Large scale PIV-measurements on the water surface of turbulent open-channel flow

An experimental open-channel laboratory study of secondary currents and surface boils is presented. Surface velocity dynamics over a large measuring field were investigated using Large Scale Particle Image Velocimetry (LSPIV). Data were analyzed for the structures of the large streamwise vortices and surface boils. The results indicate a mean multi-cellular pattern of faster and slower primary longitudinal velocities. The mean transversal velocities show a corresponding pattern. The spacing of the surface pattern of upwelling and downwelling is related to the water depth. The instantaneous pattern of the secondary currents meanders around the mean pattern. Vortex boils occur near the upwelling regions. Details of the vortex structures, in particular the growth and spreading of individual vortices when moving along the flow, are detected from instantaneous velocity maps of the water surface using a moving-camera PIV which follows the flow at the mean surface current speed

Shear velocity estimates in rough-bed open-channel flow

Shear velocity u(*) is an important parameter in geophysical flows, in particular with respect to sediment transport dynamics. In this study, we investigate the feasibility of applying five standard methods [the logarithmic mean velocity profile, the Reynolds stress profile, the turbulent kinetic energy (TKE) profile, the wall similarity and spectral methods] that were initially developed to estimate shear velocity in smooth bed flow to turbulent flow over a loose bed of coarse gravel (D-50=15cm) under sub-threshold conditions. The analysis is based on quasi-instantaneous three-dimensional (3D) full depth velocity profiles with high spatial and temporal resolution that were measured with an Acoustic Doppler Velocity Profiler (ADVP) in an open channel. The results of the analysis confirm the importance of detailed velocity profile measurements for the determination of shear velocity in rough-bed flows. Results from all methods fall into a range of +/- 20% variability and no systematic trend between methods was observed. Local and temporal variation in the loose bed roughness may contribute to the variability of the logarithmic profile method results. Estimates obtained from the TKE and Reynolds stress methods reasonably agree. Most results from the wall similarity method are within 10% of those obtained by the TKE and Reynolds stress methods. The spectral method was difficult to use since the spectral energy of the vertical velocity component strongly increased with distance from the bed in the inner layer. This made the choice of the reference level problematic. Mean shear stress for all experiments follows a quadratic relationship with the mean velocity in the flow. The wall similarity method appears to be a promising tool for estimating shear velocity under rough-bed flow conditions and in field studies where other methods may be difficult to apply. This method allows for the determination of u(*) from a single point measurement at one level in the intermediate range (036). Copyright (c) 2013 John Wiley & Sons, Ltd

Surface Water Temperature Heterogeneity at Subpixel Satellite Scales and Its Effect on the Surface Cooling Estimates of a Large Lake: Airborne Remote Sensing Results From Lake Geneva

The dynamics of spatial heterogeneity of lake surface water temperature (LSWT) at subpixel satellite scale O(1 m) and its effect on the surface cooling estimation at typical satellite pixel areas O(1 km2) were investigated using an airborne platform. The measurements provide maps that revealed spatial LSWT variability with unprecedented detail. The cold season data did not show significant LSWT heterogeneity and hence no surface cooling spatial variability. However, based on three selected daytime subpixel‐scale maps, LSWT patterns showed a variability of >2 °C in the spring and >3.5 °C in the summer, corresponding to a spatial surface cooling range of >20 and >40 W/m2, respectively. Due to the nonlinear relationship between turbulent surface heat fluxes and LSWT, negatively skewed LSWT distributions resulted in negatively skewed surface cooling patterns under very stable or predominantly unstable atmospheric boundary layer (ABL) conditions and positively skewed surface cooling patterns under predominantly stable ABL conditions. Implementing a mean spatial filter, the effect of area‐averaged LSWT on the surface cooling estimation up to a typical satellite pixel was assessed. The effect of the averaging filter size on the mean spatial surface cooling values was negligible, except for predominantly stable ABL conditions. In that situation, a reduction of ~3.5 W/m2 was obtained when moving from high O(1 m) to low O(1 km) pixel resolution

