Exercise 18
Before answering the questions at the end of this exercise, a sequence of instructions are provided to expedite your use of the TopoDrive tool. Create a representation of a topographically driven flow system by using the online TopoDrive educational tool↗ (Hsieh, 2001) to observe what controls local, intermediate, and regional flow systems; and to track groundwater flowlines and particles through the system you create. After connecting to that link and reading the brief introduction to the tool, click on ‘Go To Interactive Tool’. The menu on the black bar at the top of the page has a ‘How to Use’ tab that provides a concise quick-start guide.
Launch the Topodrive software, then choose start and enter a basin length of 5000 m and a vertical exaggeration of 1 and select OK. Click on ‘Water Table’ in the menu and when directed to draw a water table, click OK, then click high up on the left side of the left axis of the drawing panel, then move slightly down and to the right and click again, continue adding points as you move to the right, some lower and some higher, until reaching the right side of the drawing area, then double click on the last point. Select “Mesh” from the menu and select 50 rows and 50 columns, then click on properties and choose the green option of 1e-4 m/s and 20 percent porosity, click OK and use the mouse to draw a shape around the entire shape that you just created, double click on the last point and the shape will turn green to indicate those properties. You can make patterns of many different hydraulic conductivity and porosity values and you can change the value in the table where you select the properties, but for now just create a homogeneous basin.
Next, click the menu item ‘Head” and select 50 contours. The head contours will be drawn. The head at the top is determined by where you created the surface and the other boundaries are impermeable so the software calculates the contours from that information. Then select “flow” from the menu and leave the default of flow path tracking and both forward and backward, then click OK. Now click anywhere on the image and the associated flow line will be drawn – click many times to get a feel for the flow field. Next click ‘Animation’ on the menu and chose 100 days of travel per 1 second of animation with a smoothness of 10 frames per second, then click OK, then click on the image and the flow line you just drew will be animated. You can also return to ‘Flow’ and instead select ‘Particle Movement’ with and initial particle spacing of 50 m. After clicking OK, use the mouse to draw a rectangle in the upper left corner of your image, double clicking on the final point and a group of dots will appear spaced 50 m apart. Now, select “Animation’ and keep the previous settings of 100 and 10, then click OK followed by a click on the image to watch the particles move.
Did the system you created exhibit local, intermediate, and regional flow systems? If not, how might you change the water table you drew to create three system levels? If so, how might you change the water table you drew to eliminate the local and intermediate systems? Try redrawing the water table to create the conditions that your original drawing did not produce. Were you correct about what you needed to change?
Return to the ‘Properties’ menu item and add substantial heterogeneity. How does that change the flow patterns?