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Gravity Drainage Calculator

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Input Data
Enter your project data
Manning's n roughness coefficient. PVC: 0.009-0.011, HDPE: 0.010, Concrete: 0.013, Steel: 0.012. Lower n = smoother pipe = higher capacity.
Actual inner diameter (ID) of the pipe in millimeters. Use manufacturer's datasheet value, not nominal DN size. Critical for accurate hydraulic calculations.
Peak design flow rate in liters per second. For buildings: calculate from fixture units or population. For sites: use Rational Method (Q=CiA).
Pipe slope as percentage. Building drainage: 1.0-2.0%, Site drainage: 0.5-1.0%, Infrastructure: 0.15-0.5%. Minimum 0.01% for very flat terrain.
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Calculation Results
Calculated instantly from your inputs
Gravity
Gravity Drainage Calculator

This calculator analyzes gravity flow in circular drainage pipes using Manning's Equation.

MANNING'S EQUATION:
V = (1/n) × R^(2/3) × S^(1/2)
- V = Flow velocity (m/s)
- n = Manning's roughness coefficient
- R = Hydraulic radius (m) = D/4 for full pipe
- S = Slope (m/m)

FULL PIPE CAPACITY:
Qcap = A × V = (πD²/4) × V

FILL RATIO ANALYSIS:
Fill = Qdesign / Qcapacity
- < 50%: OPTIMAL - Good freeboard margin
- 50-85%: OK - Normal operating range
- 85-100%: WARNING - Limited freeboard
- > 100%: SURCHARGE - Capacity exceeded!

VELOCITY CRITERIA:
• Minimum: 0.6 m/s (self-cleansing, prevents sedimentation)
• Optimal: 0.7-2.5 m/s (green zone)
• Maximum: 3.0 m/s (erosion prevention)
• < 0.6 m/s: Sedimentation risk, increase slope
• > 3.0 m/s: Erosion risk, reduce slope or increase diameter

PIPE MATERIAL GUIDE:
• PVC (n=0.009-0.011): Smooth, economical, buildings
• HDPE (n=0.010): Flexible, trenchless applications
• Concrete (n=0.013): Infrastructure, large diameters
• Steel (n=0.012): Industrial applications

DESIGN RECOMMENDATIONS:
- Use actual inner diameter (ID), not nominal DN
- Maintain 15-20% freeboard for safety
- Check velocity at minimum flow (nighttime)
- Consider pipe aging (n increases over time)
- Verify structural capacity for burial depth

Standards:
- Manning's Equation: SI units
- Velocity limits: EN 476, ASTM C76
- Self-cleansing: 0.6 m/s minimum
- Partial flow: Most sewers run 30-70% full

Key Points:
  • Manning's Equation for gravity pipe flow
  • Partial flow analysis (typical sewer condition)
  • Velocity validation: 0.6-3.0 m/s range
  • Fill ratio monitoring with surcharge warning
  • Pipe capacity vs design flow comparison
  • Alternative pipe size comparison table
  • Material-specific Manning's n values
  • Actual inner diameter input (not nominal DN)
  • Slope percentage input with wide range
  • Design flow in L/s for practical use
  • 8-step calculation breakdown
  • Green/yellow/red status indicators
Calculation Steps:
  1. Enter all required parameters
  2. Click "Update Calculation"
  3. Review results and analysis
Formula Variables:
n Manning's Roughness (-)
D Pipe Diameter (mm)
Q Flow Rate (L/s)
S Slope (%)
A Cross-Sectional Area (m²)
R Hydraulic Radius (m)
V Flow Velocity (m/s)
Qcap Pipe Capacity (L/s)
Fill Fill Ratio (%)