Circulation patterns in open, wide and deep lacustrine embayments with different aspect ratios

Numerical simulations were carried out to examine the effect of different horizontal aspect ratios on wind-induced circulation within open, wide and deep lacustrine embayments. Two adjacent embayments in Lake Geneva, Switzerland, located between Morges and Lausanne on the north shore of the lake, were compared. These embayments were selected because of their similar exposure with respect to wind forcing. However, their aspect ratios are different, making them ideal case studies to evaluate the impact of the aspect ratio on the resulting circulation pattern. A previously validated 3D hydrodynamic model (Delft3D-FLOW) was employed to simulate currents in the lake for the year 2010. Detailed over-lake maps of wind, temperature and humidity were used as input to drive the model. The embayments were compared in terms of circulation patterns and conditions leading to gyre formation. Furthermore, we carried out a systematic comparison, focusing on the embayment aspect ratio by constructing different “synthetic” embayments with aspect ratios in the range 1 - 4. Subsequently, the flow field within these embayments was computed for typical meteorological regimes (dominant wind regimes and seasonality). In particular, the generation of gyres was compared for different cases. The results revealed that, even for large aspect ratios (~ 3), wind-induced circulation can still occur in open, wide, and deep lacustrine embayments. The results showed that, as expected, gyres are formed less frequently as the embayment aspect ratio increases. For this reason, gyres are much more probable in the Morges embayment than in Vidy Bay

Collaborative Web-GIS platform for systematic exploration of Lake Geneva

We have developed and deployed a Web-based GIS data management framework, which facilitates an effective and highly structured search, retrieval and visualisation of multi-modal scientific data, as well as its subsequent dissemination in multiple and standardised forms beneficial for both the research partners involved in the project and the general public. In the context of the long term objectives of the Élémo project, the developed methodology may be utilised for automated and systematic collection of the multifaceted scientific data and with the goal of assembling a comprehensive database encompassing all aspects of currently planned and future scientific investigations

Subgrid-Scale Dynamics of Water Vapour, Heat, and Momentum over a Lake

We examine the dynamics of turbulence subgrid (or sub-filter) scales over a lake surface and the implications for large-eddy simulations (LES) of the atmospheric boundary layer. The analysis is based on measurements obtained during the Lake-Atmosphere Turbulent EXchange (LATEX) field campaign (August-October, 2006) over Lake Geneva, Switzerland. Wind velocity, temperature and humidity profiles were measured at 20Hz using a vertical array of four sonic anemometers and open-path gas analyzers. The results indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces, suggesting that the effect of the lake waves on surface-layer turbulence during LATEX is small. The measurements allowed, for the first time, the study of subgrid-scale turbulent transport of water vapour, which is found to be well correlated with the transport of heat, suggesting that the subgrid-scale modelling of the two scalars may be coupled to save computational resources during LE

Transport and mixing in Lac Léman

Lake Geneva (Lac Léman) is the largest freshwater body in Western Europe. It is a deep peri-alpine lake whose importance stems from being not only an essential freshwater source in the region, but also a major tourist destination, a fishery and a waterway. Its dynamics have been the subject of long-term monitoring and study, and its response patterns to wind forcing (the major forcing) are relatively well understood (e.g. Lemmin et al. 2005). On the other hand, the large-scale organisation of the water circulation is less well known; the associated transport properties are even less clear. The interest in the transport of water parcels inside the lake is linked to the inflow of sediments and pollutants from the tributaries, in particular from the Rhône River, the major one in terms of water and sediment discharged into the lake. Most of the sediments entering the lake through the Rhône are believed to sink in the eastern part of the lake (Giovanoli, 1990). More recently, Halder et al. (2013) traced, using stable isotopes, water parcels from the Rhône River in the entire lake basin. This study demonstrates that the water entering the lake from its main tributary has a complex distribution inside the whole basin. How this distribution is established and evolves in time is, however, mostly unknown. The Ecological Engineering Laboratory of EPFL is trying to shed further light on this issue, by combining observational and numerical modelling tools. In particular, up to 6 Acoustic Doppler Current Profilers (ADCP’s) have been simultaneously deployed at various locations inside the lake. This data, together with available historical data, is being used to validate a hydrodynamic model of the lake, implemented using MITgcm code. Various passive-tracer release experiments were conducted using the numerical model, investigating the relative importance of wind-forcing, depth of release, stratification, and the Rhône discharge rate, for the spreading and mixing of the tracers. The preliminary numerical results confirm that the northern and southern coastal regions are preferred initial pathways for the transport of the Rhône discharge. More interestingly, the numerical simulations unmistakeably show that the transport of the Rhône River water inside the lake is highly inhomogeneous in space, and highly intermittent in time, even ignoring the discharge variability itself. This intermittency should be taken into account, in particular when interpreting point measurements, isolated in time. From a practical point of view, this is likely to have an important effect on the nutrient and oxygen availability, as well as on the concentration of pollutants. From a more fundamental point of view, this study contributes to further understanding the mixing processes in rotating, stratified flows at length scales where rotation is an important but not the only dynamic process (Rossby number small but non-zero)

