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A year is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility. In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked. Calendar year A calendar year is an approximation of the number of days of the Earth's orbital period, as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars. For the Gregorian calendar, the average length of the calendar year (the mean

Statistical Analysis of Mountain Permafrost Temperatures

Evelyn Zenklusen Mutter

The consequences of climate change are clearly visible in the Swiss Alps. In the past decades, increasing air temperatures have induced pronounced glacier retreat and permafrost thawing. Thawing permafrost in steep terrain is a potential natural hazard, as it can become unstable and trigger mass movements such as settlement, debris flows or rock fall. The WSL Institute for Snow and Avalanche Research SLF has been measuring ground temperatures and displacements in permafrost boreholes for more than a decade. This thesis aimed to analyse these ground temperatures with statistical methods, assessing possible changes, trends and physical coherencies. In a first step, temporal changes in daily ground temperatures measured in two adjacent boreholes at Muot da Barba Peider in the Eastern Swiss Alps were analysed. Statistical models, which could for example describe changes in the amplitudes or in the mean, were used to estimate possible trends. The results for the period 1996 - 2008 revealed increasing summer temperature for the upper ground layers, whereas winter temperatures had decreased. For the frozen rock below 10 m however, a general temperature increase was found. Although increasing summer temperatures were consistent with the development of the air temperatures, decreasing winter temperatures could be attributed to a thin early winter snow depth. The general warming trend in the deeper layers was assigned to increased heat transfer through the mountain ridge of Muot da Barba Peider induced by warming air temperatures and lower snow depths. These results confirm that permafrost temperatures in the Alps are influenced by factors such as snow cover, surface properties, hydrology and topography. Increasing air temperatures do not necessarily induce permafrost thawing. Special attention was paid to the so-called "active layer", the topmost ground layer above permafrost, which seasonally thaws in summer. An increase in its thickness implies a potential increase of the material which might be released in a mass movement. Typical active layer characteristics have been analysed and compared for ten different permafrost sites. Whereas over the past decade, the active layer remained rather constant at the individual sites, significant differences due to local terrain properties were visible. The comparison of the daily development of the active layer thickness with that of the thawing degree days revealed that ice-rich ground layers delay the active layer development efficiently and thus isolate the permafrost from warm air temperatures. Two of the ten sites, however, revealed abrupt active layer deepening due to lateral air and/or water flows. Further investigation of the dependence between air and ground temperature was performed. Transfer function models were used to quantify the relation between air and ground temperatures measured at 0.5 m depth at seven different permafrost sites. The results showed that the delay between daily changes in the air and ground temperature ranges from one to six days, depending on the site. The most efficient relation was found for a very coarse-blocky rock glacier site, whereas scree slopes with smaller grain sizes showed a weaker and more prolonged relation. The main heat transfer mechanism in permafrost is conduction, but due to phase change, air or water flows, heat can also be transferred non-conductively. A statistical procedure including spectral analysis, order-restricted inference and a false discovery rate procedure has been developed to detect depths and frequencies at which significant non-conductive heat transfer processes occur. The application of the procedure to two-hourly borehole temperatures measured at Muot da Barba Peider revealed significant non-conductive heat transfer for the period 2005 - 2009 only in an ice-rich layer between 1 and 1.9 m depth. To gain deeper insight into the processes occurring shorter (three-week) periods have been analysed. The results showed non-conductive heat transfer processes that could be attributed to phase changes at the freezing front and at the base of the active layer in autumn, to convective air and vapour flows through the frozen scree in winter, and to phase changes and meltwater infiltration during the thawing period in spring. Analyses of borehole temperatures also revealed special phenomena. A case study for the famous permafrost site at Flüela Pass showed that the permafrost body inside the scree slope, close to the lake has degraded from 7 m to 3.5 m thickness within four years. This rapid permafrost degradation could be attributed to a seasonal ventilation system inside the scree, leading to condensation which melts the permafrost body from below. Increasing lake water temperatures might accelerate the phenomenon. The ground temperatures analysed in this work include about one decade of measurements and therefore are too short for climate-related statements. However, the statistical analyses performed give an interesting insight into the development of permafrost ground temperatures during the past ten years and their interaction with air temperature and local properties. The presented results furthermore emphasize the strong spatial variability of the ground thermal regime in mountain permafrost.

