Drain Flow Rate & Blockage Severity Calculator
Calculate the flow rate through a circular drain pipe using Manning's equation and determine blockage severity based on the reduction in flow capacity.
Formula
Manning's Equation (full or partial flow):
Q = (1/n) × A × Rh2/3 × S1/2
Where:
- Q = volumetric flow rate (m³/s)
- n = Manning's roughness coefficient (dimensionless)
- A = cross-sectional flow area (m²)
- Rh = hydraulic radius = A / wetted perimeter (m)
- S = pipe slope (m/m)
Partial-depth geometry (circular pipe):
θ = 2 × arccos(1 − y/R)
A = (R²/2)(θ − sinθ) P = Rθ
Blockage effect:
Aeff = A × (1 − b) Rh,eff = Rh × (1 − b)0.5
Where b = blocked cross-section ratio (0–1).
Assumptions & References
- Steady, uniform, turbulent flow assumed (Manning's equation valid for fully turbulent conditions).
- Circular pipe cross-section; blockage modelled as a uniform reduction in effective flow area.
- Manning's roughness values: concrete pipe n ≈ 0.013, PVC/smooth plastic n ≈ 0.009–0.011, clay/vitrified n ≈ 0.013–0.015, corrugated metal n ≈ 0.022–0.026.
- Self-cleansing velocity threshold of 0.6 m/s per BS EN 752 and Sewers for Adoption (Water UK).
- Blockage severity thresholds based on industry guidance from WRc Sewerage Rehabilitation Manual and CCTV drain survey classifications.
- Reference: Chaudhry, M.H. (2008). Open-Channel Hydraulics. Springer. | Manning, R. (1891). On the flow of water in open channels and pipes.
- This calculator is for estimation purposes. Site-specific conditions (partial blockage shape, sediment type, surcharge) may affect actual flow rates.