Surcharged Sewer, Grease Mat, and Forecast Rain
At 14:20 on a Tuesday, you attend manhole MH-14 on a 300 mm vitrified clay gravity sewer laid at 0.4% over 1.2 km. Dry-weather flow is 38 L/s, but radar shows 18 mm of rain within six hours and a forecast peak flow of 95 L/s. MH-14 is surcharged 0.35 m above the pipe crown; MH-15 downstream is normal. A grease mat covers about 60% of the pipe area. Your meter reads H2S 15 ppm, methane 0% LEL, oxygen 20.6%. No sewage has surfaced or entered a basement yet. You have one jet/vac truck, one CCTV van, and a four-person crew. The jet/vac operator wants to start cleaning immediately from the upstream manhole.
1. Estimate the full-flow capacity of the 300 mm sewer at 0.4% slope using Manning n = 0.013, and use it to explain whether cleaning alone will prevent a discharge during the forecast rain.
Read the worked answer
Using Manning's equation: D = 0.300 m, R = D/4 = 0.075 m, A = π(0.150)^2 = 0.0707 m², S = 0.004, n = 0.013.
Q = (1/n) A R^(2/3) S^(1/2) = (1/0.013)(0.0707)(0.075^(2/3))(0.004^0.5) ≈ 0.061 m³/s = 61 L/s.
The forecast peak is 95 L/s, which already exceeds full-flow capacity; the 60% grease mat reduces capacity further. Cleaning is necessary to restore capacity, but it cannot guarantee containment. You should simultaneously prepare overflow protection (pump, sandbags, downstream monitoring), notify the supervisor/regulator if discharge is imminent, and stage the CCTV after cleaning. Do not assume a clean pipe equals adequate capacity.
2. The operator wants to jet from the upstream manhole downstream. Explain the hydraulic and safety reasons for choosing the upstream or downstream manhole, and list the controls you would apply before any worker opens MH-14.
Read the worked answer
Clean from the downstream manhole (MH-15) and jet upstream toward MH-14, pulling debris back to MH-15 for removal. Jetting from upstream would push grease and solids farther downstream and could worsen the downstream surcharge; it also puts the crew working above a surcharged, gassy manhole.
Before opening MH-14: traffic control and pedestrian exclusion; manhole guard; continuous 4-gas monitoring at the rim and at depth; forced ventilation; permit and attendant if entry is considered; no smoking/hot work; upstream/downstream isolation only if it does not create a worse backup. Keep the public away from the surcharged manhole.
3. At what point does this become a reportable sewage discharge, and what would you do if the level at MH-14 rises another 0.2 m while the crew is still setting up?
Read the worked answer
Reportable when sewage leaves the collection system: reaches a storm sewer, ditch, creek, basement, or ground surface. A contained surcharge is an internal condition but must be treated as imminent.
If MH-14 rises another 0.2 m, stop setup, evacuate the immediate area, start emergency pumping/overflow containment, notify the supervisor and regulator per the spill protocol, and warn downstream/basement occupants. Document times, levels, weather, and photos. Do not wait for surfacing.