Impact of a Tunnel on a Karst Aquifer - Springer Link

Impact of a Tunnel on a Karst Aquifer: Application on the Brunnmühle Springs (Bernese Jura, Switzerland) A. Malard, P.-Y. Jeannin and D. Rickerl

Abstract Tunnel drilling in karst regions often leads to major disturbances in the hydrogeological functioning of aquifers and flow-systems. Numerous examples are documented in Switzerland and induced significant costs, which were not or rarely anticipated (e.g.: Flims, Jeannin et al. 2009). The Ligerztunnel is one of these 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 into 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 tunnel 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 this 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 hydraulic model of the downstream part of the conduits network, which concerns the close vicinity of the safety tunnel 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.).

A. Malard P.-Y. Jeannin D. Rickerl (&) Swiss Institute for Speleology and Karst Studies, Rue de La Serre 68, 2301 La Chaux-de-Fonds, Switzerland e-mail: [email protected] Springer-Verlag Berlin Heidelberg 2015 B. Andreo et al. (eds.), Hydrogeological and Environmental Investigations in Karst Systems, Environmental Earth Sciences 1, DOI 10.1007/978-3-642-17435-3_52

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1 Introduction Communities of Twann and Ligerz (Bernese Jura, CH) obtain water from the Brunnmühle spring, which is a karst spring emerging from Malm karst aquifer. In 1980s the construction of the Ligerztunnel, which crosses the karst aquifer 100 m upstream from the spring induced serious hydrological disturbances to the karst hydraulics. Active karst conduits have been intersected in the tunnel and required the drilling of a pipe to discharge karst waters out of the tunnel, close to the spring (Bollinger and Kellerhals 2007). This considerably modified the dynamics of the karst aquifer: the seasonal regime of the spring as well as the functioning of the overflow springs located 40 m above the permanent outlet. Nowadays the construction of a safety gallery parallel to the tunnel raises the question of new hydrological disturbances on the spring. The communities already considered to replace the actual supply device by two wells drilled further upstream in the same aquifer. The question to be addressed is therefore to assess the potential disturbances of the new gallery on the spring and wells. As the study is still in course, only the steps of the proposed approach are presented in the following note. This approach is an extension of the KARSYS approach (Jeannin et al. 2013). The evaluation is based on five main steps: i. Build a geological and hydrogeological 3D model of the site in order to characterize the underground drainage zone of the system and its catchment area over the surface using the KARSYS approach; ii. Establish a speleogenetic model of the site to assess the potential organization of the karst conduits network. At the same time, the analysis of existing caves and karst features will be used to identify the inception horizons; iii. Design a hydraulic model of the system using the hydrological karst features (permanent and overflows springs, catchment areas), the artificial drainage devices (tunnel, drainage pipes) and the results of the speleogenetic and inception models; iv. Reproduce the discharge regime of the Brunnmühle permanent springs and the overflow outlets; v. Test various scenarios of hydrological disturbances on the spring regime caused by the tunnel construction. As far as possible, disturbances will be extended to pumping wells, which are being equipped as the future drinking supply for the communities;

2 Context The site is located at the foothill in the Eastern part of the Jura Mountain (canton of Bern) along Lake Biel (see Figs. 1 and 2). The geological context is composed of South-East dipping pile of Jurassic and Cretaceous limestone, underlain by Oxfordian Marls (aquiclude). The tunnel develops approximately along the strike

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Fig. 1 Site location Black line existing main tunnels; Green line existing exploration tunnel; Red line security gallery (project), Orange lines future railway and road tunnels. The Brunnmühle spring (blue) emerges at 433 m a.s.l close to Lake Biel. Other springs are visible in the Twannbach canyon (Im Moos)

of bedding planes and the Brunnmühle spring emerges at the base of the massif, close to the elevation of the lake Biel, but at the top of the limestone series. Several other springs emerge in the Twannbach canyon. Due to the existing exploration tunnel, which intersected a major karst conduit, the natural hydrological regime of the karst system was very probably modified. This must be considered in the perspective of a hydraulic simulation and impact assessment. As sketched on Fig. 2 the Brunnmühle spring emerges at the contact between the Portlandian limestone and the Purbeckian marls. It is mainly fed by the Malm aquifer (upper Jurassic limestone, from Sequanian to late Portlandian, *600 m of thickness) and partially dammed by the Purbeckian marls. All tunnels are located within the Malm aquifer, close to the groundwater table. Holiloch and Gisheren are emissive caves, which become active during highflow events. They act as overflow springs of the system, especially during the spring melt period (https://www.google.ch/maps/@47.093352,7.151663,3a,75y,331. 3h,77.88t/data=!3m4!1e1!3m2!1snPY6RkNP2cjiAKW9Pb2xEA!2e0?hl=fr). Their entrances are located 44 m above the elevation of the Brunnmühle spring indicating that the hydraulic head considerably rises during high-flow conditions.

