Les glaces de paroi : glaciologie, thermodynamique et prévision des chutes de blocs de glace sur les routes du nord de la Gaspésie (Québec, Canada)

Les glaces de paroi résultent du gel progressif des eaux de ruissellement sur une falaise. Dans le nord de la Gaspésie (Québec, Canada), soixante de ces glaces, parfois volumineuses (> 3000 m3), ont été localisées près des routes. Entre 2000 et 2012, le Ministère des Transports du Québec (M.T.Q.) a effectué 440 interventions pour déblayer les blocs de glace tombés sur la chaussée. Une meilleure compréhension des mécanismes de croissance, de fonte et d’écroulement des glaces de paroi est nécessaire pour améliorer la gestion du risque. Des échantillons de glace ont été prélevés et leur microstructure analysée par tomodensitométrie et polarisation croisée. L’évolution volumétrique de quatre glaces de paroi a été modélisée à l’aide de différents modèles thermodynamiques et validée à partir de mesures du volume de glace effectuées au LiDAR. Une méthode de prévision des chutes de blocs de glace a été développée en comparant les dates des interventions du M.T.Q. avec les données météorologiques d’Environnement Canada. Les résultats ont permis de décrire les mécanismes de croissance des quatre types de glace constituant les glaces de paroi. Les modèles ont permis de démontrer que le gel de l’eau suite au transfert de chaleur par convection de l’air est le principal responsable de leur croissance. La fonte des cascades de glace, issues du gel des eaux d’une chute, est principalement initiée par l’augmentation du débit. Celle des carapaces de glace, issues du gel d’une résurgence, est plutôt le résultat de la convection de l’air et du bilan radiatif. Le calcul des degrés-heures de gel (ou de fonte), s’est avéré efficace pour modéliser l’évolution des carapaces de glace d’orientation nord. L’ajout du bilan radiatif à ce modèle a amélioré ses performances et permis une modélisation convenable de celle exposée au sud. Enfin, les effets des variables météorologiques sur l’occurrence des chutes de blocs de glace ont été simulés à l’aide de modèles logistiques. Le calcul des degrés-jours de fonte et le suivi du meilleur modèle statistique permettent de prédire l’écroulement de certaines glaces de paroi à quelques jours près. C’est une étape importante vers une gestion plus efficace de l’aléa.The growth of rockwall icings result from the gradual freezing of runoff on a cliff. In northern Gaspésie (Québec, Canada), sixty of these sometimes voluminous ice formations (over 3000 m3) were located near roads. Between 2000 and 2012, 440 interventions to remove ice blocks from the road were conducted by the “Ministère des Transports du Québec” (M.T.Q.). A better understanding of the growth, melt and collapse dynamics of rockwall icings is needed to improve the management of this natural hazard. Ice samples were collected and their microstructure analyzed by computed tomography and under cross polarized light. The ice volume evolution of four rockwall icings was studied using various thermodynamic models and validated from ice volume measurements made with LiDAR imaging. An ice blocks fall forecasting method was developed by comparing the timing of M.T.Q. interventions with meteorological data. The results were used to describe the growth mechanisms of four different types of ice formation found on rockwalls. The models showed that freezing of the water to cold air convection is the main process responsible for their growth. In spring, the melting rate of the ice cascade or frozen waterfall clearly depends on the sensible heat carried by the increasing water flow. The melting of the ice walls formed from the freezing of seeping water coming from underground water resurgences rather is the result of air convection and the radiation heat budget (Qrad). A simple model based on the calculation of the freezing, or melting, degree-hours was also effective for simulating the evolution of the north facing ice walls. Including Qrad to this model improved its performance and allowed accurate modeling of the south facing ice wall. Finally, the effects of meteorological variables on the occurrence of ice blocks falls were simulated using logistic models. By following the trends of the melting degree-days and the best statistical model, it is possible to forecast the collapse of some of the most problematic rockwall icings and target the most hazardous periods along northern Gaspésie roads. This study is an important step towards a more effective management of this natural hazard

d13C pattern of dissolved inorganic carbon in a small granitic catchment: the Strengbach case study (Vosges mountains, France)

The transfers and origins of dissolved inorganic carbon DIC. were studied for a year in a soil–spring–stream system in the Strengbach catchment, Vosges mountains, France. This 80 ha experimental research basin is located on the eastern side of the mountains, at an altitude ranging from 883 to 1146 m.a.s.l. and is mainly covered by spruce 80%.. Brown acid and podzolic soils developed on a granitic basement, and, as a result, the DIC originates solely from CO2 generated by oxidation of soil organic matter. The d13CDIC. in catchment waters is highly variable, from about y22‰ in the springs and piezometers to about y12‰ in the stream at the outlet of the catchment. In the springs, pronounced seasonal variations of d13C exist, with the DIC in isotopic equilibrium with the soil CO that has estimated d13DIC 2 C of about y24‰ in winter and y20‰ in summer. These seasonal variations reflect an isotopic fractionation that seems only induced by molecular diffusion of soil CO2 in summer. In stream water, seasonal variations are small and the relatively heavy DIC y12‰ on average. is a result of isotopic equilibration of the aqueous CO2 with atmospheric CO2

Global warming impacts on rockfall frequency and magnitude due to changing frost distribution and frost cracking effectiveness

