Calculator Inputs
Example Data Table
| Scenario | Area | C | Intensity | Duration | Expected Output |
|---|---|---|---|---|---|
| Small paved lot | 0.8 ha | 0.85 | 70 mm/hr | 15 min | Peak Q in m³/s or L/s |
| Mixed site drainage | 3.0 ac | 0.55 | 2.2 in/hr | 20 min | Peak Q in cfs |
| IDF-driven check | 1.5 ha | 0.65 | a/(t+b)^c | 30 min | Computed intensity and peak Q |
Examples are illustrative. Use local design criteria for return period and time of concentration.
Formula Used
The Rational Method estimates peak discharge: Q = C × i × A
- Q is peak runoff rate (converted to your chosen unit).
- C is runoff coefficient (dimensionless, 0–1).
- i is rainfall intensity for the selected duration (depth/time).
- A is contributing area.
Optional IDF mode uses i = a / (t + b)^c with t in minutes, producing intensity in mm/hr when parameters are consistent.
How to Use This Calculator
- Choose an intensity method: direct entry or IDF-based computation.
- Enter drainage area and select the correct unit.
- Enter runoff coefficient based on surface type and development.
- Provide rainfall intensity (or IDF parameters) for your design criteria.
- Select output units and press Calculate Runoff.
- Use the download buttons to export the latest result as CSV or PDF.
- For IDF mode, ensure a, b, and c match your source equation.
- Use duration aligned with time of concentration where applicable.
Professional Notes on Rational Runoff
1) What the peak value represents
The Rational Method estimates the peak runoff rate at a point, not a full hydrograph. It is widely used for small catchments where travel times are short and a single design intensity can represent the critical storm period. The output supports inlet sizing, gutter checks, and preliminary pipe or culvert screening.
2) Selecting the runoff coefficient
The coefficient C reflects surface response and connected imperviousness. Typical guidance ranges from about 0.20–0.40 for lawns and open soils, 0.50–0.70 for mixed residential, and 0.80–0.95 for dense paving or roofs. For composite sites, a weighted C based on land‑use areas improves consistency.
3) Using design intensity and duration
The intensity i should match the design return period and a duration near the time of concentration. If the duration is too long, peaks are often underestimated; if too short, intensity may be unrealistically high. The IDF option lets you compute intensity from parameters using i = a / (t + b)^c, keeping t in minutes.
4) Units, conversions, and reporting
This calculator converts area to m² and intensity to m/hr so Q = C × i × A produces m³/hr, then converts to your selected unit. It also reports an optional rainfall depth and an approximate runoff volume V ≈ C × depth × area for quick documentation and QA checks.
5) Practical limits and good practice
Use the method carefully for large basins, detention routing, snowmelt, or strongly non‑uniform rainfall. When pipe networks include storage or long travel times, consider a full hydrologic model. Always document the return period, intensity source, duration basis, and any assumptions used.
FAQs
1) What area should I enter?
Enter the hydraulically connected drainage area that contributes to the inlet or point of interest. Exclude areas that bypass the point or are diverted by grading, berms, or separate collection systems.
2) How do I pick a realistic C value?
Use land cover guidance and connected imperviousness. For mixed sites, compute a weighted C: sum(area fraction × C). Confirm the result matches observed runoff behavior and local design manuals.
3) Which duration should I use?
Use a duration close to the time of concentration for the contributing drainage path. If unknown, run several durations and identify the controlling peak, then document the chosen duration and intensity source.
4) Why does the IDF option ask for a, b, and c?
Many design standards express intensity as a function of duration using parameters. Enter values that match your published IDF equation form and units, then the calculator computes intensity consistently for your selected duration.
5) Is the reported runoff volume a design volume?
No. It is a screening estimate based on C, rainfall depth over the entered duration, and area. For detention sizing, routing, and spillways, use a hydrologic method that generates a hydrograph and storage effects.
6) How do I choose the return period?
Return period depends on the asset and risk tolerance. Municipal manuals often specify different values for local roads, trunk drainage, and critical facilities. Select the required criterion and use the matching IDF intensity.
7) Why do my results differ from a spreadsheet?
Check unit assumptions first: area units, intensity units, and whether Q is in m³/s, L/s, or cfs. Also confirm the same duration and C value were used, and that the intensity matches the same IDF source.