Boundary conditions are the restrictions that limit the flow in a certain space or area. Note that each flow line begins from the upstream surface, which is at pressure γwD, and travels through the soil, constantly losing its energy and terminates at the downstream surface, which is at pressure γwd. After applying some assumptions like the Darcy’s law is valid we can write velocity of water as permeability times hydraulic gradient.

draw flow nets

As we shall see in this section, numerical methods are much more versatile, but their application is bound unequivocally to the use of a digital computer. There are many situations where one may wish to construct a flow net on the basis of piezometric data from the field. If the geologic formations are known to be anisotropic, great care must be exercised in the inference of flow directions from the equipotential data. If the complex flow net is desired, a transformed section is in order, but if flow directions at specific points are all that is required, there is a graphical construction that can be useful. In Figure 5.10 the dashed line represents the directional trend of an equipotential line at some point of interest within an xz field.

4 Saturated-Unsaturated Flow Nets

In this case the horizontal bedrock surface below the dam provides a convenient reference for head measurements. An open body of water is hydrostatic, so the hydraulic head on the sand at the bottom of the reservoir is equal to the elevation of the reservoir water. So, these locations are constant-head boundaries with a head of 10 m on the ground surface upgradient of the dam and a head of 6 meters on the downgradient side (Figure 6). The lateral portions of the aquifer are not bounded so they must be drawn far enough from the dam so that no significant leakage occurs between the reservoirs and the underlying sand at the distant ends of the system. The highest rate of seepage into the sand will be immediately up gradient of the dam with seepage decreasing with distance up gradient. If, after constructing a flow net, it appears that the diagram is not wide enough, it can be redrawn with greater lateral extent from the dam until an acceptable flow net is obtained.

  • However, in contradistinction to Snell’s law, which is a sine law, groundwater refraction obeys a tangent law.
  • (iii) The horizontal ground surfaces on each side of the dam, which are equipotential lines.
  • Figure 5.6 is a qualitatively sketched flow net for the dam seepage problem first introduced in Figure 5.3, but with a foundation rock that is now layered.

For steady-state flow, Q must be constant through the system and this can only be true if the free surface is a parabola. The parameter ω is known as the overrelaxation parameter, and it must lie in the range 1 ≤ ω ≤ 2. Once you have finished adding all the shapes, connectors and labels, you can style your flow chart. They are used to define the path that a connector takes across the drawing canvas.

Chapter 5: Flow Nets

(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. Hence, in a flow net, where all the figures are square, there draw flow nets is the same quantity of unit flow through each figure and there is the same head drop across each figure. In other words, the flow net consists of approximate squares that are called elementary squares.

  • Hydraulic head along a seepage face is equal to the elevation of the ground surface because the gage pressure along the seepage face is zero.
  • Make sure that equipotential lines meet the water table and the seepage face at an elevation that is the same as the hydraulic head of the equipotential line.
  • In this case the horizontal bedrock surface below the dam provides a convenient reference for head measurements.
  • (ii) Beneath the dam the outermost flow line will be parallel to the surface of the impermeable layer.
  • Let us consider a soil sample of length L and put it into a glass cylinder.
  • This can be done by transforming either the y axis or the x axis.

The flow net developed for the above figure will repeat across the many drains of the field in alternating mirror images. The geometric transformation from an anisotropic system to an isotropic system can be viewed as transforming the hydraulic conductivity ellipse into a circle. This can be done by transforming either the y axis or the x axis.

3 Flow Nets by Numerical Simulation

Move – Select and drag a shape that is on the drawing canvas to another position. There are many different shapes used to visualise processes in a flow chart. (ii) The junction between a permeable and an impermeable material, which is also a flow line; for flow net purposes a soil that has a permeability of one-tenth or less the permeability of the other may be regarded as impermeable. But if we change the soil in which the water is flowing, the flow net will not change. Now that we have constructed the flow net its graphical properties can be used to calculate the seepage through soil.

draw flow nets

Leave a Reply

Your email address will not be published.