LES of Gravity Currents – Roughness Elements, Non-erodible Obstacles and Slopes
Dr Talia Tokyay
Independent Researcher (formerly Assistant Professor at Middle East Technical University -METU- Ankara, Turkey)
Gravity currents are mainly generated by density differences within a fluid or between two fluids. They contain a front region usually called the head, a dissipative wake region, in which Kelvin-Helmholtz billows are shed, and a tail. The last two regions form the body of the current. The motion of the fluid behind the head depends on the slope of the bed. Gravity currents occur widely in nature. The presence of density differences of only a few percent can be enough to induce a gravity current that travels over very long distances. Predicting the evolution of turbulent gravity currents is of great interest in many areas of geophysics and engineering, in particular due to their impact on the environment. For example, erosion by gravity currents is one of the main causes of formation of submarine canyons on continental slopes and plays a determinant role in transporting sediments from shallower to deeper regions in water environments. The propagation of a gravity current is often accompanied by disastrous damage.
Large eddy simulations (LES) of these currents aim to develop insight into the flow physics of highly-turbulent Boussinesq gravity currents in lock-exchange configuration propagating over an array of large-scale roughness elements, on slopes, and interaction of constant-flux gravity currents with non-erodible submerged obstacles. Simulation of gravity current propagating over a smooth flat bed serves as a benchmark in the study. The numerical results allow us to analyze the impact of the bed topography on sediment entrainment capacity due to the passage of a compositional gravity current.
From experiments to large-eddy simulation: developing efficient arrays of hydrokinetic turbines
Dr Pablo Ouro
Dame Kathleen Ollerenshaw Research Fellow. Department of Mechanical, Aeronautical and Civil Engineering.|
The University of Manchester, United Kingdom.
Pablo is a Civil Engineer graduated in 2013 from University of A Coruna, Spain. He moved to Cardiff University (UK) to pursue his PhD in Large-Eddy Simulation of Tidal Turbines in 2017. During his doctoral studies he was awarded the JFK prize at the IAHR World Congress in 2015, the Otter trophy by CIWEM-Welsh branch in 2014 and was finalist of the Osborne Reynolds competition in 2017. Thereafter, he was appointed as Research Software Engineer at Cardiff University in the Supercomputing Wales project for 18 months until beginning of 2019 when he took his role as Lecturer in Computational Hydraulics at the Hydro-environmental Research Centre. In September 2020 Pablo moved to the University of Manchester as Dame Kathleen Ollerenshaw Research Fellow to work on digital models for offshore renewable energy. His research areas are offshore wind/tidal energy, high-fidelity computational fluid dynamics, wall-bounded turbulence, high-performance computing, and swimming fish behaviour.
Harnessing renewable energy is a key pillar for our society to become sustainable and more equative on our path to a clean, carbon-free economy. Hydrokinetic turbines are a promising technology in a development stage that can become an affordable, suitable solution for extracting kinetic energy from tidal flows and rivers.
Our research focuses on understanding the hydrodynamics of the wakes generated by vertical and horizontal axis turbines deployed in arrays, aiming to maximise their energy-generation capabilities and thus reduce costs. For this, we have undertaken laboratory experiments and performed high-resolution Large-Eddy Simulations (LES) which allowed to investigate the intrinsic complexities of turbulence within these flows.
Tropical dams and river water quality: the Zambezi River-Kariba Dam system
Ph.D. student at Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland.
Elisa graduated in Environmental Engineering at the University of Trento (IT). Her Master thesis dealt with the analysis and modelling of river thermal dynamics. After her master, she worked at University of Trento as a scientific collaborator, investigating lake surface thermal dynamics using remote sensing data. She is currently a Ph.D. student at ETH Zurich, working on the impact of dams on river water quality at low latitudes (DAFNE project https://dafne.ethz.ch/).
