CONDUIT WALL MORPHOLOGY AND KARST CONDUIT HYPORHEIC FLOW

Karst conduit hyporheic exchange allows conduits to locally gain and loose water to the matrix continually all along their course, whether or not there is a large-scale net gain or loss of water along the length of the conduit, or no net exchange. Hyporheic flow is caused by local pressure gradients produced by a variety of mechanisms, including: wall morphology, conduit sinuosity, heterogeneous matrix permeability, and others. Hyporheic flow has been recognized as a critical process in streams and may be a fundamental exchange between conduit and matrix in karst aquifers. We present results from three 2-dimensional Cartesian and radial models relating conduit wall morphology to karst conduit hyporheic flow. The models examine a reach of conduit and the surrounding matrix. For all models the upstream and downstream boundaries are periodic boundary conditions. Turbulent flow in the conduit is modeled first, and a pressure distribution along conduit walls is generated. The wall pressure distribution is then used as a boundary condition for the Darcian hyporheic flow in the porous media. Three conduit flow models are developed, the first two with Cartesian longitudinal cross-sections, and the third with radial symmetry. The first model uses a no-slip condition on the lower wall and a slip condition on the upper wall, the same condition used for a free-surface upper boundary in a surface stream. The second model uses a no-slip boundary on both walls. The third model also uses no-slip walls. Smooth, rough, and more complicated wall morphologies are considered, guided by wall surveys in phreatic karst conduits. Net gaining and losing situations are considered, as well as no net exchange. We conclude by discussing the effects of morphology driven karst conduit hyporheic flow on speleogenesis, contaminant transport, and residence time distributions.