The critical role of power intake/outlet structures in pumped storage schemes: Design challenges and optimization

A. Amini; G. De Cesare

In intake mode, the primary concerns are preventing sediment ingress into the waterways and mitigating vortex formation. Sediment intrusion can lead to erosion, blockage, and reduced turbine efficiency, while vortex formation can lead to entrainment of air, ice, or debris, pressure fluctuations, and potential risk of cavitation. • In outlet mode, key challenges include managing high velocities at the rack section, minimizing head losses, and ensuring proper jet diffusion to avoid local erosion and slope stability. 1 | Introduction Inlet and outlet structures in pumped-storage hydroelectric systems (PSH) serve a dual function, alternately acting for the lower reservoir as an inlet during pumping and an outlet during generation and conversely for the upper reservoir. In some particular configurations, two neighbouring structures can even work in opposite flow directions. This bidirectional flow condition presents several hydraulic challenges that can impact efficiency and structural integrity. Optimizing the design is critical to ensure smooth transitions between functions, minimize energy losses, and prevent problems such as cavitation, vibration, debris entrainment, abrasion of hydraulic machinery, sediment intrusion, and localized erosion in the vicinity of the structures. Guidelines published by relevant industry associations provide recommendations for the design of hydropower intakes, focusing on both hydraulic and structural considerations 1. Recent regulations have expanded to include environmental design principles, emphasizing the protection of fish, wildlife, recreational opportunities, and overall environmental quality. Despite these additional measures, maintaining maximum PSH efficiency and security remains a priority.

Design improvements for combined sediment-wood detention at mountain streams

R. Van Mol; E. Rüegg; A. Amini; G. De Cesare

Bedload transport and driftwood in steep mountain streams can cause serious damage to local communities in the vicinity of watercourses. To prevent such damage, check dams are typically built to hold back sediment and driftwood. However, by completely interrupting sediment transport, check dams are also detrimental to the ecology of mountain streams. A new type of partially permeable structure has emerged, which allows the sediment to pass during non-hazardous flows. This paper investigates the concept of partially permeable sediment and driftwood detention with the help of a physical model, based on a case study in Switzerland. This study shows that detention basins design can be improved by a combination of methods used to manage sediment in steep mountain streams and driftwood in lowland rivers. This offers an advanced way to improve flood resilience while maintaining sediment transport and ecomorphological continuity.

Restoration and Widening of the Gérine River

A. Amini; E. Rüegg; J.-M. Ribi; G. De Cesare

The Gérine River, characterized by its natural banks and an undisturbed hydrological regime, has some altered sections, such as the stretch in Marly. The preliminary project aims to widen the Gérine River to restore its lost or diminished ecological functions while also ensuring adequate flood protection.

Hydraulic analysis of Snowy 2.0 pumped storage facilities using hybrid models: validating performance and anti-vortex measures

A. Amini; E. Mazzocchi; G. De Cesare

Abstract: Within the framework of the Snowy 2.0 large-scale pumped-storage facility project in Australia, hybrid experimental and numerical modeling of Tantangara (upper reservoir) and Talbingo (lower reservoir) intake/outlet structure were carried out. The goal of the hybrid physical-numerical models was to verify the hydraulic design of the intake/outlet structures. The risk of vortex formation, velocity patterns in critical sections and the head loss coefficient were assessed under different generation and pumping scenarios. The experimental model was constructed on a scale of 1:25 while the numerical model was developed using Flow 3D software. The flow behavior in both pumping and generation modes was shown to be symmetric. The discharge is uniformly distributed between central and lateral openings, especially when the structure is functioning in inlet mode. For both operation modes the velocities are compliant with the design requirements. The design of anti-vortex measures was optimized using the experimental model. They were manufactured at a high level of detail and tested for both intakes. Vortex formation was re-assessed, and the measures were proven effective in reducing the vortices’ intensity. Overall, the hybrid experimental-numerical modeling approach for the intakes proved to be a valuable tool in gaining understanding of hydraulic behavior of these structures. It also contributed significantly to enhancing their design, particularly in preventing undesired vortex formation.

Characterization of the hydrodynamic behavior of surge shaft's orifices in SNOWY 2.0 power plant

S. L. Vorlet; A. Amini; E. Mazzucchi; G. De Cesare

The Snowy 2.0 project is a large-scale pumped storage facility located in the Snowy Mountains in the SouthEastern part of New South Wales (Australia). It is an extension of the existing Snowy scheme, linking Tantangara (upper) and Talbingo (lower) reservoirs through 27 km of tunnels as well as an underground pumped hydropower station housing six reversible Francis pump-turbine and motor-generator units. The project will provide an additional generation capacity of 2'200 MW for the Snowy 2.0 scheme and is characterized by a maximal gross head of 694.35 m, a maximum design discharge of 385 m3/s in generation mode and 280 m3/s in pumping mode. The scheme includes two surge shafts: the lower and the upper ones. This manuscript presents the results of a study conducted to assess the hydrodynamic behavior of the orifice to optimize and validate the geometry of the surge shafts by means of numerical and physical modeling. It focuses on the methodology applied to first use numerical modeling to evaluate the design and the head loss, and second conduct laboratory tests on a physical model for the results comparison and validation. Results show a good agreement between numerical simulations and laboratory tests.

Characterization of Thruster-Induced Turbulence for Fine Sediment Suspension

M. Marshall; A. Amini; G. De Cesare; A. L. Stakowian

Reservoir sedimentation is a key challenge for storage sustainability because it causes volume loss, affecting hydropower production capacity, dam safety, and flood management. A preliminary EPFL study proposed and studied an innovative device (called SEDMIX), which uses water jets to keep fine sediments near the dam in suspension and ultimately allows the sediments to be released downstream. The jets induce a rotational flow which creates an upward motion and keeps fine sediments in suspension near the dam and water intakes. The sediment can then be continuously released downstream through the power waterways at acceptable concentrations, without additional water loss or required energy. The efficiency of the SEDMIX device has been confirmed through experimental simulations and numerical analyses at EPFL This study involves updated experimental simulations to include thrusters in the device design instead of water jets, because they lead to a less complex arrangement that requires less energy to operate. The thruster-induced turbulent kinetic energy is observed and characterized using Ultrasonic Velocity Profiling (UVP) to determine optimal device configurations for effective sediment release.