The fast growing African population triggers a rising demand of water, food and energy. Such needs lead to major anthropogenic pressures on African River systems. Among others, the ongoing boom of dam constructions will affect river water quantity and quality. In my current Ph.D. project, we investigate the Zambezi River – Kariba Dam system: this case study can help to shed some light on the water quality alteration by large dams in tropical regions. In particular, we characterize Kariba Lake’s internal stratification dynamics to understand how this man-made lentic system plays a major role for the downstream Zambezi River’s thermal and oxygen regimes. In this seminar I present some results of the Kariba Lake dynamics together with the outcomes of our water quality monitoring campaign across the Zambezi River Basin. Finally, I will discuss the relations between lake dynamics, dam management and river water quality alterations.
Hydropower and local river water temperature dynamics: challenges and opportunities of a modelling approach
Dr Davide Vanzo
Researcher at the Surface Waters - Research and Management Department of the Swiss Federal Institute of Aquatic Science and Technology (Eawag), Switzerland.
Dr Davide Vanzo gained his Ph.D. in Environmental Engineering at the University of Trento (IT), working on the modelling and quantification of hydro- and thermopeaking alterations in rivers. He then joined the Laboratory of Hydraulics, Hydrology and Glaciology of ETH Zurich (CH), where he mainly focused on gravel-bed river morphodynamic modelling. At Eawag he is investigating the water thermal heterogeneity in rivers affected by hydropower production. His current research interests lie on two main topics: first, the development and application of numerical models for river eco-hydraulic problems such as pollutant and thermal transport-dispersion. The second topic concerns with the development of novel and efficient numerical solutions for the investigation of river hydro-morphodynamic processes.
Temperature is a fundamental physical property of flowing waters and it plays a key role in several ecological processes. Among others, it influences the rate of biogeochemical processes, the behavior of macroinvertebrates, and it affects different fish lifestages. Hydropower production might affect the natural thermal regime at different spatial and temporal scales. In particular, the sub-daily flow fluctuations due to hydropower production (hydropeaking) can also alter the river water temperature (thermopeaking). Understanding and modelling the water temperature variability under hydropeaking conditions, can positively contribute to a better characterization of physical habitat dynamics. In this context, numerical models are valuable tools for the assessment of water quality and ecosystem integrity. The exploitation of depth-averaged two-dimensional (2D) models has grown rapidly in last decades: however the routine application of 2D models for ecohydraulic investigations can still be inhibited by numerical challenges, such as the computational costs and the robustness in simulating unsteady, transient and shallow flows.
Forecasting Climate Change Scenarios in Myanmar Using MRI-AGCM3.2S
Dr Win Win Zin
Professor, Department of Civil Engineering, Yangon Technological University
Dr Win Win Zin graduated as Civil Engineer at Yangon Technological University, Myanmar in 1987. She received her Master degree from Karlsruhe University, Germany in 2000. She got Ph.D degree (Sandwich) from Karlsurhe University and Yangon Technological University in 2009. She is serving as a Professor at the Department of Civil Engineering, Yangon Technological University. She is enthusiastic in research and is conducting as a project leader with international collaboration. She is conductingacting projects with Delft University of Technology, IHE (Delft),University of Bonn, Kiel University and University of Tokyo. Currently she is conducting research on flood inundation mapping and climate change. Apart from her academic duties, she is positioned as member of Executive Committee, Myanmar National Committee on Large Dams (MNCOLD) as well.
In the present study, MRI-AGCM3.2S was used to simulate both the present-day climate (1981-2005) and projected climate for near future (2020-2044) and far future (2075-99) under the IPCC A1B scenario. MRI-AGCM3.2S is developed by the Meteorological Research Institute (MRI) and Japan Meteorological Agency (JMA). In this research, 84 stations are considered to calculate the rainfall percentage departure of Myanmar. 43 stations are considered to calculate the temperature change of Myanmar. The bias-correction was performed using two different techniques: linear scaling and lumped quantile mapping. Bias correction is capable of improving the GCM-simulated outputs to a certain degree. When only few stations are located within the region, these data sets do not capture the realistic distribution in this large area. In that case, it is difficult to obtain actual distribution from very limited number of observation. It is seen observed that lumped quantile mapping method is better than linear scaling method. Performance of bias correction was quantified by coefficient of determination (R2) and root mean square error (RMSE). It was seen that maximum temperature will increase 0.2 °C to 1.6 °C and 0.7 °C to 3.6 °C during (2020-2044 ) and ( 2075-2099), respectively. In some regions of Myanmar, rainfall is expected to increase in 2030s, and in most regions, rainfall is expected to increase in 2080s. Projections of future climates are the basis for climate change adaptation and disaster risk reduction planning.
