Heure | EPFL Graph Search

L’heure est une unité de mesure secondaire du temps. Le mot désigne aussi la grandeur elle-même, l’instant (l’« heure qu’il est », actuellement ), y compris en sciences (« heure solaire » employé pour temps solaire). Détermination de l’heure Histoire de la mesure du temps Heure grand public Historiquement, l’heure a été définie par l’utilisation des cadrans solaires qui marquaient par l'ombre projetée d'un style la position du Soleil dans le ciel ; il s'agissait de l'heure solaire. Actuellement, l'heure grand public correspond à un vingt-quatrième du jour ; c'est l'heure de la montre, l'heure donnée par l'horloge parlante ou l'heure GPS. Elle est liée à la course fictive d'un Soleil moyen pendant ces 24 heures. Mais d’autres divisions ou heures anciennes ont existé à différentes époques. Cependant il ne s’agit là que d’une approximation, car la période de rotation terrestre varie légèrement avec les années ; elle ralentit par l’effet des marées, et subit au

Blending a MOOCs with interactive teaching

Pierre-Yves Gilliéron, Bertrand Merminod, Geoffrey Vincent

When Massive Open Online Courses became a hot topic in Europe. The Ecole Polytechnique Fédérale de Lausanne (EPFL) decided to take the challenge and launched an internal call for proposals. At the Geodetic Engineering Lab, we saw a possibility to use contact hours for exercises and fieldwork, rather than telling stories and showing diagrams. External students were considered, but the main purpose was to modernise the first year course "Fundamentals of Geomatics" for approximately 250 civil and environmental engineering students. As the bid was successful, we started with the design of the lectures, followed by the recording sessions in a special studio. Combining photographs, sketches, speeches and quizzes into a video requires a precise script. For one hour of teaching, ten hours of preparation and processing are not always sufficient. The new formula for the course was implemented during the 2014 spring semester. The two hour session remained in the weekly program, with more assistants and other expectations. Regular students took classical mid-term and final exams. At this stage, an incidence of the teaching method on the level reached cannot be ascertained, but neither a miracle, nor a catastrophe took place. Simultaneously, the course was made available worldwide via the Coursera platform. Many people watched the short teaser. Some registered, but few of them did all the assignments and qualified for the statement of accomplishment (issued by Coursera, not EPFL). The activity level of the website was monitored. Some elements of evaluation by the students have been analysed. Reactions are widely spread, from "Bring back the professor!" to "Why not 100% MOOC?". Surely, the second edition will bring a more balanced feedback and motivate some improvements. Having opted for a blended approach, the active promotion of this course outside - Western Africa comes first to mind - depends on partnerships with local universities.

2015

Application of an Ensemble Kalman filter to a semi-distributed hydrological flood forecasting system in alpine catchments

Giovanni De Cesare, Alain Tommy Foehn, Javier García Hernández, Anne Schwob

One of the key success factor for hydrological forecasts is the establishment of initial conditions that represent well the conditions of the simulated basin at the beginning of the forecast. Real-time Data Assimilation (DA) has been shown to allow improving these initial conditions. In this article, two DA approaches are compared with the reference scenario working without DA (Control). In both approaches, discharge data at gauging station are assimilated. In the first approach, a volume-based update (VBU) compares the simulated and observed volumes over the past 24 hours to compute a correction factor used to update the soil water saturation in the upstream part of the semi-distributed hydrological model. In the second approach, an Ensemble Kalman Filter (EnKF) is implemented to account for the uncertainty in precipitation, temperature and discharge data. The comparison is carried out over 2 sub-basins of the Upper Rhône River basin upstream of Lake Geneva, where the MINERVE flood forecasting and management system is implemented. Results differ over the two studied basins. In one basin, the two data assimilation perform better than the Control simulation with the lowest error given by the VBU up to a forecast horizon of 35 hour and by the EnKF for higher forecast horizons. In the second basin, EnKF gives the lowest error over the few first hours of forecast, but then provides the weakest performance. The lowest error is given by the Control simulation, because the model already performs very well on the event without data assimilation.

2019

Wind slab formation in snow: experimental setup and first results.

Charles Fierz, Michael Lehning, Christian Gabriel Sommer

Geophys . Res. Abstr. 18: EGU2016‐11274. The formation of wind-hardened surface layers, also known as wind slabs or wind crusts, is studied. Better knowledge about which processes and parameters are important will lead to an improved understanding of the mass balances in polar and alpine areas. It will also improve snow-cover models (i.e. SNOWPACK) as well as the forecast of avalanche danger. A ring-shaped wind tunnel has been built and instrumented. The facility is ring-shaped to simulate an infinitely long snow surface (infinite fetch). A SnowMicroPen (SMP) is used to measure the snow hardness. Other sensors measure environmental conditions such as wind velocity, air temperature, air humidity, the temperature of the snow and of the snow surface. A camera is used to detect drifting particles and to measure the Specific Surface Area (SSA) at the snow surface via near-infrared photography. First experiments indicate that mechanical fragmentation followed by sintering is the most efficient process to harden the surface. The hardness increased rapidly during drifting snow events, but only slowly or not at all when the wind speed was kept below the threshold for drifting snow. With drifting, the penetration resistance increased from the original 0.07 N to around 0.3 N in about an hour. Without drifting, a slow, further increase in resistance was observed. In about six hours, the hardness of the top 1-2 cm increased to 0.5 N. During this eight-hour experiment consisting of about two hours with intermittent drifting and six hours without drifting, the density at the surface increased from 66 kg/m3 to around 170 kg/m3. In the unaffected region close to the ground, the density increased from 100 kg/m3 to 110 kg/m3.

2016

Blending a MOOCs with interactive teaching

Pierre-Yves Gilliéron, Bertrand Merminod, Geoffrey Vincent

2015