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Pfister Michael

Assoziierter Professor FH

Compétences principales

Assoziierter Professor FH

Téléphone: +41 26 429 69 22

Bureau: HEIA_D40.07

Haute école d'ingénierie et d'architecture de Fribourg
Boulevard de Pérolles 80, 1700 Fribourg, CH

Institut
iTEC - Institut des technologies de l'environnement construit

BSc HES-SO en Génie civil - Haute école d'ingénierie et d'architecture de Fribourg

Hydraulique Générale
Hydraulique Urbaine

BSc HES-SO en Géomatique - Haute école d'Ingénierie et de Gestion du Canton de Vaud

ResEaux

MSc HES-SO en Engineering - HES-SO Master

Hydraulique des barrages

Section Genie Civil - EPFL

Urban Hydraulic Systems

2024

Air–water flows

Article scientifique ArODES

Daniel Valero, Stefan Felder, Matthias Kramer, Hang Wang, José M. Carrillo, Michael Pfister, Daniel B. Bung

Journal of Hydraulic Research, 2024, 62, 4, 319-339

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Résumé:

High Froude-number open-channel flows can entrain significant volumes of air, a phenomenon that occurs continuously in spillways, in free-falling jets and in hydraulic jumps, or as localized events, notably at the toe of hydraulic jumps or in plunging jets. Within these flows, turbulence generates millions of bubbles and droplets as well as highly distorted wavy air–water interfaces. This phenomenon is crucial from a design perspective, as it influences the behaviour of high-velocity flows, potentially impairing the safety of dam operations. This review examines recent scientific and engineering progress, highlighting foundational studies and emerging developments. Notable advances have been achieved in the past decades through improved sampling of flows and the development of physics-based models. Current challenges are also identified for instrumentation, numerical modelling and (up)scaling that hinder the formulation of fundamental theories, which are instrumental for improving predictive models, able to offer robust support for the design of large hydraulic structures at prototype scale.

Améliorer la gestion des éclusées thermiques :

Article professionnel ArODES

modélisation de la température d'un cours d'eau régulé

David Dorthe, Michael Pfister, Stuart N. Lane

bulletin.ch = Fachzeitschrift und Verbandsinformationen von Electrosuisse und VSE = Bulletin SEV/AES : revue spécialisée et informations des associations Electrosuisse et AES, 2024, 4, 44-47

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Résumé:

Les éclusées impactent non seulement le débit des cours d’eau en aval des centrales, mais aussi leur température. La modélisation de cette dernière peut donc constituer un outil d’aide à la décision pour la gestion actuelle et future des cours d’eau. Une étude a désormais montré qu’une telle modélisation était à même d’atteindre un haut niveau de fidélité.

Discussion of “design considerations for high-speed flow in sewer systems”

Article scientifique ArODES

Gaetano Crispino, Michael Pfister, Corrado Gisonni, Willi H. Hager

Journal of Hydraulic Engineering, 150, 1, 07023003

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2023

Hydraulic capacity of bend manholes for supercritical flow

Article scientifique ArODES

Gaetano Crispino, David Dorthe, Corrado Gisonni, Michael Pfister

Journal of Irrigation and Drainage Engineering, 2023, vol. 149, no. 2, pp. 04022048-1-9

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Résumé:

Sewer bend manholes are frequent elements of urban drainage systems. Any deviation of straight-lined supercritical flow, as within the manhole, generates shock waves, possibly impinging at the manhole end or imposing a hydraulic jump. Then the free-surface flow regime abruptly breaks down and backwater effects occur. Thus it is important to know the maximum discharge that safely can pass across a bend manhole, as a function of its geometry, without generating collapsing flow. This study conducted calibrated numerical simulations to assess the hydraulic features of supercritical bend manholes with variable deflection angles, curvature radii, and lengths of straight downstream extension elements. The numerical model was validated previously with data from analogous physical model tests documented in the literature. The combined data from the numerical simulation and from the physical model indicated a hydraulic capacity of the bend manholes for different geometrical setups. It was demonstrated that the hydraulic capacity of a bend manhole increases with increased curvature radii and straight extension lengths, whereas the effect of the deflection angle is less significant. A multiple regression technique provided an empirical equation indicating the normalized discharge capacity of supercritical bend manholes as a function of the governing geometrical parameters, along with the approach flow filling ratio.