Survey of the combination of sediment replenishment with an artificial flood as a measure to increase habitat suitability
Laboratoire de Constructions Hydrauliques (LCH), Ecole Polytechnique Federale de Lausanne
Station 18, LCH-ENAC-EPFL, CH-1015 Lausanne, Switzerland,
Mr. Severin Stahly is currently a PhD at Ecole Polytechnique Federale de Lausanne (Switzerland). He graduated as Environmental Engineer at the Swiss Federal Institute of Technology (ETH Zurich) in 2015. In his bachelor studies he followed the major Renewable Energy Technologies and in the master studies Hydraulic Engineering and Urban Water Management. He conducted his master thesis at the University of Auckland (New Zealand) comparing sampling methodologies of grain size distributions, using traditional pebble count and semi-automatized photo-sieving methods. After completing the master studies he moved to Lausanne where he started a PhD at the Laboratory of Hydraulic Constructions (LCH) at the Ecole Polytechnique Federale de Lausanne (EPFL). In his PhD he has been focusing on the further development of the hydro-morphological index of diversity (HMID), resulting in the definition of a holistic field sampling procedure. More recently, he has diversified his research area by incorporating the study of artificial flood events downstream of dams and the effect of sediment replenishment and their impact to restore the diversity of hydraulic habitats and geomorphological patterns. Since October 2018 he has been a visiting PhD at IHE Delft Institute for Water Educationat the Department of Water Science and Engineering collaborating with Prof. Mario Franca regarding ecological response of artificial floods.
Floodplains downstream of a dam, where the natural flow regime is replaced by a constant residual flow discharge, often lack sediment supply and periodic inundation due to the absence of natural flood events. In the work presented, a flood with a one-year return period was released from an upstream reservoir combined with downstream sediment replenishment. The aim was to enhance hydraulic habitat conditions in the Sarine river downstream of the Rossens dam in Switzerland where a constant residual discharge of about 3 m3/s is released since the dam construction in 1948.
Copyright: Research group ecohydraulics, ZHAW
A novel configuration of sediment replenishment was applied and consisted of four sediment deposits distributed as alternate bars along the river banks, a solution which was previously tested in laboratory. The morphological evolution of the replenishment and of the downstream riverbed were surveyed including pre- and post-flood topography. The hydro-morphological index of diversity (HMID) was used to evaluate the quality of riverine habitats in the analyzed reach. It is based on the variability of flow depth and flow velocity. The combination of the artificial flood with sediment replenishment proved to be a robust measure to supply a river with sediment and to enhance habitat suitability. As a comparison, the same pre- and post-flood analyses were conducted at the Spol river where artificial floods have been released periodically for more than 18 years, allowing the comparison of two river systems with a different management practice.
Dynamics of gravity currents flowing up a slope
Claudia Adduce, Associate Professor Department of Engineering Roma Tre University
Via Vito Voterra 62, 00146 Rome, Italy
Claudia Adduce is Associate Professor at the Department of Engineering of Roma Tre University (Italy), where she completed her PhD in Civil Engineering in 2004. She received the Arthur Thomas Ippen Award 2019 from IAHR. During her professional career, she spent invited research stays as Visiting Professor at several universities and research institutions as the Laboratory of Geophysical and Industrial Flows (France), Instituto Superior Tecnico (Portugal), Woods Hole Oceanographic Institution (USA) and Ecole Polytechnique Federale de Lausanne (Switzerland). Her research interests and contributions are in laboratory and numerical modelling of stratified flows as gravity currents and internal solitary waves, local scouring due to turbulent water jets, eddies interacting with seamounts and islands, sloshing due to stratified fluids. Her research has been published in 30 journal papers with 862 citations and h-index of 17. She has been the leader and coordinator of several international and national projects. She is member of the Editorial Board of Environmental Fluid Mechanics and Mathematical Problems in Engineering and she is member of the Leadership Team of the IAHR-Europe Regional Division and the Leadership Team of the IAHR Experimental Methods and Instrumentation Committee.
