Void evolution in soluble rocks: Development and validation of numerical models by field evidence
Final Report Abstract
Karst aquifers are highly vulnerable to contamination due to the rapid transport of pollutants in conduits developed by the dissolution of rock. In addition, the presence of voids in the subsurface can cause severe engineering problems such as dam failure and land subsidence or collapses. The assessment of such environmental impact problems requires an adequate hydrogeological characterisation of karst terrains focusing on solution conduits and voids. Thus, this work aimed to improve our understanding of the interdependency between hydraulic and chemical processes involved in speleogenesis, and the geometry of the resulting cave patterns. Building on earlier research that focussed on generic conceptual settings, it was attempted to establish site-related models that are largely based on field observations but not intended to represent details of the evolution of specific caves. The study sites were located in the karst terrain of the Western Ukraine, as this area is well documented, displays the full sequence of karst evolutionary stages, and hosts a wide variety of karst phenomena. To examine the interrelation between hydrogeological environment and conduit development in the deep-seated settings of the Western Ukraine, simplified model settings were designed based on an existing conceptual model. A coupled continuum-pipe flow model, representing the fractured porous rock by a continuum approach and the solution conduits by a discrete pipe network, was employed for simulating conduit development under various conditions. In agreement with field observations, the evolving cave patterns were found to be characterized by pronounced horizontal passages and multiple vertical conduits at the bottom of the soluble unit but only few at the top. The frequency distribution of conduit diameters was found to be bimodal if the permeability of the rock formation is sufficiently high to allow competitive conduit development governed by the feedback between increasing flow and dissolution rates. This feedback, however, is suppressed in low-permeability formations. As a consequence, conduit development was found to be uniform rather than competitive in the corresponding model scenarios. Our results further reveal effects of the spatial extension of the discharge area, the variability of initial apertures, and the chemical saturation of the water on cave patterns evolving. More generally, our results suggest that numerical modelling is not only useful for studying the general interactions of physical and chemical processes governing karst evolution. The translation of site-related conceptual models to numerical model scenarios can provide further insight into speleogenetic mechanisms and controlling factors of karst evolution in specific types of settings.
