Formula Used
- A is wetted area (depends on depth).
- R is hydraulic radius = A / P.
- S is slope (dimensionless).
- n is roughness.
- D is diameter in meters.
- C is pipe coefficient.
- hf/L is headloss per length.
- Best for full, pressurized lines.
How to Use
- Select your unit system and flow method.
- Enter pipe diameter and, if needed, slope or headloss.
- Set depth percent for gravity drains and roughness values.
- Adjust velocity targets to match your soil and sediment risks.
- Press calculate and review flow, velocity, and checks.
- Export CSV or PDF to share design notes with others.
Example Data Table
| Scenario | Method | Diameter | Slope / Headloss | Depth | Typical Use |
|---|---|---|---|---|---|
| Yard trench drain | Gravity | 110 mm | Slope 0.01 | 80% | Paths and low spots after rain |
| Bed overflow line | Gravity | 75 mm | Slope 0.02 | 70% | Raised beds and planters |
| Pumped sump discharge | Pressure | 2 in | Headloss 0.03 | Full | Moving water to a safe outlet |
Professional Notes
Design Flow Targets for Site Runoff
Main drains in gardens manage stormwater from lawns, patios, and planting beds. Estimate contributing area, rainfall intensity, and acceptable drawdown time. For small residential sites, check peak inflow and a practical target such as 15–30 minutes after a heavy shower. Use the calculator to compare diameters while keeping velocities within limits.
Gravity Capacity Using Manning Parameters
For sloped drains that may run partly full, the Manning method links flow to wetted area, hydraulic radius, and slope. Roughness n represents pipe texture and joints; smoother interiors usually increase capacity at the same slope. Depth percentage matters because 70–90% full can pass high flow while retaining free-surface behavior. Review area and hydraulic radius to confirm the assumed depth.
Pressure Flow Checks with Available Headloss
Pumped or surcharged lines can be screened using the Hazen–Williams option. Enter headloss per length from available pump head or elevation difference, then verify velocity remains below erosion thresholds. Coefficient C reflects internal condition; aging, biofilm, or sediment can reduce capacity, so choose conservative values for longer maintenance intervals.
Velocity, Sediment Transport, and Maintenance
Drain performance depends on more than flow rate. Low velocities allow grit to settle near grates, bends, and transitions. Many practitioners aim for self-cleansing behavior by keeping velocity above a minimum target while avoiding excessive speeds that increase noise and wear. Use the warnings as an operational check, then adjust depth, slope, or diameter.
Safety Factors and Minor Loss Awareness
Real installations include entries, tees, elbows, and strainers that add losses. Apply a safety factor to reduce reported capacity, and use the optional K value to estimate minor-loss head for documentation. If your layout includes many fittings or long runs, validate results after the first season and refine inputs for future upgrades. Record final settings so crews can replicate calculations on site.
FAQs
1) Which method should I choose for my drain?
Use gravity for sloped, open-surface behavior and partly-full flow. Use pressure when the line is pumped or surcharged and you know the available headloss per length.
2) What diameter should I enter?
Enter the inside diameter. If you only know nominal size, check the manufacturer’s ID table. Small ID differences can change velocity and capacity noticeably.
3) How do I pick a Manning roughness value?
Smooth plastic is typically lower, while corrugated or rough interiors are higher. If you expect sediment, roots, or aging, use a higher n to remain conservative.
4) What does depth percent mean in gravity mode?
It represents how full the pipe is during peak flow. The calculator updates wetted area and hydraulic radius accordingly, which directly affects the Manning capacity and velocity.
5) How should I use the safety factor?
Safety factor reduces reported capacity to account for uncertainty, clogging risk, and installation variability. Typical screening ranges are 1.1–1.3, but increase it for debris-prone inlets.
6) Does the minor-loss K value change the flow result?
K is reported as an informational head estimate to document fittings and grates. If minor losses are significant, consider reducing available slope or headloss input and re-check the design.