1.2.3. Equipotential Lines and Phreatic Surface

At every point (x,z) where there is flow, there will be a value of head h(x,z). In order to represent these values, contours of equal head are drawn.

Equipotential lines are special lines in a fluid flow situation where the potential energy is the same at every point along the line. Specifically, at any point defined by coordinates (x, z), the fluid's pressure or energy level (referred to as 'head') is equal. This means that there is no flow across an equipotential line; flow only occurs parallel to the lines.

Imagine a hilly landscape where water flows downhill. The lines drawn at the same height above sea level represent equipotential lines. No matter where you measure in that specific line, the height is the same, indicating that water has potential energy at that level but isn’t moving across these lines.

A flow net is to be drawn by trial and error. For a given set of boundary conditions, the flow net will remain the same even if the direction of flow is reversed. Flow nets are constructed such that the head lost between successive equipotential lines is the same, say ∆h.

Flow nets help visualize the flow of water through soil by drawing both flow lines and equipotential lines. The construction process is iterative—meaning you start with a basic design and refine it repeatedly. You want to ensure that the loss of head (energy loss) between the equipotential lines is consistent (denoted as ∆h) and that the lines are perpendicular (orthogonal) to each other, defining the grid of the net.

Think of it like laying a grid over the water surface in a pond. Initially, you randomly place some lines, but as you observe how the water flows under different conditions, you adjust the grid until the lines perfectly show the flow direction. Each adjustment teaches you something about where the water flows and how deeply it can have energy.

The most common boundary conditions are: (a) A submerged permeable soil boundary is an equipotential line. (b) The boundary between permeable and impermeable soil materials is a flow line.

Boundary conditions are key to creating accurate flow nets. An example of a boundary condition is a submerged soil boundary, which acts like an equipotential line where water levels remain constant. Conversely, at the junction of permeable and impermeable materials, the behavior of the flow changes, creating a flow line where water can either enter or not, reflecting how water interacts with different materials.

Picture a sponge (permeable) resting on a flat surface (impermeable). The surface might hold water, but the sponge allows water to flow through it while the bottom remains dry. When drawing the flow net, you would represent the sponge's surface as equipotential while the flat surface serves as a boundary line where flow is defined.

Equipotential lines intersecting a phreatic surface do so at equal vertical intervals.

The phreatic surface is the upper surface of the groundwater in an unconfined aquifer. It intersects equipotential lines at equal vertical intervals, signifying how groundwater levels stabilize across the landscape. This consistent spacing helps scientists and engineers understand how groundwater flows and how it is influenced by surface features.

Consider a large sponge soaking in water. The water represents the phreatic surface. Just like the water level in the sponge rises evenly at different points, creating a level saturation across the sponge, the equipotential lines intersect the phreatic surface not in random patterns, but consistently, demonstrating a balance in pressure throughout.

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