Exploring the efficacy of reservoir fine sediment management measures through numerical simulations

S. L. Vorlet; M. Marshall; A. Amini; G. De Cesare

Reservoir sedimentation is one of the main challenges in the sustainable operation of large reservoirs because it causes volume loss, affecting hydropower production capacity, dam safety, and flood management. To ensure the sustainability of deep reservoirs by maintaining sediment flow continuity, it is essential to understand the mechanisms of the sedimentation process. The prediction of sediment deposition can enable adequate sediment management, including the design and implementation of prevention and mitigation measures. Different countermeasures are now being used to tackle sedimentation problems. However, many of these measures have a considerable ecologic and/or economic impact. It is therefore paramount to develop new efficient measures to ensure fine sediment transport through large reservoirs. This paper presents three innovative measures for fine sediment management in reservoirs and summarizes how state-of-the-art numerical modeling might help to assess the efficiency of these measures.

Experimental modeling of fine sediment routing: SEDMIX device with thrusters

M. Marshall; A. Amini; G. De Cesare

Reservoir sedimentation is a key challenge for storage sustainability because it causes volume loss, affecting hydropower production capacity, dam safety, and flood management. A preliminary EPFL study proposed and studied an innovative device (called SEDMIX), which uses water jets to keep fine sediments near the dam in suspension and ultimately allows the sediments to be released downstream. The SEDMIX device is composed of two rigid steel parts: one floating and one on the basin bottom holding a multi-jet manifold frame. The jets induce a rotational flow which creates an upward motion and keeps fine sediments in suspension near the dam and water intakes. The sediment can then be continuously released downstream through the power waterways at acceptable concentrations, without additional water loss or required energy. The efficiency of the SEDMIX device has been confirmed through recent experimental simulations and numerical analyses at EPFL. This study involves updated experimental simulations to include thrusters in the device design, instead of jets, because they lead to a less complex arrangement that requires less energy to operate. The experimental setup is similar to the previous experiments (utilizing a glass tank), and tests different thruster parameters to understand the resulting changes in tank turbidity (using turbidity meters).

Contribution of the Hydraulic Laboratories of the Swiss Federal Institutes of Technology to Dam Safety

G. De Cesare; F. M. Evers; B. Stocker; S. L. Vorlet; A. Amini  et al.

Bien que la construction de barrages en général et l’hydraulique des barrages en particulier soient des domaines techniques très développés, le potentiel de risque associé aux barrages justifie dans la plupart des cas la vérification et l’optimisation de la conception des installations auxiliaires par des essais de modélisation hydraulique. En outre, le transport de sédiments et de bois flottants ainsi que les écoulements multiphasiques sur les barrages et dans les réservoirs nécessitent souvent des essais de modélisation en grandeur réelle afin de mieux appréhender et comprendre les processus physiques impliqués. Les données obtenues lors de la modélisation physique sont précieuses pour le calibrage et la validation des outils de simulation numérique ainsi que pour l’établissement de normes et règlements. La recherche dans le domaine de l’hydraulique des barrages et des aspects additionnels concernant les barrages, tels que l’alluvionnement des retenues et les dangers naturels, reste très utile pour élargir les connaissances dans le domaine des barrages et améliorer leur sécurité et exploitation. Depuis leur création, les laboratoires d’ingénierie hydraulique des écoles polytechniques fédérales de Lausanne et de Zurich contribuent aux thèmes de l’ingénierie des barrages en Suisse et à l’étranger et soutiennent ainsi les connaissances spécialisées des expertes et experts suisses en la matière. Au-delà de l’importance des laboratoires d’hydraulique dans la planification, ainsi que pour l’exploitation sûre et efficace de barrages et ouvrages annexes, il convient également de souligner leur importance pour la recherche fondamentale. Selon la perception des auteurs, la plupart des publications scientifiques dans des revues spécialisées reconnues se basent toujours sur des données expérimentales collectées sur modèle physique.

Physical Modeling of Sediment Trap for Le Bez River, Switzerland

R. Van Mol; N. Schaller; A. Amini; Lukas Hunzinger; G. De Cesare

The Le Bez torrent in the Canton of Bern, Switzerland, is known for flashy and violent floods due to its watershed topography and vegetation conditions. Such floods can bring a substantial amount of sediment and wood into the small downstream village of Villeret resulting in severe flooding. To protect against such events, a sediment/wood trap system composed of two retention basins with two weirs upstream of each of them is under construction a few hundred meters upstream of the village. The downstream basin is equipped with an open check dam with a rack in front of it that aims to retain sediment and wood debris using a combination of hydraulic and mechanical control systems. To study the performance of the retention system a physical model was constructed using Froude similarity rules with a geometrical scaling factor of 1:10. The model is used to simulate different flood scenarios with return periods from 5 to 300 years with respective amount of sediment and wood income into the model. The amounts of wood and sediments transiting the model are measured are then measured to evaluate the efficiency of such trap system. Different geometrical parameters are varied to optimize the design. A vertical clearance in the rack of the open check dam of 1.8d90 was found to be sufficient to guarantee bed load transit during a 5-year flood. The retention system is working properly for a 100-year flood but during the falling limb of the hydrograph spontaneous sediment flushing was observed.

Enhancing Flood Resilience: Sediment Management in Le Bez Torrent

A. Amini; R. Van Mol; L. Hunzinger; G. De Cesare

In the canton of Bern, Switzerland, the “Le Bez” torrent is known for its flash floods, causing sediment and wood debris buildup in Villeret village settled on the alluvial fan. To mitigate this issue, a sediment/wood trap system is being implemented upstream. This system comprises two retention basins with a sill and vertical rack to retain wood debris, alongside a filter check dam equipped with hydraulic/mechanical controls to retain debris during major floods. The objective is to provide enough capacity to retain sediment during floods with over five years’ return period, striking a balance between effective retention and maintaining sediment transport during smaller discharges to prevent downstream bedload depletion. This study aims to find an efficient design of these basins using physical modelling.

