Valley | EPFL Graph Search

A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over a very long period. Some valleys are formed through erosion by glacial ice. These glaciers may remain present in valleys in high mountains or polar areas. At lower latitudes and altitudes, these glacially formed valleys may have been created or enlarged during ice ages but now are ice-free and occupied by streams or rivers. In desert areas, valleys may be entirely dry or carry a watercourse only rarely. In areas of limestone bedrock, dry valleys may also result from drainage now taking place underground rather than at the surface. Rift valleys arise principally from earth movements, rather than erosion. Many different types of valleys are described by geographers, using terms that may be global in use or else applied only locally. Format

Fonctionnement et gestion des aquifères alluviaux de haute altitude

The Haute Sarine alpine valley is located at an average altitude of approximately 1'000 to 1'200 masl. It was formed in its southern part up to the area of Gstaad to the favour of a typical North-South tectonic fault according to the main structures of the Préalpes, then followed normal fault striking East-West to end up on the rock bolt of the Vanel hill. The late and postglacial period was marked by the deposition of glaciolacustrines, glaciofluviatile and flurecent fluviatile sediments which have filled the ice free valley bottom probably starting from a period older than the Bölling-Alleröd interstadial. The depth of this overdeepening varies between 20 and more than 50 m of depth. Drillings which we carried out could document series of typical glaciolacustrine sediments. The implementation of several geophysical survey methods (resistivity, seismic refraction and reflexion, well logs and pumping tests) enabled us to analyse several sources of information which we confronted between them in order to produce a 3D model of the quaternary deposits and their hydrological properties. Most of these sediments are relatively quite permeable and build a free groundwater reservoir that we subdivided into 3 parts. The basin of Gsteig in the south is closed, the valley having been blocked by a massive landslide that probably occurred in several successive stages starting at the first climate improvements. The second part, the basin of Feutersoey has not been retained as a potential important hydrogeologic resource based on our preliminary investigations. The last basin, that of Gstaad-Saanen is the largest and thickest. It is also in this basin that most of the tourist and industrial activities of the area are concentrated. The observation network was established in order to characterize the groundwater flows enabling us at the same time to carry out piezometric measurements and chemical as well as and bacteriological analyses. We also carried out a δ18O isotope analysis campaign. The whole of these approaches highlighted the particular characteristics of the groundwater recharge. The groundwater system is mainly fed by the Sarine and the precipitations infiltration. We also located an underground karstic feeding of evaporitic type. We finally propose a groundwater management and protection concept adapted to the particular situation of the Sarine alluvial mountain aquifer system on both quantitative and qualitative aspects. The high altitude alpine groundwater catchment areas such as the Haute Sarine represent as a whole a quality resource, with high potentials yet to be defined on a larger scale, for example by applying numerical modeling and by organizing and maintaining long term observation networks that will enable us to define a sustainable groundwater resource management within the alpine area.

EPFL2000

Prevention of forest fires in a changing climatic context in Ticino, southern Switzerland

