockage processes at the spillway and the resulting water level rise in the reservoir is important for the safety evaluation of a dam. The aim of the present study is to describe how LW characteristics can influence blocking probabilities at a spillway inlet equipped with piers. By investigating the parameters linked to LW blockage like slenderness and density, or different hydraulic conditions and transport scenarios, it becomes possible to quantify the behaviour and consequences of LW interactions with spillways. Through systematic laboratory experiments, the influence of LW density on blocking probabilities of individual stems is analysed. Experiments were conducted for reservoir approach flow type, implying small magnitudes of reservoir flow velocity. The results were considered statistically as Bernoulli experiments and the methodology applied was a logistic regression. For the combinations explored, a relation between blocking probability and density, among other parameters, is studied.

2017

Entrapment of driftwood at ogee crested spillways with piers :

Conference ArODES

influence of woody debris characteristics on blocking probabilities

Michael Pfister, Paloma Furlan, Jorge Matos, Anton J. Schleiss

Actes du Colloque CFBR-SHF : «Hydraulique des barrages et des digues", 29-30 novembre 2017, Chambéry, France

Zusammenfassung:

Spillways are safety elements of dams that allow to release floods. They spill water to avoid overtopping with its potential structural damages at the dam and the downstream environment. Nevertheless it may be unsafe to assume that a flood only carries clear water. Large woody debris (LWD) are often transported by rivers into reservoirs during heavy rainfall events. There is still a lack of knowledge regarding the behaviour of LWD at spillway inlets. The accumulation and blockage of LWD at spillway inlets is a significant problem as it can change the load on the structure and also the functioning of the spillway by reducing the discharge capacity and increasing the reservoir water level. Once this point is reached, new conditions upstream are developed for the reservoir as head increase or enlargement of inundated areas. Literature provides mainly knowledge on the effect of LWD at bridges in rivers with relatively high flow velocities. However, information of the effects and consequences for reservoir approach flow conditions is generally unknown. Knowledge of the LWD blockage processes at a reservoir spillway may be vital regarding the safety evaluation of a dam. The present paper summarizes a series of laboratory experiments, where different LWD characteristics were related to blocking probabilities at an ogee crest spillway equipped with piers.

Hydraulic features of helycoidal flows in a vortex drop shaft

Conference ArODES

Michael Pfister, Gaetano Crispino, Corrado Gisonni

Proceedings of 14th IWA/IAHR International Conference on Urban Drainage, ICUD 2017, 10-15 September 2017, Prague, Czech Republic

Zusammenfassung:

Vortex drop shafts (VDSs) are special drop structures typically adopted for elevation differences larger than about 5 m (Gisonni and Hager 2012). In few cases (Volkart 1984; Bruschin and Mouchet 1985) they were utilized also to combine various sewer collectors. This uncommon concept was adopted to realize the new VDS operating in Cossonay, Switzerland. A physical model was opportunely built and tested to analyze the main hydraulic features of the entire structure. The present study provides the preliminary results focusing on the flow falling along the vertical shaft and entering the outlet structure, composed of a dissipation chamber and a tailwater tunnel.

Estimation of wave propagation velocity on a channel with smooth and rough bed

Conference ArODES

Michael Pfister, Davide Wüthrich, Ioan Nistor, Anton J. Schleiss

Proceedings of the 37th IAHR World Congress, 13-18 August 2017, Kuala Lumpur, Malaysia

Zusammenfassung:

Natural hazards such as tsunamis, impulse waves and dam-break waves are rare, but extremely destructive. In recent times, more importance was given to structures that could withstand such events, however, uncertainties still exist in the estimation of wave velocities. This project focuses on the estimation of wave front celerity in a laboratory environment for both smooth and rough beds and the results are successfully compared to previous studies and design codes. Based on the experimental data an expression for the wave celerity is presented and discussed. The influence of bed roughness on the propagation velocity is also investigated and a dependence on the bed friction is observed, pointing out the need for further studies.