2022

Force imposée sur un râtelier en amont d'un évacuateur de crue par des bois flottants, et concept pour favoriser le passage des bois flottants

Article professionnel ArODES

Michael Pfister, Loïc Bénet, Giovanni De Cesare

Wasser Energie Luft = Eau énergie air = Acqua energia aria, 2022, vol. 114, no. 4, pp. pp. 249-258

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Résumé:

L’obstruction de l’évacuateur de crue d’un barrage par des bois flottants peut réduire sa capacité hydraulique. Pour limiter cet effet, un râtelier peut être installé pour retenir les bois en amont de la section critique d’écoulement. Sur la base d’une modélisation physique, la force appliquée par les bois contre des râteliers a été mesurée. Deux types de râteliers (complet et partiel) et plusieurs positions ont été testées dans des conditions extrêmes (débits et volume de bois). La force maximale peut être exprimée sous forme d’un coefficient de trainée. Jusqu’à présent, les bois flottants bloqués doivent être enlevés après les crues. Leur contribution écologique est absente dans le cours d’eau aval. Un râtelier partiel a été développé afin de favoriser le passage des bois en maintenant la capacité hydraulique. Plusieurs apparitions de bois flottants (du bois individuel au volume extrême) sont étudiées ainsi que le positionnement du râtelier. Les expériences montrent que le passage des bois individuels est augmenté avec un râtelier partiel pour des conditions bien définies.

Plain stilling basin performance below 30° and 50° inclined smooth and stepped chutes

Article scientifique ArODES

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

Water, 2022, vol. 14, no. 23, article no. 3976

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Résumé:

Energy dissipators, such as stilling basins, are usually required at the toe of stepped chutes to achieve adequate and safe operation of the spillway. Stepped chute hydraulics has been extensively studied in last several decades, however, only limited knowledge is available on the stilling basin performance below stepped chutes. In particular, the effect of the chute slope remains unknown, despite being a central design issue. Therefore, an experimental campaign was performed using a 30° or 50° inclined smooth or stepped chute with an adjacent conventional plain stilling basin. The experimental results indicated that, within the stilling basin, the surface characteristics and the roller as well as hydraulic jump lengths are practically independent of the chute slope. This further strengthens the previous findings that stepped chutes require 17% longer dimensionless jump lengths and consequently stilling basin lengths. The experimental results also confirmed that stepped chutes generated increased extreme and fluctuating bottom pressure characteristics at the stilling basin entrance area. With increasing chute slope, the latter were found to significantly magnify. However, such increased magnitudes were not expected to provoke cavitation damage as stepped chute inflows induced bottom aeration at the basin entrance, irrespective of the chute slope.

Air–water flow in a plain stilling basin below smooth and stepped chutes

Article scientifique ArODES

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

Journal of Hydraulic Research, 2023, vol. 61, no. 1, pp. 51-66

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Résumé:

In recent decades, stepped chutes followed by a stilling basin became a standard for spillways of dams. The comprehensive knowledge of stepped chute approach flows on internal air–water flow properties of the hydraulic jump is still too limited for an appropriate design of stilling basins. Therefore, an experimental campaign was performed on a large-scale physical model of a plain stilling basin preceded by a 30° sloping smooth or stepped chute. Stepped chute approach flows induce lower deaeration rates along the jump roller as compared to smooth chute approach flows, indicating longer dimensionless jump lengths, normalized by the tailwater depth. Pronounced bottom air concentrations were observed within the first 40% of the jump length in the stilling basin downstream of the stepped chute. Thus, despite the significantly higher pressure fluctuations, this zone seems better protected against cavitation damage than in stilling basins following a smooth chute.

