Groundwater modelling in karst terrains

Groundwater models are frequently applied for practical purposes, for instance, with regard to the management and protection of water resources. To date, practical applications of such models mainly address groundwater in unconsolidated sediments such as sand or gravel, where the water moves slowly through the narrow pore spaces within the grain structure. In contrast, the dissolution of karst rocks (such as limestone) may cause a widening of fractures, thus creating much larger cavities with diameters ranging from centimetres to metres. The rapid and often turbulent flow in such solution conduits is generally not adequately considered by the prevalent groundwater models. Within this project, therefore, more recent modelling approaches accounting for the peculiarities of karst areas were tested and further developed. To this end, benchmark tests were performed and various modelling approaches were applied to well-investigated test sites. It was found that the occurrence of turbulent flow may have significant effects on the transient discharge behaviour of karst springs. The representation of these processes in the models employed was found to be inadequate, in particular, with regard to the applicability for the test sites. For this reason, a more robust and easily applicable approach accounting for non-linear (turbulent) flow was developed and implemented into the widely-used groundwater model MODFLOW (Non-Linear Flow Process; NLFP). In addition, the example applications resulting from this project demonstrate how groundwater models can support the characterization of the properties and functioning of karst aquifers and, in particular, the delineation of spring catchments, thus contributing to practical issues such as the designation of protected areas.       

Mayaud, C., Walker, P., Hergarten, S., Birk, S. (2015): Nonlinear Flow Process: A New Package to Compute Nonlinear Flow in MODFLOW. Groundwater 53 (4): 645-650. doi: 10.1111/gwat.12243 Available at: http://onlinelibrary.wiley.com/doi/10.1111/gwat.12243/abstract

Hergarten, S., Winkler, G., Birk, S. (2014): Transferring the concept of minimum energy expenditure from river networks to subsurface flow patterns. Hydrology and Earth System Sciences Discussions 11: 5831-5857. doi:10.5194/hessd-11-5831-2014 Available at: http://www.hydrol-earth-syst-sci.net/18/4277/2014/hess-18-4277-2014.html

Birk, S., Wagner, T., Mayaud, C. (2014): Threshold behavior of karst aquifers: the example of the Lurbach karst system (Austria). Environmental Earth Sciences, published online, doi: 10.1007/s12665-014-3122-z Available at: http://link.springer.com/article/10.1007%2Fs12665-014-3122-z#

Mayaud, C., Wagner, T., Benischke, R., Birk, S. (2014): Single event time series analysis in a binary karst catchment evaluated using a groundwater model (Lurbach system, Austria). Journal of Hydrology 511: 628-639. doi: 10.1016/j.jhydrol.2014.02.024 Available at: http://www.sciencedirect.com/science/article/pii/S0022169414001322

Oehlmann, S., Geyer, T., Licha, T., Birk, S. (2013): Influence of aquifer heterogeneity on karst hydraulics and catchment delineation employing distributive modeling approaches. Hydrol. Earth Syst. Sci. 17: 4729-4742. doi:10.5194/hess-17-4729-2013. Available at: http://www.hydrol-earth-syst-sci.net/17/4729/2013/

Wagner, T., Mayaud, C., Benischke, R., Birk, S. (2013): Ein besseres Verständnis des Lurbach-Karstsystems durch ein konzeptionelles Niederschlags-Abfluss-Modell. Grundwasser 18 (4): 225-235. doi:10.1007/s00767-013-0234-4. Available at: http://link.springer.com/article/10.1007%2Fs00767-013-0234-4

Mayaud, C, Wagner, T., Benischke, R., Birk, S. (2013): Understanding changes in the hydrological behaviour within a karst aquifer (Lurbach system, Austria). Carbonates and Evaporites, published online, doi: 10.1007/s13146-013-0172-3. Available at: http://link.springer.com/article/10.1007%2Fs13146-013-0172-3

Reimann, T., Birk, S., Rehrl, C., Shoemaker, W. B. (2012): Modifications to the Conduit Flow Process Mode 2 for MODFLOW-2005. Ground Water 50 (1): 144-148. doi:10.1111/j.1745-6584.2011.00805.x. Self-archived version available here.

Reimann, T., Rehrl, C., Shoemaker, W. B., Geyer, T., Birk, S. (2011): The significance of turbulent flow representation in single-continuum models. Water Resources Research 47, W09503, doi:10.1029/2010WR010133. Self-archived version available here.

Groundwater flow in porous media is usually considered to follow the linear Darcy law. However, flow can become non-linear or turbulent if the specific discharge is high. This is known to occur, for example, within the solution conduits of karst aquifers or in the vicinity of pumping wells. As most of the existing groundwater models such as MODFLOW-2005 are based on the linear Darcy law, there is a lack of tools accounting for the non-linear flow conditions encountered in these settings. For this reason, a new MODFLOW package (Non-Linear Flow Process; NLFP) simulating non-linear flow following the Forchheimer equation was developed and implemented in MODLFOW-2005 (Mayaud et al. 2014). This package is based on an iterative modification of the linear conductance used by MODFLOW. The resulting effective Forchheimer conductance decreases with increasing specific discharge and thus mimics the effect of the non-linear term of the Forchheimer equation. NLFP can be used only with the Block Centered Flow package (BCF) but supports the different layer types,boundaries conditions, and solvers as well as the wetting capability of MODFLOW. The current version computes non-linear flow only within preferential flow layers; the vertical conductance remains unchanged. NLFP can be downloaded here.