The distribution of freezing and thawing within rock masses is time varying (day to day or season to season) and controls the effectiveness of the frost cracking processes from the surface until various depths. These processes are major contributors to the development of rock instabilities. By altering the thermal regime of rockwalls, global warming could have a major impact on rockfall dynamic by the end of the 21st century. This study seeks to improve our understanding of the influence of this warming on (i) the distribution of freezing and thawing within rock masses, (ii) the effectiveness of frost cracking and (iii) the frequency and magnitude of rockfalls. Thermistor sensors inserted in a 5.5-m horizontal borehole and a weather station were installed on a vertical rockwall located in the northern Gaspé Peninsula (Canada). This instrumentation was used to calculate the surface energy balance of the rockwall and to measure and model its thermal regime at depth over a period of 28 months. Combining locally recorded historical air temperature data with simulated future data (scenarios RCP4.5 and RCP8.5) made it possible to extend the rockwall thermal regime model over the period 1950–2100. The effectiveness of frost cracking over this 150-year period has been quantified using a thermomechanical model. Depending on the scenario, warming of 3.3°C to 6.2°C is expected on the northern Gaspé Peninsula by the end of the 21st century. This rapid warming is likely to decrease the maximum depth reaches by the seasonal frost by 1–2 m and shorten its duration by 1–3 months. The frequency of freeze–thaw cycles could increase twelvefold in January. Frost cracking effectiveness should intensify around 70 cm in depth and disappear beyond that (RCP4.5) or diminish starting at 10 cm in depth (RCP8.5). In areas subject to seasonal freeze–thaw cycles, decimetric rockfall frequency could grow considerably in winter but be significantly reduced in fall and spring. Furthermore, frost cracking would cease contributing to the development of larger magnitude instabilities

Addition of tethered nonaromatic carbon nucleophiles to chemoselectively activated amides

Abstract: In an effort to develop new ways of synthesizing polycyclic alkaloids, we successfully added silyl enol ethers, allylsilanes, and enamines to iminium ions generated from amides. Because of their higher oxidation state, such iminiums show a yet unexploited advantage of potential double cyclizations over standard Mannich monocyclizations. We report herein the first example of tethered nonaromatic carbon nucleophiles adding to activated amides for the generation of enaminals of various ring sizes, with endo- or exo-cyclic nitrogen

Contract Flexibility and Dispute Resolution in African Manufacturing

This paper examines the contractual practices of African manufacturing firms using survey data collected in Burundi, Cameroon, Côte d’Ivoire, Kenya, Zambia, and Zimbabwe. Descriptive statistics and econometric results are presented. They show that contractual flexibility is pervasive and that relational contracting is the norm between manufacturers, their suppliers, and their clients. The existence of long-term relations between firms helps them deal with contract non-performance through negotiation. Confrontational methods such as lawyers and courts are used only by large firms and when negotiations fail. Whenever confrontation can be avoided, business is resumed. Of the six studied countries, incidence of breach and the use of lawyers and courts are highest in Zimbabwe which is also the country with legal institutions that best support business. Our favored interpretation is that good legal institutions incite firms to take more chances, thereby encouraging trade and leading to more cases of breach and more recourse to courts and lawyers. A high frequency of contract non-compliance should thus not be interpreted as a sign of imperfect legal institutions.

Credit constraints in manufacturing enterprises in Africa

We investigate the question whether firms in the manufacturing sector in Africa are credit constrained. The fact that few firms obtain credit is not sufficient to prove constraints, since certain firms may not have a demand for credit while others may be refused credit as part of profit maximising behaviour by banks. To investigate this question, we use direct evidence on whether firms had a demand of credit and whether their demand was satisfied in the formal credit market, based on panel data on firms in the manufacturing sector from six African countries. More than half the firms in the sample had no demand for credit. Of those firms with a demand for credit, only a quarter obtained a formal sector loan. In line with expectations, our analysis suggests that banks allocate credit on the basis of expected profits. However, controlling for credit demand, outstanding debt is positively related with obtaining further lending while micro or small firms are less likely to get a loan than large firms. The latter effect is strong and present in the regression, despite including several variables typically referred to as explaining why small or ‘informal’ firms do not get credit. The role of outstanding debt is likely to be a reflection of inefficiency in credit markets, while the fact that size matters is consistent with a bias as well, although we cannot totally exclude that they reflect transactions costs on the part of banks. Finally, we could not detect any differences between countries in the effects of these factors in the credit allocation rule, although financial deepening is found to explain most of the country-specific fixed effects, shifting the probability of obtaining credit across the firm distribution.

Do African manufacturing firms learn from exporting?

In this paper, we use firm-level panel data for the manufacturing sector in four African countries to estimate the effect of exporting on efficiency. Estimating simultaneously a production function and an export regression that control for unobserved firm effects, we find both significant efficiency gains from exporting, supporting the learning- byexporting hypothesis, and evidence for self-selection of more efficient firms into exporting. The evidence of learning-by-exporting suggests that Africa has much to gain from orientating its manufacturing sector towards exporting.

Do African Manufacturing Firms Learn from Exporting?

In this paper, we use firm-level panel data for the manufacturing sector in four African countries to estimate the effect of exporting on efficiency. Estimating simultaneously a production function and an export regression that control for unobserved firm effects, we find both significant efficiency gains from exporting, supporting the learning-byexporting hypothesis, and evidence for self-selection of more efficient firms into exporting. The evidence of learning-by-exporting suggests that Africa has much to gain from orientating its manufacturing sector towards exporting.