Lake Geneva: A natural laboratory for transport processes

Dynamics in Lake Geneva are analysed in view of recent observational and numerical advances. The classical linear internal wave view of Lake Dynamics is complemented by the description of near-shore, vortex-like nonlinear motions. The development of these structures is linked to bottom friction. Preliminary results from Lagrangian particle tracking simulations suggest that their existence is important for the transport of scalars within the lake

Nonlinear dynamics of the near-shore boundary layer of a large lake (Lake Geneva)

We examine near-shore and pelagic current variability in Lake Geneva, a large and deep lake in western Europe, using observations from several measurement locations and a three-dimensional numerical model for the period 2014-2016. Linear internal seiche modes excited by wind forcing clearly appear as peaks in the energy spectra for measurements in off-shore locations. In contrast, spectra from the near-shore data, where currents interact withthe lake bed, reveal a negligible contribution of internal seiches to the total kinetic energy. A similar contrast is seen in the spectra obtained from the numerical model at the same locations. Comparing the contribution of the different terms in the vertically-averaged momentum equation from the modeling results shows that the nonlinear advective term dominates in the near-shore boundary layer. Its contribution decays with distance from shore. The width of this near-shore boundary layer, which may extend for several kilometers, seems to be mainly determined by local topography. Both field measurements and modeling results indicate that nonlinear dynamics are of primary importance in the near-shore boundary layer

Achieving high-resolution thermal imagery in low-contrast lake surface waters by aerial remote sensing and image registration

A two-platform measurement system for realizing airborne thermography of the Lake Surface Water Temperature (LSWT) with ~0.8 m pixel resolution (sub-pixel satellite scale) is presented. It consists of a tethered Balloon Launched Imaging and Monitoring Platform (BLIMP) that records LSWT images and an autonomously operating catamaran (called ZiviCat) that measures in situ surface/near surface temperatures within the image area, thus permitting simultaneous ground-truthing of the BLIMP data. The BLIMP was equipped with an uncooled InfraRed (IR) camera. The ZiviCat was designed to measure along predefined trajectories on a lake. Since LSWT spatial variability in each image is expected to be low, a poor estimation of the common spatial and temporal noise of the IR camera (nonuniformity and shutter-based drift, respectively) leads to errors in the thermal maps obtained. Nonuniformity was corrected by applying a pixelwise two-point linear correction method based on laboratory experiments. A Probability Density Function (PDF) matching in regions of overlap between sequential images was used for the drift correction. A feature matching-based algorithm, combining blob and region detectors, was implemented to create composite thermal images, and a mean value of the overlapped images at each location was considered as a representative value of that pixel in the final map. The results indicate that a high overlapping field of view (~95%) is essential for image fusion and noise reduction over such low-contrast scenes. The in situ temperatures measured by the ZiviCat were then used for the radiometric calibration. This resulted in the generation of LSWT maps at sub-pixel satellite scale resolution that revealed spatial LSWT variability, organized in narrow streaks hundreds of meters long and coherent patches of different size, with unprecedented detail