Les variations de niveau des réservoirs de la Volga et leurs conséquences sur l'environnement

Les réservoirs volgiens ont pour but de retenir les eaux de fonte de printemps pour les redistribuer dans le cadre de l'année hydrologique (avril-mars). Cette fonction implique des variations de niveau des lacs de retenue et ceci est à l'origine de conséquences négatives. Le remplissage de printemps peut être une menace hydrologique sérieuse pour l'agriculture. Le déstockage qui suit est mené en saison de navigation (il intéresse donc tous les secteurs impliqués dans l'aménagement) ou en hiver (il n'intéresse plus que l'hydroélectricité). D'après les règles établies, le déstockage devrait être mené selon un rythme précis et devrait être régulier d'une année sur l'autre pour ne pas porter préjudice à la pèche dans les réservoirs et pour ne pas entretenir l'érosion sur les rives. Depuis vingt ans, la pèche attribue toutefois ses mauvais résultats au non-respect des règles. D'autres ministères se plaignent du même fait. Les règles sont-elles respectées? Seule l'étude des niveaux réels des réservoirs permet de répondre à cette question. Elle fait apparattre depuis vingt ans des remplissages de printemps anarchiques, d'importantes et imprévisibles variations de niveau en saison de navigation, des déstoclcages massifs d'hiver. Cette pratique est partiellement responsable du développement en URSS d'un puissant mouvement de contestation écologique.Since 1962, the Volga River has become a series of cascades of eight reservoirs. Initially, the main object of the development was hydro-electricity but fishing, irrigation and transport became more and more important. Sine 1962, the function of Volgian reservoirs has been to retain spring water for its redistribution during the navigation season - for all the sectors involved in the management - and during the winter - only for hydro-electricity. This function implies lake level variations, which can generate negative environmental effects.According to Soviet economic principles, each Ministry manages its own equipment, which means that the Volga development is controlled from Moscow by different Ministries. The Ministry of Energy, controlling dams and turbines, has an enormous power. Hence, there are rules for defending the interests of the other economic sectors. The Ministry of Energy, allegedly, is concerned only about its own plans and does mot comply with the rules, which naturally it denies. These ruses have mot been published but their main lines are well-known as they are largely referred to in scientific publications concerning the Volga management problem. The only way to check is to compare the rules with the available hydrological data on lake levels.The most significant part of annual Volgian runoff proceeds from the spring melting of snow. This water must be used to fill the Volgian reservoirs and to satisfy the Lower-Volga artificial flood (110 km3). Actual data about Kuybyshev dam water discharge shows that the lower-Volga water requirements are mot satisfied. The true data about Kuybyshev and Rybinsk lakes shows disorganized fillings : the reservoirs are filled up to different levels every year, which contributes to activate bank erosion and prevents the constitution of aquatic vegetation. Bank erosion supplies a lot of sediments which, certain years, obstruct access to storm shelters or, sometimes, to harbours (Dimitrovgrad, for example). The lack of aquatic vegetation reduces the possibilities of fish reproduction.By raising the level of groundwaters around the reservoirs, this spring filling jeopardizes agriculture, so it is mot allowed to full reservoirs over the maximum water level. Yet, the data shows that the Kuybyshev reservoir, right in the middle of the agricultural zone of the USSR, was filled more than 16 years out of 26.The drawdown must be carried out with moderation and according to a specific rhythm particularly before winter in order to force the fish to leave the shallows. During the winter it is better to avoid massive drawdowns. The study of winter levels reveals very deep drawdowms. In This case the lake surfaces reduce and the tee collapses down to the lake bottom causing very important prejudice to the aquatic fauna.According to the numerous scientific publications by Soviet fishing Institutes, the non-observation of the rules accounts for the poor production of fish. (500-600 kg a year per km2 on Kuybyshev and Kybinsk lakes) and for the increase of bank erosion. The study of the lake levels shows that the Volgian water is used only in the interest of hydro-electricity and without any care for the other users (agriculture, fishing, navigation) or for the effects on the environment. The Energy lobby is so strong that 25 years of scientific protest bas not changed anything.Such inconsistent management bas played a great part in the determination of an environmental movement In the USSR. Over the last forty years significant areas of territory have been submerged under the large reservoirs (on the Volga alone more than 300,000 people have been resettled on account of submersion) so the scientific protest met with the approval by a show of hostility to hydro-energy. Since the instauration of the « glasnost », the protest against the « hydro-lobby » has taken a symbolical value, a facet of the opposition to the « old order », and the poor management of Volga development largely contributed to this

