Calculations are based on Manning's formula:
Q = A * V = A * (1/n) * R^(2/3) * S^(1/2)
Where:
Q - Flow [m³/s]
A - Cross-sectional area of flow [m²]
V - Flow velocity [m/s]
n - Manning's roughness coefficient [-]
R - Hydraulic radius [m] = A/P
P - Wetted perimeter [m]
S - Hydraulic slope [-]
Manning's roughness coefficients
- PVC/PP pipes: n = 0.010
- Vitrified clay pipes: n = 0.013
Minimum slopes
For sewer networks, the minimum slope is most often taken as the reciprocal of the diameter, i.e. 1/DN. For example, for a DN200 pipe, this is 1/200, which is 0.5% (5‰).
Minimum slopes for drainage pipes of domestic and combined sewerage:
| Drainage pipe diameter DN [mm] | Minimum slope [%] |
|---|
| 100 | 2,0 |
| 125 | 2,0 |
| 150 | 1,5 |
| 200 | 1,0 |
| 250 | 0,8 |
| 300 | 0,67 |
Minimum slopes for drainage pipes of storm sewerage
| Drainage pipe diameter DN [mm] | Minimum slope [%] |
|---|
| 125 | 1,0 |
| 150 | 0,8 |
| 200 | 0,5 |
| 250 | 0,4 |
Maximum allowable slopes of sewer pipes
| Material | Diameter | Slope |
|---|
| Ceramic, concrete and plastic pipes | ≤ 150 | 15,0 |
| 200 | 10,0 |
| ≥ 250 | 8,0 |
| Cast iron pipes | ≤ 150 | 40,0 |
| ≥ 200 | 25,0 |
