Sewer network design

Civil Engineering (B.E./B.Tech) · Water and environment · free study notes for exams, placements and interviews

In short

Sewer network design is the hydraulic and layout design of the underground pipe system that collects wastewater (and in some cases storm water) from houses, institutions and industry and carries it by gravity to a treatment plant or disposal point. It fixes the quantity of flow, the pipe size, slope, velocity, depth of cut, materials and appurtenances such as manholes, drop manholes and siphons. It matters because an undersized or flat sewer silts up and overflows, while an oversteep one erodes and needs deep excavation — both are expensive public-health failures. In India it is governed by the CPHEEO Manual on Sewerage and Sewage Treatment (Ministry of Housing and Urban Affairs) along with IS codes, and it is examined in GATE Civil (Environmental Engineering), university Environmental Engineering-II papers, and in interviews for AMRUT/Jal Jeevan/Namami Gange consultancy and municipal engineer roles.

Explained

Systems of sewerage and the network layout

Separate system: two independent networks — sanitary sewers for sewage, storm drains for rainfall. Sewers are small (self-cleansing easier), treatment plant load is steady. Standard choice for Indian towns because of the intense monsoon.
Combined system: one conduit carries sewage plus storm water. Cheaper in congested old cities (one trench), but pipes are huge, velocities are low in dry weather (silting), and storm overflow carries raw sewage to the river.
Partially separate system: roof and courtyard drainage is admitted into the sanitary sewer; street runoff goes to open drains. A practical Indian compromise.
Layout patterns: *perpendicular* (short mains straight to the river — for storm drains), *interceptor* (a trunk sewer along the river intercepting the perpendicular ones), *radial* (flow outward from the centre to several outfalls), *fan* (all flow converging to one outfall/STP), *zone* (separate systems for different elevation zones, with pumping from the low zone).
Terminology: *house/building sewer* → *lateral* → *sub-main (branch)* → *main* → *trunk sewer* → *outfall sewer* → STP. An *intercepting sewer* takes the dry-weather flow of a combined/storm drain to treatment.
Design period: laterals and branches are normally designed for the ultimate/saturation development (hard to replace); trunk sewers, outfall and pumping mains for about 30 years; pumping machinery for 15 years and treatment units in stages of about 15 years (CPHEEO guidance — check the current edition, as staging recommendations are periodically revised).

Estimating the design flow

Sanitary sewage = 75–80% of the water supplied. CPHEEO takes 80% of the per-capita supply as the dry-weather flow, the remainder being lost to consumptive use, gardening and leakage.
Per-capita water supply (CPHEEO/IS 1172): 135 lpcd for towns with piped supply and sewerage, 150–200 lpcd for metropolitan cities with sewerage and fire-fighting demand, 70 lpcd for piped supply without sewerage, about 40 lpcd where only stand-posts/spot sources exist. So sewage ≈ 108 lpcd for a 135-lpcd town.
Infiltration: groundwater entering through joints and cracks; allowed as a flat rate per unit length or per hectare (CPHEEO gives ranges such as roughly 5,000–50,000 litres/ha/day depending on groundwater and workmanship) — use the figure specified in the project DPR rather than a remembered number.
Variation of flow: *average flow* = population × per-capita sewage. Peak factor on average flow (CPHEEO): 3.0 up to 20,000; 2.5 for 20,001–50,000; 2.25 for 50,001–7,50,000; 2.0 above 7,50,000. Minimum flow ≈ 1/3 to 1/2 of the average.
Design rule: size the sewer to carry the peak flow flowing just full (not under pressure), and check that at the minimum flow the velocity is still self-cleansing or that the depth is at least ~0.2–0.3 of diameter.
Industrial and institutional flows are added as measured/estimated discharges, not from population; trade effluent must meet the sewer-admission limits before discharge.
Storm flow (Rational method): Q = (1/360)·C·i·A with i from the intensity–duration–frequency curve for the city, duration = time of concentration (inlet time + time of travel in the drain). Return period 1–2 years for residential drains, 2–5 years for commercial/important areas.