Michael Reinhard

Switzerland is not a place commonly associated with forest fires, unlike many areas of the nearby Mediterranean Basin. However, in this Alpine country, forest fires occur every year and destroy forests that fulfil various functions, the most socially and economically important of which is protection from natural hazards. Canton Ticino is the most affected by forest fires. The protective function is of vital importance for local communities. Most of Ticino's population lives in valley bottoms, where most transportation infrastructures are built, and without forests these are vulnerable to mass flows. In Ticino, forest fires occur mostly during the winter/spring season when dry conditions occur due to low precipitation and strong Foehn events. At this time, deciduous trees have shed their leaves and there is an important seasonal accumulation of dry litter on the forest floor. Fuel load has also increased over recent decades due to the abandonment, since about 1960, of traditional agro-forestry activities that removed dead wood, dry leaves and forage grass from the forest. Fuel in the forest is particularly flammable during the dry winter and leads to very high fire risks. Climate change has contributed to increasingly fire-prone conditions over the past decades and may also have had an effect on forest composition. Most Ticinese forest fires are anthropogenically triggered and negligence is the primary cause of fires. The only natural cause of fire is lightning, which affects forests particularly during summer, when heavy storms occur. This dissertation aimed at gaining a better scientific understanding of the interactions between landscapes, humans and forest fires in the context of climate change, in order to prescribe recommendations to achieve forest fire prevention and diminish fire related risks. We focused on the mechanisms leading to forest fires and adopted an integrated approach, which necessitated the use of a variety of methods: i) climate trend analyses, ii) flammability tests using a standardised ignition apparatus and a thermo-gravimetric analyser, iii) qualitative social science methods using focus groups and semi-structured interview techniques, iv) quantitative social science methods by means of a questionnaire. The results suggest that climate has changed in recent decades with more frequent and intense fire-prone conditions. Minimum and maximum temperatures have both increased significantly and relative humidity has decreased significantly for all daily measures during fire season. The present study suggests that fire-prone conditions are likely to become even more acute. Climate also has an important impact on the composition of the forests and the state of vegetation, as well as the condition of the fuel load. The most flammable species (Castanea sativa and some laurophyllous species) tend to occur near human settlements and this is of importance for management. Some of these species are expected to increase due to climate change. People involved with the forest fire prevention system perceived the actual fire policy as generally well working and well planned, and they perceive preparedness as strong, although discussions showed that the prevention of accidental fires is not addressed by the current fire policy. Effective prevention requires accurate information about the social aspects of forest fires, which until now has not been available in Ticino. Mechanisms leading to fires are not very well understood by locals, although many people identify correctly some technical aspects of forest fires. Depth of understanding is related to geography, gender, age, profession, marital status, income and life-style. Differences between these socio-demographic groups and geographical patterns suggest that prevention needs to be better targeted and that the population cannot be treated as a homogeneous entity. We conclude the dissertation with a series of recommendations for local authorities and people involved in the forest fire policy setting. These recommendations concern climate, vegetation flammability and forest fire prevention in Ticino.

EPFL2006

Contribution à l'étude du comportement thermo-hydraulique du stockage de chaleur en aquifère

Stéphane Dupasquier

In this work, a parametric study is done with computational methods for groundwater flow and heat transport in order to describe the dominant physical phenomena in an Aquifer Thermal Energy Storage (ATES). Optimum operating conditions and geological parameters are deduced from it. The intended use of the ATES is the domestic water or space heating. The stock is seasonal and functions without heat pump. Parametric study is organized as a series of reference simulations relating to the thickness of the aquifer and the injection flow rate. The other parameters and operating conditions of are kept constant. Other simulations are done and compared to the reference simulations in order to describe the influence of injection temperature, hydraulic conductivity, anisotropicity, injection frequencies, phases of rest, outflow rate and doublet wells. The cells of natural convection are strongly affecting the performances of an ATES. The thermal losses by conduction trough bedrock and caprock is an unfavourable factor as well, but to a lesser extent however. The increase of inflow rate always improves the efficiency, whereas an optimum aquifer thickness exists for each inflow rate. This optimum is characterised by weak natural convection phenomena. A greater thickness means strong density-viscosity effects, while a thinner aquifer forms a horizontally stretched stock, with increased conductive losses through the bedrock and cap rock. A maximum thermal recovery rate exists for a given flow rate. This maximum increases with inflow rate. The injection temperature behaves on the stock in the same way as the aquifer thickness for a given inflow: there is an optimal temperature. However it is in general preferable to increase the injection temperature instead of the inflow rate. In terms of optimization of the thermal power at disposal, the most favorable scenario is an increasing flow during the discharge. The precise scenario is different for each problem. The performances of a ATES are the complex result of the many parameters which enter in plays. One will recommend an inflow rate of at least 100m33/d during 6 months, and even rather 500 - 1000m3/d with an injection temperature of 80°C at least. An injection temperature of 60°C will have to be associated with an inflow of 1000m3/d at least. The geological targets proposed in Switzerland are the quaternary alluvial deposits filling the former glaciated valleys, fluvioglacial deposits and alluvial fans.

EPFL2001