Peak Discharge Input Form
Use weighted runoff inputs and design factors for a practical drainage estimate.
Example Data Table
These examples show how land cover and design factors shift peak flow.
| Scenario | Area (ha) | Weighted C | Intensity (mm/hr) | Climate | Safety | Detention | Net Peak (m³/s) |
|---|---|---|---|---|---|---|---|
| Small paved yard | 3.2 | 0.76 | 110 | 1.05 | 1.10 | 0.95 | 0.82 |
| Mixed campus block | 12.0 | 0.57 | 95 | 1.10 | 1.15 | 0.92 | 1.83 |
| Industrial drainage zone | 28.5 | 0.71 | 125 | 1.15 | 1.20 | 0.88 | 7.44 |
Formula Used
Weighted runoff coefficient: Cw = fimp × Cimp + (1 − fimp) × Cperv
Base peak discharge: Qbase = 0.00278 × Cw × I × A
Adjusted design peak: Qadj = Qbase × Fclimate × Fsafety × Fdetention
Net peak after additional losses: Qnet = Qadj × (1 − L)
Equivalent full-flow pipe diameter: D = √(4Q ÷ πV)
Here, A is area in hectares, I is rainfall intensity in millimeters per hour, Q is discharge in cubic meters per second, and L is the extra loss percentage written as a decimal.
How to Use This Calculator
- Enter the drainage area and choose its unit.
- Set impervious share plus separate runoff coefficients for impervious and pervious surfaces.
- Enter rainfall intensity from the selected design storm or IDF curve.
- Add time of concentration and the storm duration you are checking.
- Apply climate, safety, detention, and loss adjustments that suit your design basis.
- Optionally add a target conveyance velocity to estimate an equivalent circular diameter.
- Submit the form and review the result card placed above the input form.
- Use the CSV or PDF buttons to export the result summary for reports.
Frequently Asked Questions
1. What method does this calculator use?
It uses the rational method with a weighted runoff coefficient. That makes it useful for small catchments, paved sites, yards, roofs, and preliminary drainage checks where design intensity is already known.
2. Why are there separate pervious and impervious coefficients?
Mixed drainage areas rarely behave like one surface. Separate coefficients create a weighted runoff value that better reflects parking, roofing, landscaping, and open soil within the same catchment.
3. What rainfall intensity should I enter?
Use the design intensity from your local IDF data for a duration near the time of concentration. That keeps the input aligned with how the rational method is normally applied.
4. What does the detention factor do?
It reduces flow when onsite storage, grading, or drainage controls soften the peak. A value below one lowers the adjusted peak, while one means no detention benefit is assumed.
5. Why include both safety and climate factors?
They serve different purposes. Safety covers design uncertainty and conservatism, while climate accounts for heavier future rainfall assumptions or updated resilience targets in planning studies.
6. Is the pipe diameter output a final design size?
No. It is only an equivalent full-flow diameter from continuity. Final design still needs roughness, slope, inlet control, tailwater, freeboard, code limits, and hydraulic grade checks.
7. When should I avoid this calculator?
Avoid it for large watersheds, routed hydrographs, reservoir operation, floodplain storage, or channels needing unsteady analysis. Those cases require more detailed hydrologic and hydraulic modeling.
8. Can I use acres and inches per hour?
Yes. The calculator converts acres to hectares and inches per hour to millimeters per hour before computing discharge, so the engineering equations stay internally consistent.