1 Introduction
It is not possible to see groundwater flowing through the subsurface of the earth, so the most valuable asset a groundwater hydrologist can attain is the ability to visualize the distribution of hydraulic head and flow paths in three-dimensional space. This comes easily to a few people, but many people need to immerse themselves in viewing and thinking about two-dimensional examples of steady-state flow in the form of flow nets before the visualization of flow systems becomes intuitive. The effort invested in mastering visualization of flow systems is worth the reward. Figure 1 provides a link to a video of dyed water moving through porous material. Flow nets can reveal the distribution of forces driving the flow and the flow paths.
Figure 1 – Click to view a video of dye moving through a sand tank. A few snapshots from the video are pictured here: Time 1 is before dye is introduced to the flow system, so one cannot see the flow lines; Time 2 is after red dye is injected in a shallow well and has migrated to the stream outlet; Time 3 is after green dye is injected into a deep well and has migrated upward and to the right; and, Time 4 is after the green dye has reached the gravel lens and then rapidly migrated to the stream outlet. Viewing the video elucidates the flow better than these snap shots. (Sand Tank constructed by Students at the University of Wisconsin, Stevens Point. Filmed at IGWMC of Colorado School of Mines. Edited and narrated by Eileen Poeter.)
Graphical flow net analysis is useful in every hydrogeologist’s toolbox because it can: 1) provide a quick and useful understanding of flow systems when a computer is not available; 2) help a numerical model user assess their intuition regarding the results of a numerical groundwater flow model; 3) and facilitate understanding of basic concepts of groundwater flow for those new to groundwater work. However, mastery of basic numerical model construction is also a must for the modern groundwater hydrologist.
As shown in Figure 2, a flow net provides an image of equipotential lines (black dotted lines) and flow lines (blue arrows) in a groundwater flow system. This book explains graphical construction of groundwater flow nets. Before using flow nets to understand flow systems, it is best to begin by understanding graphical construction of two-dimensional, steady-state flow nets. It is useful to have knowledge of a few items pertinent to flow nets including: hydraulic head, boundary conditions, Darcy’s Law, and the groundwater flow equations. Another Groundwater Project book (Woessner and Poeter, 2020) discusses these items in detail, however brief reviews of these items are provided in this book.
Figure 2 – Images of flow nets: a) groundwater moving in a vertical cross section under a dam; and, b) a plan view of groundwater flow from a lake to a pond surrounded by bedrock outcrops.