Calculator Inputs
Enter watershed and design storm values. Results appear above this form after submission.
Example Data Table
Use this sample to understand typical input combinations and expected output ranges.
| Basin | Area (km²) | C | I (mm/hr) | Loss (%) | Safety | Estimated Unit Peak (m³/s/km²) |
|---|---|---|---|---|---|---|
| Urban Block A | 2.50 | 0.78 | 95 | 6 | 1.15 | 22.640 |
| Mixed Catchment B | 8.20 | 0.55 | 70 | 10 | 1.10 | 13.750 |
| Industrial Yard C | 1.40 | 0.88 | 110 | 5 | 1.20 | 31.920 |
| Suburban Basin D | 12.00 | 0.48 | 62 | 12 | 1.08 | 10.210 |
Formula Used
1) Rational Method Peak Discharge
Qp = 0.278 × C × I × A
2) Loss-Adjusted Peak
Qloss = Qp × (1 − Loss% / 100)
3) Conveyed Peak
Qconv = Qloss × (Channel Efficiency / 100)
4) Final Design Peak
(Qconv + Baseflow) × Safety Factor
5) Unit Peak Discharge
qu = Final Design Peak / Area
Meaning of variables: Qp is peak discharge in m³/s, C is runoff coefficient, I is rainfall intensity in mm/hr, and A is catchment area in km².
The constant 0.278 converts the rational method into SI units when intensity is entered in mm/hr and area in km².
How to Use This Calculator
- Enter the watershed name, project name, and selected return period.
- Provide the catchment area in square kilometers.
- Enter the runoff coefficient based on land use and surface conditions.
- Input the design rainfall intensity for the chosen storm duration.
- Add impervious percentage, hydraulic loss factor, and channel efficiency.
- Include optional baseflow and a suitable safety factor.
- Press the calculate button to display results above the form.
- Review the chart, interpretation, and export the outcome as CSV or PDF.
Frequently Asked Questions
1) What is unit peak discharge?
Unit peak discharge is the peak flow divided by drainage area. It helps engineers compare flood response across basins of different sizes using one normalized measure.
2) Why is the runoff coefficient important?
The runoff coefficient represents how much rainfall becomes direct runoff. Paved and compacted surfaces usually produce higher values than vegetated or permeable areas.
3) When should I use the rational method?
The rational method is commonly used for small to medium drainage areas, especially for storm sewers, culverts, site grading, and preliminary hydraulic design checks.
4) What does rainfall intensity mean here?
Rainfall intensity is the average storm rate for the selected duration and return period. It should come from local IDF curves or approved design standards.
5) Why include a safety factor?
A safety factor adds conservatism for uncertainty in rainfall, land cover, hydraulic losses, and future development. It helps prevent undersized drainage infrastructure.
6) How do losses affect discharge?
Losses reduce the effective flow reaching the outlet. They can represent detention, infiltration, storage, obstruction, or other attenuation mechanisms in the system.
7) Can I compare two catchments with this value?
Yes. Unit peak discharge is especially useful for comparing hydrologic response between watersheds because it removes the direct influence of total drainage area.
8) Is this calculator suitable for final approval design?
It is best for planning, screening, and preliminary engineering checks. Final design should always follow local codes, hydrology manuals, and project-specific review.