Abstract:

Karst aquifers develop where an enlargement of fractures due to dissolution creates highly permeable conduits. These conduits are embedded in the much less permeable fissured system of the surrounding rock. The hydrogeological characterisation of these heterogeneous, dualistic flow systems requires a deep understanding of the processes involved in karstification. During the last two decades many numerical models have been developed to simulate conduit evolution in karst terrains and to understand and analyze the mechanisms of speleogenesis. In this study, conduit development within a soluble unit of a multi-layer aquifer system is examined by process-based numerical modeling. The dual flow system is adequately represented by a coupled continuum-pipe flow model; the flow model is coupled to a module calculating dissolution rates and the corresponding widening of conduits depending on flow conditions. The simplified model scenarios are largely based on field observations compiled from the gypsum karst terrain of the Western Ukraine. It is demonstrated that the hydraulic conductivity of the rock formation is a crucial factor that controls the frequency distribution of conduit diameters in hypogene speleogenesis. If the permeability of the rock formation is sufficiently high, conduit development is found to be competitive and leads to bimodal aperture distributions. Otherwise flow in low-permeability formations is suppressed and as a consequence, there is a smooth transition from scarcely developed proto-conduits to well-developed conduits rather than a clear and distinct separation. This work further examines the influence of the variability of the initial apertures on dissolutional growth of fissures and the evolving cave patterns. The initial apertures were not spatially correlated and log normally distributed. The influence of the aperture variability was investigated in several scenarios. It is found that in an ensemble average sense the degree of heterogeneity determines the temporal development of the cave patterns, i.e. higher aperture variability generally decelerates the karstification process. The aperture variability, however, appears to be of minor relevance regarding the general structure and geometric properties of the evolving cave patterns.