Flow structure and grain motion assessments of large river widening in a physical model using ultrasonic Doppler velocity measurements

J.-N. Saugy; A. Amini; G. De Cesare

Local river widening aims to reduce the flood risk and enable the self-morphodynamic development of the river. However, a large amount of transported sediments settles due to the flow velocity reduction in the widening. Assessing the flow and the grain motion is therefore a key factor to the sustainability of a local river widening project. The grain motion depends on the ratio between tractive forces and resisting forces, which can be evaluated through the local shear stress. The most common method to estimate shear stress in uniform flows is to determine the shear velocity based on the logarithmic distribution of velocity over depth. This involves knowing the velocity profile and in turn the flow structure. In the present study, morphodynamic tests are conducted to explore the hydrodynamics and the grain motion of a local river widening in the framework of the 3rd correction of the Rhone River, the largest flood protection project in Switzerland so far. The ultrasonic velocity profiler method is used to measure velocity profiles at two selected cross-sections. The obtained velocity profiles allow for the assessment of local shear stress on the mobile riverbed. The results show a non-uniform distribution of the flow and the shear stresses. The flow conditions at the preferential channel are more favorable to grain motion compared to those at the river's edges. [GRAPHICS] .

Plunge pool scour and bank erosion: assessment of protection measures for Ilarion dam by physical and numerical modelling

R. Van Mol; C. Mörtl; A. Amini; S. Siachou; A. Schleiss  et al.

The 130 m high Ilarion Dam is built on the Aliakmon River in northern Greece. Its two spillways are equipped with ski jumps to deflect high-velocity jets towards the centre of the plunge pool. Significant scouring occurred in the pool during two major floods in recent years. To investigate reliable protection measures, a 1:55 scale physical model and a numerical model with FLOW-3D® were created. The hydrodynamic behaviour of the flow in the plunge pool and in the spillways as well as the trajectory of the jets could be determined. The numerical model showed, that the flow in the plunge pool is highly asymmetric and a rotational flow forms with only one of the two spillways is operational. This rotational flow results in locally effective flow rates three times greater than the flow rate from the spillway. Simulations confirm, that one solution to this problem is the symmetrical operation of both spillways.

Morphological evolution of the Rhone River in the Martigny bend

N. Schaller; S. L. Vorlet; A. Amini; G. De Cesare

Sustainable water management is a principal matter to ensure safety of waterways (Berga, 2016). In this framework, the correction of the Martigny bend is a priority measure of the 3rd Rhone River correction in Switzerland, which aims at increasing the flood protection in the region while ensuring sustainable conditions for the Rhone River (Arborino and Jordan, 2014). The project foresees widening the bed of the river to the regime width and lowering the bed in the bend (Saugy et al., 2022). A numerical model is built to represent the priority measure, in order to assess the impact of the project on the sediment transport capacity and the morphological evolution in the long term.

Underground flow section modification below the new m3 Flon metro station in Lausanne

L. Repnik; S. L. Vorlet; M. Seyfeddine; A. Amini; R. Dubuis  et al.

Rapid urbanization is increasing the demand for public transport in Switzerland’s major cities. The new m3 metro line in Lausanne is planned to circulate by 2030. Its construction will require a modification of the underground flow section of the Flon River vaulting at the Flon metro station. The Flon River drains a natural watershed, as well as the combined sewage network of the city of Lausanne. Two prior modifications of the vaulting geometry at the Flon station were carried out to accommodate the infrastructure of the m2 and LEB metro lines. The research aim was to assess the proposed vaulting geometry underneath the m3 by means of a hybrid hydraulic modelling approach combining a numerical and physical model in order to evaluate the new capacity limit of the system. A design discharge of 90 m3/s was defined, corresponding to a 100-year return period. A 3D numerical model at prototype scale was developed in the commercial software packages ANSYS Fluent and Flow-3D to simulate a multiphase flow. The physical model was built at a reduced scale of 1:20 based on Froude similarity. Taking into account the precission of the measurements on the physical model and the accuracy of the numerical results, they all lye withing the assumed limitations. Reagrding the engineering project, the results indicate that the design discharge cannot be maintained under free-surface flow conditions with the proposed vaulting geometry. The main limitation for the validation of the results is the lack of in-situ stage-discharge data.

Three-Dimensional Numerical Modelling of Al-Salam Storm Water Pumping Station in Saudi Arabia

A. Amini; M. Wickenhäuser; A. Koliji

The Al-Salam Pump Station will be constructed in the framework of Jeddah Storm water Masterplan (JSWM) in Saudi Arabia. The station discharges the incoming flow from the Al-Salam storm tunnel to the Red Sea. Its structure includes a circular deep wet well (22m diameter and 20m depth below the ground) equipped with six pumps, one weir feed channel (6 m length, 10m width) equipped with four pumps, two pumping channels and two gravity discharge channels. Simulations have been carried out using CFD numerical modelling (Flow-3D) to verify the hydrodynamic performance of such a structure. The hydraulic analysis focuses on aspects such as flow velocity fields and distribution, water elevations, shear stresses and vorticities. The numerical simulations allowed to verify the hydraulic operation and design/performance criteria. These criteria include the allowable freeboard, minimum shear stress to transport sand particles and organic materials, the critical submergence of pumps for different hydrological scenarios, and finally the pre-swirl angle of the pumps. Simulation results show uniform velocity distribution in the wet well as well as in the pumping channels and provide plausible information to confirm design criteria fulfilment.

Experimental investigations on plunge pool protection measures for Ilarion Dam in Greece

R. Van Mol; A. Amini; K. Mangina; G. De Cesare; A. Schleiss

Ilarion Dam is a 130 m high earthfill dam at Aliakmon River in northern Greece. The dam was commissioned in 2010. It has two spillway tunnels with ski jumps at the outlets. The two spillways are designed to release a total discharge of 5'500 m3/s during the probable maximum flood (PMF). During the flood events in 2013 and 2015, where only one spillway was working, scour holes have formed inside the plunge pool and erosion was observed at its banks and further down the tailrace channel. After a series of preliminary numerical investigations on the hydrodynamic behavior of the plunging jets, a physical model was built to assess the stability of prims protection carpets mitigate further scour of the plunge pool.

Bedload transport assessment on a physical model of a large river widening using ultrasonic Doppler velocity measurements

J.-N. Saugy; A. Amini; G. De Cesare

The 3rd correction of the Rhône River is the largest flood protection project in Switzerland so far. Covering a length of 162 km, it aims to protect some 100,000 people and prevent flood damages, which could raise up to 10 billion Swiss francs for major events. It also intends to revitalize the river and its surrounding area. A natural or revitalized river presents a dynamic morphology that can be assessed using bathymetric survey, bedload transport and hydrodynamic flow behavior. The present case study aims to investigate the behavior of the bedload transport for the future Rhône at the Verney widening using physical model. A valid bedload assessment requires distributed water depths and velocities. The used dataset consists of Lidar scans and recorded streamwise and crosswise velocity profiles. The analysis is performed at two selected cross sections based on the ultrasound Doppler velocity profile method. The velocity profiles allow assessing the local shear stress on the mobile river bed. The different velocity profiles are compared, and the results discussed.

