Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing

To prevent the recurrence of a disastrous eruption of carbon dioxide (CO2) from Lake Nyos, a degassing plan has been set up for the lake. Since there are concerns that the degassing of the lake may reduce the stability of the density stratification, there is an urgent need for a simulation tool to predict the evolution of the lake stratification in different scenarios. This paper describes the development of a numerical model to predict the CO2 and dissolved solids concentrations, and the temperature structure as well as the stability of the water column of Lake Nyos. The model is tested with profiles of CO2 concentrations and temperature taken in the years 1986 to 1996. It reproduces well the general mixing patterns observed in the lake. However, the intensity of the mixing tends to be overestimated in the epilimnion and underestimated in the monimolimnion. The overestimation of the mixing depth in the epilimnion is caused either by the parameterization of the k-epsilon model, or by the uncertainty in the calculation of the surface heat fluxes. The simulated mixing depth is highly sensitive to the surface heat fluxes, and errors in the mixing depth propagate from one year to the following. A precise simulation of the mixolimnion deepening therefore requires high accuracy in the meteorological forcing and the parameterization of the heat fluxes. Neither the meteorological data nor the formulae for the calculation of the heat fluxes are available with the necessary precision. Consequently, it will be indispensable to consider different forcing scenarios in the safety analysis in order to obtain robust boundary conditions for safe degassing. The input of temperature and CO2 to the lake bottom can be adequately simulated for the years 1986 to 1996 with a constant sublacustrine source of 18 l s−1 with a CO2 concentration of 0.395 mol l−1 and a temperature of 26 °C. The results of this study indicate that the model needs to be calibrated with more detailed field data before using it for its final purpose: the prediction of the stability and the safety of Lake Nyos during the degassing proces

Utilisation thermique des eaux superficielles

Les eaux superficielles suisses renferment d’immenses réserves d’énergie thermique renouvelable, dont une fraction pourrait servir à chauffer et refroidir les infrastructures proches. Une telle utilisation pourrait avoir des impacts, notamment via les rejets d’eau réchauffée ou refroidie. En se basant sur de nombreuses études, cet article détaille ces impacts et propose des recommandations concrètes visant à les minimiser et à garantir une exploitation durable

Effects of Lake–Reservoir Pumped-Storage Operations on Temperature and Water Quality

Pumped-storage (PS) hydropower plants are expected to make an important contribution to energy storage in the next decades with growing market shares of new renewable electricity. PS operations affect the water quality of the connected water bodies by exchanging water between them but also by deep water withdrawal from the upper water body. Here, we assess the importance of these two processes in the context of recommissioning a PS hydropower plant by simulating different scenarios with the numerical hydrodynamic and water quality model CE-QUAL-W2. For extended PS operations, the results show significant impacts of the water exchange between the two water bodies on the seasonal dynamics of temperatures, stratification, nutrients, and ice cover, especially in the smaller upper reservoir. Deep water withdrawal was shown to strongly decrease the strength of summer stratification in the upper reservoir, shortening its duration by ~1.5 months, consequently improving oxygen availability, and reducing the accumulation of nutrients in the hypolimnion. These findings highlight the importance of assessing the effects of different options for water withdrawal depths in the design of PS hydropower plants, as well as the relevance of defining a reference state when a PS facility is to be recommissioned

Lake surface temperatures in a changing climate: a global sensitivity analysis

We estimate the effects of climatic changes, as predicted by six climate models, on lake surface temperatures on a global scale, using the lake surface equilibrium temperature as a proxy. We evaluate interactions between different forcing variables, the sensitivity of lake surface temperatures to these variables, as well as differences between climate zones. Lake surface equilibrium temperatures are predicted to increase by 70 to 85% of the increase in air temperatures. On average, air temperature is the main driver for changes in lake surface temperatures, and its effect is reduced by ~10% by changes in other meteorological variables. However, the contribution of these other variables to the variance is ~40% of that of air temperature, and their effects can be important at specific locations. The warming increases the importance of longwave radiation and evaporation for the lake surface heat balance compared to shortwave radiation and convective heat fluxes. We discuss the consequences of our findings for the design and evaluation of different types of studies on climate change effects on lakes

THERMISCHE NUTZUNG VON OBERFLÄCHENGEWÄSSERN

Les eaux superficielles suisses renferment d’immenses réserves d’énergie thermique renouvelable, dont une fraction pourrait servir à chauffer et refroidir les infrastructures proches, remplaçant combustibles et électricité. Une telle utilisation repose sur des techniques éprouvées et de nombreux systèmes sont en fonctionnement ou planifiés en Suisse et à travers le monde. Le principe consiste à utiliser l’eau d’un lac, d’une rivière ou d’une nappe phréatique pour en extraire ou y rejeter de la chaleur, selon les besoins. Les lacs profonds et les grandes rivières sont particulièrement adaptés à cet objectif. La technique implique une modification de la température de l’eau utilisée: généralement un refroidissement en hiver et un réchauffement en été. Les rejets de l'eau utilisée peuvent potentiellement avoir des conséquences physicochimiques, mais aussi écologiques pour les organismes et écosystèmes aquatiques. Différentes études indiquent que le risque principal se situe dans la sensibilité de plusieurs espèces aux températures trop chaudes – typiquement au-dessus de 25 °C. C’est cependant essentiellement le changement climatique qui met les écosystèmes sous cette pression. Un réchauffement additionnel pourrait ainsi péjorer la situation des espèces vulnérables (p. ex. les truites) au profit d’autres (p. ex. les carpes). Par ailleurs, une altération de la température peut perturber le développement, le comportement et la reproduction des organismes, avec au final d’éventuels effets sur la composition et le fonctionnement de l’écosystème. Les refroidissements modérés générés lors de l’utilisation pour le chauffage sont souvent peu critiques. Une conception réfléchie des systèmes d’extraction de chaleur et de froid permettrait de minimiser les impacts possibles et d’exploiter ces ressources thermiques de manière durable

