Seminars

seminar room safl

Every other week during the academic year, SAFL hosts prominent figures in environmental science and fluid mechanics. They come from all over the US and the world to share their insight and inspire us to tackle important questions in the field. These seminars are free and open to the public. Join us to learn about the latest research advancements and network with contacts in the field.


SAFL seminars are held on Tuesdays from 3:00 to 4:15 p.m. unless otherwise noted. Join us in the SAFL Auditorium or via Zoom.

 
Spring 2024 Seminar Series
Tuesday, Jan 23-Katey Anthony
Tuesday, Feb 6th-No Seminar 
Tuesday, Feb 20th-Neal Iverson
Tuesday, March 12- Jennifer Stucker 
 
Tuesday, March 26th-Mike Shelley
Tuesday, April 9th-Sergio Fagherazzi
Tuesday, April 23rd-Ruben Juanes
Tuesday, May 7th-Walter Musial

Recordings
We will record seminars and post them here when given permission by the speaker. To see if a recording is available, scroll down this page to "Past Seminars."

Seminar Notifications
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Upcoming Seminars

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Past Seminars

Water Quality and Stratified Flow: How Density Currents Mitigate the Effects of Point and Nonpoint Loadings

Inaugural Heinz G. Stefan Fellowship Award Ceremony

Keynote Speaker: Dennis Ford, President and Principal Hydrologist, FTN Associates (and one of Dr. Stefan's first graduate students!)

2016 Award Recipient: Maria Garcia-Serrana, PhD Candidate, Department of Civil, Environmental, and Geo-Engineering, University of Minnesota (advisor John Gulliver)

 

Lake Catherine is a run of the river hydropower reservoir located on the Quachita River southeast of Hot Springs, AR in central Arkansas.  It is located immediately downstream of Lakes Hamilton and Quachita which are also hydropower projects.  Lake Quachita is a large Corps of Engineers impoundment that provides flood control and storage for the system of three lakes and is characterized by excellent water quality.  Cold hypolimnetic releases from Lake Quachita pass through both Lake Hamilton and Lake Catherine as density currents. The surface waters of Lake Catherine are typically much cooler than the upstream lakes.  Since Lakes Catherine and Hamilton have no storage, they are normally operated in series so that releases from Lake Hamilton coincide with releases from Lake Catherine.  The flow regime in Lake Catherine is a complex, dynamic combination of riverine and lake water movements.  During the summer months, the peaking hydropower operations result in reverse currents that move surface water back upstream after hydropower shuts down.

The water quality of Lake Catherine is impacted by 18 point sources, which enter the Lake either directly or indirectly, and nonpoint source runoff from urban and industrial areas.  By any measure of eutrophication, Lake Catherine should be characterized by poor water quality but it’s not because of complex, stratified flow dynamics.  The City of Hot Springs WWTP discharges directly into Lake Catherine immediately below Lake Hamilton through a submerged outfall.  The buoyant plume is quickly mixed with the cold water releases from Lake Hamilton and, after mixing, passes through Lake Catherine as a density current with minimal exposure to light.  An industrial discharger located near the middle of the lake uses a submerged multi-port high rate diffuser to quickly mix the highly dense effluent with ambient lake water, which eventually falls back to lake bottom and moves as a density current.  To minimize water quality impacts, the diffuser is operated as a hydrograph controlled release with a unique design that aspirates upstream dilution water to maximize dilution and keep the diffuser operating efficiently.  Near the dam, an electric generating plant draws cool, hypolimnetic water for cooling and discharges it back to the surface as a buoyant thermal plume.

Over the years, there has always been public interest with the water quality of Lake Catherine because of the large number of point/non-point sources to the Lake. To address the public, the State Legislature asked the Arkansas Department of Environmental Quality (ADEQ) to conduct a water quality study in 2013.  ADEQ concluded that it was safe to drink the water, eat the fish, and enjoy primary contact recreation. Our 30 years of study indicate how complex stratified flow dynamics mitigate the many point and non-point sources to the lake.