Hydraulic design: velocity, slope and partial flow

Flow is treated as steady uniform open-channel flow even though it varies, because the flow changes slowly compared with the short travel time through a reach, so the depth stays close to normal depth.
Self-cleansing velocity: the velocity that keeps grit and organic solids moving. Design minimum 0.6 m/s at design peak flow, preferably 0.8 m/s or more; this must be achieved at least once a day. Derived from Shields' criterion (the Camp/Shields relation, see formulas).
Maximum velocity (non-scouring / limiting): about 3 m/s for ordinary conditions — higher velocities abrade the pipe invert, especially with grit. RCC/stoneware allow about 2.5–3 m/s; cast iron and steel more.
Grade/slope: fixed by velocity, not by ground slope alone. A useful field rule for the minimum gradient is 1 in D, with D the pipe diameter in mm (e.g. 1 in 150 for a 150 mm sewer, 1 in 600 for a 600 mm sewer) — larger sewers need flatter slopes for the same velocity because R is bigger.
Minimum sizes: 150 mm diameter for house/lateral sewers in India (not less than 100 mm for a building connection), because of the risk of blockage by rags and solids.
Partially full flow: at depth d in a pipe of diameter D, the flow and velocity are obtained from the hydraulic elements (proportionate) diagram. Key results: velocity is maximum at d/D ≈ 0.81 and discharge is maximum at d/D ≈ 0.95; at d/D = 0.5 the velocity equals the full-bore velocity and q/Q = 0.5; at d/D = 0.8, q/Q ≈ 0.98 and v/V ≈ 1.14.
Derivation outline (circular section): for a central angle 2θ, A = (R²/2)(2θ − sin2θ), P = 2Rθ, so R_h = A/P; put R_h into Manning's equation to get v/V = (R_h/R_full)^{2/3} and q/Q = (A/A_full)(v/V). That single step produces the whole proportionate-flow table.
Freeboard/surcharge: sanitary sewers are designed to run just full at peak; storm drains may be allowed to surcharge briefly. A sewer never runs under pressure except in an inverted siphon or a rising main.

Materials, appurtenances and structural aspects

Pipe materials: stoneware/salt-glazed (vitrified clay) for small laterals — corrosion-proof but brittle; RCC (IS 458, NP2/NP3/NP4 classes) for most gravity sewers; PVC-U and HDPE for small to medium sizes — light, corrosion-free, long lengths, now common; GRP/FRP for large non-pressure sewers; cast iron/DI/steel for pumping mains, siphons and above-ground crossings; brick masonry for large old sewers of any shape.
Shapes: circular (standard, best hydraulically when full), egg-shaped/ovoid (better self-cleansing at low flow in combined sewers), horseshoe and rectangular for very large conduits.
Manholes (IS 4111): for inspection, cleaning and ventilation at every change of direction, size, grade and at junctions. Typical spacing 30 m for ≤300 mm sewers, 45–75 m for 300–900 mm, up to 90–120 m for large sewers; minimum internal size ~0.9 m for shallow manholes up to ~0.9 m deep, ~1.2 m dia for 0.9–2.5 m depth, and ~1.5 m dia with an access shaft beyond that. Benching sloped to the channel; steps/ladder rungs for entry.
Drop manhole: used where a branch enters much higher than the main or where the ground falls steeply — energy is dissipated inside the manhole instead of scouring the invert; generally provided when the drop exceeds about 0.6 m.
Other appurtenances: *gully trap* (water-sealed trap at a house connection, prevents sewer gas and foul air entering), *catch basin* (storm inlet chamber with a silt trap), *street inlet* for storm water, *flushing tank* for dead-end laterals, *grease and oil trap* for kitchens and garages, *sand/grit chamber*, *ventilating column*, *lamp hole*, *clean-out*, *storm-water overflow (leaping weir, siphon spillway)* in combined systems, *inverted siphon* (depressed sewer running under pressure to cross a stream, metro or utility — always in two or more barrels so each can be cleaned, and sized for self-cleansing velocity at low flow).
Depth and bedding: minimum cover of about 0.9–1.2 m under roads to protect from traffic load and to receive house connections; load analysis by the Marston theory, bedding factor from the bedding class; backfill in layers.
Testing and commissioning: *water (hydrostatic) test* on gravity sewers between manholes with about 1.5 m head over the crown at the upper manhole, leakage within the specified limit; *air test* as a quick alternative; *ball/straightness test* and alignment by mirror/CCTV; pressure mains tested to 1.5 times the working pressure.

