Calculator Inputs
Example Data Table
| Case | Method | k (m/s) | H (m) | L (m) | A (m²) / b (m) | Nf | Ne | Q (m³/s) | Q (L/s) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Darcy | 1.0e-5 | 3 | 30 | A=2.0 | — | — | 2.0e-6 | 0.002 |
| 2 | Flow net | 8.0e-6 | 5 | — | b=40 | 3 | 15 | 3.2e-5 | 0.032 |
| 3 | Darcy | 3.0e-6 | 2 | 20 | A=1.5 | — | — | 4.5e-7 | 0.00045 |
Formula Used
For saturated, steady seepage through porous media:
Q = k × i × A
- k = hydraulic conductivity (m/s)
- i = hydraulic gradient = H/L
- A = cross‑sectional area normal to flow (m²)
For 2D seepage under structures using a flow net:
Q = k × H × (Nf/Ne) × b
- Nf = flow channels
- Ne = equipotential drops
- b = width represented (m)
How to Use This Calculator
- Select a method: Darcy’s law for measured gradients, or flow net when Nf and Ne are known.
- Enter hydraulic conductivity with the correct unit and magnitude.
- Provide head difference between upstream and downstream conditions.
- For Darcy: enter seepage path length and define the flow area (rectangular or direct).
- For flow nets: enter Nf, Ne, and the representative width of the section.
- Click Calculate seepage flow to view results under the header.
- Use CSV or PDF downloads to save results for reports.
Professional Notes
Seepage flow rate in excavation and foundation works
Seepage estimates support dewatering sizing, stability checks, and temporary works planning. The discharge Q is reported in m³/s, L/s, and m³/day to match pump curves, sump storage, and haul-off planning. For small excavations, even a few liters per second can govern trench safety, filter selection, and discharge permits.
Hydraulic conductivity selection and field data
Hydraulic conductivity k can be taken from lab permeameter tests, in‑situ tests, or typical ranges. Clayey soils often fall near 10⁻¹¹ to 10⁻⁹ m/s, silts commonly near 10⁻⁹ to 10⁻⁶ m/s, and clean sands may reach 10⁻⁶ to 10⁻³ m/s. Use conservative values when stratification, fissures, or construction disturbance may increase seepage.
Choosing Darcy versus flow net inputs
Darcy’s law is practical when you can define a seepage path length L and a flow area A. Flow nets are preferred for seepage beneath sheet piles, cutoff walls, or dams when the geometry is complex but Nf and Ne are reliably counted from a drawn flow net. The flow‑net ratio Nf/Ne directly scales discharge and provides a quick sensitivity lever during design iterations.
Interpreting results for pumping, drainage, and reporting
Convert discharge to L/s for pump selection, and to m³/day for daily water handling forecasts. Compare computed seepage velocity v (Darcy method) with filter criteria to reduce piping risk. If the computed hydraulic gradient i is unusually steep, validate boundary heads and flow length. Maintain a record of assumptions, units, and conversions to avoid order‑of‑magnitude errors in site meetings.
Example data and documentation practice
The example below illustrates a typical Darcy case used for a preliminary dewatering check. Update k and geometry once test results and as‑built dimensions are available, then export a PDF for the calculation file. Re‑run the estimate after excavation staging changes, cutoff installation, or groundwater level shifts.
| Example | k (m/s) | H (m) | L (m) | A (m²) | Q (m³/s) | Q (L/s) |
|---|---|---|---|---|---|---|
| Darcy | 1.0e-5 | 3 | 30 | 2.0 | 2.0e-6 | 0.002 |
FAQs
1) What is seepage flow rate?
It is the volume of water passing through soil per unit time under a head difference. This calculator reports discharge in m³/s and provides common field conversions for practical use.
2) When should I use Darcy’s law?
Use Darcy when the seepage path length and cross‑sectional area are reasonably defined from drawings or measurements. It is most reliable for uniform flow through a known section.
3) When is the flow net method better?
Use flow nets for seepage under structures or cutoffs where the geometry is complex but the flow net can be constructed. Discharge depends on k, total head, and the Nf/Ne ratio.
4) Which unit should I enter for hydraulic conductivity?
Enter the unit that matches your source (lab, in‑situ, or reference table). The calculator converts to m/s internally, which reduces mistakes when comparing cases and exporting results.
5) Why does the calculator warn about steep gradients?
Very large i = H/L can indicate unrealistic heads, too short a flow path, or a wrong unit. Steep gradients may also increase piping risk, so they deserve a quick engineering review.
6) Are these results suitable for final design?
They are suitable for preliminary sizing and comparison. Final design should use site‑specific k values, verified groundwater boundaries, staged geometry, and any applicable standards or review requirements.
7) How do I save results for reports?
After calculating, use the CSV for spreadsheets and the PDF for a calculation appendix. Both exports include the method and key converted results for traceable documentation.