|Groundwater modelling with MicroFEM • Lesson 5A: Profiles and Leaky aquifers|
Load the model of Lesson 4.
Step 3: Drawdown cone in profile
Right-click in the Map / Fit to scale
[Drawing mode] / [F10] / [F11]
First a yellow straight line is displayed in the map: the Profile line. Then the profile itself is displayed, showing the drawdown cone (yellow curve).
Shift the profile window out of the map area.
Adjust the size if necessary. The default position of the profile line in the Map is West-East through the centre of the model. A left mouse click at a location in the map sets the position of the left end of the line. Similarly a right-mouse click positions the right end of the line. The graph in the profile is updated immediately. The end points can be placed anywhere, also outside the model boundaries.
Enlarge the Profile window.
The x-axis always starts with zero at the left-hand side.
You can zoom-in to display details. Use "Fit to scale" to reset the scale. When "Zoom In" is unchecked (right-click in profile to uncheck), you can also left-click in the Profile. The "Profile options" window appears with a "Scale" and "Colors" tab.
Menu bar: Options / Profile ...
The Profile graph can be exported to the Clipboard.
Menu bar: Export / Copy to clipboard / Profile
Now you can paste the Profile graph in PowerPoint, Word, etc.
The Profile window and the Profile line can be removed with the same [F10] and [F11] buttons (or keys).
Step 4: Leaky aquifer
For a leaky aquifer we have to allow vertical flow in an aquitard above the aquifer. This requires a fixed head boundary at the top of the model and a resistance between the aquifer head and this fixed head. The fixed head is assigned to H0, the vertical resistance to C1. The vertical resistance of an aquitard is defined as the saturated thickness of a (homogeneous) aquitard divided by its vertical conductivity. It is expressed in days (and the reciprocal of leakance).
The vertical flux in the aquitard is proportional to the difference in head (H0-H1). More precisely, it is computed as (H0–H1)/C1. The downward leakage (m3/d) for a nodal area is computed as a*(H0-H1)/C1, where "a" is the nodal area (m2). The resistance of a completely impervious aquitard is infinitely large.
One of the MicroFEM "rules" is that when a resistance is infinite, we enter the value "0" (zero).
That explains why in our original fully-confined model we kept the C1 at zero. When C1 is zero, there will be no vertical flow in the top aquitard and the value of H0 has no effect on the computed fluxes or heads.
In our model we assume a vertical resistance of 1500 days. When we keep H0 at zero (the same as the fixed head aquifer boundary) the computed heads in the aquifer show the drawdown by the well.
[Input mode] / C1 / 1500 / [F5]
Menu bar: Calculate / Go calculate
[Walking mode] / [F5] / Draw your conclusions / [F5]
The model water balance shows that the boundary inflow is reduced to 35 m3/d. Almost all water discharged by the well now comes from leakage through the aquitard.
[Drawing mode] / H1 / [F7] / interval = 0.01 / [OK]
Repeat the previous actions when you did not notice that the Minimum head of all nodes is –0.720 m. Apparently, changing from fully confined to semi-confined reduced the drawdown in the well from 0.840 to 0.720 m.
Step 5: No-flow boundaries
In step 2 we set fixed-head boundaries for the confined aquifer. Since the leaky aquifer is (mainly) recharged by the top aquitard, we can (alternatively) choose a no-flow boundary condition.
[Walking mode] / [Ins] / [Del] / [F7]
[Input mode] / H1 / [Nonfixed head for marked nodes (F8)] / [Del]
Calculate heads, check the water balance and find the head in the well.
Draw your conclusions, then go to Lesson 5B.