2.6 The “Hear See Do” of Flow Nets

The Chinese philosopher Confucius (551 BC to 479 BC), stated “I hear (read) and I forget. I see and I remember. I do and I understand.” The parenthetical “read” has been added here to adjust to our heavily text-based world 2500 years after the time of Confucius. In short, his point is that we learn more readily when we move beyond reading about something, to seeing it, and then on to doing it. Previous sections of this book provide the opportunity to “hear” (read).

One opportunity to “see” flow nets is provided using videos of flow in a Hele-Shaw cell. Like a flow net, a Hele-Shaw cell represents a planar section of a flow system. A Hele-Shaw cell consists of two parallel transparent walls with a small gap between them that is filled with a viscous liquid that moves in response to a pressure difference along the plates. The pattern of flow between the walls is equivalent to groundwater flow in a plane. A Hele-Shaw model allows us to view flow lines but does not show lines of equipotential, so one needs to envision those when viewing a Hele-Shaw model. Videos of flow in a Hele-Shaw cell can be accessed through the links in Figure 14. The first video, Figure 14a, illustrates flow in a system similar to the one shown in the video link provided in Figure 15 which illustrates drawing of a flow net. The second video, Figure 14b, is a flow system with recharge at the top of a land mass and discharge to both sides. The third video, Figure 14c, uses the same model as the second system but the left side is blocked at first so initially all flow exits on the right. Midway through video 14c, the right side is closed and the left side is opened so the transient shift of the flow lines can be observed.

Figure 14 – Hele-Shaw cells display flow paths for a) Click to view flow under a dam, b) Click to view flow of recharge from a ridge to a surrounding water body, c) Click to view flow of recharge from a ridge first to the lake on the right and then to the lake on the left. (Videotaping of Hele-Shaw model simulations at the University of Hong Kong by J. Jiao and W.Z. Liang. Editing and voice over by Eileen Poeter.)

Another opportunity to “see” flow nets is provided by the video of a flow net being sketched with pencil and paper that can be accessed through the link provided in Figure 15. The video provides an opportunity to see that drawing a flow net is a process of trial-and-error, so it is useful to have an eraser available.

Figure 15 – Click to view a video illustrating drawing a flow net.

As Confucius indicated, learning is improved when we “do”. Thus, it is useful to draw flow nets when striving to understand groundwater flow. Actively drawing flow nets brings attention to aspects of flow system that may be unclear to those who are new to the study of groundwater, so the act of drawing can clarify groundwater concepts. The exercises in Section 3 provide an opportunity to draw some flow nets and then view a finished flow net.