Blocking probability of driftwood at ogee crest spillways with piers :

Conference ArODES

influence of woody debris characteristics

Michael Pfister, Paloma Furlan, Jorge Matos, Anton J. Schleiss

Proceedings of the 37th IAHR World Congress, 13-18 August 2017, Kuala Lumpur, Malaysia

Zusammenfassung:

Spillways are designed to evacuate floods in a safe way. They should safely release water in order to avoid dam overtopping with its related damages. Nevertheless, it may be dangerous to assume that a flood is only carrying “clear” water. Large woody debris (LWD) and sediments are often transported by rivers into reservoirs during heavy rainfall events. There is still a lack of knowledge regarding the behaviour of LWD at spillway inlets equipped with piers and gates. The accumulation and blockage of LWD at spillway inlets are a significant problem as they can reduce the discharge capacity of the spillway and consequently, an uncontrolled increase of the water level in the reservoir may occur. Literature provides mainly knowledge on the effect of LWD at bridges in rivers with relatively high flow velocities. However, the latter is hardly applicable for reservoir approach flow conditions. Knowledge of the LWD blockage processes at spillways is important regarding the safety assessment of a dam. The present paper summarizes a series of preliminary laboratory experiments, where the influences of different LWD characteristics are linked to blocking probabilities at an ogee crest spillway equipped with piers. The results highlight the influence of repeatability of events and density of LWD on blocking probabilities under different hydraulic conditions.

Discharge capacity of junction manholes with bottom drops or top offsets

Conference ArODES

Michael Pfister, Gaetano Crispino, Corrado Gisonni

Proceedings of the 37th IAHR World Congress, 13-18 August 2017, Kuala Lumpur, Malaysia

Zusammenfassung:

An adequate operation of combined sewer systems is related to the efficiency of sewer manholes. Sometimes, sewer manholes are designed to convey safely storm water discharges with a return period shorter than the infrastructure life time. In other cases, the manhole discharge capacity is sufficient but the flow structure established in the manhole leads to unacceptable free-surface flow conditions. This poor performance may easily occur in junction manholes, which merge two or more upstream branches into a single downstream collector. The hydraulic behavior of junction manholes under sub- and supercritical flows has been thoroughly studied in the past, mainly on an experimental basis. However, former studies are all referred to a standard junction manhole layout, with a constant diameter assigned to both up- and downstream branches along with a flat manhole invert. Contrarily, diversified set-ups are usually employed in practical cases. Upstream sewer branches are generally characterized by various cross-section profiles. Moreover, the good design practice recommends settling the concerned branches by aligning the branch tops, in order to avoid backwater effects. Given the above, advancement in design procedures for junction manholes is strongly required, mainly with regard to the discharge capacity of junctions characterized by generalized configurations. The present study considers the results of experimental campaigns performed on different physical models of junction manholes. A communal structure with a junction manhole approached by two circular upstream branches, with variable diameters, is considered. Then, junction angles and upstream branch diameters are varied, resulting in different manhole set-ups with inlet bottom drops or top offsets. Selected overload experimental runs are performed to investigate the hydraulic conditions under which the manhole discharge capacity is exceeded. The analysis of the experimental data allowed in outlining a preliminary comparison of discharge capacity of junctions with generalized layouts.

2016

Example of wave impact on a residential house

Conference ArODES

Michael Pfister, Davide Wüthrich, Anton J. Schleiss

Proceedings of 4th IAHR Europe Congress: Sustainable hydraulics in the era of global change, 27-29 July 2016, Liège, Belgium

Zusammenfassung:

Waves impacting against structures can create damages and devastation. This topic regained interest after some recent catastrophic events and the present paper investigates the main phases of a wave impacting against a residential house commonly observed in areas subject to tsunami hazards. The project is based on an experimental approach and both dry bed surges and wet bed bores were tested. Through some visual observations and high speed cameras, important run-up heights in the vertical direction were observed for all configurations; these were more intense for wet bed bores. For the largest waves a full overflow of the structure was observed and the presence of the structure provoked a change of regime and the upstream propagation of a bore.

