The classic approach is to represent a river with a "Fluid-Transfer Boundary Condition" (3rd kind / Leakage / Cauchy BC), which requires a predefined reference head value (h_ref) combined with a conductance parameter (In-/Out-transfer rate).
The following steps will help you to include a river in a simple FEFLOW model:
#01 - The river line should be included in the Supermesh before the mesh generation as a polyline.
This will make it much easier to refine the mesh near the river, required to focus on the zone of interaction between surface water and groundwater - see Fig. 1.
Fig. 1 - Supermesh view with a polygon model area and a line inside it
#02 - Choose a suitable mesh generator like Triangle
Choose a suitable mesh generator like Triangle, to refine the line feature (activate "Refine line" option in the Mesh generator Properties and specify the parameters (Line Gradation, Line Target Size) - see Fig. 2.
Fig. 2 - Around the river line the mesh needs to be refined to make it easier to investigate and model the surface water-groundwater interaction
#03 - Line options
The initial line feature can be used to select the nodes on the river line (double click on Lines under the Supermesh option in the Maps panel and choose Select by Map Lines in the Selection toolbar) - see Fig. 3.
Fig. 3 - Picture Select the Lines option
This line selection can be done with multiple line features and also in later stages of the model building, (e.g. if you realize that a river line was not added to the model, you can always import another line as a map). Please note that the mesh needs to be rebuilt and already assigned. Parameters can be modified by the remeshing process, therefore, we recommend including every possible geometry-changing object (lines, points, polygons) at the very start of the model setup.
Specify the Snap distance( ) in which the nodes will be selected close to the line feature and click on the lines in the map view or use the Select by all Map Geometries button () in the Selection toolbar - see Fig. 4.
Fig. 4 - Picture Choose Select by Map Line and Select by All Map geometries
You should have created a node selection that will be used to assign boundary conditions on those nodes - see Fig. 5.
Fig. 5 - Picture: Selected nodes on the river line to assign boundary conditions
Fluid-Transfer Boundary Conditions can be assigned to Element Faces (in 3D models) or Edges. In the first case, Element Faces can be horizontal or vertical - see Fig. 6.
Fig. 6 - Picture Correctly assigned connected Fluid-Transfer BC-s
#04/A - 1D representation of rivers
If the river is really thin compared to the model width, one line of Fluid-Transfer BC-s should be enough to represent the river on the top of the model (Fig. 7). In this case, the same node selection is required on the 2nd slice to assign the same h_ref value of the Fluid-Transfer BC-s to the nodes on both slices 1 and 2 (If you have deeper river beds, this method can be extended to the underlying slices). - see Fig. 8.
Fig. 7 - Picture Fluid-Transfer BC-s representing the river line from top view
Fig. 8 - 3D view of the model showing the carved-out area (in green) representing the connected faces of Fluid-Transfer BC-s throughout the first 3 Slices
The inflow/outflow corresponding to the Fluid-transfer boundary condition is calculated from the relevant area, the transfer rate, and the difference between reference and groundwater levels. The transfer rate is a conductance term related to the conductivity of the clogging layer. FEFLOW distinguishes between two different transfer rates for infiltration from surface water (Transfer rate in) and exfiltration to surface water (Transfer rate out). According to the gradient direction, FEFLOW automatically chooses the correct value - see Fig. 9.
Fig. 9 - Picture 3D view of the several layer deep river representation and the In Transfer Rate assigned to elements shown via red color, the rest of the model is purple leaving the In Transfer rate at the default 0
#04/B 2D areal representation of rivers
Fluid-transfer BCs can be assigned to faces on top of the model for representing wide rivers - see Fig. 10.
Fig. 10 - Picture Fluid-Transfer BC-s representing a wide river line
The In and Out Transfer rate should be assigned to the corresponding elements - Fig. 11.
Fig. 11 - Picture The In-and-Out Transfer rate
Keep in mind that only In- and Out-transfer rates assigned to the areas of the rivers (green Elements in Picture 11, for example) will be used. Values assigned where no Fluid-Transfer BC is prescribed will not affect the calculations, as they won't be used.
#05 - Lakes
The interaction between groundwater and lakes can also be simulated with this method, assigning Fluid Transfer BC-s in the region of the lake and choosing the In and Out Transfer rate for that area.
Please keep in mind that lakes near rivers can be highly influenced by the river water level changes, therefore, it is a usual procedure to get a measured value from the river and extend that Fluid-transfer BC value all over the lake area - Fig. 12.
Fig. 12 - Picture Lakes represented as Fluid Transfer BC-s near a small river
There is another possibility to represent surface water in FEFLOW: a native coupling between FEFLOW and MIKE 1D engines for integrated groundwater / surface water modelling through the plug-in piMIKE1D.
There are several ways to represent surface water and groundwater interactions inside FEFLOW. You can use the described methods individually or combine them to have a complex groundwater model including the surface waters of the region.
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