| Scenario | Inputs | Area (m²) | Velocity (m/s) | Comment |
|---|---|---|---|---|
| Drip mainline | Q=0.8 L/s, D=25 mm | 0.000491 | 1.63 | Balanced velocity for many runs |
| Soaker supply | Q=0.2 L/s, D=20 mm | 0.000314 | 0.64 | Gentle speed, lower noise |
| Open ditch | Width=0.30 m, Depth=0.10 m, Q=0.01 m³/s | 0.030000 | 0.33 | Low velocity reduces bank wear |
| Swale estimate | Manning: n=0.030, S=0.01, 0.40×0.15 m | 0.060000 | ≈1.03 | Estimate only; verify in the field |
- Velocity from flow rate:
V = Q / A, whereQis volumetric flow rate andAis cross‑sectional area. - Pipe area:
A = πD² / 4, using the inside diameterD. - Rectangular channel area:
A = w × d, widthwand water depthd. - Manning estimate (open channel):
V = (1/n)·R^(2/3)·S^(1/2), hydraulic radiusR = A/P, slopeS, roughnessn. - Reynolds number (optional):
Re = V·Dh/ν, hydraulic diameterDhand kinematic viscosityν.
- Choose Q ÷ A if you know the measured flow rate.
- Select pipe for pressurized lines or channel for open flow.
- Enter sizes using any unit; the tool converts internally to metric.
- Review velocity, Reynolds regime, and the guidance note.
- Export your latest result as CSV or PDF for records.
Why velocity matters in garden irrigation
Flow velocity influences pressure loss, noise, and long‑term reliability. When velocity is too low, sediment and biofilm can settle in laterals and filters. When velocity is too high, friction losses rise sharply and fittings experience higher stress. Many garden supply lines perform well around 0.6–2.0 m/s, depending on pipe material, run length, and pump behavior.
Matching pipe size to a target speed
Use the Q ÷ A method when you know the actual flow from a meter, pump chart, or timed bucket test. The calculator converts common flow units, computes the cross‑sectional area, then returns velocity in m/s and ft/s. If velocity is above your comfort range, increasing diameter is often more effective than throttling, because a larger area reduces speed and lowers friction loss for the same flow.
Reynolds number and what it signals
Reynolds number (Re) is a quick indicator of flow behavior using velocity, a characteristic diameter, and kinematic viscosity. Laminar flow is uncommon in typical irrigation mains; most systems are transitional to turbulent. Turbulent flow increases mixing, but also amplifies energy loss in long runs and around elbows, valves, and tees. Use Re alongside the guidance note to decide whether to keep speeds moderate.
Open channels, swales, and Manning estimates
For ditches and swales, the Manning option estimates velocity from channel geometry, slope (S), and roughness (n). Rough grass, stones, and vegetation raise n, lowering velocity for the same slope. The calculator also estimates discharge as Q = V × A, which helps compare a planned swale to incoming runoff. Because real channels vary, treat Manning results as a planning estimate and validate on site.
Practical limits and maintenance checks
If you see high velocities, verify that your entered diameter is the true inside diameter, not nominal size. Check for partially closed valves, clogged filters, or undersized fittings that can create localized high speeds. After adjusting inputs, export CSV or PDF to document your final settings, then re-check velocities after seasonal changes or pump upgrades.
FAQs
1) What is the difference between velocity and pressure?
Velocity is water speed through an area. Pressure is stored energy pushing water through restrictions. Higher velocity often increases friction loss, which can reduce pressure at emitters.
2) How do I lower velocity without changing the pump?
Increase the pipe diameter, split flow into parallel lines, or shorten high-flow sections. These changes raise flow area, reducing speed and usually lowering friction losses.
3) When should I use Q ÷ A instead of Manning?
Use Q ÷ A for pressurized pipes or when you know flow rate. Use Manning for open channels where slope and roughness govern the flow behavior.
4) What kinematic viscosity value should I enter?
For clean water near room temperature, 1e‑6 m²/s is a practical default. Cooler water is slightly higher, and warmer water is slightly lower.
5) How do I choose a Manning n value?
Smoother surfaces have lower n values. Clean lined channels are lower, while grassed swales, stones, and vegetation increase n. If unsure, start higher for conservative estimates.
6) Why does slope (S) change the Manning velocity so much?
Manning velocity scales with the square root of slope. Small slope increases can noticeably raise velocity, which affects erosion risk and the discharge capacity of the channel.