Waves generated by ship convoy :

Conference ArODES

comparison of physical and numerical modeling with in-situ measurements

Michael Pfister, Théodora Cohen Liechti, Giovanni De Cesare, Anto J. Schleiss, Robin Amacher

Proceedings of the 4th IAHR European Congress, 27-29 July 2016, Liège, Belgium

Zusammenfassung:

A part of the domestic waste of the city of Geneva (Switzerland) is transported with ship convoys on the Rhone River to the waste incineration station. These convoys generate waves, which partially endangers the stability of the river banks and the riparian fauna. To reduce the dominant wave peaks, a flap was added at the stern of the barge. The efficiency of that flap was tested in physical and numerical model tests, and then compared to in-situ measurements. This case study focuses on a discussion of the appropriateness of the two models, by describing their accuracy for the present case. It indicates that the physical model reproduces the wave heights almost correctly, but does not re-produce adequately the dominant frequencies. In contrast, the numerical model damps the wave heights significantly, but gives correct dominant frequencies.

Piano key weir for enlargement of the west fork of Eno river reservoir

Conference ArODES

Michael Pfister, B. M. Crookston, L. Crowley

Proceedings of 6th IAHR International Symposium on Hydraulic Structures and Water System Management, 27-30 June 2016, Portland, OR, USA

Zusammenfassung:

The West Fork of Eno River Reservoir Dam is located near Hillsborough, North Carolina, USA. The earthen embankment dam provides an impoundment used by the town for water supply. The project was originally completed in 2000 with forethought by the owner for an expansion, which was anticipated in the original design. Currently, the design of that expansion to raise the reservoir level for increased water supply is underway. Following a detailed analysis of viable upgrades, a piano key (PK) weir (anticipated as the first to be constructed in the USA) was selected by the owner due to its effectiveness in meeting project requirements. Presented herein are the site-specific characteristics of the existing auxiliary chute spillway, the proposed PK weir, and additional modifications to the chute. This paper also presents site-specific information including the existing auxiliary chute spillway, the new PK weir, and additional modifications to the auxiliary chute. The results include the comparison of four PK weir hydraulic design methods for sizing the spillway. Details and results of a CFD model are also included. Finally, additional significant design considerations are presented, which include: anticipated hydraulic conditions in the downstream chute and identified existing structure deficiencies; estimated spillway design flood hydraulic force distributions taken from the CFD model; the PK key weir stability summary; and other important project aspects including embankment modifications and seepage control measures. It is anticipated that this project will be of interest to those concerned with dam safety and will further encourage the consideration of PK weirs in the USA.

Experimental study of large scale plunging jets

Conference ArODES

Michael Pfister, Grégory Guyot, Mathieu Rodriguez, Jean-Philippe Matas, Alain H. Cartellier

Proceedings of the 6th International Symposium on Hydraulic Structures, Hydraulic Structures and Water System Management, 27-30 June 2016, Portland, OR, USA

Zusammenfassung:

The lack of knowledge regarding air entrainment caused by large jets in hydraulic structures, such as downstream Pelton turbines, has led EDF (Electricité de France) to carry out a test series to optimize the La Coche Power plant. The data extracted from this work have been used in the frame of a research project. A first experiment, performed on a large jet impacting the free surface of an open channel, has confirmed that physical phenomena concerning the behavior of large scale plunging jets are still not well understoodThe main parameters that have been measured are: the penetration depth, the entrained air flow rate, the average bubble size below the free surface and the rising slope of the bottom boundary of the bubble plume. Globally, when comparing the data collected downstream the jet impact zone with the predictions proposed in the literature, the different relations are not scalable with the jet dimension. Moreover it appeared that these parameters are intimately linked to the jet state upstream the impact point. Consequently a second experimental set-up was designed to measure the dynamic pressure and the void fraction inside the jet. These results have shown that even an elementary parameter such as the breaking length is not well predicted by existing formula. Indeed, high frequency videos have proved that in certain cases the jet is still continuous while literature predicts a broken jet. In addition, the experimentations show that the jets are flapping during the fall and thus impact the free surface over an enlarged region.