Sewer ventilation, corrosion and safety

Why gases form: sewage turns septic in long, flat or surcharged sewers; sulphate-reducing bacteria in the slime layer produce H₂S, which escapes to the crown space.
Crown (crown-of-pipe) corrosion: *Thiobacillus* bacteria on the moist crown oxidise H₂S to sulphuric acid, which attacks cement concrete from the top down. Remedies: keep velocity self-cleansing, ventilate well, limit detention time, use PVC/HDPE/GRP or epoxy/PE-lined concrete, avoid long rising mains, and add chemicals (lime, chlorine, ferrous salts, peroxide) in problem stretches.
Dangerous sewer gases: H₂S (toxic, rotten-egg smell that disappears at high concentration — hence deadly), methane (explosive), carbon monoxide, carbon dioxide and nitrogen (asphyxiants, oxygen deficiency).
Manhole entry is a confined-space operation: test for oxygen and gases, ventilate, two persons outside, harness, lifeline and breathing apparatus. The Prohibition of Employment as Manual Scavengers and their Rehabilitation Act, 2013 bans manual cleaning of sewers and septic tanks without prescribed protective gear and devices; mechanised desilting (jetting-cum-suction machines, robotic cleaners) is now the required practice. The NAMASTE scheme (replacing/expanding earlier self-employment schemes) supports mechanisation and worker safety — programme details change, so verify current guidelines.
Pumping stations: needed in flat terrain and for low zones. Wet-well/dry-well arrangement, non-clog centrifugal or submersible pumps, screens before the pump, standby units, and the wet-well storage sized to limit pump starts and to avoid septicity (short detention).

Codes, approvals and current Indian practice

CPHEEO Manual on Sewerage and Sewage Treatment (Part A Engineering, Part B Operation and Maintenance, Part C Management) — the governing design document for Indian municipal sewerage.
IS codes: IS 1172 (water/sewage requirements), IS 458 (RCC pipes), IS 783 (laying of concrete pipes), IS 4111 (ancillary structures in sewerage — Part 1 manholes), IS 1742 (building drainage), IS 5329 (sanitary pipework), IS 15328 (UPVC sewer pipes) and IS 14333 (HDPE sewerage pipes), SP 7 (NBC 2016, Part 9 Plumbing Services) for building drainage.
Regulatory frame: Water (Prevention and Control of Pollution) Act 1974 and the Environment (Protection) Rules — effluent standards for discharge of treated sewage into inland surface water; CPCB/SPCB consent; MoEFCC STP discharge norms (BOD, TSS, faecal coliform; limits were revised and litigated through the NGT, so quote the notification in force for the project rather than a memorised figure).
Programmes where this work happens: AMRUT 2.0, Swachh Bharat Mission (Urban) 2.0, Namami Gange (sewerage under NMCG), state urban infrastructure projects, often on hybrid-annuity or DBOT contracts.
Modern practice: sewer network modelling in SewerGEMS, InfoWorks ICM, SWMM; GIS-based asset mapping; CCTV inspection and trenchless rehabilitation (CIPP lining, pipe bursting, microtunnelling/HDD for crossings); decentralised/condominial sewerage and small-bore/settled sewerage for dense low-income areas.

Key points

Separate system – sewage and storm water in separate conduits; preferred in India due to monsoon intensity.
Sewage quantity – 80% of water supplied, plus infiltration, plus industrial flow.
Peak factor – 3.0 (≤20,000), 2.5 (20,001–50,000), 2.25 (50,001–7,50,000), 2.0 (>7,50,000) on average flow.
Design discharge – peak flow carried with the sewer running *just full*, by gravity, never under pressure.
Self-cleansing velocity – minimum 0.6 m/s at design flow, preferably 0.8 m/s; must occur at least once daily.
Maximum velocity – about 3 m/s to prevent abrasion of the invert by grit.
Minimum diameter – 150 mm for laterals in India, to resist blockage.
Minimum gradient rule – 1 in D (D in mm); large sewers manage flatter grades because R_h is larger.
Maximum velocity at d/D ≈ 0.81; maximum discharge at d/D ≈ 0.95 in a circular sewer.
At d/D = 0.5, v/V = 1.0 and q/Q = 0.5 — half depth carries half the full-bore discharge at the same velocity.
Manning's n – about 0.013 for RCC/stoneware sewers, 0.009–0.011 for PVC/HDPE.
Manhole spacing – ~30 m (≤300 mm), 45–75 m (300–900 mm), 90–120 m (large); at every change of size, grade, direction or junction.
Drop manhole – dissipates energy where a branch enters high or the ground slopes steeply (drop above ~0.6 m).
Inverted siphon – depressed sewer under pressure to cross an obstruction; two or more barrels for cleaning.
Crown corrosion – H₂S → H₂SO₄ by *Thiobacillus* on the moist crown; attacks concrete; controlled by velocity, ventilation and liners.
Gully trap – water-sealed trap between the house drain and the sewer, blocking foul gas.
Testing – hydrostatic test with ~1.5 m head over the crown between manholes; plus air and straightness tests.
Codes – CPHEEO Manual on Sewerage; IS 1172, IS 458, IS 783, IS 4111, IS 1742; Water Act 1974; Manual Scavengers Act 2013 (no manual sewer entry).