Third correction of the Rhône River in Switzerland: physical and numerical modelling of the “Martigny bend” priority measure

J.-N. Saugy; A. Amini; G. De Cesare; S. André; P. Bourqui  et al.

The Rhône River, second largest in Switzerland, has already been corrected twice (1863-1893; 1930-1960) to ensure safety and economic development in the Rhône valley. However, in October 2000, the highest flood of the century occurred showing the limitations of the two first corrections and the need for a third one. The Rhône bend, located near the city of Martigny, is a priority project because of the high damage potential. Given the complexity of the hydraulic and morphological phenomena, physical and numerical modelling are used to validate and optimize the project design. To obtain the best results, the two models must be used together and the results cross-compared. Hence, they are checked and adjusted in parallel. The first tests, which were purely hydraulic, showed a good agreement between them. The extreme flood succeeded to pass through the enlarged cross-section with an acceptable water level. After these encouraging results, the morphodynamic tests are ongoing.

River morphology and sediment transport processes numerical modeling applied to the fluvial environment of the Rhône River in Valais

F. Molné; G. De Cesare; J. L. Garcia Rodiguez; A. Amini; J.-N. Saugy

Within the frame of the Third Rhône Correction, the river bend in Martigny is considered a top priority measure for flood protection. During the preliminary project, physical and numerical modeling tests were considered necessary. For the latter, the software BASEMENT has been employed, using the Shallow Water Equations and the Meyer-Peter and Müller transport equations. The scenario simulated consists of a 10-year return period flood applied during 19 days and 20 hours to force the formation of a natural bed, which will be used as an initial state for further scenarios. The results show the morphological evolution of the river and confirm the heterogeneity of the flow. Moreover, a cross-comparison with a physical model has been carried out for a better understanding and with the objective to validate the numerical model for its application at other areas of the Third Rhône Correction project.

Effect of the winglet on reduction of blade tip vortex from elliptical hydrofoil

S. Maeda; T. Sano; M. Iino; M. Farhat; A. Amini

It is well known that the performance of the turbomachinery (losses, vibrations, noise, and erosion) is often affected by the trailing vortex from the blade tip (Tip Leakage Vortex; TLV) of the impeller or propeller, and sometimes cavitation occurs in the center of the vortex core (Tip Vortex Cavitation; TVC). Many researches on the vortex characteristics of isolated hydrofoil by numerical simulations or experiments have been done so far, however, the research on understanding the mechanism of the vortex control technique is not enough, and it is important to mitigate the effect of TLV or TVC, to enhance the performance of turbomachinery. In this study, numerical simulation and experimental investigation were carried out to clarify the effect of winglet, attached to the elliptical hydrofoil (NACA16020). Five different winglet patterns were tested, changing bending angle and direction. Experiments were carried out using the high-speed cavitation tunnel in Ecole Polytechnique Federale de Lausanne (EPFL). As for numerical simulation, it was carried out in the same condition of the experiment. As a result, it was clarified that TVC decreased as increasing the bending angle of winglet, and disappeared in the case of 90 degree bent toward pressure side. And it is clarified that the vortex strength, in other word, vortex core pressure is affected by the interaction of the leading edge vortex and the tip vortex, and in case of pressure side 90 degree bended winglet, both vortices were mutually affected and cancelled the strength of these vortices.

Physical and numerical model studies of the Martigny bend as part of 3rd correction of the Rhone River

A. Amini; J.-N. Saugy; G. De Cesare; S. André; K. Essyad  et al.

The 3rd correction of the Rhone River (R3) is the largest flood protection project in Switzerland so far. Covering a length of 162 km, it aims to protect some 100,000 people and prevent flood damages, which could raise up to 10 billion CHF for major events. The present study aims to investigate the behavior of the future Rhone at the “Martigny bend” using both physical and numerical models. The correction of the "Martigny bend" is a priority measure of the R3 project, due to the high flood hazard risk and significant potential damages. Once cali-brated based on physical model results, the numerical model will act as an extension of the phys-ical model, and will be maintained and exploited in further design and construction phases. It allows to test possible project evolutions and to analyze its performance after the completion of the works. This paper focuses only on pure hydraulic tests.

Numerical Simulations of an Innovative Water Stirring Device for Fine Sediment Release: The Case Study of the Future Trift Reservoir

A. Amini; A. Chraïbi; P. Manso

Reservoir sedimentation and consequently lack of storage volume and perturbation of the operation of intakes and bottom outlet is a key challenge affecting both hydropower production as well as dam safety and flood management. In the framework of a peer-reviewed research project (Jenzer-Althaus, 2011) an innovative countermeasure, called SEDMIX, was proposed allowing to keep in suspension or re-suspend the fine particles near the power water intakes, thanks to an optimized arrangement of four water jets producing an upward whirling flow like produced by a mixer. With such a system, the suspended particles can be conveyed downstream at acceptable rates through the power waterways during the normal operation of the hydropower plant. Although experimental studies have shown the very promising efficiency of such a device in simple cases and by numerical simulations in a laboratory reservoir, SEDMIX performance has not been investigated yet in a real-life reservoir under prototype conditions. The aim of this study is therefore, to analyze the performance of a real-sized SEDMIX operating in the future Trift reservoir via numerical analyses. This study allows to validate or to improve SEDMIX optimal configuration experimentally determined. The numerical simulations are performed for different positions and heights for the SEDMIX device. The performance of SEDMIX in each position has been evaluated and tested for different jet discharges. The analysis of the numerical simulation results shows that the presence of SEDMIX does create a vortex flow pattern and sediment movement upward. The sediment volume fraction in the higher layers of the reservoir increases and consequently the evacuated volume of fin sediments increases for simulation using the SEDMIX device comparing those without the device.

Anatomie d’un coude

G. De Cesare; J.-N. Saugy; A. Amini; R. Sprenger

À Martigny, le Rhône prend un virage à 90 degrés vers le nord-ouest. Un changement de cap qui influence fortement la dynamique du fleuve, surtout en période de crue. Afin de mieux en connaître les effets, la Plateforme de constructions hydrauliques de l’EPFL construit un gigantesque modèle réduit de ce tronçon et confronté ses observations aux données obtenues par des modélisations numériques.

