Drawing Flow Nets

drawing flow nets

In a matter of moments, the chart is going to be displayed in the main window along with the value of the shape factor and the critical point . With the help bookkeeping of the slider bars aforementioned, you can easily adjust the flow net lines and tune, so that you can be sure you receive the results that interest you.

Weirs designed and constructed on the basis of Bligh’s theory also failed due to undermining of the subsoil. As a result it was thought essential to study the problem of weirs on the permeable foundations more elaborately.

drawing flow nets

Several minor changes were made on the plotter to make it more suitable for ground-water studies. In summary, Darcy’s law and Ohm’s law were shown to lead to the Laplace equation.

Flow Net Soil Water

The surface of seepage is defined as the surface between the free surface and the actual level of the water in the well. Figure 7 shows the Thiem curve, free surface, surface of seepage, and the water level in a pumping well. Figure 6—Block diagram of an ideal flow net system showing equipressure lines and orthogonal flow lines.

The resistance in the paper is sufficiently uniform so that it may be considered analogous to a homogeneous aquifer. Manufacturing tolerance is plus or minus 8 percent along the length of the roll and plus or minus 3 percent along the width of the roll. These tolerances are within the realm of accuracy achievable for ground-water plots. Variations can be detected readily if the probe indentations are close enough, and irregularities can be overcome by drawing a smooth curve through the points. There is no satisfactory method yet known to vary the resistance in a portion of the paper by any controlled amount. Either the black or the silver side of the paper may be used for plotting.

How To Construct A Flow Net For Seepage Analysis: Laplaces Equation And Methods

Also, let Δq represent the discharge passing through the flow channel, per unit length of structure . A flow net is a graphical representation of two-dimensional steady-state groundwater flow through aquifers. Note In flow net problems we assume that the permeability of the soil is uniform throughout the soil’s thickness. The conducting-sheet analog field plotter is restricted to problems of a two-dimensional nature. It may be applied to three-dimensional problems only for obtaining approximate solutions.

  • Mathematically, the process of constructing a flow net consists of contouring the two harmonic or analytic functions of potential and stream function.
  • Computer groundwater models are based on the geologic and hydrologic field data collected during drilling, geotechnical sample analysis and aquifer testing.
  • The most important addition to the plotter was the installation of a 4,000-ohm variable resistor at each end of the positive and negative leads.
  • Damnasht is a small-sized hydraulics application whose main purpose is to aid individuals in drawing flow nets for sheet piles, as the name hints at.
  • It is the simplest and the quickest method of flow net construction.
  • The appearance of the entire flow net should be watched and not that of a part of it.

If the left-band tank is lowered, and the level of water in the right-hand tank is maintained, water will flow downwards through the soil (Fig. 16). After completing part of a flow net it is usually possible to tell whether or not the final diagram will be correct. This ledger account 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 electric current is analogus to the velocity of flow and the homogeneous conductor is analogus to the homogeneous fluid. This method only for a practical method of drawing a flow net for a particular set of boundaries. The field plotter cannot be applied accurately to areal plots of three-dimensional, radial, gravity-flow problems, because electric flow has no analog for a variable third dimension in fluid flow.

• The stream lines are so spaced that rate of flow Q is same between each successive pair of lines. From the derivations and two inferences can be drawn for a flownet with elementary square fields. Before commencing the calculation, the available accounting published flow nets must be studied and duly analyzed so as to get a rough idea of the nature of the flow net. From the drawn flow net, Nf and Nd can be easily counted, and hence, the seepage discharge can be easily computed by using Eqn.

On a ten-turn helical-wound resistance element, the arm (“slider”) moves the complete length of the helix on a ten-turn screw. Position of the arm is indicated on a double dial (trade name “Duodial”), which has 100 units on drawing flow nets an inner dial and 10 units on an outer dial. In this manner the voltage is divided into 1,000 separate readable units. The three posts on the back of the power and voltage divider unit are leads from the voltage divider.

drawing flow nets

Equation 9 is the Laplace equation and must be satisfied by all steady-state flow problems. Slichter was the first writer to recognize that the Laplace equation holds for fluid flow. Another satisfactory method for solving flow problems is the resistor grid, which comprises a simple electrical system composed of numerous resistors arranged in a grid. Each resistor is adjusted and the voltage drop is measured across each resistor until the grid voltage matches the hydrologic data.

What Is Seepage Water?

The flow and equipotential lines are drawn by modifying W. The modification allows adjustable spacing between the grid lines and attains greater accuracy. Even a part of the flow domain can be analyzed to obtain the grid that further simplifies the analysis. The beginner usually makes the mistake of drawing too sharp transitions between straight and curved sections of flow lines or equipotential lines. Keep in mind that all transitions are smooth, of elliptical or parabolic shape. The size of the squares in each channel will change gradually. The base of the dam and the side of the cutoff pile represents first stream line or flow line.

They By Drawing Equipotential Lines The Flow Net Is

When no further drawdown occurs, the cone of depression is considered to be in equilibrium and a steady state of flow exists. One approach to describing fracture flow was developed by Snow . He developed an equation to calculate equivalent hydraulic conductivity for a set of planar fractures. The flow lines and equipotential lines meet at right angles to one another.

Theory of flownets provides a remarkable solution to the problem. First of all, smooth curves representing the flow lines that meet the specified requirements are first marked. Among the various methods of flow net construction, the most convenient method is the Graphical method. Phreatic line is a seepage line as the line within a dam section below which there are positive hydrostatic pressures in the dam. The hydrostatic pressure on the phreatic line itself is atmospheric. Other sides of the squares are set equal to the widths as determined above. Irregularities are smoothed out, and the next flow line DF is drawn joining these bases.

Figure 2 shows the wiring diagram for an analog field plotter. SEEP2D can be used for either confined or unconfined steady state flow models. For unconfined models, both saturated and unsaturated flow is simulated. The phreatic surface can be displayed by plotting the contour line at where pressure head equals zero. Anisotropy, which is the opposite of “isotropy,” is a term used to denote preferential flow direction in soils and other geologic materials. If soil consisted of perfectly spherical grains, flow rates would be isotropic – the same in all directions, other factors being equal.

Draw flow lines such that the flow is equally divided between adjacent lines. Then, the equipotential lines are drawn such that they cut or intersect the flow lines at right angles. It must be ensured that the equipotential lines are drawn such that the fields form approximate curvilinear squares. This method requires a lot of erasing to get the proper shape of a flow net and also consume a lots of time. A graphical consists of drawing steamlines and equipotential lines such that they cut orthogonally and form curvilinear squares.

Analytical Method:

Single or multiple well data may be plotted if it is remembered that the greatest error develops near the well. The model depicts a two-dimensional surface, whereas the actual well involves three dimensions. It follows that error in the plot increases with drawdown. In other words, the more the well develops a three-dimensional aspect, the greater will be the error in the two-dimensional plot. Figure 4—Vertical section showing error in plot caused by failure to account for drawdown in well. Dashed lines are extensions of equipotential and equipressure points.

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