Why Flow Net Is Important?


draw flow nets

F) Name a field instrument that you could use to monitor the pore pressure at any point. First identify the hydraulic boundary conditions. 8.3, the upstream bed level GDA represents 100% potential line and the downstream bed level CFJ, 0% potential line.

draw flow nets

From the drawn flow net, Nf and Nd can be easily counted, and hence, the seepage discharge can be easily computed by using Eqn. 3 can be solved if the boundary conditions at the inlet and exit are known. It is pore water pressure and it acts vertically upward direction due to residual pressure head. Make second trial adjustment of constructed flownet. Is parallel to impermeable boundaries or to constant head boundaries. 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 .

What Is Use Of Flow Net?

If required more trials may be taken to draw the flownet finally. • The stream lines in flow net show the direction of flow and the equinoctial lines join the points the equal velocity potential Φ. • The streamlines ψ and equipotential lines Φ are mutually perpendicular to each other. The flow net in Figure 4.4c with 1.2 flow channels is an example.

  • The flow line and equipotential lines should be orthogonal and form approximate squares.
  • Before commencing the calculation, the available published flow nets must be studied and duly analyzed so as to get a rough idea of the nature of the flow net.
  • The boundary conditions must be duly established.
  • Depict flow lines as arrows pointing in the direction of groundwater flow, i.e., in the direction of declining hydraulic head.
  • A flow net is a graphical representation of two-dimensional steady-state groundwater flow through aquifers.
  • As an example, suppose that it is necessary to draw the flow net for the conditions shown in Fig.

Contours of total head and flow vectors can be plotted. An option is also available for computing flow potential values at the nodes. Together with the equipotential lines , the flow lines can be used to plot a flow net. Each square obtained by intersection of flow lines and equipotential lines is called a field.


In hydrology, seepage flow refers to the flow of a fluid in permeable soil layers such as sand. The fluid fills the pores in the unsaturated bottom layer and moves into the deeper layers as a result of the effect of gravity. The soil has to be permeable so that the seepage water is not stored.

The flow net can be understood as the graphical representation of the flow of water through a mass of soil. The uplift pressure at any point within the soil mass can be found using the undermentioned https://personal-accounting.org/ formula. These points are mathematical artifacts of the equation used to solve the real-world problem, and do not actually mean that there is infinite or no flux at points in the subsurface.

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 draw flow nets head equals zero. Equipotential lines are like contour lines on a map which trace lines of equal altitude. In this case the “altitude” is electric potential or voltage. Equipotential lines are always perpendicular to the electric field.

Flow nets must satisfy the boundary conditions of flow field. Quantity of water flowing through each flow channel is the same. The potential drop in any two consecutive equal potential lines is same/constant. Flow lines and equal potential lines are smooth curves.

How To Draw A Flow Net For Soil?

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. Theory of flownets provides a remarkable solution to the problem. Equipotentials are established by the boundary conditions before the flow net is started.

Total head is lost by the time it reaches downstream end. Naturally downstream ground surface represents equipotential line with zero head. 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.

The equipotential lines are further extended downward, and one more flow line GHJ is drawn, representing the step . Starting from the upstream end, divide the first flow channel into approximate squares. The size of the squares should change gradually. Some of the squares may, however, be quite irregular. The flow line and equipotential lines should be orthogonal and form approximate squares. The horizontal and vertical component of the hydraulic gradient are, respectively.

Rate Of Seepage Loss Q

The long flow line is indicated by the impervious stratum NP. The groundwater flow direction is along the flow lines. Depict flow lines as arrows pointing in the direction of groundwater flow, i.e., in the direction of declining hydraulic head. A flow net consists of two sets of lines, flowlines and equipotential lines. Flowlines or streamlines are the loci of the paths of flow of individual water particles. Equipotential lines pass through points of equal pressure. All intersections between the streamlines and equipotential lines are at right angles.

Time-dependent flow net equations are limited in engineering applications to simple boundary conditions. The geometry of transient flow nets does not change with time, as only the numerical values assigned to equipotential lines and flow lines change with time. Construction of a flownet is often used for solving groundwater flow problems where the geometry makes analytical solutions impractical. The method is often used in civil engineering, hydrogeology or soil mechanics as the first check for problems of flow under hydraulic structures like dams or sheet pile walls. Flow nets are drawn based on the boundary conditions only. They are independent of the permeability of soil and the head causing flow. The space formed between two flow lines and two equipotential lines is called a flow field.

Properties Of Flow Net

Portions of this flow net have been subdivided one or more times to give greater detail. Apparent directions of needed corrections of this flow net are indicated by the arrows. Directions of these intersecting lines are shown at several random points in Figure 4.2a. Unconfined flow in single permeability sections. Confined flow in single permeability sections. B) Label the recharge and discharge areas of the flow net. Differentiate among progressive die, compound die and combination die.

draw flow nets

Either flow lines or equipotential lines are smoothly drawn curves. They indicate the path followed by the seepage water. Other set of curves are called equipotential lines.

To construct a flow net for a site, measure the hydraulic head in wells across the site following the groundwater gauging procedures detailed previously in this section. Interpolate the hydraulic head between wells assuming that the change in head is linear between neighboring wells. Connect points of equal hydraulic head to depict the equipotential lines. Choose equipotential line intervals such that the drop in head between adjacent lines is constant. The equipotential lines represent the height of the water table or potentiometric surface above mean sea level or other datum plane.

The flow net is not applied to sharply diverging flow , as the actual flow pattern is not represented by the flow net. Internal erosion is the formation of voids within a soil caused by the removal of material by seepage. Piping is induced by regressive erosion of particles from downstream and along the upstream line towards an outside environment until a continuous pipe is formed. The coefficient of permeability of a soil describes how easily a liquid will move through a soil. It is also commonly referred to as the hydraulic conductivity of a soil. This factor can be affected by the viscosity, or thickness of a liquid and its density. V. In case more point to be located say P, from vertical line QP at any distance x from F.

The flow lines and equipotential line are intersecting lines at 90 degree to each other. A) What is the vertical permeability of the dam, if the horizontal permeability has been determined to be 3.6 x 10-7 m/s? B) Using the incomplete sketch given to you as a separate sheet, construct a flownet to enable the seepage losses to be determined. C) Hence determine the seepage losses (i.e. flow rate) in m3 / day, through the dam if the elevation of water behind the dam is 16.8 m above the toe drain. D) Find the pore pressure at an elevation of 5 m from the base of the dam along its centreline. E) Name a field instrument that would allow you to measure the horizontal permeability of soil.