Computational modelling of fine sediment release using SEDMIX device with thrusters

A. Onate-Paladines; A. Amini; G. De Cesare

Problem: Reservoir sedimentation occurs in dams worldwide, reducing the live storage available in the reservoirs. Possible solution: Jenzer-Althaus 2011) testes a water stirring device (called SEDMIX) that keeps sediments in suspension, enhancing its release through the power intakes of the dam reporting a high efficiency. Background studies: Numerical simulations for a prototype for the Trift reservoir have been carried out in the past by Amini et al. (2017) and Chraïbi et al. (2018), obrtaining good results for sediment evacuation and determining the optimal location and dimensions of the device. Objective of the current work: Numerically test the performance of SEDMIX at the Trift reservoir implementing thrusters instead of the previous configuration with water jets using ANSYS 2019 R1 software.

Short-and-long-term storage volume recovery in a reservoir using a combined mixing-dredging device

A. Amini; A. Onate-Paladines; B. Mongiardino; F. M. Binder; G. De Cesare  et al.

Innovative methods to release fine sediments from reservoirs developed at EPFL, Switzerland

G. De Cesare; P. Manso; S. Chamoun; A. Amini; A. Schleiss

Electricity generation, water supply, flood protection, flow regulation and navigation are amongst the main services provided by reservoirs. Sedimentation affects the sustainability of reservoirs, by reducing their storage capacity, and increases the negative impacts of dams on downstream rivers due to sediment impoverishment. For these reasons, reservoir sedimentation must be considered in dam planning, design, commissioning and operation. Typically, fine, mostly suspended sediments enter reservoirs during flood events, glacier melt periods or during operation of upstream infrastructure. This article describes innovative methods developed at the Platform of Hydraulics Constructions (PL-LCH) of Ecole Polytechnique Fédérale de Lausanne (EPFL) to cope with the accumulation of fine sediments within alpine reservoirs in Switzerland.

Kariba plunge pool rehabilitation

I. Stojnic; C. Ylla Arbos; A. Amini; G. De Cesare; A. Schleiss

The Kariba Dam is a 128 m high arch dam located on the Zambezi River, between Zambia and Zimbabwe. It forms the Kariba lake, with a surface of 5400 km2 and a capacity of 185 km3. In the 20 years following dam construction, heavy rainfalls required frequent spilling operations at high discharges. Excess water from the upstream reservoir is conveyed to the downstream river through six gated spillways with a section of 9.1 m in height and 8.8 in width, which generate free-falling jets with high impact velocities of around 21 m/s. Intensive operation of the spillways progressively scoured a deep and steep-sided hole with its current bottom at about 70-80 m below the original riverbed and about 80-90 m below the normal tailwater level. Between 2010 and 2012, a comprehensive study including both physical and numerical modeling was carried out to investigate the plunge pool situation, with the aim to assess and control any potential future scour development (LCH/AVE Report 04/12). The results of the study indicated unacceptable scour evolution, and therefore several countermeasures were proposed and tested. The study concluded that plunge pool geometry reshaping in the downstream direction was both an efficient and technically plausible solution to limit future erosion. Different reshaping geometries were tested, and based on pressure and velocity measurements at selected locations, a final configuration of the plunge pool was proposed. In order to perform the necessary excavation works for plunge pool reshaping, a cofferdam is foreseen downstream of the pool. Furthermore, a major geological fault will be treated during excavation by means of a concrete structure. The objectives of the present study are i) to validate the design of the cofferdam; ii) to assess dam safety if only partial excavation of the plunge pool is carried out; iii) to investigate the role of the cofferdam in energy dissipation, and iv) to identify relevant hydrodynamic loads for the design of the fault treatment structure.

Effect of spilling from adjacent orifices on the velocity field and pressures in front of needle stop-logs

I. Stojnic; C. Bron; A. Amini; G. De Cesare

In this study, the effect of spilling from neighboring sluice gates on stop-logs was experimentally investigated on a 1:65 physical scale model comprised of a double arch dam, a reservoir area, six spillway orifices and two stop-logs. Two types of measurement were performed: 2D velocity field in approach flow zone at the central horizontal plane of the orifices using two 0.5 MHz long range UVP transducers and dynamic pressures acting on the needle stop-logs at nine relevant positions using piezo-resistive pressure transmitter. The results showed that the pressure acting on the needle stop-logs followed hydrostatic distribution and velocity magnitudes in the zone of the needle stop-log are low and mostly unaffected by the spilling of the neighboring sluice gates.

Innovative methods to release fine sediments from seasonal reservoirs

G. De Cesare; P. Manso; S. Chamoun; S. Guillén Ludeña; A. Amini  et al.

The papers present three innovative solutions for fine sediment manage- ments in seasonal reservoirs, which are at different stages of maturity. Operational reservoir stirring using jet-like inflows may facilitate sediment routing, at no addi- tional cost and without water losses. Forced stirring, using the SEDMIX device, has shown very promising results in laboratory and from computational modelling and is now entering the phase of proof-of-concept at prototype scale. Sediment venting through the bottom outlets has been object of extensive research, allowing this solution to be consistently integrated in design criteria from now onwards. Future work is required to complete the development of the first two solutions, both in terms of physical principles (inlet/outlet jet-flows) and prototype scale-up (SEDMIX). Regarding venting, compiling data of field operations is the next logical steps in view of obtaining adequate feedback on the water volumes consumed as function of venting efficiency, as well as downstream impacts in terms of released sediment concentration.

A universal formula for generalized cardinal B-splines

A. Amini; R. Madani; M. Unser

Improvement of EIA methods for large reservoirs by using network thinking analysis approach: a case study of Azad dam, Iran

M. Nikravan; M. Azizi; M. Payami; M. Sadeghi; A. Zarrati  et al.

Dam construction and large reservoir projects are essential for sustainable economic developments especially in developing countries like Iran. Such big projects, could have major social and environmental impacts. At this point, Environmental Impact Assessment (EIA) in dam construction is needed in order to study and address the social, environmental as well as economic issues. In the framework of a joint research project between master students of Amirkabir University of Technology in Tehran, Iran, and Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, integrated matrix oriented tools were analysed and improved in order to answer the complex challenge of EIA of a large reservoir and pump-storage hydropower plant. For this purpose, a modified Leopold and ICOLD matrix is used. Furthermore, the Gomez and Probst network thinking method is applied, in order to identify the involved factors and improve the understanding of the complex problem. This methodology is applied to the case of the Azad pump-storage hydropower plant in Iran. This case study highlights better the advantages which is provided in the combined model. The results show that using network thinking analysis approach could enhance the matrix methods and helps in improving the quality of EIA report for large reservoirs and dams.