POTENTIEL DES LACS ET RIVIÈRES SUISSES

Les eaux superficielles suisses renferment d’immenses réserves d’énergie thermique renouvelable, dont une fraction peut servir à chauffer et refroidir les infrastructures proches. Cet article présente une estimation du potentiel thermique des principaux lacs et rivières suisses, compare ce potentiel à la demande régionale et résume les considérations et difficultés techniques inhérentes à l’utilisation de cette énergie thermique

The role of double diffusion for the heat and salt balance in Lake Kivu

Double diffusion in lakes and oceans can transform vertical gradients into staircases of convectively mixed layers separated by thin stable interfaces. Lake Kivu is an outstanding double-diffusive natural laboratory with > 300 such steps over the permanently stratified deep basin. Here, we use 315 microstructure profiles (225 measured in Rwanda and 90 in the DRC) to shed light on the heat and salt balances of Lake Kivu. Comparing profiles from 2011 and 2015 reveals warming of 8.6 mK yr−1 below 80 m depth and negligible changes in salinity. The double-diffusive layering is coherent over horizontal distances of 20–30 km and remained unchanged between 2011 and 2015, indicating little variability. The mean estimated dissipation within mixed layers is 1.5 × 10−10 W kg−1. If unshaped Batchelor microstructure spectra are interpreted as nonturbulent, the rescaled dissipation of 0.44 × 10−10 W kg−1 corresponds to a vertical heat flux of 0.10 W m−2, which agrees with the molecular heat flux through the adjacent stable interfaces. Using estimates of upwelling, temporal changes of temperature and salt, and vertical double-diffusive fluxes, we established heat and salt balances, which require lateral heat and salt inputs. For salt, lateral input of freshwater at the main gradients balances upwelling. For temperature, however, the divergence of the vertical double-diffusive fluxes can only be balanced by horizontal inputs supplying cool water above and warm water below the main gradients. This suggests that lateral inputs of water at various depths are the main drivers for this unique double-diffusive phenomenon in Lake Kivu

Combined effects of pumped-storage operation and climate change on thermal structure and water quality

The assessment of ecological impacts of pumped-storage (PS) hydropower plants on the two connected water bodies is usually based on present climatic conditions. However, significant changes in climate must be expected during their long concession periods. We, therefore, investigate the combined effects of climate change and PS operations on water temperature and quality, as well as extent and duration of stratification and ice cover, using a site in Switzerland. For this purpose, a coupled two-dimensional hydrodynamic and water quality model for the two connected water bodies is run with 150 years long synthetic stochastic meteorological forcing for both current and future climate conditions under two PS and two reference scenarios. The results show relevant synergistic and antagonistic effects of PS operations and climate change. For example, hypolimnion temperatures in September are projected to increase by < 0.6 °C in a near-natural reference scenario and by ~ 2.5 °C in an extended PS scenario. Ice cover, which occurs every year under near-natural conditions in the current climate, would almost completely vanish with extended PS operation in the future climate. Conversely, the expected negative impacts of climate change on hypolimnetic dissolved oxygen concentrations are partially counteracted by extended PS operations. We, therefore, recommend considering future climate conditions for the environmental impact assessment in the planning of new or the recommissioning of existing PS hydropower plants

A review on hot cathode ionisation gauges with focus on a suitable design for measurement accuracy and stability

project 16NRM05 'Ion gauge'A literature review starting from 1950 is given on the design and geometry of ionisation gauge types with hot cathodes. Considerations on the material of the electrodes and of surface effects are included. The review focuses on the design issues for measurement accuracy, linearity, repeatability, reproducibility, and stability of sensitivity. Also, the attempts to reduce the lower measurement limit are reviewed to some extent.publishersversionpublishe

Effects of alpine hydropower operations on primary production in a downstream lake

Abstract.: During the past century, the construction of hydropower dams in the watershed of Lake Brienz has significantly altered the dynamics of turbidity, which has important implications for lake productivity. To assess these effects, we measured in situ carbon assimilation rates and ambient light intensities over 18months. Based on experimental data, a numerical model was developed to assess gross primary production under present light conditions and those under a hypothetical case without upstream dams. Light conditions for the hypothetical ‘no-dam' situation were estimated from pre-dam Secchi depths and simulated ‘no-dam' particle concentrations. Current gross primary production is low (~66 gC m−2 yr−1), and could increase ~44% if the lake was less turbid. Disregarding nutrient retention in reservoirs, we estimate gross primary production would be ~35% lower in summer and ~23% higher in winter in the absence of reservoirs. The annual primary production (~58 gC m−2 yr−1) would decrease ~12% compared to the current primary production with dams. According to model calculations, hydropower operations have significantly altered the seasonal dynamics, but have little effect on annual primary production in Lake Brien