3 Approach The first step of the KARSYS approach (Jeannin et al. 2013) consists in building a geological 3D model of the site depicting the geometry of the Malm aquifer. Figure 3 depicts the geological structure of the site. The existing and projected

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Fig. 2 Schematic cross-section of the site, the Jurassic and Cretaceous units plunge toward the South-East. The Brunnmühle spring emerges at the contact between the portlandian limestone and the purbeckian marls

Fig. 3 Perspective view of the geological 3D model of the site (8km along the east-western axe and 6km along the north-southern axe); the tunnel (green pipe) develops along the bedding planes below the Purbeckian marls (red layer)

tunnels (green pipes) all develops within the Malm limestone, i.e., Purbeckian Marls (red layer on Fig. 3). Hydrological features (mainly springs, but also boreholes and caves indicating the position of the water table) are added to the model in order to build a hydrogeological 3D model of the site. The model shows that the Brunnmühle


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Fig. 4 Preliminary catchment area of the Brunnmühle karst system; the area is 66 km2 wide but presents divergent parts with the adjacent karst systems

spring emerges at the lowest outcrop of Malm limestone, confirming the damming effect of the Purbekian marls (red layer). Knowing the geometry of the aquifer and assuming an almost horizontal water table at low water conditions, the extension of groundwater bodies, the underground drainage zone, and the respective catchment area feeding the system can be delineated (Fig. 4). The second step of the approach is to set up a speleogenetical model of the site in order to hypothesize the organization of the karst conduits within the massif.

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The density and the geometry of existing caves and karst evidences in this area will be analyzed and compared with the geological model in order to assess the main horizons of karstification (called ‘‘inception horizon’’, Filipponi 2009) along some specific stratigraphic limits or due to tectonic disturbances. Such model makes it possible to assess the supposed organization of the karst network. This approach is known as the KarstALEA method (Filipponi et al. 2012). The aim of the third step of the approach is to generate a pipe network according to the 3D hydrogeological and speleogenetical models, and to make a first guess of the flow parameters of the generated pipes. A series of hydraulics and speleogenetical principles are used to generate a conduit network linking the catchment area according to its topography and infiltration characteristics to the main outlets (springs) of the system. The generator provides files directly compatible for SWMM 5.0 (Rossman 2004), which is a robust pipe-flow modeling tool. For the fourth step, the model will be calibrated using first existing head and discharge data acquired in the neighboring of the spring. The hydraulic characteristics and the topology of the pipes in the vicinity of the spring will be calibrated using a series of various hydraulic situations (e.g., overflow conditions of the respective springs, measured heads, and discharge rates). This network will be included into the pipe network generated at catchment scale. This model will be calibrated using a recharge model assessing infiltration on various parts of the catchment area. The fifth step will include the simulation of the intersection of various karst conduits (pipes) by the projected tunnels. The first simulations show that the effect can range from almost insignificant to very considerable depending on the topological position of the intersected karst conduit.

4 Discussion One main uncertainty of the proposed approach is related to the lack of discharge rate measurements (short or incomplete hydrographs) of the different outlets. This is especially true for artificial tunnels, where discharge rates have not been measured correctly until now (April 2014). Another difficulty is that no data are available for all springs before the construction of the first tunnel, making a comparison with the natural situation almost impossible.

5 Conclusion The KARSYS approach was then applied to the Brunnmühle karst system in order to define the underground drainage pattern and the catchment area of the spring. In a second step a speleogenetical model was established in order to design the


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organization of the karst conduits within the massif. Then, these parameters are being injected within a hydraulic model—SWMM 5.0 to simulate the regime of the Brunnmühle spring and its overflow springs in the Twannbach canyon. According to the speleogenetical model, several scenarios of potential disturbances will be tested to assess the effects on the springs’ regime, as well as on the future borehole. As a preliminary discussion, it appears that simulating such systems may be quite complicated as data and measurements do not exist for all outlets. These data and measurements may also be obsolete as artificial galleries (or drainage) probably modified the natural functioning of the aquifer. As the study is still in progress, results could not be discussed yet. Only the approach was presented here. Results will be presented during the conference and compared to the initial approach.

References Bollinger D, Kellerhals P (2007) Umfahrungstunnel Twann (A5): Druckversuche in einem aktiven Karst. Bull angew Geol 12/2:49–61 Filipponi M (2009) Spatial Analysis of Karst Conduit Networks and Determination of Parameters Controlling the Speleogenesis along Preferential Lithostratigraphic Horizons. Ecole polytechnique fédérale de Lausanne (EPFL), Suisse, PhD dissertation. 305 p Filipponi M, Schmassmann S, Jeannin PY, Parriaux A (2012) KarstALEA: Wegleitung zur Prognose von karstspezifischen Gefahren im Untertagbau—Forschungsprojekt FGU 2009/003 des Bundesamt für Strassen ASTRA, unpubl. rep. Schweizerischer Verband der Strassen- und Verkehrsfachleute VSS, Zürich, Schweiz Jeannin PY, Eichenberger U, Sinreich M, Vouillamoz J, Malard A et al (2013) KARSYS: a pragmatic approach to karst hydrogeological system conceptualisation. Assessment of groundwater reserves and resources in Switzerland. Environ Earth Sci 69(3):999–1013 Jeannin PY, Häuselmann P, Weber E, Wildberger A (2009) Impact assessment of a tunnel on two karst springs, flims, Switzerland. In: Proceedings of the 15th international congress of speleology—Kerrville, vol 3. Texas, United States of America, p 1537 19-26 July 2009 (contributed Papers) Rossman LA (2004) Storm Water Management Model. User’s manual version 5.0, EPA, unpubl. rep. U.S. Environmental Protection Agency, Cincinnati, OH