(GRS-750) HYMNE Hydrogeological Modelling at a Regional Scale

A. Schneider, N. Conen, A. Gehrke, J. Hilbert, M. Knodel, K.-P. Kröhn, M. Lampe, L. Larisch, B. Lemke, A. Nägel, T. Schön, F. Salfelder, M. Stepniewski, G. Wittum, H. Zhao

Contract No. 02E11809A and 02E11809B

The code d³f++ was developed for the modelling of density-driven flow and nuclide transport in the far field of repositories for radioactive waste in deep geological formations. It can be used in porous media as well as in fractured rock or clay, for modelling salt and heat transport and for models with free groundwater surfaces. 

The growing demands on groundwater flow and transport modelling require constant further development of the computer codes. Every potential repository site in Germany has been affected by permafrost conditions within the last 100,000 years. With the aim of enhancing d³f++ for the modelling of permafrost phenomena, the implemented equations were extended to include models for thawing and freezing processes. The new development was successfully tested by two benchmarks from the INTERFROST project.

Regional models with a free groundwater surface lead to a growing importance of an efficient model calibration. Therefore, a new tool for inverse modelling was implemented, tested and applied to a first large-scale 3D model. The level set method for modelling free groundwater surfaces was further stabilized, especially for large-scale models with density influence (1,000 km²). An ongoing process is the improvement of the robustness and efficiency of the multigrid solver. Parts of the code were restructured for this purpose.

As an alternative method for phreatic groundwater flow, a phase field model was introduced in d³f++, based on the Richards equation, that was generalized for densitydependent problems. Additionally, this model was adapted to make it also applicable to models with discrete, low-dimensional fracture networks.

Drainage through large networks of receiving waters plays a decisive role in the water balances of almost all regional models. It mainly depends on the water level, but often only the discharge of the main rivers is known. For this reason, a model based on the Saint Venant equation was implemented to enable the simulation of the water flow in the rivers.

An important part of the work was the application of d³f++ to two large-scale 3d models, the Äspö Site Descriptive Model (SDM), and the former candidate site Kraví Hora in the Czech Republic.