Formulas and examples

Average sewage flow Q_avg = P × q × 0.8 / (24 × 3600 × 1000) m³/s — P = population, q = per-capita water supply (lpcd)
Design (peak) flow Q_design = Peak factor × Q_avg + Q_infiltration + Q_industrial
Manning's equation V = (1/n) R^2/3 S^1/2 ; Q = A·V — V in m/s, R = hydraulic radius A/P (m), S = slope (m/m), n = roughness coefficient
Full circular sewer A = πD²/4, P = πD, so R = D/4 ; V_full = (1/n)(D/4)^2/3 S^1/2
Chezy V = C√(R·S) ; Darcy–Weisbach h_f = f L V²/(2gD)
Partially full (central angle 2θ, radians) A = (D²/8)(2θ − sin 2θ) ; P = (D/2)(2θ) ; d = (D/2)(1 − cos θ)
Proportionate elements v/V = (R/R_full)^2/3 ; q/Q = (a/A)·(v/V)
Self-cleansing (Shields/Camp) V_s = (1/n) R^1/6 √[ k (G_s − 1) d_p ] ; τ = γ·R·S — τ = tractive force (N/m²), γ = 9810 N/m³, G_s = specific gravity of particle, d_p = particle size (m), k ≈ 0.04 (grit) to 0.8 (organic)
Slope needed for self-cleansing at partial depth S' = S × (R_full/R)^4/3 × (n'/n)² (with n' = n, S' = S·(R_full/R)^4/3)
Rational method (storm) Q = C·i·A/360 — Q in m³/s, C = runoff coefficient, i = rainfall intensity (mm/h), A = catchment area (hectares)
Rainfall intensity i = a/(t + b) or i = KT^x/(t + b)ⁿ — t = duration = time of concentration (min)
Pumping power P = ρ g Q H / η (watts) — H = total head (m), η = overall efficiency

Hydraulic elements of a circular sewer (Manning, n constant)

d/D	a/A	r/R	v/V	q/Q
0.2	0.142	0.482	0.615	0.088
0.3	0.252	0.684	0.776	0.196
0.5	0.500	1.000	1.000	0.500
0.7	0.748	1.185	1.120	0.838
0.81	0.868	1.217	1.140 (max v)	0.989
0.95	0.981	1.146	1.095	1.075 (max q)
1.0	1.000	1.000	1.000	1.000

Example 1 — Design flow for a town. Population 60,000; water supply 135 lpcd; infiltration 10,000 L/ha/day over 150 ha; no industry.

Average sewage = 60,000 × 135 × 0.8 = 6,480,000 L/day = 6,480 m³/day → 6,480/86,400 = 0.075 m³/s.
Peak factor for 60,000 population (50,001–7,50,000 band) = 2.25 → peak sanitary flow = 2.25 × 0.075 = 0.169 m³/s.
Infiltration = 10,000 × 150 = 1,500,000 L/day = 1,500 m³/day = 1,500/86,400 = 0.0174 m³/s.
Design flow = 0.169 + 0.0174 ≈ 0.186 m³/s (186 L/s).
Minimum flow = (1/3) × 0.075 ≈ 0.025 m³/s — check self-cleansing at this value.

Example 2 — Size a sewer and check velocity. Design peak flow 0.186 m³/s; RCC sewer, n = 0.013; available ground slope allows S = 1 in 600 (0.001667). Find diameter running just full.

Q = (1/n)·(πD²/4)·(D/4)^2/3·S^1/2 = (π/(4·0.013·4^2/3))·D^8/3·√0.001667
Constant: 4^2/3 = 2.5198; π/(4 × 0.013 × 2.5198) = 3.1416/0.13103 = 23.98
√0.001667 = 0.04082 → Q = 23.98 × 0.04082 × D^8/3 = 0.9789 D^8/3
0.186 = 0.9789 D^8/3 → D^8/3 = 0.1900 → D = 0.1900^3/8 = 0.536 m → adopt the next commercial size, D = 600 mm.
Check at D = 0.6 m: A = π(0.6)²/4 = 0.2827 m², R = 0.15 m, R^2/3 = 0.2823. V_full = (1/0.013) × 0.2823 × 0.04082 = 0.886 m/s (> 0.6 m/s, < 3 m/s ✔) Q_full = 0.2827 × 0.886 = 0.2505 m³/s — comfortably above 0.186 m³/s.