EPFL2012

Alexandre Sarrasin (1895-1976) et l'esthétique de l'ingénieur

Philippe Mivelaz

Alexandre Sarrasin (1895-1976) studied at Zürich Federal Swiss Institute of Technology (ETH) from 1913 to 1918. Despite First World War's conflicts surrounding the neutral Switzerland, the intellectual climate inside ETH was cosmopolitan and liberal. As unique educational institution directly depending from the Federal government, the ETH played an important role in shaping modern Switzerland where country planning and infrastructure building were the central power's main tasks. Engineers were important figures of this process of modernization. They embodied the values of the industrial revolution: rationality, efficiency and the huge power of transformation of the country and its society. Sarrasin opened an engineer's office in Lausanne in 1921 and in Brussell two years later. He established himself in the capital of Belgium in 1927 while ruling at the same time his office in Lausanne. He came back to Switzerland when the German army invaded Belgium. The office in Bruxells was maintained up to the early 1960's. Branches were opened in Sion and Geneva during the 1950's. Sarrasin's work concerned foundations, buildings, bridges and some dams also. Nearly all these constructions works were made of reinforced concrete, his favorite material. This thesis is dedicated to the construction works, especially the bridges in Valais, Sarrasin's native district, and the aesthetic question in engineer's design. Of the bridge of Branson (1924- 1925) near Fully, his first important commission, he wrote that "The elegance of the design is due to its simplicity. The quietness of its lines is fully harmonized with the landscape. Nothing was planned for the decoration. The bridge is the simple realization of a mathematical truth". This quotation highlights the issue of form for engineers and especially in Sarrasin's works: the affirmation of simplicity and "mathematical truth", that is to say the expression of the structural mechanics, as the engineer's aesthetic's fundamental the relation with the landscape The question of form was not only thought of in terms of efficiency and economy, or lack of decoration. The development of structural mechanics as engineer's science gave new conceptual tools allowing new designs. The first professor of civil engineering at the ETH, Karl Culmann (1821-1881), was the graphical static's father. This applied science was inspired by descriptive geometry and graphical calculation methods developed in France during the 18th and 19th century. To the construction's classical point of view distinguishing between elements bearing weight (for example pillars) and others being supported (for example lintels), graphical static opposed a spatial vision of forces interactions into the structure. New materials allowed bringing these forces into play with tensed cables, compressed or extended frames. At the beginning of the 20th century, reinforced concrete appeared as modern material along side with iron and steel. Created by builders in the 19th century, its construction method was investigated by engineers making it a universal material used by everyone, independent of patents. This assimilation was made by elaborating construction norms that had sometimes inhibitive effects on the creativity of engineers. Other engineer looked for new forms fitting the material's caracteristics. Robert Maillart (1872-1940) was one of them. He had a great influence on Sarrasin's work. The first half-century of modern engineers which started with the creation of the ETH in 1855, was a period of great innovations in science and technology. Most construction works were made of metal and were built for the railways and designed with the graphical static, creating a new architecture, but the question of aesthetic became an issue only at the beginning of the 20th century. The first reaction rose against the overwhelming presence of technology in surroundings : its impact on the landscape or on the city's image was depicted as negative. At the same time, other observers – architects, art historians – recognized in industrial constructions an aesthetical value. Some engineers wanted a come back to traditional forms as stone vaults. During the first quarter of century, several metallic bridges had been considered as fragile and replaced by stone works. Bridges were designed as monuments. On the other side, some engineers argued for the beauty of modern materials and technology, opposed to the picturesque point of view, represented by those who wanted preserving the landscape. The First World War was not only the beginning of the 20th century. It seemed also to be the moment where engineers lost their innocence. Aesthetical questions and the impact on the landscape were matters of public debates. During the same period engineering became a profession free of contractors. That had probably the effect that engineers developed their own language. Sarrasin exactly spent his study years during this period of changes. Matters of aesthetic belonged to the themes treated by his professor of structural mechanics, Arthur Rohn (1878- 1956), who considered that a bridge should not only be built rationally, but should also express his structural system. François Schüle (1860-1925), who was involved in developing constructive norms for reinforced concrete, was his professor of material's technology. He represented the scientific side of the profession of engineer. The works of Sarrasin embodied this double requirement for rationality in using materials and the will to integrate the construction works into the landscape and into the environmental context. Valais was the landscape taken into account. This mountainous district entailed contradictory representations of the need of modernization and the preservation of landscape and traditions. On one side, this region was shown as a society preserved from modernity with unchanged traditions, on the other side, the Valais quickly took part in the national economy. The landscape changed deeply with the draining of the main valley of the Rhône river, the construction of rail and road nets, industry and hydroelectric plants. Sarrasin's works were the products of this economic revolution which transformed the Valais region; they wanted at the same time to be a testimony of this story. One of the interesting aspects of the attitude of this engineer is the way he used references to ancient constructions. Beamers and arches reconsidered in respect with modern technology by use of reinforced concrete, created a dialogue with the landscape or with stoned vaults thrown upon the Alps deep abysses.

EPFL2007

Assessment of the water resource in Tambarga (Burkina Faso) based on stable isotope analysis

Tabea Schutter

Assessment of available water is the basis that allows for sustainable management of the water resource. Special need concerns the assessment of groundwater resources, about which, often little is known. The present study aims to explore the water resource, with a focus on groundwater, of a small catchment (3.5km2), situated in a semi-arid zone in Burkina Faso (West Africa). A 5-year data set of stable isotopes of water, groundwater level and other hydrologic data is used to gain insight into the water resources in this small catchment. The main effort is devoted to the analysis of isotope data, with the secondary goal to asses the potential of the use of stable isotopes of water as an innovative tool in hydrology. Analyzed are temporal and spacial variations of the isotopic signature in groundwater, surface water and rain. Isotope data is compared to groundwater level data. Isotope data allowed to distinguish two different categories of wells. First, wells where evaporation dominated compared to recharge at the end of the wet season through the dry season, and wells that have constant isotope ratios throughout the year. Evolution of the isotope signature of the former suggests that these wells get disconnected from the aquifer at some point of the year. Comparison of isotope ratios of different sampling sites (springs, wells, pumps, surface water and rain) revealed that the upstream springs diverge from the other sites in their isotopic signature, and thus either have a different origin or were subject to different processes. Negligible diurnal variations observed in isotopic signatures of wells prompt that diurnal fluctuations observed in groundwater levels are due to non isotope fractionation processes. Periods deduced from seasonal changes in isotopic signatures match periods of rising and decreasing groundwater levels. Though groundwater levels show inter-annual variability, especially in their peak levels, this was not reflected in the isotope data. Isotope data confirmed conclusions gained from other data (recharge periods), added additional information (diurnal fluctuations of wells are non isotope fractionation processes), and pointed out the fact that upstream spring water distinguishes in some form or another from other groundwater.

2014