Complete Guide to Stormwater Runoff Calculation
The Stormwater Runoff Calculator estimates peak flow, volume, and detention needs using industry-standard methods. Critical for urban drainage, flood mitigation, and sustainable design. Pair with Drainage Pipe and Culvert tools.
Why Calculate Runoff?
Urbanization increases impervious area → higher runoff → flooding. Accurate estimation prevents:
- Pipe surcharge
- Erosion
- Pollution
- Infrastructure damage
Rational Method
\[ Q = C i A / 360 \]
Q (m³/s), A (ha), i (mm/h)
SCS Curve Number
\[ Q = \frac{(P - 0.2S)^2}{P + 0.8S} \]
S = 25400/CN - 254
Modified Rational
Trapezoidal hydrograph for detention sizing.
Step-by-Step
- Define area and land use
- Select method
- Input rainfall data
- Compute Q or V
- Size pond if needed
Example: Rational
A=5 ha, C=0.65, i=80 mm/h:
- Q = 0.65 × 80 × 5 / 360 = 0.72 m³/s
CN Values
| Surface | CN (AMC II) |
|---|---|
| Paved | 98 |
| Lawn | 61–79 |
| Woods | 55–77 |
Detention Pond Sizing
Volume = area under inflow - outflow hydrograph.
Best Practices
- Use local IDF curves
- Include 10–25% safety
- Check Tc with Kerby/Kirpich
- Model multiple storms
Common Mistakes
- Wrong C or CN
- Ignoring future development
- No outlet control
Advanced Topics
- SWMM modeling
- Green infrastructure
- Climate change factors
Conclusion
Accurate stormwater runoff design protects communities. Our calculator delivers instant, code-compliant results. Integrate with pipe sizing for complete drainage systems. Design resilient infrastructure!