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Institut für Wasser- und Umweltsystemmodellierung - IWS



"A double-continuum approach for two-phase flow in macroporous media: Parameter study and applications"

In recent years, the failure of hill slopes has frequently had fatal consequences for both population and environment. The stability of natural slopes is influenced by a number of complex and coupled hydrological, subsurface hydraulic and soil mechanical processes which occur on very different time and spatial scales. The DFG (German research foundation) project group ’Großhang’ conducts research at the slope ’Heumöser Hang’ in Ebnit, Vorarlberg (Austria). The objective of this project is to develop a simulation tool which combines the coupled processes flow and deformation in order to improve predictions on the initiation of slope movements. Pressure and fluid dynamics of the slope subsurface are driven by the interacting flow components surface runoff, subsurface flow and fast infiltration via macropores. The time scale for these components is small, ranging from minutes to days. When the top soil layers of slopes consist of a network of macropores the time scale for the infiltration process is even smaller. Large amounts of water reach deeper soil regions rapidly and even bypassing of low permeable layers can occur. The subsequent pressure response of the system triggers elastic and viscoplastic deformation processes in the slope on a larger time scale (days to months). In the case of heavy precipitation events and surface ponding fast infiltration via macropores causes buoyancy effects in the bottom parts of the slope. It is assumed that these forces can trigger large deformations quickly which reduce the slope stability. A useful simulation tool for subsurface flow in macroporous slopes therefore must take into account macropore flow and the complex interactions with the surrounding matrix. To this end the modeling software MUFTE-UG is extended to a double-continuum model for two-phase flow (Stadler et al., 2008).

The objective of this thesis is to examine the complex interaction between ma- trix and macropores by means of the existing double-continuum model. Special focus is put on the process of fast infiltration and the bypassing of impermeable layers via macropores. The Forchheimer law as non-linear motion equation is implemented into MUFTE-UG to examine the effect on macropore flow. The obtained results can help to enhance the understanding of subsurface flow dynamics in macroporous slopes and serve as a basis for future work on this research topic.