Elastic deformation due to tangential capillary forces \ud

A sessile liquid drop can deform the substrate on which it rests if the solid is sufficiently “soft.” In this paper we compute the detailed spatial structure of the capillary forces exerted by the drop on the solid substrate using a model based on Density Functional Theory. We show that, in addition to the normal forces, the drop exerts a previously unaccounted tangential force. The resultant effect on the solid is a pulling force near the contact line directed towards the interior of the drop, i.e., not along the interface. The resulting elastic deformations of the solid are worked out and illustrate the importance of the tangential force

Magnetically self-regulated formation of early protoplanetary discs

The formation of protoplanetary discs during the collapse of molecular dense cores is significantly influenced by angular momentum transport, notably by the magnetic torque. In turn, the evolution of the magnetic field is determined by dynamical processes and non-ideal MHD effects such as ambipolar diffusion. Considering simple relations between various timescales characteristic of the magnetized collapse, we derive an expression for the early disc radius, r \simeq 18 \, {\rm AU} \, \left({\eta_{\rm AD} / 0.1 \, {\rm s}} \right)^{2/9} \left({B_z / 0.1\, {\rm G}} \right) ^{-4/9} \left({M / 0.1 \msol} \right) ^{1/3}, where $M$ is the total disc plus protostar mass, $\eta_\mathrm{AD}$ is the ambipolar diffusion coefficient and $B_z$ is the magnetic field in the inner part of the core. This is about significantly smaller than the discs that would form if angular momentum was conserved. The analytical predictions are confronted against a large sample of 3D, non-ideal MHD collapse calculations covering variations of a factor 100 in core mass, a factor 10 in the level of turbulence, a factor 5 in rotation, and magnetic mass-to-flux over critical mass-to-flux ratios 2 and 5. The disc radius estimates are found to agree with the numerical simulations within less than a factor 2. A striking prediction of our analysis is the weak dependence of circumstellar disc radii upon the various relevant quantities, suggesting weak variations among class-0 disc sizes. In some cases, we note the onset of large spiral arms beyond this radius.This research has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 Grant Agreement no. 247060 and no. 306483). We acknowledge financial support from ”Programme National de Physique Stellaire” (PNPS) of CNRS/INSU, France

Impact of the Hall effect in star formation and the issue of angular momentum conservation

This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this recordWe present an implementation of the Hall term in the non-ideal magnetohydrodynamics equations into the adaptive-mesh-refinement code RAMSES to study its impact on star formation. Recent works show that the Hall effect heavily influences the regulation of the angular momentum in collapsing dense cores, strengthening or weakening the magnetic braking. Our method consists of a modification of the two-dimensional constrained transport scheme. Our scheme shows convergence of second-order in space and the frequency of the propagation of whistler waves is accurate. We confirm previous results, namely that during the collapse, the Hall effect generates a rotation of the fluid with a direction in the mid-plane that depends on the sign of the Hall resistivity, while counter-rotating envelopes develop on each side of the mid-plane. However, we find that the predictability of our numerical results is severely limited. The angular momentum is not conserved in any of our dense core-collapse simulations with the Hall effect: a large amount of angular momentum is generated within the first Larson core, a few hundred years after its formation, without compensation by the surrounding gas. This issue is not mentioned in previous studies and may be correlated to the formation of the accretion shock on the Larson core. We expect that this numerical effect could be a serious issue in star formation simulations.We acknowledge financial support from “Programme National de PhysiqueStellaire” (PNPS) of CNRS/INSU, CEA, and CNES, France, and from an International Research Fellowship of the Japan Society for the Promotion of Science

Eigenvalues of the truncated Helmholtz solution operator under strong trapping

For the Helmholtz equation posed in the exterior of a Dirichlet obstacle, we prove that if there exists a family of quasimodes (as is the case when the exterior of the obstacle has stable trapped rays), then there exist near-zero eigenvalues of the standard variational formulation of the exterior Dirichlet problem (recall that this formulation involves truncating the exterior domain and applying the exterior Dirichlet-to-Neumann map on the truncation boundary). Our motivation for proving this result is that (a) the finite-element method for computing approximations to solutions of the Helmholtz equation is based on the standard variational formulation, and (b) the location of eigenvalues, and especially near-zero ones, plays a key role in understanding how iterative solvers such as the generalized minimum residual method (GMRES) behave when used to solve linear systems, in particular those arising from the finite-element method. The result proved in this paper is thus the first step towards rigorously understanding how GMRES behaves when applied to discretizations of high-frequency Helmholtz problems under strong trapping (the subject of the companion paper [P. Marchand et al., Adv. Comput. Math., to appear])

