| dc.description.abstract | With increasing pressure on water resources through climate change and anthropogenic influences, adequate freshwater management is essential. Important managing tools require efficient infiltration of freshwater into the subsurface. Fast, High Volume Infiltration (FHVI) is a novel infiltration technique that, if employed at its full potential, offers fast and efficient infiltration of freshwater into the subsurface. However, to date, the mechanisms and conditions enabling FHVI are not fully understood and its application is mainly based on trial and error. To overcome this barrier, research is initiated to accurately model FHVI. Due to the high injection pressures employed with FHVI, it is expected that non-linear flow behavior occurs around the injection well. Therefore, insight into this non-linear flow behavior is an adequate first step in understanding FHVI.
This research focusses non-linear Forchheimer flow in porous media similar to those in which FHVI is frequently applied (fine to course sands). An experimental setup that allows hydraulic head vs. superficial velocity measurements is used to analyze non-linear flow behavior in a variety of sandy porous media. Analysis is performed using the Forchheimer equation coefficients, statistical properties of the grain size distribution and the macroscopic parameters of the porous medium.
First, 11 fairly uniformly distributed calibrated reference porous media are tested. The porous medium with the smallest median grain size (U 0.2-0.5 [mm]) yielded the highest flow resistance with Forchheimer coefficients a and b being equal to 2.2369·10-2 [day m-1] and 1.8249·10-6 [day2 m-2], respectively. The porous medium with the largest median grain size yielded the lowest flow resistance with Forchheimer coefficients a and b being equal to 9.268·10-5 [day m-1] and 8.2308·10-8 [day2 m-2], respectively. In case of the reference porous media, flow resistance increased with decreasing median grain size.
Secondly, mixtures of the calibrated reference sand were prepared to investigate the effect of the shape of the grain size distribution on non-linear flow in porous media. The composites were prepared with their median grain size being equal to those of the reference porous media, but with a larger standard deviation or with the presence of one or multiple gaps in their distribution. It was found that the wider the grain size distribution, the larger the flow resistance. Especially the presence of a large fraction of fine material seemed to enhance the flow resistance. The most interesting results were obtained for the gap-graded porous media. For gap-graded porous medium N-U3, the Forchheimer coefficients a and b increased with 160.2 [%] and 200.8 [%], respectively, with respect to the fairly uniformly distributed reference porous medium U 0.4-0.8 [mm] with a similar median grain size¬.
Thirdly, the applicability of 7 existing empirical relations, for non-linear flow in porous media, to describe the experimental results of this study was assessed. In case of the reference porous media, it appeared that good fits were obtained by both the expressions by Kovács (1981) (for d50 < ±1[mm]) and Macdonald et al (1979) (for d50 >±1[mm]). The empirical relations proved to be inadequate to describe non-linear flow in non-uniformly distributed porous media (the composite porous media) as the empirical relations do not take the shape of the grain size distribution into account. It is recommended that modifications to these empirical relations are made if employed for non-linear flow in non-uniform porous media.
The outcome of this study is very useful within the context of FHVI. Great insight into non-linear flow in a variety of porous media is obtained. A subsequent step will be the application of the experimental data to a numerical model. | |