Gravity currents are flows, generated by a density difference between two fluids, caused by gradients of temperature, salinity or particles in suspension. Gravity currents can occur both in nature as oceanic overflows, sea breeze fronts, avalanches, turbidity currents, and in industrial processes as accidental gaseous releases or buoyancy-driven ventilation processes.
These currents flow over complex boundaries as salt wedges occurring in estuaries, propagating upslope over a complex bathymetry and affecting the river flow, sea breeze penetrating inland and interacting with an upsloping topography and the gravity currents flowing up a slope and produced by internal solitary waves breaking at the continental shelf. Along their path, gravity currents entrain ambient fluid with a decrease in density. The presence of a bottom upslope can affect the interfacial mixing with implications on sediment transport. Numerical models not resolving small-scale mixing processes use entrainment parameterizations affecting the evolution of gravity currents and need small-scale experiments as benchmark to reproduce interfacial mixing. To this purpose, a series of laboratory and numerical experiments (LES) simulating gravity currents flowing up a slope are performed.
A first series of laboratory and numerical experiments are carried out for low upslopes, by varying the aspect ratio of the initial volume of dense fluid in the lock R, to investigate the mixing processes between the dense current and the ambient fluid. As the gravity current moves up a slope, the dense layer becomes thinner, and an accumulation region of dense fluid in the initial part of the tank occurs. The current speed decreases as the bed upslope increases, and for the steepest upslopes, the gravity current stops before reaching the end of the tank. Entrainment and mixing in a lock-release gravity current are investigated by using different entrainment parameters and an energy budget method. The entrainment parameter depends on both Froude, Fr, and Reynolds, Re, numbers. In addition, the entrainment decreases as the steepness of the bottom increases. Irreversible mixing does not change during the slumping phase, while, during the following phases of motion, it decreases as th and R increase. When a gravity current starts to develop, mixing strongly depends on the evolution of Kelvin-Helmholtz billows. At a later stage, when these structures lose their coherence, three-dimensional features of the flow appear more evident and cause mixing. The analysis of the friction velocity suggests that for high th the tail region of the gravity current plays a key role in the transport of sediments.
A second series of laboratory experiments simulating both full-depth and partial-depth gravity currents flowing up a slope and spanning a wide range of bottom upslopes is performed. The presence of a steep upslope affects the flow causing an earlier transition between the different flow regimes. A relation predicting the length of the slumping phase depending on both th and the depth-ratio of the dense and ambient fluids is developed. For steep upslopes, the back flow is due to the dense current reflected by the upsloping bottom behaving more like an obstacle than as an upsloping bed. In addition, the entrainment parameter is deeply affected by the flow dynamics: it decreases as th increases till to reach a minimum value, beyond which, the gravity current “feels” the upslope as an obstacle
Bridging disciplines to explore the water food energy nexus
Dr. Caitlin Grady, Assistant Professor, Department of Civil and Environmental Engineering & Research Associate in the Rock Ethics Institute, Penn State University
Contact Information: ; www.gradylab.psu.edu
Dr. Caitlin Grady is currently an Assistant Professor of Civil and Environmental Engineering and Research Associate in the Rock Ethics Institute at Penn State. Previously she served as a Management Analyst and negotiator for the U.S. Department of State, an Energy Policy Analyst for the U.S. Department of Energy and a Legislative Assistant for The U.S. House of Representatives. She received her bachelor’s degree from Virginia Tech in Science and Technology Studies, and her master’s and doctoral degree from Purdue University in the Agricultural and Biological Engineering Department and the School of Civil and Environmental Engineering, respectively. Dr. Grady’s research interests revolve around the transdisciplinary nature of water resources, particularly within the water, energy, and food security international development community.