Computational hydraulic modelling of fine sediment stirring and evacuation through the power waterways at the Trift reservoir

A. Amini; P. Manso; S. Venuleo; N. Lindsay; C. Leupi  et al.

Taking advantage of the withdrawal of some glaciers, several new dams will eventually be constructed in Switzerland in the coming years as a part of the 2050 energy strategy. The Trift dam, located in Berner Oberlands in Switzerland, is one of these new projects. Reservoir sedimentation is however one of the main challenges for long-term sustainable operation of dam reservoirs that requires mitigation measures. Settling of suspended sediment may reduce reservoir live storage available for hydropower production and hamper operation of the bottom outlets. Jenzer-Althaus (2011) has experimentally tested a stirring device (hereafter called SEDMIX) as a mitigation measure to prevent reservoir sedimentation. The device induces an adequate level of upwind turbulence preventing sediment from settling near the dam, keeping them in suspension for progressive evacuation through the power intake during normal operation of the hydropower plant. This innovative system can be potentially installed in several reservoirs in Switzerland and abroad to avoid reservoir siltation due to fine sediments mainly transported by turbidity currents. The present study investigates the implementation of the SEDMIX device in the Trift reservoir by means of computational hydraulic modelling. The study proves the efficiency of the SEDMIX device in evacuating fine sediments during normal operation of the hydropower plant. The effect of other parameters such as the orientation and discharge of the SEDMIX jets, as well as the characteristics of the suspended sediments such as initial concentration and grain size, on the efficiency of the SEDMIX device will be evaluated in the future.

Investigation of venting turbidity currents in the Rudbar-Lorestan reservoir in Iran

A. Amini; S. Venuleo; S. Chamoun; G. De Cesare; A. Schleiss  et al.

Venting of turbidity currents is known as an efficient measure to prevent reservoir sedimentation and is applied in many reservoirs globally. It has several economic and ecological advantages compared with other evacuation methods. In the literature, numerous researchers mentioned the importance of venting, but mostly qualitatively. Given the complexity of the phenomenon and the presence of many parameters affecting the efficiency of venting, case studies offer good insight into solutions for specific reservoirs where there is a high probability of occurrence of turbidity currents. In the present study, the efficiency of venting turbidity currents is investigated for Lorestan Reservoir in Iran. For that purpose, 3D numerical simulations of turbidity currents over 11 km and venting operation are performed using ANSYS Inc. software. The results of the present research allow to optimize venting operations in the Lorestan Reservoir and reservoirs having similar general characteristics. Recommendations are drawn on outlet opening timing and discharge allowing to vent the greater amount of sediments while minimizing the water release.

Wave atlas for Swiss lakes: modeling design waves in mountainous lakes.

A. Amini; B. Dhont; P. Heller

The present study aims to provide a comprehensive wave atlas for Swiss lakes. The atlas is an adequate tool that contains wave characteristics of extreme events derived from numerical calculations. Swiss lakes are surrounded by mountains, and the wind field over the lakes is highly influenced by the region's topography. Therefore, it is important to apply the winds with appropriate direction distribution. The numerical weather prediction model COSMO-2 is used for defining the wind directional distribution, whereas the spectral wave model SWAN is used for wave simulations. The evaluation results are ultimately published on a geographical information internet platform, where a wave rose correlates the wave height to the event frequency for each point on a lake. An accurate knowledge of wave parameters is essential for the design of new shore protection measures, beach erosion control and harbor silting studies.

Assessment of wave risk for Swiss lakes: Numerical simulation of waves of different return periods

A. Amini; P. Heller

New hydraulic gallery to improve operation conditions at the Emosson hydro scheme

N. Nilipour; M. van Mark; A. Amini; P. Monnay

A new hydraulic gallery was constructed to increase operational resilience and mitigate risks of Emosson hydropower scheme in the Swiss-French Alps. One of the main constraints of the operation is to avoid the overflow of the so called "Corbes" surge chimney during pumping, if the upstream valve is closed. While this scenario is very rare and happened only three times since the commissioning of the dam in 1973, it is considered as a critical issue for operation of the scheme due to the high risk of this event. Different solutions have been studied to avoid the overflow of the surge chimney. However, the safest solution was to create a new gallery bringing water from the top of the surge chimney to the Emosson Reservoir. However, the execution of the new so-called "Corbes" Gallery took place in challenging alpine conditions with difficult access and a very tight schedule.

Untersuchung von Renaturierungskonzepten am Seeufer Gals, Bielersee

A. Schiefer; C. Iseli; A. Amini; G. De Cesare

Analyse des ökologischen Aufwertungspotentials am Seeufer Gals (Bielersee, Schweiz), welche mithilfe einer hydrodynamisch- numerischen Modellierung mit der Software MIKE 21 und Bestimmung von Makrophyten vorgenommen wurde. Unter der Annahme von zwei unterschiedlich starken Windszenarien wurden die windinduzierten signifikanten Wellenhöhen und die welleninduzierten Strömungen für verschiedene Varianten simuliert. Die Ergebnisse der Modellierungen zeigten, ob die in den Varianten geplanten Massnahmen eine Reduzierung der Wellenhöhen und der Strömungen verursachen. Das Hauptziel war, Bedingungen zu schaffen, welche den Habitatsansprüchen von Makrophyten zusagen und somit eine Neuansiedlung von Makrophytenarten an dem Seeufer Gals möglich machen und gleichzeitig die Wellenbelastung auf das Ufer zu verringern, um eine fortschreitende Erosion zu vermeiden. Mithilfe einiger Massnahmen konnten erfolgsversprechende Ergebnisse erzielt werden, die zu einer Verbesserung der aktuellen Situation führen würden. Es wurde ebenfalls gezeigt, dass Makrophyten für die Beurteilung eine bedeutende Rolle spielen, doch eine Bewertung nur anhand von ihnen sehr schwierig ist, da sie nicht nur auf Wellen- und Strömungsbedingungen, sondern auf weitere Faktoren und zudem extrem schnell auf deren Veränderungen reagieren. Die Studie wurde im Rahmen einer Masterarbeit an der TU Dresden im Landschaftswerk Biel-Seeland durchgeführt.

