Convergent flow observed in a laboratory‐scale unsaturated fracture system 1

LaViolette

2004

[1] Under conditions of unsaturated flow, others have shown experimentally that fracture intersections can direct flow to a single exiting fracture. In addition, they have been found to gather water from above to release as a pulse below. We formulate a simple model where these two behaviors are embedded within a network. With slow steady inflow distributed randomly along the top of the network, the system self organizes to form avalanches of water that can penetrate to great depths. When all intersections split their outflow, flow diverges with depth and develops into a self-organized dynamical state where the distribution of avalanche sizes follows a power-law over many decades. As the fraction of intersections that direct outflow singly is increased, spatial structure passes from divergent through braided to a fully convergent, hierarchical flow regime where avalanche size is minimized along one-dimensional slender pathways.

Section: Introductionmentioning

confidence: 99%

Self organized spatial‐temporal structure within the fractured Vadose Zone: Influence of fracture intersections

LaViolette

2004

“…Unsaturated flow experiments in fracture networks indicate that intersections can direct flow to a single exiting fracture [ LaViolette et al , 2003]. In addition, they have been found to gather water from above to release as a pulse below [ Wood et al , 2002].…”

Section: Introductionmentioning

confidence: 99%

Self organized spatio‐temporal structure within the fractured Vadose Zone: The influence of dynamic overloading at fracture intersections

LaViolette

2004

Self Cite

Under low flow conditions (where gravity and capillary forces dominate) within an unsaturated fracture network, fracture intersections act as capillary barriers to integrate flow from above and then release it as a pulse below. Water exiting a fracture intersection is often thought to enter the single connected fracture with the lowest invasion pressure. When the accumulated volume varies between intersections, the smaller volume intersections can be overloaded to cause all of the available fractures exiting an intersection to flow. We included the dynamic overloading process at fracture intersections within our previously discussed model where intersections were modeled as tipping buckets connected within a two‐dimensional diamond lattice. With dynamic overloading, the flow behavior transitioned smoothly from diverging to converging flow with increasing overload parameter, as a consequence of a heterogeneous field, and they impose a dynamic structure where additional pathways activate or deactivate in time.

“…Projection of our results to a regular network of equal aperture vertical and horizontal fractures suggests substantial differences between wetting and nonwetting flows. The absence of entry into the horizontal fractures by the nonwetting phase would preclude the following phenomena observed for wetting phase flow: macroscopic flow convergence [ LaViolette et al , 2003], pathway switching [ Glass et al , 2002, 2003], and temporal fluctuations in outflow over a range of scales [ Glass et al , 2002]. Instead, we expect that the nonwetting phase would move rapidly along narrow pathways through the vertical fractures, establishing small pools at each intersection along the way.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…Recent experiments considering unsaturated flow in hydrophilic fracture networks have shown that capillary barriers formed at simple intersections between vertical and horizontal fractures can act to divert flow [ LaViolette et al , 2003], impose a temporal signal [ Glass et al , 2002; Wood et al , 2002], and/or block lateral spreading [ Glass et al , 2003]. While not fully understood, this complex behavior clearly results from a sudden change in the relative importance of capillary forces that rises from the difference in void geometry between an intersection and the contributing fractures.…”

Section: Introductionmentioning

confidence: 99%

Influence of a simple fracture intersection on density‐driven immiscible flow: Wetting vs. nonwetting flows

Nicholl

et al. 2004

[1] The influence of a single fracture intersection on density-driven immiscible flow is compared between wetting (water into air) and nonwetting (Trichloroethylene into water) flows. At low supply rates, the intersection acted as a hysteretic gate to pulsed flow of the wetting phase, but had minimal influence on nonwetting phase flow. For both cases, increasing the supply rate led to the formation of continuous fluid tendrils that crossed the intersection without interruption. The wetting experiment returned to pulsed flow as the supply rate was decreased, while the nonwetting experiment maintained a continuous flow structure. Results suggest a fundamental difference between wetting and nonwetting phase flows in fracture networks.