The role of fracture intersections in fluid flow in unsaturated fractured rock networks

There is an urgent need for better predictive models of unsaturated flow and transport in fractured rock vadose zones. In order to develop such models, it is first necessary to improve our understanding of the processes that control fluid flow. Recent experimental evidence suggests that the capillary heterogeneity associated with fracture intersections can act to impose temporal and spatial structure on flow at the fracture-network scale. The fundamental questions that this dissertation addresses relate to the importance of fracture intersections in imposing such structure. Unsaturated flow in fractured rock is investigated in a series of laboratory experiments designed to elucidate the critical role of fracture intersections. The body of work presented herein demonstrates that fracture intersections, although generally ignored in most conceptual models, often generate flow behavior that is not anticipated or predicted with commonly employed modeling approaches. The action of fracture intersections to impose spatial-temporal phenomenon such as flow convergence with depth, pathway switching, fluid cascades and flow integration are documented in a series of meso-scale laboratory experiments. Smaller-scale experiments provide the first direct evidence that a single fracture intersection can impose temporal structure by integrating slow steady flow into less frequent larger pulses. This behavior occurs because the interplay between capillary and gravitational forces at a fracture intersection can lead to the formation of capillary barriers capable of blocking or diverting flow. The geometry of the fracture intersection provides critical control on the strength of the capillary barrier and the volume of dynamic storage released during flow integration by the fracture intersection. The body of work presented herein has identified, described and defined the critical role of fracture intersections in unsaturated flow in fractured rock. Serious flaws exist in the current conceptual models and additional research is identified that will lead to advancements in the understanding of the role of fracture intersections on fluid flow across a wide range of geologic scales in fractured rock.