Hydraulique des canalisations :

Livre ArODES

cinquième séminaire

Michael Pfister

2022, Glattbrugg, Suisse : VSA, 143 p.

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Partial driftwood rack at gated ogee crest :

Article scientifique ArODES

trapping rate and discharge efficiency

Loïc Bénet, Giovanni De Cesare, Michael Pfister

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

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Résumé:

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

Article scientifique ArODES

Stéphane Terrier, Michael Pfister, Anton J. Schleiss

Water, 2022, vol. 14, no. 153

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Résumé:

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

Article scientifique ArODES

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

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

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Résumé:

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

Article scientifique ArODES

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

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

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Résumé:

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 :

Article scientifique ArODES

blocking sensitivity of labyrinth weirs in channel and reservoir applications

Taylor Vaughn, Brian M. Crookston, Michael Pfister

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

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Résumé:

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 ? :

Article scientifique ArODES

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

Nathan Bongard, Luca Rossi, Michael Pfister

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

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Résumé:

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

Article scientifique ArODES

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

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

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Résumé:

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.

Reservoir level rise under extreme driftwood blockage at ogee crest

Article scientifique ArODES

Loïc Bénet, Giovanni De Cesare, Michael Pfister

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

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Résumé:

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.

Bemessungsbeispiele von Tosbecken unterhalb von Treppenschussrinnen

Article scientifique ArODES

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

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

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Résumé:

Advances of roller compacted concrete (RCC) in the construction of concrete dams has significantly influenced the development of stepped spillways. The latter are now more frequently implemented in the rocky abutments of embankment dams. The energy dissipation downstream of stepped chutes is mostly ensured by stilling basins. Such stilling basins are typically designed according to guidelines developed for basins below smooth chutes. A systematic experimental research study revealed that these design guidelines are not applicable for stilling basins with stepped chute approach flows. The aim of this paper is thus to present a practical design guideline for stilling basins downstream of stepped chutes based on two design examples that highlight the significant differences with smooth approach flows.

2020

Hydraulic engineering of Dams

Livre ArODES

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

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

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Résumé:

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.

Bemessung von klassischen Tosbecken unterhalb von Treppenschussrinnen

Article professionnel ArODES

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

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

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Résumé:

Advances in dam construction techniques with roller compacted concrete (RCC) significantly increased the number of constructed stepped spillways. Stilling basins are often implemented as an energy dissipator at their end. Although stepped chute hydraulics was extensively investigated in the last decades, only fragmentary design information is currently available for stilling basins preceded by stepped chutes. Therefore, an extensive experimental campaign was performed to study the hydraulic behavior of classical stilling basins downstream of stepped chute. The results of this study indicate that design guidelines for stilling basins developed for smooth chute spillways, and currently used in engineering practice for stepped spillways, are not applicable for stepped chute approach flows. The aim of this paper is thus to present new practical design guidelines for such stilling basins.

Hydraulique des canalisations :

Livre ArODES

quatrième séminaire

Michael Pfister

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

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

Article scientifique ArODES

Davide Wüthrich, Michael Pfister, Anton J. Schleiss

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

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Résumé:

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

Article professionnel ArODES

Michael Pfister, Loïc Bénet, Giovanni De Cesare

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

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Résumé:

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

Article professionnel ArODES

Michael Pfister

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

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Hydraulic design of classical stilling basins downstream of stepped chutes

Article professionnel ArODES

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

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

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Résumé:

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

Article scientifique ArODES

Davide Wüthrich, Michael Pfister, Anton J. Schleiss

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

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Résumé:

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 :

Article professionnel ArODES

documentation technique de la norme SIA 190:2017

Jean-Louis Boillat, Michael Pfister

SIA D 0264 : SIA 190,

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Résumé:

Les réseaux d’égouts sont généralement de type ramifié, à fonctionnement gravitaire. Afin d’éviter les instabilités hydrauliques, leur dimensionnement impose un écoulement à surface libre, capable de maintenir une circulation d’air sans mise en charge de la conduite. Ces réseaux sont caractérisés par la présence de nombreuses singularités, telles que coudes et raccordements, dont la réalisation est souvent standardisée, mais aussi d’ouvrages particuliers nécessitant un dimensionnement ad hoc, tels que jonctions, puits de chute ou siphons. Le présent document passe en revue les méthodes classiques du dimensionnement hydraulique des canalisations, dans le respect de la norme SIA 190:2017. Il se concentre en particulier sur les conditions d’écoulement susceptibles de réduire la capacité théorique d’une canalisation, liées en particulier à la turbulence de l’écoulement et à l’entraînement d’air. Les bases essentielles, décrites dans ce document, ne constituent cependant pas une référence unique à l’application de la norme SIA 190:2017. La littérature spécialisée doit également être consultée pour la résolution de cas particuliers.

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

Article scientifique ArODES

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

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

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Résumé:

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

Article scientifique ArODES

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

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

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Résumé:

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

Chapitre de livre ArODES

Paloma Furlan, Michael Pfister, 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

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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

Article scientifique ArODES

Gaetano Crispino, Corrado Gisonni, Michael Pfister

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

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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

Article scientifique ArODES

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

Environmental Fluid Mechanics,

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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

Article scientifique ArODES

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

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

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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

Article scientifique ArODES

Davide Wüthrich, Michael Pfister, Anton J. Schleiss

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

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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

Article scientifique ArODES

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

Journal of Hydraulic Research,

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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

Article scientifique ArODES

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

Journal of Hydraulic Engineering, 2018, 144, 11

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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.

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

Article scientifique ArODES

Gaetano Crispino, Michael Pfister, Corrado Gisonni

Journal of Hydraulic Research, 2018

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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.

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

Article scientifique ArODES

Davide Wüthrich, Ioan Nistor, Anton Schleiss, Michael Pfister

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

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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.

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

Article scientifique ArODES

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

Journal of Hydraulic Research, 2018

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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

Article scientifique ArODES

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

Journal of Hydraulic Research, 2018

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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

Article scientifique ArODES

Davide Wüthrich, Michael Pfister, Ioan Nistor, Anton Schleiss

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

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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

Article scientifique ArODES

Davide Wüthrich, Michael Pfister, Ioan Nistor, Anton Schleiss

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

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2017

Bodenöffnungen :

Article professionnel ArODES

Ergänzende Betrachtungen

Michael Pfister, Jean-Marc Ribi

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

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Résumé:

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.

Hydraulique des canalisations :

Livre ArODES

troisième séminaire

Michael Pfister

2017, Fribourg : HEIA-FR, xiii, 135 p.

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

Chapitre de livre ArODES

David Dorthe, Michael Pfister

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

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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

Article scientifique ArODES

Stefan Felder, Michael Pfister

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

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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

Article scientifique ArODES

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

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

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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

Article scientifique ArODES

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

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

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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.

Time-based failure analysis of compressed riverbank riprap

Article scientifique ArODES

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

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

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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.

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

Article scientifique ArODES

Penghua Teng, James Yang, Michael Pfister

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

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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.

Mobile riverbed scour downstream of a piano key weir

Article scientifique ArODES

Stefan Jüstrich, Michael Pfister, Anton J. Schleiss

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

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Résumé:

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

Article scientifique ArODES

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

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

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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

Article professionnel ArODES

Michael Pfister

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

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Free overfall weirs are hydraulically efficient and reliable in operation. With this, the overfall capacity depends on the hydraulically active developed length of the weir. Folded types of weir, such as the piano key weir (PKW), provide an over-proportionally developed length and are thus especially efficient, in particular for relatively small energy heads. The compact construction of the PKW in addition allows its economic and rapid creation on crest of gravity dams. Therefore, the PKW lends itself if and when the discharge capacity of existing flood spillways is to be increased or in case a subsidiary overflow is to be emplaced under cramped space conditions. The hydraulic functional capability of the PKW has been tested and optimised by means of several model studios. In addition, several PKW have been built and have confirmed their reliability under flood conditions.