Linking scales of sediment dynamics from sand grains to the synoptic

Joe Calantoni, Naval Research Laboratory

In the Sediment Dynamics Section at the U.S. Naval Research Laboratory we perform basic and applied research focused on understanding seafloor, estuarine, and riverine sediments. We are motivated by the need to predict the dynamical properties of sediments. A multi-disciplinary team of scientists and engineers works in a collaborative environment to simulate, model and observe phenomena in both the laboratory and field at scales from the motions of individual sand grains immersed in fluid up to tens of kilometers and several days. Simulation and modeling efforts are focused on a new probabilistic paradigm to bridge the gap from grain scale physics to large-scale morpohdynamics. We propose to utilize a hierarchy of computationally intensive, high fidelity simulations to populate a probabilistic framework to make predictions across a range of cascading length and time scales. The success of our approach relies on rigorous validation of our high fidelity simulations using detailed laboratory and field measurements of fluid-particle turbulence at the scales of interest. Recent advances in optical imaging techniques have made it possible to make highly resolved three-dimensional measurements of fluid-particle turbulent interactions in the laboratory with spatial and temporal resolutions at or near the Kolmogorov scale. Work is ongoing to transition these technologies for use in the field. Synoptic field observational efforts are focused on combining remote sensing with in situ measurements to provide both validation for emerging predictive capabilities and optimization for data assimilation and boundary conditions for operational forecasting. We will present an overview of results from our modeling efforts along with relevant laboratory and field observations.

Visualizing Change – Communicating Environmental Change at Different Scales to Public Audiences

Pat Hamilton, Director of Global Change Initiatives, Science Museum of Minnesota

The Science Museum of Minnesota has made communicating about the implications of a human-dominated environmental change a priority through its Global Change program. In addition to the museum’s environmental exhibits, Patrick Hamilton reaches audiences beyond the walls of the institution to help a wide range of audiences better comprehend the reality and implications of humanity as the driving agent of environmental change at all scales – global, regional, local.

Evolution and instabilities of helical vortices in rotor wakes

Andras Nemes, Postdoctoral Scholar, St. Anthony Falls Laboratory, University of Minnesota

The primary feature in the near wakes of open rotor systems including wind turbines, helicopter rotors and marine propellers are helical vortex structures formed by the rotating blades. In all these application these helical vortices constitute an undesirable characteristic of the wake – contributing to increased turbulence in the far wake, fatigue loading on downstream structures, noise generation and dangerous flight aerodynamics. In this talk we review some recent research into helical vortices and present water channel experiments investigating the evolution of the vortex structures behind a model rotor using particle image velocimetry. Measurements recover the primary features of the near wake featuring tip and root vortices and their evolution as a function of tip speed ratio of the rotor. The dynamics of the tip vortex configuration is then related to the instability mechanisms of helical vortex filaments finding that the symmetry breakdown is dominated by the mutual inductance of neighboring helices. The destabilisation of the helical vortices is shown to be related to the growth rates predicted by inviscid linear stability analysis. Finally, results of phase-locked and Lagrangian measurements of the tip vortices are shown which reveal the presence and mode structures of shortwave instabilities on the vortex core.

High-Fidelity CFD and Applications in Environmental Fluid Flow Study

Lian Shen, Benjamin Mayhugh Associate Professor, Dept. of Mechanical Engineering, and Associate Director of Research, St. Anthony Falls Laboratory, University of Minnesota

With the growth in computer power, computational fluid dynamics (CFD) has been playing an increasingly important role in the study of fluid mechanics with a wide range of applications in environmental and industrial problems.  High-fidelity CFD, which is built on highly-accurate numerical schemes based on flow physics and utilizes high-performance computing on massively parallel computers, is especially valuable for faithfully capturing flow physics of complex problems and revealing the underlying dynamics.  In this talk, I am going to present a suite of numerical tools developed in my group for the simulations of nonlinear waves, atmospheric boundary layers over water bodies, and turbulence in upper oceans and lakes.  A number of our current projects related to flows in natural environment will also be introduced briefly.