Junction chamber at vortex drop shaft :

Conference ArODES

case study of Cossonay

Michael Pfister, David Dorthe, Gaetano Crispino, Thierry Fuchsmann, Corrado Gisonni

Proceedings of the 6th International Symposium on Hydraulic Structures, Hydraulic Structures and Water System Management, 27-30 June 2016, Portland, OR, USA

Zusammenfassung:

The drainage network of the city of Cossonay (Switzerland) is currently being adapted for future needs. In particular, it is required to drain increased storm discharges due to a population augmentation, and to provide an adequate concept to overcome unfavorable geotechnical conditions. Vortex drop shafts are sewer manholes commonly applied in steep urbanized topographies to connect conduits across large elevation differences. In Cossonay, the existing 48 m high vortex drop shaft, with a diameter of 1.5 m, allowed the storm discharge to flow from the city to a watercourse issued at half of the valley height. The discharge capacity was initially assumed as 4.1 m3/s, but frequent pulsations and choking phenomena implied a reduced effective capacity of around 3.0 m3/s. A new planned vortex drop shaft will collect the supercritical inflows of four collectors in the old City Centre and spill them through a shaft of roughly 120 m height, restituting the flow at the valley bottom. It was pre-designed using FLOW-3D simulations to estimate the hydraulic features of the incoming flows and to predict the hydraulic behavior of the upper elements (before the water enters the shaft). The simulation thus included a novel junction chamber type and the steep inlet channel before the spiral intake. The numerical simulations provided a first layout of the structure that was then validated by physical model tests. The physical model was built at the Laboratory of Hydraulic Constructions (LCH) of École Polytechnique Fédérale de Lausanne (EPFL).

Effect of deflector aerator on stepped spillway flow

Conference ArODES

Michael Pfister, Stéphane Terrier, Anton J. Schleiss

Proceedings of the 6th International Symposium on Hydraulic Structures, Hydraulic Structures and Water System Management, 27-30 June 2016, Portland, OR, USA

Zusammenfassung:

Systematic physical model tests are performed on a stepped spillway equipped with a bottom aerator at the beginning of the stepped part. A deflector is used to issue a jet in order to initiate air entrainment into the flow. A horizontal slot located in the vertical face of the first step allows for air supply underneath the flow. The cavity subpressure was measured to ensure optimal aerator performance, namely atmospheric pressure conditions. The air discharge entrained below the jet is measured to derive the aerator air entrainment coefficient. The local air concentrations are spatially measured downstream of the aerator at regularly spaced profiles, allowing the investigation of air transport and detrainment as well as the average and bottom air concentrations. The present paper focuses on the resulting spatial distribution of air concentration for five deflector geometries. The chute angle, step height, approach flow Froude number, and approach flow depth were kept constant so that the differences occur mostly on the jet length and air entrainment coefficient. The flow depth and the air concentration rapidly converge towards quasi-uniform flow values downstream of the aerator.

2015

Head losses in sewer junction

Conference ArODES

Michael Pfister, Corrado Gisonni

E-Proceedings of the 36th IAHR World Congress, 28 June- 3 July 2015, The Hague, the Netherlands

Zusammenfassung:

Abstract: The functionality of sewer networks is strongly affected by the correct operation of their appurtenances; the dendritic structure of urban drainage systems implies that junction manholes represent a crucial hydraulic structure, allowing two conduits merging into one. Hydraulic features of combining flows become quite complex when supercritical flows are involved, as in the case of steep urban context, with consequent formation of shockwaves and surging phenomena. Former studies conducted by Gisonni and Hager resulted in an optimized layout of sewer junctions operated under supercritical approach flow conditions. Recently, an extensive experimental campaign was performed on a physical model with generalized geometrical conditions, including various conduit diameters. Furthermore, physical model tests have been used to implement and validate a numerical model, aiming to explore a wider range of junction angles, which were limited to 45°and 90°for the physical model. In particular, the numerical model focused on the flow condition where both approach flows are supercritical. Based on the dataset constituted from both physical and numerical model results, comprehensive equations are proposed for the prediction of energy losses at junction manholes with different upstream and lateral conduit diameters, with particular reference to supercritical combining flows.