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

Article scientifique ArODES

Michael Pfister, Blake Tullis, Anton J. Schleiss

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

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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%.

Hydraulique des canalisations :

Livre ArODES

deuxième séminaire

Michael Pfister

2015, Fribourg : HEIA-FR, xiv, 121 p.

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Hydraulic behaviour of junction manholes under supercritical flow conditions

Article scientifique ArODES

Corrado Gisonni, Michael Pfister

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

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Wave-reducing stern flap on ship convoys to protect riverbanks

Article scientifique ArODES

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

Naval Engineers Journal, 2015, no. 127-1

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Résumé:

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

Article scientifique

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

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

2014

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

Article scientifique ArODES

Michael Pfister, Corrado Gisonni

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

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Résumé:

Former studies on combining flows resulted in an efficient layout of sewer junctions operated under supercritical approach flow conditions. Straight extensions allowed a reduction in the shock wave heights generated by the merging flows, so that the global discharge capacity was significantly increased. Herein, an extensive experimental campaign is presented on a physical model with the aforementioned layout, although with generalized geometrical conditions now including various conduit diameters. The effects of the main parameters governing the energy losses for combining flows were ascertained to enhance the information available from the literature. The results and their analysis provide a basis for the prediction of energy losses at junction manholes with different upstream and lateral conduit diameters and various flow conditions.

Trajectories and air flow features of ski jump-generated jets

Article scientifique ArODES

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

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

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Résumé:

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.

Two-phase air-water flows :

Article scientifique ArODES

scale effects in physical modeling

Michael Pfister, Hubert Chanson

Journal of Hydrodynamics, 2014, vol. 26, pp. 291-298

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Résumé:

Physical modeling represents probably the oldest design tool in hydraulic engineering together with analytical approaches. In free surface flows, the similitude based upon a Froude similarity allows for a correct representation of the dominant forces, namely gravity and inertia. As a result fluid flow properties such as the capillary forces and the viscous forces might be incorrectly reproduced, affecting the air entrainment and transport capacity of a high-speed model flow. Small physical models operating under a Froude similitude systematically underestimate the air entrainment rate and air-water interfacial properties. To limit scale effects, minimal values of Reynolds or Weber number have to be respected. The present article summarizes the physical background of such limitations and their combination in terms of the Morton number. Based upon a literature review, the existing limits are presented and discussed, resulting in a series of more conservative recommendations in terms of air concentration scaling. For other air-water flow parameters, the selection of the criteria to assess scale effects is critical because some parameters (e.g., bubble sizes, turbulent scales) can be affected by scale effects, even in relatively large laboratory models.

History and significance of the Morton number in hydraulic engineering

Article scientifique

Pfister Michael, Willi H. Hager

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

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

Article scientifique

Pfister Michael, Corrado Gisonni

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

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

Article scientifique

Pfister Michael, Hubert Chanson

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

2013

Hydraulique des canalisations :

Livre ArODES

séminaire VSA/EPFL

Michael Pfister

2013, Fribourg : HEIA-FR, xii, 136 p.

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

Article scientifique

Pfister Michael, 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

Article scientifique

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

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

2024

Modelling of the thermodynamic regime downstream of Rossens dam during two floods

Conférence ArODES

David Dorthe, Michael Pfister, Stuart Lane

Proceedings of the 10th International Symposium on Hydraulic Structures (ISHS), Zurich, 17-19 June 2024, Zurich, Switzerland

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Résumé:

Floods are of critical importance for the safety of a dam and its hydraulic structures. Besides, floods play a central role for the ecosystem downstream of a dam. If the powerhouse is located run-off-the-river, then a residual flow reach occurs. The latter is eighter subjected to rather small residual discharges or pronounced floods, both representing extremes of a natural discharge regime. The binary regime of floods versus residual flow as typically occurring downstream of dams’ influences, among others, the streambed exchanges, stream temperature, food availability for the aquatic species, bedload transport and channel morphology of the subsequent river. Floods have been increasingly used during the past as a mitigation measure on such regulated rivers. Monitoring river parameters during artificial floods helps understand their influence and utility. In such a context, river temperatures were measured within the residual flow reach of the Sarine River downstream of Rossens dam (Switzerland) during two floods in October 2020 and May 2022. The evolution of the latter provides insight into the thermal processes occurring during and after a flood, which might be compared to the residual flow regime. Beyond the temperature measurements, the latter were also modelled with a thermo- and hydrodynamic 1D process-based stream model. A reasonable agreement between model and measurement resulted. Results indicate that such models can capture the thermal processes guiding stream temperature at a reach- scale during flood events and hence be a basis of effective artificial flood design.

2023

Blocking probability of individual logs at racks in rivers

Conférence ArODES

Riccardo Zaccardi, Michael Pfister

Proceedings of the 40th IAHR World Congress "Rivers - Connecting Mountains and Coasts", 21-25 August 2023, Vienna, Austria

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Résumé:

Driftwood racks are installed in rivers to hold back arriving logs at a hydraulically acceptable place, so that the downstream reach is protected from potential flooding. The bar spacing is a central rack parameter and was considered herein with physical modelling. Single driftwood logs (uncongested arrival) were supplied serially and their blocking or passage was counted and set in relation to the relative rack bar spacing and to the approach flow conditions. The data analysis, also considering approaches published in literature, provided an empirical estimation to predict the blocking probability.

Optimal hydraulic design of supercritical bend manholes

Conférence ArODES

Gaetano Crispino, Davide Dorthe, Corrado Gisonni, Michael Pfister

Proceedings of the 40th IAHR World Congress "Rivers - Connecting Mountains and Coasts", 21-25 August 2023, Vienna, Austria

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Résumé:

The flow pattern of supercritical bend manholes can result very complex due to the possible occurrence of the choking flow condition. This failure condition limits the maximum discharge conveyable by a bend manhole. At a design stage, it is, therefore, convenient to know the maximum capacity of the inspected bend manhole geometry and compare it to the design discharge carried out by the approach pipe. The present paper aims to give to the practitioners an immediate design tool to estimate the hydraulic capacity of supercritical bend manhole and, consequently, select the best appropriate geometrical setup. This tool consists of an empirical equation, according to which the bend manhole capacity is computed as a function of axial curvature radius, the length of the straight extension at the end of the manhole, the bend angle and the approach partial filling ratio. The accuracy was proved by carrying out a detailed experimental campaign based on the utilization of a CFD 3D numerical model.

2022

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

Conférence ArODES

Michael Pfister, Fabienne Favre Boivin, Bruno Spahni

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

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Résumé:

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

Conférence ArODES

Santiago Sandoval, Fabienne Favre Boivin, Michael Pfister

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

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Résumé:

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

Conférence ArODES

Gaetano Crispino, David Dorthe, Corrado Gisonni, Michael Pfister

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

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Résumé:

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

Conférence ArODES

Michael Pfister, Loïc Bénet, Giovanni De Cesare

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

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Résumé:

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

Conférence ArODES

Michael Pfister, Jean-Marc Ribi

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

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Résumé:

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 :

Conférence ArODES

an introduction

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

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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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 :

Conférence ArODES

an experimental study

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

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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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?