Water, food, and energy security remain the top development challenges of the decade, and perhaps the century. In recent decades, billions of people have obtained access to more food, better nutrition, electricity, improved water, and basic sanitation facilities worldwide. The negative consequences of lack of resources are enormous and include environmental degradation, political and economic insecurity, and social strife. On the other side of the spectrum, resource over-extraction has the potential to cause widespread interlinked climatological, human health, and global environmental consequences.
This presentation will showcase boundary spanning research on quantifying and understanding management strategies in the Water-Food-Energy Nexus. First, this presentation will explore bridging disciplines using tools derived from network theory and urban metabolism models to explore resource consumption in 65 major U.S. cities. Beyond the United States we will also discuss recent work on the next generation of a worldwide water, food, and energy integrated framework seeking to understand the state of multiple resources to improve resource management, economic outcomes, population livelihood, and public health.
Multi-scale fluvial remote sensing: from large scale planning to restoration monitoring
Herve Piegay, Fluvial Geomorphologist
Research Director at the National Center for Scientific Research (CNRS)
University of Lyon
Herve Piegay, research director at the National Center of Scientific Research, got his Ph.D. in 1995 on the interactions between riparian vegetation and channel geomorphology. Since 1995 he is continuing his studies at the University of Lyon (Ecole Normale Superieure of Lyon), France. He is a fluvial geomorphologist involved in integrated sciences for rivers, strongly interacting with hydraulic engineers, freshwater ecologists and practitioners (Water Agencies, Regions, Ministry of Ecology, French agency for biodiversity, Compagnie Nationale du Rhone, EDF). He is strongly involved in river management, planning and restoration, developing methodological frameworks and tools using GIS and remote sensing. He has contributed to more than 200 papers in peer-review journals and book chapters and has coordinated several edited books such as Tools in Fluvial Geomorphology – Handbook for ecologists and practitioners with M.G. Kondolf (2003, 2015), Gravel-bed rivers 6 : From process understanding to river restoration with H. Habersack and M. Rinaldi (2007) or fluvial remote sensing for science and management with P. Carbonneau (2012). He got in 2018 the Linton Award of the Bristish Society for Geomorphology.
Fluvial remote sensing is becoming a very critical issue to better understand river processes and changes, target management actions at regional scale, diagnose river status and promote adaptive strategies through intensive channel monitoring. A few examples from South-east France are introduced and discussed.
Sediments in suspension in a river, should they stay or should they go?
Carmelo Juez, Researcher
Institution: Instituto Pirenaico de Ecologia, Consejo Superior de Investigaciones Cientificas (IPE-CSIC), Campus de Aula Dei, Zaragoza, Spain.
He graduated as Industrial Engineer at Universidad de Zaragoza (Spain) in 2010. Afterwards, he conducted a Master degree in Fluid Mechanics, focusing on computational fluid-dynamics techniques. In 2010, he joined the Computational Hydraulics Group at Universidad de Zaragoza and he worked in both research and consulting projects, as well as a software developer for geomorphological flows (sediment transport in fluvial environments and landslides in steep areas). Additionally, efficient computation with up-to-date techniques, such as OMP parallelization and GPU, were also considered during his work. Finally, he obtained his PhD degree at Universidad de Zaragoza in 2014 and he was a post-doctoral associate at the Universite Catholique de Louvain (Belgium) prior to moving to Lausanne in 2015. His research at Ecole Polytechnique Federale de Lausanne (Switzerland) has been focused on assessing the hydraulic and morphological impact of restoration activities in rivers (e.g. bank lateral embayments or sediment replenishment). More recently, he has diversified his research area by incorporating the study of urbanization processes at the river basin scale. Urbanization processes imply changes in the land use and ultimately, the alteration of the hydraulic and geomorphological properties of the river basin.
River restoration works often include measures to promote morphological diversity and enhance habitat suitability. One of these measures is the creation of macro-roughness elements, such as lateral cavities and embayments, in the banks of channelized rivers. However, in flows that are heavily charged with fine sediments in suspension, such as glacier-fed streams and very low-gradient reaches of large catchment rivers, these lateral cavities may trap these sediments.
