Improvements in the coupling interface between FEFLOW and MIKE 11

Since 2005 the coupling interface IfmMIKE11 has been available. The interface module couples FEFLOW to MIKE11 using the FEFLOW InterFace Manager (IFM). Its functionality covers the exchange of boundary condition data during runtime and the synchronization of the temporal and spatial discretization. The system has been applied to both simple benchmark cases and complex field conditions. In 2006 the coupling module was successfully extended for the coupling of polder areas and forelands. This option is also available in the present version of the module. Furthermore, this version includes a better representation of the exchange area between the river and groundwater body. This paper focuses on this topic. In FEFLOW rivers are mostly represented by Cauchy boundary nodes. The area enclosed by these boundary nodes describes the exchange area normally being applied to calculate the exchange discharge between the river and groundwater body. In most applications rivers are described by polylines along more than one slice (horizontal exchange) or by a boundary area only in the top slice (vertical exchange). The area can be changed during the simulation using a free and movable top slice. Using a phreatic or fixed top slice the exchange area will be held constant during the simulation. In none of these cases does the area represent the real exchange area, which depends both on the river profile and the water depth. To overcome this problem a new IFM function has been developed in FEFLOW, which enables the user to define not only the surface water reference level at a single boundary node, but also the exchange area represented by the node as well as a nodal transfer rate. This function can now be optionally used in IfmMIKE11. In that case, the cross section areas are used from the MIKE11 model. It also takes into account the case in which the groundwater drops below the bottom of the river. Finally, the coupling of mass transport will be introduced. For this coupling also some changes of the MIKE11 interface were necessary. In effect the coupling had to be changed from having the hydrodynamic and the transport equations being solved sequentially to solving them step wise.