Surface and subsurface flow systems are unified continua that often need to efficiently operate together in a water resource distributed modelling framework. Watershed modelling studies targeting surface water flood and storm events, or groundwater flooding events involving complex, strong surface water - groundwater interactions require access to powerful, physically-based simulation tools.
A new integrated model has been designed by allowing the two state-of-the-art groundwater model FEFLOW and surface water model MIKE 21 Flow Model FM to communicate into a single, dynamically coupled modelling system. The tool features the ability to model coupled variably-saturated and overland flows, as well as coupled heat transfer and salinity transport, by taking advantage of optimal parallelization based on hybrid OpenMP, MPI, and multi-GPU programming models. The contrasted flow dynamics of the surface and subsurface environments operate in an efficient manner using unstructured, flexible non-conforming meshes for the two continua. An area-weighted interpolation strategy allows the joint use of process-tuned meshes with optimal resolution, while a sub-timing approach with independent dynamical stepsize control supports the continua dynamics coupling and synchronization.
The tool is applied in the lower Havel polder region (Germany), a place flooded by a complex weir group and used as a retention area to reduce peak discharges in the lower Elbe region part. Recent flooding events initiated modelling work efforts in order to optimize the polder and the weirs, and support renaturation planning including reconnection of old river branches and removal of dike protections. With the new integrated model it is now possible to analyze these interaction processes with much more details.