Example 3 — Partial-flow check at minimum flow (same 600 mm sewer). Minimum flow = 0.025 m³/s → q/Q = 0.025/0.2505 = 0.0998 ≈ 0.10.

From the hydraulic elements table, q/Q = 0.10 corresponds to d/D ≈ 0.21, with r/R ≈ 0.50 and v/V ≈ 0.63.
v = 0.63 × 0.886 = 0.56 m/s — marginally below 0.6 m/s, so either steepen the grade or accept it, because the *peak* flow velocity governs the self-cleansing requirement (0.886 m/s is achieved daily).
Slope required to make v = 0.6 m/s at this depth: S' = S × (0.6/0.56)² = 0.001667 × 1.148 = 0.001914 ≈ 1 in 522.

Example 4 — Storm drain by the Rational method. Catchment 25 ha: 10 ha roofs/paved (C = 0.80), 15 ha residential with gardens (C = 0.45). Inlet time 8 min, travel time 7 min. IDF relation i = 1000/(t + 20) mm/h.

Weighted C = (10 × 0.80 + 15 × 0.45)/25 = (8.0 + 6.75)/25 = 14.75/25 = 0.59
t_c = 8 + 7 = 15 min → i = 1000/(15 + 20) = 1000/35 = 28.57 mm/h
Q = C·i·A/360 = 0.59 × 28.57 × 25/360 = 421.4/360 = 1.17 m³/s
Size as a circular drain at S = 1 in 400 (0.0025), n = 0.013: Q = 23.98 × √0.0025 × D^8/3 = 23.98 × 0.05 × D^8/3 = 1.199 D^8/3 → D^8/3 = 1.17/1.199 = 0.9758 → D = 0.991 m → adopt 1,000 mm. Check V_full: R = 0.25, R^2/3 = 0.3969 → V = (1/0.013)(0.3969)(0.05) = 1.53 m/s ✔.

Example 5 — Self-cleansing by tractive force. A 300 mm sewer (n = 0.013) must move grit of 1 mm size, G_s = 2.65, k = 0.04. Find the slope needed when running full.

V_s = (1/0.013) × (0.075)^1/6 × √(0.04 × 1.65 × 0.001)
(0.075)^1/6: ln 0.075 = −2.590; /6 = −0.4317 → e^−0.4317 = 0.6494
√(0.04 × 1.65 × 0.001) = √(6.6 × 10⁻⁵) = 0.008124
V_s = 76.92 × 0.6494 × 0.008124 = 0.406 m/s (so the 0.6 m/s rule is the stricter criterion here)
Slope for V = 0.6 m/s: S = [V·n/R^2/3]² = [0.6 × 0.013/0.075^2/3]². 0.075^2/3 = 0.1779 → (0.0078/0.1779)² = (0.04384)² = 0.001922 → 1 in 520; the thumb rule 1 in 300 is steeper and therefore safe.
Tractive force check at S = 1/520: τ = 9810 × 0.075 × 0.001922 = 1.41 N/m² (typically 1–2 N/m² is taken as adequate for grit).

Separate vs combined system

Aspect	Separate	Combined
Number of conduits	Two	One
Pipe size	Small sanitary sewer	Very large
Self-cleansing	Easier (steady flow)	Poor in dry weather
Capital cost	Two trenches, higher pipe cost	Lower laying cost in congested roads
Treatment plant load	Uniform, concentrated sewage	Highly variable, dilute in monsoon
Pollution risk	Low	Storm overflow discharges raw sewage
Indian suitability	Preferred for new towns	Old city cores only

