Sketching a Groundwater Flow Net

Draw the boundaries of the soil mass, including the inflow and outflow boundaries. Now that you have a grasp of the fundamentals, let’s explore the process of drawing a flow net. While specific steps may vary depending on the complexity of the problem, the general approach remains consistent. A flow net can also be constructed for two-dimensional flow in a plan view. An important assumption for graphical construction of a flow net in a plan view is the absence of areally distributed recharge, such as infiltration of precipitation to the flow system.

Gabriel Freitas is an AI Engineer with a solid experience in software development, machine learning algorithms, and generative AI, including large language models’ (LLMs) applications. Graduated in Electrical Engineering at the University of São Paulo, he is currently pursuing an MSc in Computer Engineering at the University of Campinas, specializing in machine learning topics. Gabriel has a strong background in software engineering and has worked on projects involving computer vision, embedded AI, and LLM applications. For instance, creating a flow net to study a coastal aquifer can reveal how freshwater from inland areas interacts with saline coastal waters.

Flow net diagrams are a crucial tool in soil mechanics, used to visualize the flow of seepage or groundwater through a porous medium, such as soil or rock. In this section, we will explore the basics of flow net diagrams, their importance, and the steps involved in drawing them. The next step is to draw flow lines along paths where you envision groundwater will flow, ensuring they are perpendicular to equipotential lines on the boundaries (Figure Box 4-2). There is no recharge from the impermeable upper boundary, so the uppermost flow line forms the water table. The flow line should meet the 25 m equipotential line (constant head boundary) at a right angle.

Step 2: Determine the Hydraulic Head

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Foundation Design:

These simulations can enhance the precision of flow net drawings by comparing results and adjusting for discrepancies. This combined approach ensures more accurate predictions of seepage and pressure distributions, crucial for designing safe and efficient infrastructure projects. By following these steps and guidelines, engineers can create accurate and informative flow nets that provide valuable insights into the behavior of water flow in soil mechanics. The flow of water is driven by a difference in hydraulic head, which is a measure of the potential energy of water. The hydraulic gradient, the slope of the hydraulic head, dictates the direction and velocity of water flow. A Flow net is a graphical representation of flow of waterthrough a soil mass.

We know the hydraulic head at the ground surface is equal to the elevation of the ponded water (0.8 m). We assume the pressure is atmospheric in the drain (that is, the water flowing to the drain discharges at the end of the drain without backing up water in the drain). Hydraulic head is the sum of pressure head in terms of a height of a column of water and elevation. Atmospheric pressure is used as the zero-reference point for quantifying pressure so, at the drain, the pressure is zero and the hydraulic head is equal to the elevation.

• The groundwater flow equation is derived and discussed in another Groundwater Project book (Woessner and Poeter, 2020).
• When designing foundations for structures, it is essential to consider the potential impact of groundwater seepage.
• Now that you have a grasp of the fundamentals, let’s explore the process of drawing a flow net.
• Explore real-world case studies and analyze how flow nets are used to solve practical engineering challenges.
• Two requirements need to be kept in mind when drawing the equipotential and flow lines in order to obtain an accurate solution to the groundwater flow equation.

(13 minutes)A discussion of the material in this video is provided in Section 2.6 “The “Hear See Do” of Flow Nets” of the Groundwater Project book “Graphical Construction of Groundwater Flow Nets”. Section 2.6 can be read online, or the entire book can be read online or a PDF of the book can be downloaded. Mastering the art of flow net construction involves applying fundamental principles of hydraulics and soil properties.

Box 5 – Drawing Flow Nets for Anisotropic Systems

A flow channel is a region enclosed by two adjacent equipotential lines and two adjacent flow lines. The number of flow channels provides insight into the distribution of seepage flow. Flow nets are used to analyze the stability and seepage characteristics of embankments, which are earth mounds used for various purposes, such as roads, dams, and levees. By understanding the flow patterns, engineers can optimize the embankment design to prevent erosion, seepage, and potential failure. Flow lines represent the path of flow along which the water will seep through the soil.

Equipotential Lines:

Where k is the hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional area. In order to draw the flow net, it is first essential to find the location and shape of the phreatic line or the top flow line separating the saturated and unsaturated zones. The accuracy of the computation of hydraulic quantities, such as discharge and pore water pressure, does not depend much on the exactness of the flow net. The total head drop, H, is estimated as 0.6 m (that is, the difference between the 0.8 m head at the ground surface and the average 0.2 m head along the drain). The average head along the drain is estimated as 0.2 m because the head at the top of the drain is 0.25 m, the center is 0.2 m, and the bottom is 0.1 m.

Understanding the Fundamentals of Flow Nets

While sketching the flow line, care should be taken to make flow fields as approximate squares throughout. The flow line and equipotential lines should be orthogonal and form approximate squares. In the construction of underground storage tanks, understanding how water might seep into or out of the storage area is vital. Drawing a flow net helps design effective protective barriers or drainage systems that ensure the tanks remain secure and usable over time.

• This is especially crucial for maintaining water quality and preventing seawater intrusion.
• They are used to model the movement of water through soil and rock formations, essential for groundwater management and engineering projects.
• For instance, creating a flow net to study a coastal aquifer can reveal how freshwater from inland areas interacts with saline coastal waters.
• Using knowledge of Darcy’s Law and the fact that flow is parallel to no-flow boundaries, one flow path can be drawn along the concrete dam from the upgradient to the downgradient reservoir (Figure 6).

In this case, it is determined that 18 head drops create curvilinear squares. So, the contour interval is determined by dividing the total head drop of 10 m by 18 to obtain a value of 0.56 m head drop between each pair of equipotential lines. This establishes the values of the contour lines and knowing that they must meet the water table at the elevation equal to their values further constrains the position of the lines. For example, the first equipotential line to the right of the upper reservoir will have a value of 24.44 m and so the intersection of the equipotential line and the water table should be at that elevation. Flow net diagrams can be constructed using various methods, including graphical methods, numerical methods, and analytical methods. Graphical methods involve manually drawing the flow lines and equipotential lines, while numerical methods involve using software to solve the governing equations.

Analytical methods involve using mathematical equations to derive the flow net diagram. A homogeneous and isotropic groundwater system is one in which the hydraulic conductivity is the same at every location and does not vary for different directions of flow. Hydraulic conductivity is a measure of the ease with which water can pass through a material and is discussed in another Groundwater Project book (Woessner and Poeter, 2020). The groundwater flow equation is based on Darcy’s Law and conservation of mass. The groundwater flow equation is derived and discussed in another Groundwater Project book (Woessner and Poeter, 2020).

Such systems may also have a seepage face, where groundwater seeps out along a sloping section of ground surface. The position of the water table and the length of the seepage face need to be adjusted along with the flow and equipotential lines while drawing the flow net. Because the water pressure is equal to the atmospheric pressure at the water table, the equipotential lines need to intersect the water table at the elevation equal to the value of the equipotential line label. This video illustrates the process of drawing a groundwater flow net below a dam from a headwater reservoir on the left to a tailwater reservoir on the right. A cutoff wall protruding below the base of the dam increases the flow path length, decreasing the flow velocity, thus reducing the potential for the groundwater flow to erode the porous material at the toe of the dam.

The cost of creating a flow net diagram in soil mechanics can vary widely, depending on the complexity of the problem, the software used, and the expertise of the engineer or researcher. In general, the cost of creating a draw flow nets flow net diagram can range from a few hundred dollars for a simple problem to tens of thousands of dollars for a complex problem. Additionally, the cost of creating a flow net diagram may be higher if specialized software or expert consultants are required.