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After completing part of a flow net it is usually possible to tell whether or not the final diagram will be correct. This line must now be redrawn in its corrected position and the procedure repeated again, amending the first flow line if necessary, until a satisfactory net is obtained. As an example, suppose that it is necessary to draw the flow net for the conditions shown in Fig. The boundary conditions for this problem are shown in Fig. 2.9b, and the sketching procedure for the flow net is illustrated in Figs c, d, e and f of Fig.

how to draw a flow net

The value of the flow rate across these lines will be displayed in the Slide2 Interpret program. We will simulate ponded water to the left and right of the sheet piling. We will stipulate a water elevation of 13 m, which equals the elevation of the top of the sheet pile.


Irregular points (also called singularities) in the flow field occur when streamlines have kinks in them (the derivative doesn’t exist at a point). An equivalent amount of flow is passing through each streamtube (defined by two adjacent blue lines in diagram), therefore narrow streamtubes are located where there is more flow. We can also attempt to replicate the flow through the actual structure using physical models. There are two major techniques for solving Laplace’s equation. The first is an approximation known as flownet sketching, and the second is the finite difference method.

What are the four properties of flow net?

  • Rate of Seepage loss (Q)
  • Seepage Pressure (Ps)
  • Uplift Pressure (Pu)
  • Exit Gradient (iexit)

Mathematically, the process of constructing a flow net consists of contouring the two harmonic or analytic functions of potential and stream function. These functions both satisfy the Laplace equation and the contour lines represent lines of constant head (equipotentials) and lines tangent to flowpaths (streamlines). Together, the potential function and the stream function form the complex potential, where the potential is the real part, and the stream function is the imaginary part.

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The flow vectors are displayed on the model as shown in the image below. By default, the entire model is assigned the properties of Soil (Material 1). You can add more than one label to a connector – at the source end, the target end, and in the middle. If it is a floating connector, the end closest to the waypoints you moved will ‘float’ around the perimeter of your shape.

The sum of the volumetric downwards flow is equal to the volumetric upwards flow between the sheet pile walls. In Slide2, a Discharge Section is a user-defined polyline segment, across which steady-state, volumetric flow rates, normal to the segments, will be calculated during groundwater seepage analysis. Before we can set the boundary conditions, we must generate the finite element mesh. We will use the 6 noded triangles and an approximate number of elements of 1500 to generate the mesh. (i) Buried surfaces (e.g. the base of the dam, sheet piling), which are flow lines as water cannot penetrate into such surfaces.

Rate of Seepage loss (Q)

The method consists of filling the flow area with stream and equipotential lines, which are everywhere perpendicular to each other, making a curvilinear grid. The inference from Equations (4a) and (4b) is that the velocity of flow (v) is normal to lines of constant total head, as illustrated in Figure 1 The direction of v is in the direction of decreasing total head. The head difference between two equipotential lines is called a potential drop or head loss. Another example of flownet are shown in Figures 3. Figure 2 shows a flownet for a sheet pile wall, and Figure 3 shows a flownet beneath a dam.

  • In Slide2, a Discharge Section is a user-defined polyline segment, across which steady-state, volumetric flow rates, normal to the segments, will be calculated during groundwater seepage analysis.
  • If it is a fixed connector, the end will remain attached to that specific connection point.
  • A Flow net is a graphical representation of flow of water
    through a soil mass.
  • It is the preferred method of analysing flow through soils for geotechnical engineers.

Any differential equation requires knowledge of the boundary conditions in order to be solved. Since the boundary conditions of the majority of “real” structures are complex, an analytical or closed-form solution cannot be obtained for these structures. Using numerical techniques such as finite difference, finite element, and boundary element, it is possible to obtain approximate solutions. In this tutorial, we will determine the quantity of seepage entering a cofferdam using finite element groundwater seepage analysis in Slide2. This example is based on problem 2.3 from Craig (2012) in which the seepage quantity entering a cofferdam was calculated.

boundaries conditions.

Check these sections to ensure that repeated bisection results in a point for a precise flownet. 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 equipotential lines are further extended downward, and one more flow line GHJ is drawn, representing the step (4).

  • 3 can be solved if the boundary conditions at the inlet and exit are known.
  • Other sides of the squares are set equal to the widths as determined above.
  • To calculate the flow quantities in the coffer dam, we will define a discharge section.
  • The horizontal and vertical component of the hydraulic gradient are, respectively.
  • There are many different shapes used to visualise processes in a flow chart.
  • (d) With compasses determine the position of the next flow line; draw this line as a smooth curve and complete the squares in the flow channel formed.

Properties and application of flow net are
explained in this article. If the flow fields in the last flow channel are inconsistent with the actual boundary conditions, the whole procedure is repeated after taking a new trial flow line. The volumetric flow into the dam is 1.98e-6 m3/s. The model, therefore, gives the same result within the number of significant digits given.

The groundwater is flowing around the impermeable sheet pilings with high flow rates directly below the pilings. Since we are only looking at groundwater flow, the material properties don’t matter – we will keep the default properties but change the names of the materials. Waypoints are shown as small circles on a selected connector. They are used to define the path that a connector takes across the drawing canvas. You can create many different types of diagrams with and our online diagram editor.

  • The interface boundary, is neither an equipotential line or a flow line.
  • An equivalent amount of flow is passing through each streamtube (defined by two adjacent blue lines in diagram), therefore narrow streamtubes are located where there is more flow.
  • The rate of flow between any two flow lines is constant.
  • The area between two flow lines is called a flow channel (Figure 1).

Short labels on shapes make it easier to understand a diagram quickly. When you move the shape to a new position, the connector ends will automatically move around the shape to ensure the shortest distance. Move – Select and drag a shape that is on the drawing canvas to another position. To select multiple shapes, hold down Shift or Cmd and click on them.

Let us consider an element of soil of size dx, dz through which flow is taking place. Run flownet –help to see all possible command line argument options. Founded in 1996, Rocscience is a world leader in developing 2D and 3D software for civil, mining, and geotechnical engineers. As engineers ourselves, we know the importance of having reliable and easy-to-use software. That’s why we constantly develop and refine our programs to make expert solutions that work for you. We will construct a flow net as was done in the Craig’s solution.

how to draw a flow net

Darcy’s law describes the flow of water through the flow net. Since the head drops are uniform by construction, the gradient is inversely proportional to the size of the blocks. Big blocks mean there is a low gradient, and therefore low discharge (hydraulic conductivity is assumed constant here).