The critical zone in a combined 2D-3D model is the transition zone between the two model types. Every two-dimensional horizontal model assumes (based on Dupuit) exclusively horizontal flow. Hence, the partial 3D model must be large enough that the vertical velocity (direction z) in the transition zone is equal 0.

Specifying equal potential heads over the full height at the boundary nodes of the 3D area is a useful method for the transition condition. It means that the potential head at the nodes is unknown and must be computed during the calculation. But there is a second condition that defines a constant potential head for all nodes, one below the other (see identification GLEI). Mathematically, in the equation system, the single nodes are summed and represented by one representative node. The other nodes are eliminated.

The program automatically uses this procedure. Comparing the boundary of the 3D area (3DRA) with the 2D model, it can identify the 3D model as complete or partial. Equal potential head can never be used for the boundary nodes of a complete 3D model. Instead, explicit boundary conditions of 1st order (potential head), or rather of 2nd order (mass flow rate), are required.

 

Transition condition between the 2D and the 3D area

 

Realization in SPRING