Conférence ArODES

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

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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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

Conférence ArODES

Paloma Furlan, Michael Pfister, 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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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

Conférence ArODES

Paloma Furlan, Michael Pfister, Jorge Matos, Anton Schleiss

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

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Résumé:

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 :

Conférence ArODES

influence of woody debris characteristics on blocking probabilities

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

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

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Résumé:

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

Conférence ArODES

Gaetano Crispino, Michael Pfister, Corrado Gisonni

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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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 :

Conférence ArODES

influence of woody debris characteristics

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

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

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Résumé:

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

Conférence ArODES

Gaetano Crispino, Michael Pfister, Corrado Gisonni

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

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Résumé:

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

Velocity profile measurements in bore waves

Conférence ArODES

Davide Wüthrich, Giovanni De Cesare, Anton J. Schleiss, Michael Pfister

Proceedings of 10th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering (ISUD 10), 28-30 September 2016, Tokyo, Japan

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Résumé:

Hydrodynamic waves are an unsteady flow motion generated by rapid water level rise. In nature, such events can be found in dam-break waves, impulse waves and tsunamis. These phenomena are rare, but highly destructive. The present study is based on an experimental approach and it investigates the hydrodynamic behavior of bores propagating on wet bed in terms of height and velocity profiles. The waves are investigated using Ultrasonic distance Sensors (US) to measure the wave height and average front velocity; the instantaneous velocity profiles were obtained through an Ultrasonic Velocity Profiler (UVP), installed in the bottom of the channel, with an emitting frequency of 2 MHz and inclined with an angle of 20° in the upstream direction. The acoustic scattering was increased using a hydrogen bubble technique with an anode and a cathode installed in the upstream part of the channel. The probe was sampled with a frequency varying from 13.5 to 55 Hz depending on the maximum wave height. A sensitivity analysis of the main influential factors was carried out, pointing out the need for a compromise between quality and quantity for these highly unsteady flows. Results showed some interesting logarithmic profiles typically associated with open channel flows for all wave configurations.

Example of wave impact on a residential house

Conférence ArODES

Davide Wüthrich, Michael Pfister, Anton J. Schleiss

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

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Résumé:

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 :

Conférence ArODES

comparison of physical and numerical modeling with in-situ measurements

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

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

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Résumé:

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.

Shock wave patterns in supercritical junction manholes with inlet bottom offsets

Conférence ArODES

Gaetano Crispino, Corrado Gisonni, Michael Pfister

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

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Résumé:

Urban floods are frequently provoked by the failure of drainage systems. Sometimes, the malfunctioning of drainage systems arises from uncertain or complicated hydraulic conditions developing into sewer manholes. Junction manholes represent basic elements for sewers as they enable two or more upstream branches to merge into a single downstream branch. Initial researches mainly investigated the hydraulic behaviour of subcritical junctions with constant branch diameters and an even invert. However, the junction of supercritical flows is the most complex process, mainly due to the likely occurrence of shock waves, breakdowns or choking flows. Furthermore, the sewers laying is commonly characterized by the branch tops alignment. Bottom offsets thus control the manhole inlets when upstream branch heights differ from the downstream one. In this regard, a physical model investigation was carried out at the Laboratory of Hydraulic Constructions (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL). The model reproduced a 45◦ junction manhole with aligned up and downstream branch tops and variable upstream branch diameters. The experimental tests focused on the scenario with supercritical flows entering the junction manhole. First outcomes in terms of flow behaviour and shock waves formation are described.

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

Conférence ArODES

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

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

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Résumé:

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

Conférence ArODES

Grégory Guyot, Mathieu Rodriguez, Michael Pfister, 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

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Résumé:

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 :

Conférence ArODES

case study of Cossonay

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

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

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Résumé:

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

Conférence ArODES

Stéphane Terrier, Michael Pfister, 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

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Résumé:

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.

Surface turbulence on bores and surges propagating on smooth and rough beds

Conférence ArODES

Davide Wütrich, Michael Pfister, Pedro Manso, George Constantinescu, Anton J. Schleiss

Proceedings of the 6th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16), 10-13 May 2016, Ottawa, Canada

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Résumé:

Hydrodynamic waves are characterised by unsteady, highly turbulent and irregular behaviours. In nature, such phenomena can be observed in dam breaks, impulse waves and tsunamis. Both wet bed bores and dry bed surges were experimentally reproduced on smooth and rough surfaces. For all tested scenarios a front propagating in the channel was observed, with secondary waves with various frequencies occurring behind the propagating front. Such phenomena are probably a consequence of the non-hydrostatic pressure distribution around the bore front. The present study investigates in both time and frequency domains, the distribution of the frequencies observed on top of the bore. The latter represent the surface fluctuations of a bore behind the front. Measurements allowed to characterise the eddy-size distribution for all configurations. Results also showed that the most recurring frequencies are between 0.5 and 2 Hz. Furthermore, it was shown that the first part of the wave was characterised by high-amplitude oscillations associated with greater energy content.

