The rational method is simple but sensitive to runoff coefficient and rainfall intensity. It is best suited for small catchments with short response time.
Recommended workflow
- Delineate the drainage area clearly.
- Select runoff coefficient based on land use, slope, soil, and imperviousness.
- Estimate time of concentration and select matching rainfall intensity from IDF curve.
- Calculate peak flow using consistent units.
- Check inlet, culvert, and downstream drainage capacity.
Quick design table
| Item | Check |
|---|---|
| C value | Do not copy blindly; adjust for slope and surface condition. |
| Intensity | Use duration equal to time of concentration. |
| Applicability | Avoid using it for large complex basins without justification. |
The method is quick, but the input selection should still be documented in a calculation note.