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Institut für Wasser- und Umweltsystemmodellierung - IWS



"Macroscopic Trapping of Water during Drainage and the Role of Material Interfaces"

Upscaled flow models of drainage in porous media on larger scales usually imply an equilibrium assumption for the representative elementary volume (REV) of the model meaning that water content inside the REV is at steady state. However, heterogeneous substructure in REV may cause flow behavior with typical time scales comparable to typical time scales for water flux over the whole medium. Such effects cause tailing and retardation of water content. This requires different modeling concepts like a dual continuum model. Such a situation may occur during drainage, if fine material is enclosed into coarse and the typical drainage time scale for drainage of the coarse material is small compared to that of the fine. Then, water in the fine material does not drain due to the reduced conductivity of the surrounding material. Here, we investigate this effect in a drainage experiment in a sand column made by cubes of 1x1x1 cm3. Beside the outflow, the 3D transient spatial water distribution was observed using Neutron Tomography. The retention of water in the fine material was observed and different concepts to estimate the equilibrium time could be compared. During the experiment the interfaces between the sand cubes showed a different drainage behavior compared to the inner part of the cubes and have thus to be treated as separate material. By carrying out the simulations the drainage of the interfaces and the capturing of water in the inner fine material could be reproduced as well as the estimated time scales of the drainage.