SPRING has various possibilities for defining the thickness of an aquifer. The model checking is able to determine the thickness with the help of one of the following data combinations:

explicit definition of the thickness using the attribute MAEC or MAEK (then an iteration is not possible)

the difference between upper and lower boundary (attributes OBER - UNTE) (2D only)

difference between the ground level (GELA) and the lower boundary (UNTE or UNTK) considering the thickness of the impermeable layer (UNDU), thickness = (GELA - UNDU) -UNTE or UNTK)

difference between the nominal potential heads and the lower boundary (attributes EICH – UNTE or UNTK)

 

Calculating the thickness (M) from existing data

The value for the thickness is assigned to elements or nodes. The input data , UNDU and EICH are values assigned to nodes. Therefore, they must be interpolated first from node wise values to element wise values, when the thickness is assigned to elements (provided that each node has one value). Hence, the interpolation can finally lead to less exact results for the thickness.

It is useful to define the upper boundary in case of a natural or artificial (building) upper restriction of the aquifer. It is possible to define an upper boundary in partial areas of the model. If the aquifer is covered by a relatively impermeable layer, the thickness of this layer and the ground level are the best input data. Then leakage processes can be taken into account automatically. For 3D models, an upper limit can be defined by generating an additional layer with very low permeability.

The definition of nominal potential heads (measured groundwater level) is only necessary if a calibration is done. Nevertheless, further usage of EICH after the calibration is recommended, because the resulting initial thickness is the best input for a possible iteration of thickness. If the user has defined values for upper boundaries as well as for nominal potential heads, the program calculates two values for the thickness and finally chooses the smaller one.

After running the model checking, the thereby-computed thickness is used for the iteration of the free surface in horizontal 2D models.

In a fashion similar to the calculation of the initial thickness, during the model checking the maximum possible thickness is calculates for each element, in order to decide whether confined or unconfined conditions prevail. The maximum thickness is calculated as the differences between:

the upper and lower boundaries (attributes OBER – UNTE)

the ground level and the lower boundary considering the thickness of the impermeable layer, thickness = (GELA - UNDU) -UNTE or UNTK

the ground level and the lower boundary (attributes GELA – UNTE or UNTK)

 

Based on these initial and maximum possible thicknesses the calculation of the free surface is done during the flow calculations.