A NON-STEADY AND NON-UNIFORM HYBRID MODEL FOR KARST AQUIFER CHARACTERIZATION

Groundwater flow in karst aquifers is focused mainly in the highly permeable conduit drainage network. To investigate karst systems on local and regional scale, it is possible to interpret artificial induced signals, e.g. large scale pumping tests in a borehole intercepted a conduit in the vicinity of the spring. For process-based interpretation of such pumping tests, it is necessary to consider the time and space dependent response of the hydraulic head in the karst conduit. Therefore, a model approach to describe non-steady and non-uniform flow in karst conduits is of great effort.

Hence, we adapted the USGS code MODBRNCH, a coupling of a free-surface flow model with MODFLOW, to account for karst aquifers. The modified model ModBraC solves the St. Venant equations for open-channel flow. Modifications comprise mainly the implementation of a variable time step, the introduction of the Preissmann slot for pressurized flow as well as the adaptation of the boundary conditions.

To proof the correct functioning of ModBraC, evaluation tests were performed in order to investigate: (1) flow in filled conduits, (2) water transfer between matrix and conduits, (3) wave propagation for free-surface flow, and (4) storage in the conduits. If possible, the results were compared with current steady modeling approaches, e.g. MODFLOW-2005 Conduit Flow Process (CFP) as well as with analytical solutions. Evaluation tests demonstrate the correct implementation of the free-surface flow equations in ModBraC. The comparison with steady modeling approaches based on the water balance shows a large difference between both and reflects the importance of transient storage in karst conduits.

To demonstrate the abilities of ModBraC, we performed a parameter study in a synthetic catchment stressed by pumping. Parameters as well as pumping data were based on a field experiment conducted by Maréchal et al. (2008). By use of ModBraC, the influence of several parameters on the observed discharge was investigated, which lead to a more authoritative parameter assignation. Finally, we can conclude that the St. Venant equations are appropriate to describe flow in fully as well as in partly filled karst conduits. Hence, the hybrid model ModBraC can be used to describe the transient behavior of karst aquifers.