2.1 Approaches to Constructing Flow Nets

A flow net can be constructed using a number of approaches. All of the approaches generate equipotential and flow lines in one way or another. The graphical construction approach provides an approximate result that is often sufficient for practical purposes. Computer approaches are more versatile, but require knowledge in use of software.

Graphical Construction: This approach requires only paper and pencil, and fundamental understanding of flow net characteristics. Non-ideal conditions such as heterogeneity and anisotropy are challenging, and in such cases, numerical models are a better tool. The procedure for drawing graphical flow nets is described in Section 2.2.

Analytical Element Approach: This approach can be used in more complex systems and requires a computer and software. A computer code is used to solve the groundwater flow equations for hydraulic head and stream functions to generate both equipotential line and flow lines at any location within the model domain.

Numerical Approach: This approach also requires a computer and software. Numerical approaches can be used in the most complex systems. Numerical models can be constructed quickly to create steady-state, two-dimensional flownets. A continuous solution is not provided by numerical models because heads are calculated for discrete locations within the model domain. A very precise calculation requires closely spaced points for head calculation. The only way to know if the grid is fine enough, is to solve the same problem using a finer grid and obtain the same answer as calculated by the coarser grid. After computing the discrete distribution of hydraulic head, a flow path tracking model (also known as a particle tracking model) is used to compute flow lines in accordance with Darcy’s Law.

Although computer models can be used to simulate complex flow systems, it is beneficial for groundwater professionals to acquire the ability to sketch flow nets with pencil and paper to master understanding of flow nets and to enhance understanding of the basic concepts of groundwater flow. The ability to sketch flow nets can be used for rapid initial assessment of flow systems in the field and at impromptu project meetings. However, mastery of numerical model construction is also a must for the modern groundwater hydrologist.