Consequently, the morphological changes may be affected, and the functionality of the restoration interventions may be compromised. In this seminar, a research study will be presented that aimed at evaluating the influence of these bank lateral embayments on the transport of fine sediments in the main channel. Multiple laboratory tests with different geometrical configurations of lateral embayments were tested with uniform flow charged with sediments. Surface PIV, sediment samples and temporal turbidity records were collected all through the experiments. The results of these experiments led to identify the channel geometry and the shallowness of the flow as the governing parameters of the sediment trapping efficiency of the bank lateral embayments. Furthermore, the morphological resilience to flow fluctuations of the fine sediment deposits settled inside the bank lateral embayments was also assessed. It was observed that the morphological resilience of the sediment deposits is strongly dependent on the flow field and the mass exchange between the main channel and the lateral embayments. This mass exchange is modulated by the geometry of the cavities and the magnitude of the hydrographs applied. The presentation will conclude with a comparative study of the performance of two numerical schemes based on the Shallow Water Equations: a 1-st order numerical scheme and an arbitrary order WENO-ADER scheme. The results obtained indicate that the 1-st order numerical scheme fails in reproducing the complexity of the flow present in a channel with bank lateral embayments (vortex shedding, gravitational waves) even when refining the mesh. On the contrary, the high order numerical scheme can cope with such flow features and it can thus be an attractive solution for modelling environmental flows in such bank lateral embayments.
Turbidity currents: Engineering and geological implications
Dr. Sequeiros currently works in the Shell Integrated Gas team in Rijswijk within the Metocean discipline. He has more than 10 years in the oil & gas and dredging industry. He received his bachelor’s degree in civil and hydraulic engineering from the National University of La Plata, Argentina. He has been interested in gravity flows since completing his Doctorate at the of the University of Illinois on sediment transport/erosion associated to turbidity currents.
Turbidity currents belong to the larger family of density currents, with the presence of sediment held in suspension by fluid turbulence differentiating them from other density currents driven by differences in temperature or concentration of dissolved substances. The weight of the suspended particles drives the water flow, instead of the water driving the sediment particles as it is for example the case of sediment transport in rivers. They are arguably the main mechanism of sediment transport from shallow to deep waters in submarine environments, resulting in the incremental development of sedimentary deposits called turbidites with potential for hydrocarbon reservoirs. Additionally, the study of turbidity currents has significant application for subsea and pipeline engineering as they can cause major damage to submarine telecommunication cables, pipelines, instrumentation, and equipment.
Picture from Dr. Sequeiro
Published on the front page of Sedimentology Vol 57 Issue 6. https://onlinelibrary.wiley.com/toc/13653091/2010/57/6 in relation to the paper https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-3091.2010.01152.x
Complex future: A story of water and the surfaces it shapes
Civil and Environmental Engineering Department, Faculty of Engineering, University of Auckland.
Heide Friedrich leads the Water-worked Environments Research Group (water.auckland.ac.nz) and is the Deputy Head (Research) in the Department of Civil and Environmental Engineering at the University of Auckland, New Zealand. She has over 15 years’ experience, both in industry and academia, having worked and lived in Germany, Taiwan, UK, Australia and NZ. Her main research focus is on studying the physical processes in natural aquatic environments, such as rivers, and how water interacts with and shapes its surroundings.
Historically, hydraulic engineering was one of science’s leading edge disciplines. Design standards have somewhat simplified the work of a hydraulic engineer in the last century, but the fundamentals of hydraulic engineering are still governed by extreme complexity. More recently, we have seen hydraulic engineering laboratories moving away from the more traditional hydraulic structure projects, to areas such as general fluid mechanics and ecohydraulics. Yet, physical hydraulic modelling is also making a comeback in recent times. I present, from a hydraulic engineering laboratory viewpoint, the change that is presently taking place on how we study large-scale water resources processes, thus tackling problems of vital importance to a prosperous society, often associated with natural hazards, e.g. flood risk, river morphological changes, tsunami resilience. I present work from my own research group to show the challenges we deal with. Our future looks rosy, especially if we team up with the various water-associated stakeholders.
Fine-sediment bedform pattern (left); near-boundary flow measurement (middle); sediment-laden flow (right).