Where it is used

Urban local bodies and water boards — Delhi Jal Board, BWSSB, CMWSSB, MCGM, HMWSSB: design, bid-checking and O&M of sewer networks and pumping stations.
Consultancies and EPC contractors — L&T Construction, Tata Projects, VA Tech Wabag, WAPCOS, Mott MacDonald, AECOM, SMEC: DPRs, hydraulic modelling and construction supervision for AMRUT 2.0, Namami Gange and SBM-U sewerage packages.
Job roles — junior/assistant engineer (public health engineering) in state PHED and municipal cadres via GATE/state PSC, design engineer (water & wastewater), WASH specialist, O&M manager for a sewerage DBOT concession.
Building and township services — plumbing and drainage design to NBC 2016 Part 9 for housing projects, IT parks, airports and SEZs, including internal drainage, gully traps and connection to the municipal sewer.
Industrial estates — common effluent collection networks with corrosion-resistant HDPE/GRP pipework and sewer-admission limits for trade effluent.
Rehabilitation and smart O&M — CCTV survey, GIS asset registers, trenchless relining of old brick sewers in Kolkata, Mumbai and Chennai; SCADA-monitored sewage pumping stations.

Interview questions

Q: Why is sewage taken as 80% of the water supplied? A: About 20% of supplied water is lost to consumptive use, gardening, car washing, evaporation and distribution leakage and never reaches the sewer; CPHEEO therefore fixes 80% as the dry-weather flow.
Q: What are the minimum and maximum velocities in a sanitary sewer, and why? A: Minimum about 0.6 m/s (preferably 0.8) so grit and organic solids do not settle, and maximum about 3 m/s so the invert is not abraded by suspended grit.
Q: At what depth of flow is the velocity in a circular sewer maximum? A: At about d/D = 0.81, because beyond that the wetted perimeter grows faster than the area, so the hydraulic radius — and hence velocity — falls.
Q: If a sewer runs half full, what fraction of its full-bore discharge does it carry? A: Exactly half, and at the same velocity as when running full, because the hydraulic radius at half depth equals D/4.
Q: Why is a separate system preferred in Indian cities? A: Monsoon rainfall is intense and seasonal, so a combined sewer would have to be enormous yet would silt up for most of the year and spill raw sewage during storms.
Q: What is crown corrosion and how do you prevent it? A: H₂S released from septic sewage is oxidised by *Thiobacillus* bacteria on the moist pipe crown into sulphuric acid, which eats the concrete; prevent it with self-cleansing velocities, good ventilation, short detention and PVC/HDPE/GRP or lined pipes.
Q: Why is an inverted siphon built with more than one barrel? A: Dry-weather flow in a single large barrel would be too slow to be self-cleansing, so two or more barrels are used — one carries the low flow at proper velocity while the others come in at higher flows and can be isolated for cleaning.
Q: When would you provide a drop manhole? A: Where a branch sewer joins the main at a much higher invert, or the ground slopes steeply, typically for drops over about 0.6 m, so the falling flow does not scour the main or cause splashing and odour.
Q: Why does a 1 m sewer need a flatter gradient than a 150 mm sewer for the same velocity? A: Velocity by Manning depends on R^2/3, and R = D/4 when full, so a larger diameter gives a much bigger hydraulic radius and needs less slope — hence the 1-in-D rule.
Q: Why do we design laterals for ultimate development but trunk sewers for 30 years? A: Laterals are small, cheap to upsize at the time of laying but very disruptive to replace under built-up streets; trunk sewers are costly, so staging them over about 30 years is economical.
Q: How is a newly laid gravity sewer tested? A: By a water test between consecutive manholes with about 1.5 m head over the crown at the upstream end, checking that leakage is within the specified limit, together with air and straightness/ball or CCTV tests.
Q: Is manual cleaning of sewers legal in India? A: No — the Prohibition of Employment as Manual Scavengers and their Rehabilitation Act, 2013 bars hazardous manual entry; cleaning must be mechanised (jetting-cum-suction, robotic units) with prescribed protective gear and confined-space protocol.

Common mistakes

Using the average flow instead of the peak flow for sizing, or forgetting to add infiltration and industrial discharge.
Applying the wrong peak factor band — reading 3.0 for a city of 5 lakh instead of 2.25.
Taking R = D/2 for a full circular sewer instead of R = D/4, which throws the velocity out by about 59%.
Mixing units in the Rational method — area in km² or m² instead of hectares, or intensity in cm/h, when the 1/360 constant demands ha and mm/h.
Checking self-cleansing velocity only at full flow and never at the minimum/initial flow, or vice versa — both checks are required.
Reporting the computed diameter (0.536 m) as the design value instead of rounding up to a commercially available size.
Confusing a gully trap (water seal at the house connection) with a grease trap or a catch basin, and placing manholes only at fixed spacings while ignoring the mandatory points — every change of direction, grade, size and every junction.