2015

Karahnjukar dam spillway :

Conférence ArODES

comparison of operational data and results from hydraulic modelling

Sigurdur M. Gardarsson, Michael Pfister, Andri Gunnarsson, Gunnar G. Tomasson

Proceedings of Hydro 2015, 26-28 October 2015, Bordeaux, France

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Résumé:

The Karahnjukar dam spillway is the spillway for the Halslon reservoir, the main reservoir for the 690 MW Karahnjukar Hydroelectric Project in East Iceland. The spillway consist of a 140 m long overflowing weir with a capacity of 2250 m3/s which discharges into a side channel, followed by a 450 m long chute. The chute terminates at a canyon edge where the jet drops some 100 m into a narrow gorge downstream of the Karahnjukar dam. During the design phase of the project, the flow conditions in the spillway were modelled in the hydraulic model tests in 1:45 scale model at the Laboratory for Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich. The power plant went into operation in 2008 and since then considerable experience has been obtained from operation of the spillway. In particular, flow depths and air discharge measurements have been obtained during spilling periods. In this paper, these measurements are presented and compared with results from the hydraulic model tests. In particular, flow depths are discussed as well as aeration effectiveness.

Head losses in sewer junction

Conférence ArODES

Corrado Gisonni, Michael Pfister

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

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Résumé:

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

Conférence ArODES

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

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

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Résumé:

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

Conférence ArODES

Stéphane Terrier, Michael Pfister, Anton J. Schleiss

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

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Résumé:

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 :

Conférence ArODES

developments until 1985

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

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

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Résumé:

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

Conférence ArODES

Michael Pfister, Anton J. Schleiss

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

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Réalisations

2022

Coastal Engineering Journal Citation Award

2022 ; Award

Collaborateurs: Pfister Michael

The CEJ Citation Award recognizes an influential work that has provided significant scientific and engineering impacts to interdisciplinary fields. It is determined annually by the total number of citations on all journal papers over the past five years.

Davide Wüthrich, Michael Pfister, Ioan Nistor & Anton J. Schleiss: Experimental study on the hydrodynamic impact of tsunami-like waves against impervious free-standing buildings, 60:2, 180-199, 2018 (winner of the CEJ Award, 2018).

2019

Willi H. Hager Best Reviewer Award, IAHR Journal of Hydraulic Research

2019 ; Award

Collaborateurs: Pfister Michael

2019

2018

Coastal Engineering Journal, Annual Best Paper

2018 ; Award

Collaborateurs: Pfister Michael

2018

2017

Member of Academic “111 Project”, Ministry of Education and State Administration of Foreign Experts Affairs, P.R. China

2017 ; Award

Collaborateurs: Pfister Michael

2017

2011

Outstanding Reviewer Recognition, ASCE Journal of Hydraulic Engineering

2011 ; Award

Collaborateurs: Pfister Michael

2011

PEOPLE@HES-SO
Annuaire et Répertoire des compétences

Pfister Michael

Assoziierter Professor FH

Compétences principales

Air-water flows

Spillway hydraulics

Model and scale effects

Urban drainage hydraulics

Assoziierter Professor FH

PEOPLE@HES-SO Annuaire et Répertoire des compétences

Pfister Michael

Assoziierter Professor FH

Compétences principales

Air-water flows

Spillway hydraulics

Model and scale effects

Urban drainage hydraulics

Assoziierter Professor FH

PEOPLE@HES-SO
Annuaire et Répertoire des compétences