Air entrainment and pressure development in skimming flow on an abrupt slope change on stepped spillways

Conference ArODES

Michael Pfister, Mohammad Javad Ostad Mirza, Jorge Matos, Anton J. Schleiss

E-Proceedings of the 36th IAHR World Congress, 28 June- 3 July 2015, The Hague, the Netherlands

Zusammenfassung:

Numerous stepped spillways were built to date, namely on RCC gravity dams or on valley flanks besides embankment or rockfill dams, while some others are currently under design or construction. In some cases, slope changes may be required on stepped chutes, due to the topography, leading to reduced excavation and related economic benefits. A quite limited number of stepped spillways have been built with slope changes, whereas no systematic scientific investigation for designing such type of configuration has been conducted to date. Thus, there is a lack of information with regards to the main flow features along abrupt slope changes on stepped spillways. The present study is focussed on an experimental investigation of the air entrainment and pressure development on skimming flow on stepped spillways, in the vicinity of an abrupt slope change. Physical modelling was conducted in a relatively large scale facility, where detailed air-water flow measurements were gathered upstream and downstream of a 50˚-30˚slope change. In addition, dynamic pressure measurements were obtained on both vertical and horizontal faces of several steps in the vicinity of the slope change. The results on the air entrainment and pressure development are presented and discussed. A substantial influence was observed on the air entrainment and pressure development pattern, in comparison with typical results for constant sloping stepped spillway flows.

Comparison of chute aerator effect on stepped and smooth spillways

Conference ArODES

Michael Pfister, Stéphane Terrier, Anton J. Schleiss

E-Proceedings of the 36th IAHR World Congress, 28 June- 3 July 2015, The Hague, the Netherlands

Zusammenfassung:

Bottom chute aerators are installed to prevent cavitation damages and they have been studied in detail on smooth spillways. In parallel, stepped spillways became widespread in the past decades. Research has shown that stepped spillways may be endangered even more by cavitation than smooth spillways, particularly for high unit discharges. As a consequence and besides issues of energy dissipation, the unit discharge of stepped spillways is usually limited to lower values than on smooth spillways. In order to overcome that limitation, flow aeration–mainly at the beginning of the chute–is necessary. Until now only fragmentary guidelines exists for the design of such aerators. Systematic tests with bottom chute aerators on stepped spillway are performed on a physical model. A deflector is used to separate the jet from the bottom in order to produce slight negative air pressures. A horizontal slot located in the vertical face of the first step allows for air supply underneath the flow. In addition of the global air entrainment by the aerator, the local air concentrations are spatially measured downstream of the deflector. The resulting air concentration distribution allows the investigation of air transport and detrainment as well as the streamwise average and bottom air concentration. The present paper discusses a test with a typical aerator and compares it with a reference test without aerator and a test on a smooth chute with a similar aerator.

Labyrinth weirs :

Conference ArODES

developments until 1985

Michael Pfister, Willi H. Hager, Blake P. Tullis

E-Proceedings of the 36th IAHR World Congress, 28 June- 3 July 2015, The Hague, the Netherlands

Zusammenfassung:

The weir is a fundamental structure in hydraulic engineering, serving to retain a water body, to control a water level, facilitate flow diversion, or to measure discharge. Under particular site conditions, the cross-sectional width at the weir location is limited so that either higher overflow depths or a compressed weir expansion are set. A form of the latter arrangement is the so-called labyrinth weir, which is composed of rectangular, trapezoidal or triangular plan shaped weirs, so that the geometrical crest length is increased. Along with the recently developed Piano Key Weir, labyrinth weirs represent economically and hydraulically sound alternative for increasing spillway discharge capacity. The present paper describes their historical development, reviews the main advances until the 1980s, summarizes current design guidelines, and presents the main individuals having participating in their development.

Discharge capacity of PK-weirs considering floating wooden debris

Conference ArODES

Michael Pfister, Anton J. Schleiss

Proceedings of 25th ICOLD Congress on Large Dams, 14-19 June 2015, Stavanger, Norway