Reactivation of Klingnau reservoir sidearm: Numerical simulation of sediment release downstream

A. Amini; P. Heller; G. De Cesare; A. Schleiss; N. Kräuchi  et al.

The Klingnau reservoir, situated in north Switzerland, suffers from sedimentation in its left enlargement. The dam is placed in the Aare River about 1 km upstream of its confluence with the Rhine. Due to natural conservation and flood protection reasons, it has been sought for decades to slowdown the sedimentation process in the reservoir. For this purpose, 20’000 m3 of deposited material will be excavated by suction dredge and injected downstream into the Rhine. The objective of the present study is firstly to spot the best place for the material injection downstream. It is then aimed to assure that this injected material will not deposit along the Rhine River down to the Birsfelden dam. The latter is situated 60 km downstream of Klingnau and is the last reservoir on the Rhine River in Swiss territory. To accurately simulate the sedimentation process, one and two-dimensional numerical models are applied. It is confirmed that the majority of injected material stays in suspension and can be transported through the Rhine River down to Birsfelden.

Comprehensive numerical simulations of sediment transport and flushing of a Peruvian reservoir

A. Amini; P. Heller; G. De Cesare; A. Schleiss

Numerical modeling of sediment transport in reservoirs, especially for sudden events such as flushing, is still a challenging research topic. In the present study, sediment transport and sediment flushing are simulated for a Peruvian reservoir. Situated in the Peruvian Andes, the watershed is affected by high erosion rates and the river carries high amounts of suspended sediment whose estimated annual volume is about 5 Million m3. This study aims to investigate the reservoir sedimentation using different numerical models. A one-dimensional (1D) sediment transport model, a horizontal two-dimensional (2D) hydraulic model and a vertical 2D model are used for this purpose. Annual sedimentation, full drawdown flushing, and sediment concentration in power intakes are particularly investigated.

Expertise hydraulique d’un projet lacustre

J.-L. Boillat; F. Hachem; A. Amini; R. Amacher

Dans la phase d'étude de la création de plages et de l'agrandissement de l'adjacent Port de la Nautique sur le Léman à Genève, la complémentarité des modèles numérique et physique a permis de converger vers une solution optimale pour cet aménagement complexe.

Physical and numerical modeling of harbors and shore protection measures / Modélisation physique et numérique de mesures de protection portuaires et d’aménagement de plages

F. Hachem; A. Amini; J.-L. Boillat

The expansion project of the ‘’Port de la Nautique’’ and the creation of adjacent new beaches on the ‘’Lac Léman’’ in Geneva is currently under survey. In the conceptual and study phase, numerical- and physicalscaled models have been exploited at the Ecole Polytechnique Fédérale de Lausanne (EPFL). The objectives are to analyze the courantology and the wave’s field inside the protected zone of the harbor and their influence on the stability of the new beaches. The numerical modeling is driven by two models at different scales. The large scale model provides the boundary conditions to the small one which is used to estimate the circulation of water in the new harbor and to predict the erosion and deposition processes inside the harbor and along the beaches. The physical model is built using distorted scale factors inside an experimental wave tank that covers the same area as the small numerical model. It aims to optimize the project and to confirm the numerical results concerning notably the wave heights inside the harbor and the sediment transportation processes all along the beaches. The results obtained by the complicity of the numerical and physical simulations helped to converge toward an optimal design locations, orientations, and types of the protective dikes of the port as well as the geometric configuration and number of groins and the choice of the suitable backfill grain size for the new beaches.

Numerical Modeling Of Oil-Water Multiphase Flow Contained By An Oil Spill Barrier

A. Amini; A. Schleiss

To enhance the understanding of hydrodynamics of oil spill containment booms, numerical simulations of different booms were carried out using the state-of-the-art Computational Fluid Dynamics program, FLUENT. Single and double barriers containing oil over water surface were modeled and the flow characteristics in the vicinity of rigid and flexible barriers with different drafts were studied. The numerical results were compared with experimental observations and measurements. The simulated velocity field was in good agreement with precise experimental measurements. The extension of turbulence wake was discussed for single and double barriers. Finally, using a multiphase flow model the effect of oil on flow pattern was studied.

Entrainment of floating granules behind a barrier

A. Amini; J.-L. Boillat; A. Schleiss

To simulate the retaining capacity of an oil barrier in an uniform flow field, experiments were carried out in a laboratory flume at Ecole Polytechnique Federale de Lausanne (EPFL), Laboratory of Hydraulic Constructions (LCH) by using Light Expanded Clay Aggregates (LECA) and plastic particles. It was demonstrated that under appropriate assumptions for the effects of buoyancy and gravity forces, the Shields approach is suitable to predict both the entrainment of suspended granules behind a barrier and the start of leakage underneath. The phenomenon was also simulated numerically with a multiphase model using a CFD code, Fluent, and the results were compared to those of the physical experiments. The "Eulerian model" multiphase model of FLUENT was selected to simulate the phenomenon. The numerical model successfully predicts the evolution of the slick shape behind the barrier for various flow conditions. The amount of LECA that leaked from the barrier agreed well with the experimental observations.

Behavior of rigid and flexible oil barriers in the presence of waves

A. Amini; A. Schleiss

Although oil barriers have been used for many years to contain slicked oil in open seas, the effect of waveson them has been rarely considered. In the present study, we investigated the response of rigid and flexible oil containment barriers in the presence of currents with and without waves. Two-dimensional experiments with both rigid and flexible oil containment barriers were carried out in a laboratory flume equipped with a pneumatic wave generator. The initiation of containment failure for various conditions were analyzed and compared.

Velocity profiles and interface instability in a two-phase fluid: investigations using ultrasonic velocity profiler

A. Amini; G. De Cesare; A. Schleiss

In the present study the velocity profiles and the instability at the interface of a two phase water-oil fluid were investigated. The main aim of the research project was to investigate the instability mechanisms that can cause the failure of an oil spill barrier. Such mechanisms have been studied before for a vast variety of conditions. Although the velocity field in the region behind the barrier can influence the failure significantly, it had not been measured and analyzed precisely. In the present study the velocity profiles in the vicinity of different barriers were studied. To undertake the experiments, an oil layer was contained over the surface of flowing water by means of a barrier in a laboratory flume. The ultrasonic velocity profiler method was used to measure velocity profiles in each phase and to detect the oil–water interface. The effect of the barrier geometry on velocity profiles was studied. It was determined that the contained oil slick, although similar to a gravity current, can not be considered as a gravity current. The oil–water interface, derived from ultrasonic echo, was used to find the velocity profile in each fluid. Finally it was shown that the fluctuations at the rearward side of the oil slick head are due to Kelvin–Helmholtz instabilities.

