2.9 Create and Investigate Topographically Driven Flow Systems

A software tool called “TopoDrive’ can be used to create and simulate topographically-driven flow systems. The flow system is a vertical cross-section with no-flow boundaries on the left, right, and bottom sides. The top boundary is the water table and is specified by the user. This flow system is different from the flow systems encountered earlier in the book in two ways. First, in the case of flow beneath an impermeable dam (Section 2.3, Figure 11), the head is uniform along the bottom of upstream reservoir (10 m) and the downstream reservoir (6 m). By contrast, in the topographically-driven flow system, the head along the water table is equal to the elevation of the water table, and therefore can vary along the boundary (unless the water table is flat). Second, in the case of flow in an unconfined system with a water table (Section 2.5, Box 4), the position of the water table is initially unknown and is determined while the flow net is constructed. Here, in the topographically-driven flow system, the position of the water table is specified. Given this specification of the water table elevation, in order to satisfy Darcy’s Law and conserve mass, water must flow across the water table boundary either into or out of the system.

The TopoDrive software uses a numerical model to solve the groundwater flow equations, then draws equipotential lines at a constant contour interval throughout the system. Once the equipotential lines are drawn, you select a point(s) from which you want the program to trace a flow line(s). Because the flow lines are drawn from arbitrary starting points selected by the user, the flow tubes are not likely to carry the same groundwater discharge.

Figure 19 provides a link to a video demonstrating the use of topodrive. For written instructions on using TopoDrive, click here to go to Box 6. The online version of TopoDrive can be accessed at https://tdpfonline.net, where readers can create their own flow systems.

video demonstration of using the TopoDrive software

Figure 19Click here for a video demonstration of using the TopoDrive software.

The TopoDrive software was created by Hsieh (2001) and updated by Hsieh (2020) to function on modern web browsers. Once one learns how to use TopoDrive, the best use of it is for checking one’s groundwater system intuition. In order to do that, create a system using the instructions provided in Box 6, and before clicking the “Head” button which will draw the equipotential lines, develop a mental (or hand sketched) picture of the general appearance of the equipotential lines, then click the “Head” button to see the solution. If the head distribution is much different than what you envisioned, consider why your intuition differed from the correct solution so that you can work toward improving your intuition. Similarly, before clicking a point to initiate flow path tracking, try to visualize where the flow path will go, then click the point to see the solution. Again, if the flow path is much different than what you envisioned, consider why your intuition differs from the correct solution. In summary, rather than using the software to obtain a solution, use the software to evaluate and improve your intuition of groundwater flow systems.

License

Graphical Construction of Groundwater Flow Nets Copyright © 2020 by The Authors. All Rights Reserved.