|Groundwater modelling with MicroFEM • Lesson 6B: Two-aquifer system and Grid manipulation|
Assume that a well pumps from the top aquifer of a leaky two-aquifer system.
Start by clicking the "Lesson 6.fen" file and set the number of aquifers to 2 / [OK]
When we have a single aquifer model on the screen, we can add a new aquifer at the base of the present aquifer.
[Alter-grid mode] / [F11] / 2 / [OK]
Four cells are added to the "Param" table, with the following codes: C2, T2, H2 and Q2. The water table in the top of the aquitard is assumed fixed at 12 m. The resistances of the aquitards is 800 (top aquitard) and 200 days (between aquifers). The transmissivities of the aquifers is 600 (top aquifer) and 300 m2/d (base).
[Input mode] / Select C1 / 800 / [F5] / T1 / 600 / [F5] / C2 / 200 / [F5] / T2 / 300 / [F5]
The water table in the top of the aquitard is assumed fixed at 12 m. The model boundary condition is no-flow all around.
[Input mode] / Select H0 / 12 / [F5]
The well is located at x = 3645, y = 2600 and discharges 600 m3/d from the top aquifer. We want a well node at the exact position.
[Drawing mode] / [F6] / spacing = 10000 / offset X = 3645 / offset Y = 2600 / [OK]
A cross hair appears showing the well location (why?). Zoom into that area.
[Alter-grid mode] / [Ins]
Point at the node closest to the cross hair, click and shift the node.
Use [+] a few times to further zoom in and bring the node further in position.
Now Use [-] a few times to see part of the grid again.
Relocate some neighboring nodes to make the elements more or less equilateral again.
When you are satisfied [Ins] to switch "relocate nodes" off.
[Input mode] / current node at cross hair node / Select Q1 / 600 / [Enter] / Select Label 1 / well / [Enter]
Right click in the map / Fit to scale
Menu bar: Calculate / Options ... / Set "Error of heads" to 1E-8 / [OK]
Menu bar: Options / Save settings
This stopping criterion will now automatically be chosen for future MicroFEM sessions. The settings are stored for the active user, and so depend on the user who has logged on to the system. The default settings can be restored at any time using: Options / Defaults
[Walking mode] / [F5]
You probably have to enlarge the water balance table to see both layers. Notice there is both upward and downward flow in the C2 aquitard.
Close the water balance table.
Draw the head contours of the 1st and 2nd aquifer in different colors.
What are the Minimum heads? And the largest drawdowns?
Check nodes at the boundary and note that drawdown in the 2nd aquifer >= than in the top aquifer.
Draw head contours of the 2nd aquifer with a very small interval to find out what part of the model boundary is most effected by the well, and how much.
For the computation of heads it is sufficient when T-values (transmissivities) and c-values (vertical resistances) are known. Up to now we did not use K-values (hydraulic conductivities) and layer thicknesses. This latter information is only required when we want to compute 3D-flowlines or flow vectors, when we want to see sections of the model, and when we want the transmissivity of unconfined aquifers to be a function of the water table height.
For now, we will only add uniform thicknesses to all layers.
Menu bar: Project / Project manager ... / add unit to project [green +] / New / [OK] / Close project manager
In the table, next to the "Param" tab, a new tab has appeared "Thick". It contains 5 cells.
[Input mode] / top level = 12 / [F5]
Top level is the height of the top of the model (water table) above reference level.
Similarly enter thicknesses for all (4) layers: from top to base = 10, 20, 2 and 12 m.
Save the model
Menu bar: Files / Save all
Have a closer look at the Project manager. Thickness data are stored in a separate thi-file.
[Drawing mode] / [F12] / try left and try right mouse clicks / [F12]
The [F12] button is only available when the thickness unit is added to the model.