Interaction of Fracture and Conduit Flow in the Evolution of Karst Aquifers

Karst aquifers in their initial state consist of a net of fractures with largely differing aperture widths. As a most simple example we investigate the evolution of a karst aquifer where a wide fracture with aperture width A0 = 0.03 cm is embedded into a dense net of narrow fractures of aperture widths a0 < A0. The aim of this work is to investigate the influence of the hydraulic coupling between these fractures to the evolution of the karst aquifer. The modelling domain consists of a confined aquifer, which is divided into a square network consisting of narrow fractures. In its center a straight wide fracture leads from the input at hydraulic head h to the output at head zero. We have computed the breakthrough times of this aquifer as a function of a0. For a0 = 0 the breakthrough time is that of an isolated one-dimensional fracture. As a0 is increased the breakthrough times drop until at about a0 > 0.02 cm they are reduced significantly by almost one order of magnitude. This is caused by the following mechanism. As the central tube gets enlarged into a funnel like shape from its entrance water from its tip is injected into the fine net of fractures. Therefore more aggressive solution enters into the central fracture and enhances dissolutional widening there. By this way aquifers with wide fractures embedded into a continuum of fine fractures experience accelerated karstification.