Last reviewed 16 Sept 2026 · 7 min read
Quantity of sanitary sewage
(About 80% of the water supplied reaches the sewers — CPHEEO; the rest is lost in gardening, evaporation, consumption.)
Add infiltration of groundwater into sewers (depends on water table, pipe material, joints) and industrial or institutional flows.
Variation of flow
Sewage flow varies hourly (peaks a little after water-demand peaks, lag in the sewers), daily and seasonally.
Peak factors for sewers (CPHEEO, based on contributory population):
| Population | Peak factor |
|---|---|
| Up to 20 000 | 3.0 |
| 20 000 to 50 000 | 2.5 |
| 50 000 to 7.5 lakh | 2.25 |
| Above 7.5 lakh | 2.0 |
( = population in thousands.)
- Sewers are designed for the peak flow at the end of the design period and checked for self-cleansing velocity at the minimum (present) flow.
- Minimum flow is commonly taken as a fraction (about one-third to one-half) of the average flow.
- Design period for sewers is long (about 30 years), since relaying is costly; pumping machinery and treatment units shorter (about 15 years).
Quantity of storm water
in m³/s; = rainfall intensity (mm/h) for a duration equal to the time of concentration; in hectares; = runoff (impermeability) coefficient.
Composite coefficient:
Runoff coefficients (indicative)
| Surface | |
|---|---|
| Roofs (watertight) | 0.70 – 0.95 |
| Asphalt and concrete pavements | 0.80 – 0.95 |
| Brick/stone pavements | 0.70 – 0.85 |
| Gravel roads | 0.25 – 0.60 |
| Lawns, gardens, parks | 0.05 – 0.30 |
| Wooded areas | 0.01 – 0.20 |
Time of concentration
- Inlet time — time for overland flow to reach the first inlet (commonly 5–30 min depending on slope and surface).
- Time of flow — travel time in the drain = length ÷ velocity.
Rainfall intensity
Intensity–duration–frequency relations are used where available. Classical formulas (British Ministry of Health, in mm/h, in min):
The design frequency (return period) is chosen by the importance of the area — higher for commercial centres and critical areas than for residential streets.
Hydraulic design of sewers
For a circular sewer flowing full: .
Typical : about 0.013 for concrete and stoneware sewers; about 0.010–0.011 for smooth plastic pipes; about 0.015–0.017 for brick sewers.
Velocity limits
- Self-cleansing velocity — the minimum velocity that prevents deposition of solids. Commonly adopted minimum values: about 0.6 m/s for sanitary sewers at peak flow (with a check at present peak flow) and about 0.75–1.0 m/s for storm and combined sewers (grit is heavier).
- Camp's (Shields) formula:
( ≈ 0.04 for starting motion, up to 0.8 for complete cleansing of sticky material; = Darcy friction factor; , = specific gravity and size of particles.)
- Non-scouring (maximum) velocity — to prevent erosion of the sewer: about 2.5–3 m/s for concrete and stoneware (higher for cast iron); steep ground needs drop manholes to keep velocities down.
Minimum size and depth
- A minimum sewer diameter (commonly 150 mm for laterals, larger in big cities) is used to avoid choking.
- Sewers are laid deep enough to receive house connections from basements/ground floors and to be below water mains; minimum cover protects against traffic loads.