Velocity measurements in the vicinity of an oil spill barrier using LSPIV method

M. Altinakar; N. Yildirim; S. Kumcu; G. Tayfur; M. Gogus  et al.

Dynamics of low-viscosity oils retained by rigid and flexible barriers

A. Amini; E. Bollaert; J.-L. Boillat; A. Schleiss

Large reservoirs and greenhouse gas emissions – A network thinking analysis

A. Amini; S. Kantoush; B. Rosier; M. Geiges; A. Schleiss

Contractile floating barriers for confinement and recuperation of oil slicks

A. Amini; A. Schleiss

Droplet Entrainment and Slick’s Characteristics in the Oil Retained by a Flexible Spill Barrier

A. Amini; J.-L. Boillat; E. Bollaert; A. Schleiss

Contractile floating barriers for confinement and recuperation of oil slicks

A. Amini

Marine oil spills can cause serious environmental damages to natural resources and to those whose sustenance depends upon these resources. Unfortunately experience shows that even the best efforts have not prevented occasional occurrences of major accidents on the sea. As long as massive oil spills are probable, special techniques and equipments will remain essential to facilitate spill cleanup in coastal regions. Oil spill containment booms are the most commonly adopted techniques to collect and contain oil on the sea surface, or to protect specific areas against slick spreading. Recently, an anti-pollution boom called the Cavalli system, has been designed with the intention of preventing the spread of spilled oil by trapping it inside a flexible floating reservoir and improving the pumping operation by decreasing the reservoir surface, and consequently increasing the oil layer thickness. Although flexible barriers have become increasingly common as a cleanup facility, there is no more than inadequate elaborate knowledge about their behavior. According to an extensive literature review, most of existing researches, either physical or numerical, have been done for rigid barriers. The main motivation for introducing the present research project is to study the efficiency and operational limits of the Cavalli system. However, the objectives are not constrained to this particular case. The present investigation focuses on the behavior of flexible barriers containing spilled oil. Previous researches of containment booms, even for the case of rigid barriers, have been mainly carried out in calm water. Accordingly, the main concentration is devoted to the response of a flexible barrier in presence of sea waves. Both experimental and numerical approaches were pursued to evaluate the efficiency limits and behavior of flexible barriers. Two-dimensional experiments have been carried out in a laboratory flume 6.5 m long, 1.2 m deep, and 12 cm wide. Flexible and rigid barriers containing rapeseed oil were examined, with and without waves. As the first step, the behavior of a flexible barrier in currents without waves was studied and compared to that of a rigid barrier. The key challenge was to contain the oil behind a flexible barrier that can freely deform in the water flow. This could be achieved using a slitted side skirt on the boom where it faces the lateral wall of the flume. The failure mode observed for rapeseed oil was entrainment failure. The initial failure velocity of different experimental conditions was studied and an empirical relationship was suggested in order to assess the maximum permissible oil-water relative velocity as a function of barrier draft and oil characteristics. The geometrical characteristics of the contained slick were examined and empirical equations were proposed to predict the slick length and headwave thickness as a function of contained oil volume. The second and more significant step was to conduct experiments with a flexible floating barrier in presence of five different waves. The analysis focused on the relationship between the failure velocity and the wave parameters with an emphasis on the behavior of flexible barriers. Likewise, empirical equations were proposed for the prediction of the initial failure velocity and geometrical characteristics of the slick. A type of drainage failure, namely, surging drainage was observed in the presence of waves. It was shown that the wave steepness and oil layer thickness are the dominant parameters in such failure. It was noticed that by decreasing the wave period or increasing the wave height, interfacial waves became more aggressive and consequently failure initiated at a lower velocity. Flexible barriers were more sensitive to the variations of wave characteristics. Applying appropriate time and length scales, a critical wave period of 6 s and wave height of 0.5 m were proposed for the prototype. Accurate measurements of velocity profiles and flow patterns in the vicinity of barriers with different conditions by means of Ultrasonic Velocimetry Profiling (UVP) and Large-Scale Particle Image Velocimetry (LSPIV) methods provided a reasonable understanding of the hydrodynamics in the vicinity of the barrier. The characteristics of the headwave at the upstream end of the oil slick were deliberately compared to those of a gravity current. It was concluded that despite geometrical similarities, these two phenomena are quite diverse. Furthermore, the oil-water interface was traced by detecting the maximum ultrasonic echo intensity, and velocity profiles in water and oil phases were independently obtained. To enhance the understanding of the mechanisms associated with oil containment failure, numerical simulations of multiphase flow were carried out using FLUENT code, applying the finite volume method (FVM). Comparisons between the obtained flow pattern and velocity field derived from numerical simulations and precise experimental measurements confirmed the capability of the numerical model to simulate the multiphase flow. The turbulence wake downstream of rigid and flexible barriers was simulated with and without the presence of oil phase. The simulations revealed the effect of contained oil on flow pattern and consequently the drag force acting on the barrier. Simulations of a full-scale barrier proposed a drag coefficient, Cd, of 1.90 for rigid barriers. Contrarily a constant value for the drag coefficient cannot be attributed to flexible barriers, since its deformations do not allow it to form similar shapes at different velocities. Last but not least, comparing the drag force on a rigid barrier with that of a flexible barrier towed by the same velocity demonstrated the fact that the forces acting on the skirt could be appreciably reduced by allowing flexibility.

Experimental investigation of oil spill containment behind a flexible barrier

A. Amini; E. Bollaert; J.-L. Boilat; A. Schleiss

Fluid-Structure Interaction Analysis Applied to Oil containment Booms

A. Amini; M. Mahzari; E. Bollaert; A. Schleiss

Experimental investigation of oil spill containment by granules contained behind a rigid barrier

A. Amini; E. Bollaert; J.-Boillat; A. Schleiss

Preliminary design criteria for oil spill confinement booms

A. Amini; E. Bollaert; J. Boillat; A. Schleiss