Implications of river channel self-organization for climate and landscape evolution

Colin Phillips, Postdoctoral Scholar, St. Anthony Falls Laboratory, University of Minnesota

Evaluating the role of meteorological climate (storms) in the evolution of mountain landscapes requires understanding how coarse-grained rivers remove eroded rock and debris via water-driven sediment transport. Short hydrological records and limited observations preclude rare events making it difficult to determine the role of a changing climate on the frequency and magnitude of sediment transporting floods. To understand the effects of extreme events on river sediment transport we focus on a detailed case study of sediment motion and channel geometry in the Mameyes River located in Northeast Puerto Rico, a river with frequent flash floods and extreme discharge variability. Sediment transport and channel surveys suggest that despite large floods the river channel remains adjusted to be near the threshold for coarse sediment motion. We demonstrate that the results from the Mameyes River are general by analyzing channel geometry and stream-flow records from 186 coarse-grained rivers across the United States. We find that channels adjust their geometry such that floods slightly exceed the stress required to transport bed sediment – regardless of widely-varying climatic, tectonic, and lithologic controls. Remarkably, the distribution of fluid stresses associated with floods is consistent, indicating that self-organization of near-critical channels filters the climate signal evident in discharge. This effect blunts the impact of extreme rainfall events on landscape evolution. Coupling these findings with recent experimental results suggests a simplistic treatment of climatic forcing in long-term mountain landscape evolution models.

Simulating the hydrologic impact of distributed flood mitigation practices and tile drainage in an agricultural catchment

Nicholas Thomas,Postdoctoral Scholar, University of Iowa

In 2008 flooding occurred over a majority of Iowa, damaging homes, displacing residents, and taking lives. In the wake of this event, the Iowa Flood Center was charged with the investigation of distributed flood mitigation strategies to reduce the frequency and magnitude of floods in Iowa. This work focused on the application of a numerical model to quantify the impact of flood mitigation strategies in an agricultural watershed. Variability in peak flow impact was a product of antecedent soil moisture, 24-hour design storm total depth, and initial structural storage capacity. The highest peak flow reductions occurred in scenarios with dry soil, empty project storage, and low rainfall depths. Peak flow reductions were estimated to dissipate approximately 2 km downstream of the small watershed outlet. Additional investigation into tile drainage illustrated the hydrologic impact of the commonly applied agricultural practice. A numerical tracer analysis identified the contribution of tile drainage to stream flow which varied through an annual cycle as a product of meteorological forcing. Beyond the analysis of individual agricultural features, this work assembled a framework to analyze the feature at the field scale for implementation at the watershed scale. It showed large scale simulations reproduce field scale results well. The product of this work was, a systematic hydrologic characterization of distributed flood mitigation structures, and pattern tile drainage systems facilitating the simulation of each practice in a physically-based coupled surface-subsurface model.

Field applications of hydrogeochemical modeling: What we can learn about complex interactions between transport, geochemistry, and biology

Gene-Hua Crystal Ng, Assistant Professor, Department of Earth Sciences, University of Minnesota

The spread of contaminants through our groundwater systems involve complex interactions among physical, geochemical, microbial, and plant ecological processes.  Reactive-transport models provide a tool for evaluating these dynamical links.  Two examples will be presented that demonstrate how field observations and reactive-transport modeling together generate insights into the multiple factors controlling contaminant fate.  The first example looks at secondary water quality impacts at a crude oil spill site near Bemidji, MN, which has served as a long-term research site for over 35 years.  Its uniquely extensive dataset was used to develop a reactive-transport model that describes secondary plumes in aquifers - such as those of iron and methane - that are triggered as the primary hydrocarbon contaminant biodegrades.  The second example describes on-going research on the Iron Range in northeast Minnesota examining groundwater's influence on how high sulfate concentrations in lakes and streams impact wild rice.  A suite of newly collected physical and geochemical field measurements are integrated with a reactive-transport model to evaluate how the regional groundwater system and surface water interact with the stream bed hyporheic zone where wild rice grows.  In both examples, a process-based model represent coupled interactions not readily measured, while data are crucial for ensuring that models simulate field-scale relevant processes. 

