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Résumé |
Tunnel drilling in karst regions often leads to major disturbances in the hydrogeological functioning of
aquifers and flow-systems. Numerous examples in Switzerland do exist and induced significant costs,
which were not or rarely anticipated (e.g.: Flims, Jeannin et al. 2009). The Ligerztunnel is one such
example. The tunnel was built a few hundreds of meters upstream from the Brunnmühle spring, which
contributes to the drinking water supply of communities of Twann and Ligerz. During the construction, a
major karst conduit with a huge discharge rate was intersected in a side exploration tunnel. Overflowing
water was diverted in the Twannbach canyon. In the main section, smaller conduits were found and
drained outside by pipe leading water close to the Brunnmühle spring. Actually, authorities want to add a
safety gallery parallel to the main tunnel. In this view, SISKA is in charge of evaluating the hydrological
disturbances on the spring regime. The paper presents the approach applied to assess the potential effect of
the drilling of a new tunnel near to a group of karst springs and pumping wells. The approach combines
available spatial information and a hydraulic model. The KARSYS approach is first applied on the system
in order to set up a 3D geological and hydrogeological model of the karst aquifer and the related systems.
The spatial distribution of karst conduits within the massif is assessed based on a speleogenetical and
inception horizons model (KarstALEA method). Inferring from these models, a karst conduits network is
generated. The downstream part of the conduits network, which concerns the vicinity of the tunnels
project, is precisely calibrated using head and discharge data. Flow in this conduits network is then
simulated using SWMM 5.0 in order to reproduce the hydrological responses of the different outlets
(permanent springs, drainage devices, overflow springs, etc.). |
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