Sewer Siltation: Low-Flow Velocity Under a Fixed Outfall Invert
A 600 mm diameter RCC sewer, 500 m long, is laid at a uniform gradient of 1 in 400 between two manholes and carries the entire flow of a town. Manning's n is 0.013. The peak design discharge is 300 L/s, while the minimum dry-weather flow reaching the head of the reach at about 03:00 h is 60 L/s. The minimum self-cleansing velocity adopted by the utility is 0.90 m/s. The invert of the downstream manhole is fixed by an existing outfall structure and permits a total fall of only 1.2 m over the 500 m reach. Two years after commissioning, maintenance staff report heavy silt and grit deposition in the last 150 m, and hydrogen sulphide odour at the downstream manhole. The Chief Engineer wants to know whether the sewer is under-designed and what can be done without demolishing the reach.
1. Check the full-flow capacity and velocity of the 600 mm sewer at 1 in 400 and compare it with the 300 L/s peak discharge.
Read the worked answer
Section properties: A = π(0.6)²/4 = 0.2827 m²; P = π × 0.6 = 1.885 m; R = A/P = 0.150 m
Full-flow velocity (Manning): V = (1/n) R^(2/3) S^(1/2) = (1/0.013) × 0.150^(2/3) × (0.0025)^0.5
= 76.92 × 0.2823 × 0.05 = 1.086 m/s
Full-flow discharge: Q = 1.086 × 0.2827 = 0.307 m³/s = 307 L/s
This is only 2% above the 300 L/s peak - the pipe is effectively running full at peak. Using the proportional-flow relation for a circular section, at q/Q = 300/307 = 0.977 the depth ratio is about d/D = 0.87 and v/V ≈ 1.13, so v ≈ 1.23 m/s. Velocity is fine, but running at 0.87 D at peak is poor practice: the standard design target is about 0.8 D at peak, so that the pipe has reserve capacity for infiltration, future connections and surcharge effects, and so that ventilation and the release of sewer gases are not impeded.
The pipe is therefore marginally under-designed for the peak, but only just - there is no serious capacity failure. That confirms the reported problem must lie at low flow, not at peak.
2. Estimate the velocity at the minimum nightly flow of 60 L/s and explain the siltation and odour complaints.
Read the worked answer
Proportional flow ratios for a circular sewer (from the standard hydraulic-elements table): at q/Q = 60/307 = 0.195, the corresponding depth is d/D ≈ 0.30 and v/V ≈ 0.78.
v_min = 0.78 × 1.086 = 0.85 m/s, which is below the 0.90 m/s self-cleansing requirement.
Two mechanisms then operate together, and both match the field evidence:
- Deposition. At 0.85 m/s, the tractive force at the invert is not enough to move grit and heavier solids, which settle progressively. Because the last 150 m is the flattest, slowest and most downstream part of the reach, and where the flow is already attenuating, that is exactly where the deposits accumulate. Note also that a part-full pipe loses velocity disproportionately: the hydraulic radius at d/D = 0.30 is only about 0.68 of the full-flow value, because the wetted perimeter includes the pipe walls well above the water line.
- Septicity. With d/D ≈ 0.30, a large wetted wall area is exposed above the water line and stays wet but unaerated. Anaerobic conditions in the slime layer generate H₂S, which then oxidises to sulphuric acid on the crown, attacking the cement matrix. The odour at the downstream manhole is the visible signature of ongoing crown corrosion.
So the sewer is not wrongly sized for peak discharge; it is wrongly sized for the ratio between peak and minimum flow, a classic combined-sewer problem.
3. Given that the outfall invert allows only 1.2 m of fall over 500 m, recommend remedial measures and state what must be verified before they are adopted.
Read the worked answer
The available fall of 1.2 m over 500 m is a gradient of 1 in 417, i.e. flatter than the 1 in 400 already provided, so steepening the reach is physically impossible. Reducing the diameter is also excluded: a 500 mm pipe at 1 in 400 would pass only about 190 L/s at full flow, well below the 300 L/s peak. The practical answer is therefore operational, plus a limited structural intervention:
- Flushing. Provide automatic flushing tanks at the head of the reach and at the upstream manhole of the last 150 m, releasing a wave at night when the dry-weather flow is at its minimum. This is the standard fix for a low-flow-reach and can be installed without touching the pipeline.
- Remove the solids before they arrive. Grit chambers and properly sized silt traps upstream, plus a ban on the connection of storm runoff and construction waste, attack the cause rather than the symptom.
- Periodic jetting and desilting of the last 150 m under a maintenance contract, with records, until the flushing regime proves itself.
- Crown protection. Because H₂S attack is already indicated, inspect the crown by CCTV, and line or replace the affected length with sulphate-resisting or HDPE-lined pipe where the concrete has softened.
- Partial duplication. A small-diameter dry-weather-flow sewer laid at a steeper gradient within the available fall, with the 600 mm acting as the storm/peak carrier, is technically attractive but costly and should be the last resort.
Before issuing the scheme, verify: the actual night flow by gauging over at least a week (the 60 L/s may be overstated if infiltration is included), the silt load and its composition, the CCTV survey of the crown, and the structural condition of the last 150 m. If gauging shows the true minimum flow is much lower than 60 L/s, duplication of the low-flow capacity becomes the correct rather than the last-resort option.