INFLUENCE OF TOPOGRAPHICALLY DRIVEN CONVECTION ON HEAT-FLOW IN THE SWISS ALPS - A MODEL STUDY

A 2-D thermohydraulic model for two sites in Switzerland is presented. Geologically both sites belong to Alpine nappe units and consist of g enerally low permeable rocks with hydraulic conductivities in the rang e of 10-(10)-10(-8) m s(-1). Elevation of topography attains 2.4-3.1 k m, and the mountain water table exhibits a correspondingly high relief . The geothermal regime of the area is characterized by relatively hig h values of measured heat how on the order of 80-100 mW m(-2). The res ults reveal well developed subsurface fluid circulation systems in bot h cases. The vertical fluid velocities reach some cm/yr, which is high er by more than one order of magnitude than the how rates in hat terra in. In mountainous terrain, flow systems may penetrate to depths of 2- 3 km below sea level. The discharge zones in areas with high relief an d steep slopes are rather narrow and concentrate at and near the valle y bottoms. Such zones are the regions of heating where positive distur bances may reach up to 1.5-1.8 times the basal heat flow value. In the recharge areas at ridges, heat how may in turn become quite low. The calculated thermohydraulic models have been validated by measured temp erature distributions in drillholes. Reasonably good agreement between the measured and the calculated temperatures exists for both sites. ( C) 1998 Elsevier Science B.V. All rights reserved.