Unsaturated flow through a small fracture-matrix network. 1. Experimental observations

The behavior of unsaturated flow was investigated in a laboratory model. A constant and uniform supply of chemically equilibrated water was introduced to the upper surface of three artificial fractures in a surrogate fracture network consisting of a thin wall of uncemented limestone blocks. Water was collected from the lower boundary via fiberglass wicks placed at the bottom of each artificial fracture. Eight experiments were conducted to evaluate the repeatability of flow under nearly identical conditions and to characterize general patterns in flow behavior. Collected data revealed that flow generally converged to a single fracture in the bottom row of blocks. Periods of pathway switching were observed to be more common than periods with steady, constant flow pathways. We noted the importance of fracture intersections for integrating uniform flow and discharging a "fluid cascade," where water advances rapidly to the next capillary barrier creating a stop and start advance of water through the network. Under very similar initial moisture and boundary conditions, flow in the system was less repeatable than expected. The results of this simple experiment suggest that the interaction of multiple fracture intersections in a network creates flow behavior not generally recognized in popular conceptual and numerical models, (i.e., convergence of flow, pathway switching, and fluid cascades).