Flow Net Calculator

Flow net estimates for seepage and uplift. Compare channels, drops, permeability, gradients, and safety factors. Improve drainage assessments for dams, weirs, sheet piles, foundations.

Enter flow net inputs

Enter soil permeability along the main seepage direction.
Leave equal to kh for isotropic soil.
Use upstream head minus downstream head.
Count complete channels between adjacent flow lines.
Count equipotential intervals across the full head loss.
Use drops remaining from the chosen point to exit.
Representative seepage path length near the downstream exit.
Enter 1 for per-meter discharge.
Used to estimate seepage velocity through voids.
Needed for critical gradient against piping.
Use the representative void ratio of the foundation soil.
Use 9.81 for fresh water unless a different value applies.

Example data table

This sample shows a preliminary seepage review for a hydraulic structure foundation.

Case kh (m/s) kv (m/s) H (m) Nf Nd Remaining drops Le (m) q per m (m³/s) iexit FS
Dam apron section A 0.00015 0.00010 8.0 4 16 3 0.60 0.000212 0.8333 1.1647
Sheet pile section B 0.00008 0.00005 6.0 3 18 4 0.85 0.000089 0.3922 2.4762
Weir base section C 0.00020 0.00012 5.5 5 20 2 0.75 0.000301 0.3667 2.6455

Formula used

These equations support preliminary flow net interpretation for seepage under hydraulic structures and embankments.

kₑ = √(kₕ × kᵥ)
q = kₑ × H × (Nf / Nd)
Q = q × width
Δh = H / Nd
Residual head = H × (remaining drops / Nd)
Uplift pressure = γw × residual head
i_exit = Δh / Lₑ
i_c = (G_s - 1) / (1 + e)
Factor of safety against piping = i_c / i_exit
Seepage velocity = q / n
Engineering note: The discharge formula assumes a correctly drawn flow net with approximate curvilinear squares. For detailed design, confirm geometry, boundary conditions, stratification, and field data.

How to use this calculator

  1. Prepare a drawn or interpreted flow net for the section you are checking.
  2. Enter horizontal and vertical permeability values for the foundation soil.
  3. Input the total upstream to downstream head loss.
  4. Count the total number of flow channels and potential drops.
  5. Choose the location where you want uplift pressure and residual head.
  6. Enter the remaining downstream drops from that location.
  7. Input the exit length near the downstream toe or exit point.
  8. Provide porosity, soil specific gravity, and void ratio for piping checks.
  9. Click the calculate button to show the result summary above the form.
  10. Use the CSV or PDF buttons to save the calculated results.

Frequently asked questions

1. What does a flow net calculator estimate?

It estimates seepage discharge, residual head, uplift pressure, exit gradient, and piping safety from a prepared flow net and supporting soil properties.

2. Why are Nf and Nd important?

The ratio Nf/Nd controls seepage discharge. More flow channels increase discharge, while more potential drops reduce the head loss per field.

3. When should I use different kh and kv values?

Use different values when the soil is anisotropic, such as layered sand or silty deposits. The calculator applies the geometric mean for effective permeability.

4. What is residual head in this tool?

Residual head is the remaining hydraulic head at a selected point, based on the number of equipotential drops still left to the downstream exit.

5. How is uplift pressure found?

Uplift pressure equals unit weight of water multiplied by residual head. The result is reported in kPa when γw is entered in kN/m³.

6. What does the exit gradient tell me?

It indicates how steep the hydraulic gradient is near the seepage exit. High values can signal erosion or piping risk at the downstream toe.

7. Is this calculator enough for final design?

No. It is best for preliminary checks. Final design should also consider actual geometry, filters, cutoffs, stratification, seepage modeling, and site investigation data.

8. Can I save the results for reports?

Yes. After calculation, use the CSV button for spreadsheet work or the PDF button for a clean report-ready summary.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.