Model investigation of Lake Superior circulation and biogeochemistry

Katsumi Matsumoto, Professor, Department of Earth Sciences, University of Minnesota

Lake Superior is the largest lake in the world by surface area and provides essential resources to the surrounding cities. Yet there are a number of gaps in knowledge regarding its physical and biogeochemical properties. Here I use a ROMS-based numerical model of Lake Superior in an attempt to fill some of those gaps.

Air-Sea Gas Exchange in High Winds

Christopher Zappa, Lamont Associate Research Professor, Lamont-Doherty Earth Observatory, Columbia University

Poor understanding of the complex physical controls of air-sea exchanges under high winds, in particular with respect to uptake and release of greenhouse gases, remains one of the major uncertainties in biogeochemical models and climate predictions. Adequate characterization of gas transfer across the air-sea interface is not only essential to quantify local and global sinks and sources of CO2 but also to budget many other trace gases that influence Earth’s radiation.

At high wind speed, breaking waves become a key factor to take into account when estimating gas fluxes. Breaking results in additional upper ocean turbulence and generation of bubble clouds. Efforts have been made towards including the effect of bubble mediate transfer to reduce the uncertainties around gas transfer velocity K estimates at high wind speed. These parameterizations model the transfer velocity due to breaking as a function of fractional whitecap coverage (W) or windsea Reynolds numbers.

Limitations in these gas transfer models arise from the large scatter in W parameterizations that is not yet fully understood. Recent work has focused on linking W variability to wave field statistics in addition to wind speed. Other limitations arise from the lack of observations at high wind speed. While gas transfer coefficients and W have regularly been obtained under winds of up to ~15 m s-1, few gas transfer measurements at wind speeds of 15 to 30 m s-1 exist, and almost none with coincident wave physics observations.

The High Wind Gas exchange Study (HiWinGS) offers a diverse data set that presents the unique opportunity to gain new insights on the poorly understood aspects of air-sea interaction under high winds.  The HiWinGS cruise took place in the North Atlantic during October and November 2013. Wind speeds exceeded 15 m s-1 25% of the time amounting to a total of 189 hours of wind speeds above 15 m s-1 of which 48 hours wind speeds greater than 20 m s-1.  On October 25th, wind speeds exceeded 25 m s-1 with gusts of 35 m s-1 during the St Jude storm. Continuous measurements of turbulent fluxes of heat, momentum and gas were taken from the bow of the R/V Knorr. Visible imagery was acquired from the port and starboard side of the flying bridge during daylight hours at 20Hz and directional wave spectra were obtained when on station from a wave rider buoy. Additional wave field statistics were computed from a laser altimeter as well as from a Wavewatch III hindcast.

Taking advantage of the range of physical forcing and variable wave conditions sampled during HiWinGS we investigate how W and K vary with sea state, contrasting pure windseas to swell dominated periods. We distinguish between wind seas and swell based on a separation algorithm applied to directional wave spectra as described in [Hanson and Philips, 2001]. For mixed sea, system alignment is considered when interpreting results.  

The role of bubble-mediated transfer depends on gas solubility. The four gases (CO2, DMS, acetone, and methanol) sampled during HiWinGS ranged from being mostly waterside controlled to almost entirely airside controlled. While bubble-mediated transfer appears to be small for moderately soluble gases like DMS, the importance of wave breaking turbulence transport has yet to be determined for all gases regardless of their solubility. This will be done by correlating measured gas transfer velocities to estimates of active whitecap fraction (WA) and turbulent kinetic energy dissipation rate (ε). WA and ε are estimated from moments of the breaking crest length distribution derived from the imagery, focusing on young seas, when it is likely that large-scale breaking waves (i.e., whitecapping) will dominate the TKE dissipation rate.