High-frequency estimates on boundary integral operators for the Helmholtz exterior Neumann problem

We study a commonly-used second-kind boundary-integral equation for solving the Helmholtz exterior Neumann problem at high frequency, where, writing Γ for the boundary of the obstacle, the relevant integral operators map L2(Γ) to itself. We prove new frequency-explicit bounds on the norms of both the integral operator and its inverse. The bounds on the norm are valid for piecewise-smooth Γ and are sharp, and the bounds on the norm of the inverse are valid for smooth Γ and are observed to be sharp at least when Γ is curved. Together, these results give bounds on the condition number of the operator on L2(Γ); this is the first time L2(Γ) condition-number bounds have been proved for this operator for obstacles other than balls

Contact angles on a soft solid: from Young's law to Neumann's law

The contact angle that a liquid drop makes on a soft substrate does not obey the classical Young's relation, since the solid is deformed elastically by the action of the capillary forces. The finite elasticity of the solid also renders the contact angles different from that predicted by Neumann's law, which applies when the drop is floating on another liquid. Here we derive an elasto-capillary model for contact angles on a soft solid, by coupling a mean-field model for the molecular interactions to elasticity. We demonstrate that the limit of vanishing elastic modulus yields Neumann's law or a slight variation thereof, depending on the force transmission in the solid surface layer. The change in contact angle from the rigid limit (Young) to the soft limit (Neumann) appears when the length scale defined by the ratio of surface tension to elastic modulus $\gamma/E$ reaches a few molecular sizes

Polycyclic aromatic hydrocarbons (PAHs) in the atmospheres of two French alpine valleys: sources and temporal patterns

International audienceAlpine valleys represent some of the most important crossroads for international heavy-duty traffic in Europe, but the full impact of this traffic on air quality is not known due to a lack of data concerning these complex systems. As part of the program "Pollution des Vallées Alpines" (POVA), we performed two sampling surveys of polycyclic aromatic hydrocarbons (PAHs) in two sensitive valleys: the Chamonix and Maurienne Valleys, between France and Italy. Sampling campaigns were performed during the summer of 2000 and the winter of 2001, with both periods taking place during the closure of the "Tunnel du Mont-Blanc". The first objective of this paper is to describe the relations between PAH concentrations, external parameters (sampling site localization, meteorological parameters, sources), and aerosol characteristics, including its carbonaceous fraction (OC and EC). The second objective is to study the capacity of PAH profiles to accurately distinguish the different emission sources. Temporal evolution of the relative concentration of an individual PAH (CHR) and the PAH groups BghiP+COR and BbF+BkF is studied in order to differentiate wood combustion, gasoline, and diesel emissions, respectively. The results show that the total particulate PAH concentrations were higher in the Chamonix valley during both seasons, despite the cessation of international traffic. Seasonal cycles, with higher concentrations in winter, are also stronger in this valley. During winter, particulate PAH concentration can reach very high levels (up to 155 ng.m-3) in this valley during cold anticyclonic periods. The examination of sources shows the impact during summer of heavy-duty traffic in the Maurienne valley and of gasoline vehicles in the Chamonix valley. During winter, Chamonix is characterized by the strong influence of wood combustion in residential fireplaces, even if the temporal evolution of specific PAH ratios are difficult to interpret. Information on sources given by PAH profiles can only be considered in qualitative terms

Origin of line tension for a Lennard-Jones nanodroplet

The existence and origin of line tension has remained controversial in literature. To address this issue we compute the shape of Lennard-Jones nanodrops using molecular dynamics and compare them to density functional theory in the approximation of the sharp kink interface. We show that the deviation from Young's law is very small and would correspond to a typical line tension length scale (defined as line tension divided by surface tension) similar to the molecular size and decreasing with Young's angle. We propose an alternative interpretation based on the geometry of the interface at the molecular scale