1 research outputs found
Analogue Fracture Experiments and Analytical Modeling of Unsaturated Percolation Dynamics in Fracture Cascades
Infiltration and recharge dynamics in fractured aquifer systems often strongly
deviate from diffuse Darcy–Buckingham type flows due to the existence of a complex
gravity-driven flow component along fractures, fracture networks, and fault
zones. The formation of preferential flow paths in the unsaturated or vadose zone
can trigger rapid mass fluxes, which are difficult to recover by volume-effective
modeling approaches (e.g., the Richards equation) due to the nonlinear nature
of free-surface flows and mass partitioning processes at unsaturated fracture
intersections. In this study, well-controlled laboratory experiments enabled the
isolation of single aspects of the mass redistribution process that ultimately affect
travel time distributions across scales. We used custom-made acrylic cubes (20 by
20 by 20 cm) in analog percolation experiments to create simple wide-aperture
fracture networks intersected by one or multiple horizontal fractures. A high-precision
multichannel dispenser produced gravity-driven free-surface flow (droplets
or rivulets) at flow rates ranging from 1 to 5 mL min−1. Total inflow rates were kept
constant while the fluid was injected via 15 (droplet flow) or three inlets (rivulet
flow) to reduce the impact of erratic flow dynamics. Normalized fracture inflow
rates were calculated and compared for aperture widths of 1 and 2.5 mm. A higher
efficiency in filling an unsaturated fracture by rivulet flow observed in former
studies was confirmed. The onset of a capillary-driven Washburn-type flow was
determined and recovered by an analytical solution. To upscale the dynamics and
enable the prediction of mass partitioning for arbitrary-sized fracture cascades,
a Gaussian transfer function was derived that reproduces the repetitive filling of
fractures, where rivulet flow is the prevailing regime. Results show good agreement
with experimental data for all tested aperture widths.Open-Access-Publikationsfonds 2019peerReviewe