Sewer Pipe Flow Capacity Calculator
Calculate the flow capacity of circular sewer pipes using Manning's equation. Supports full-flow and partial-flow (depth-based) conditions.
Typical values: Concrete=0.013, PVC=0.009, Clay=0.013, Cast Iron=0.012
Formulas Used
Manning's Equation (Full Flow):
V = (1/n) × R2/3 × S1/2
Q = V × A
Where:
- V = flow velocity (m/s)
- n = Manning's roughness coefficient (dimensionless)
- R = hydraulic radius = A / P (m)
- S = pipe slope (m/m)
- Q = volumetric flow rate (m³/s)
- A = cross-sectional flow area (m²)
- P = wetted perimeter (m)
Full-Flow Circular Pipe:
- Af = πD² / 4
- Pf = πD
- Rf = D / 4
Partial-Flow Circular Pipe (depth y):
- θ = 2 × arccos(1 − 2y/D) [central angle in radians]
- Ap = (D² / 8) × (θ − sinθ)
- Pp = (D / 2) × θ
- Rp = Ap / Pp
Assumptions & References
- Steady, uniform, turbulent flow is assumed throughout.
- The pipe is circular in cross-section and flows under gravity (open-channel conditions).
- Manning's equation is valid for fully turbulent flow; accuracy decreases at very low velocities.
- Maximum discharge in a circular pipe occurs at approximately 94% of full depth (y/D ≈ 0.94), not at full flow, due to the reduction in hydraulic radius near the crown.
- Maximum velocity occurs at approximately 81% of full depth (y/D ≈ 0.81).
- Recommended minimum self-cleansing velocity: 0.6 m/s (AS/NZS 3500, EN 752).
- Typical Manning's n values: PVC = 0.009–0.011, Concrete = 0.012–0.015, Vitrified Clay = 0.012–0.014, Cast Iron = 0.012–0.013.
- References: Chaudhry, M.H. (2008) Open-Channel Hydraulics; AS/NZS 3500.3; EN 752; ASCE Manual of Engineering Practice No. 36.