Hydraulic design of storm sewers + drains using Manning's equation — open-channel + circular pipe formulas, partial-flow ratios (Q/Qfull, V/Vfull, d/D), self-cleansing velocity, maximum permissible velocity, hydraulic grade line, surcharge analysis, head loss at junctions + bends, freeboard requirements.
For a circular pipe of diameter D at slope S with Manning's n, compute full-pipe Q + V and the design values at d/D = 0.80 (CPHEEO standard partial-flow design). Use 'design n' (0.015 for new RCC) — accounts for biofilm + sediment over service life.
Manning's equation is to drainage what Hardy-Cross is to water distribution — the irreplaceable workhorse. Every pipe size, every slope, every velocity check passes through Manning's. Misuse it and the network either floods or wastes concrete.
The equation itself is simple: V = (1/n) × R^(2/3) × S^(1/2). The art is in the coefficients + the design conventions.
Manning's n is the most-debated input. The 'as-new' value for smooth concrete pipe (0.013) is fine for the day the pipe is laid. By year 5, biofilm + sediment + minor cracks push it to 0.015. For design, use 0.015 — designing to 0.013 means the pipe is hydraulically inadequate by year 5.
Design depth ratio: 80 % depth-to-diameter (d/D = 0.80) is the Indian + global convention. Full-pipe design pressurises the line under any surge + can lift manhole covers. At 80 % depth, you get ~97 % of full-pipe capacity with 14 % higher velocity (hence better self-cleansing) and headroom for surge.
Self-cleansing velocity = 0.6 m/s at design flow. Below this, sediment settles + accumulates over years. The slope required to achieve 0.6 m/s sets the minimum pipe gradient — typically 1 in 250 for 300 mm; 1 in 600 for 1000 mm. Steeper slopes are fine; shallower are not.
Maximum velocity is the upper limit before pipe abrasion + erosion damage start: 3-4 m/s for concrete, 2.5 for brick, 1.5 for unlined earth. Open-channel concrete-lined drains tolerate up to 5 m/s. Beyond these limits, you need abrasion-resistant lining or energy dissipators.
HGL (Hydraulic Grade Line) is the running water surface elevation — must stay BELOW the road invert at every inlet, else water surcharges + backs up + floods the street. Compute HGL by summing pipe friction loss + junction K-losses upstream from outfall. Modern software (SewerCAD, SWMM) automates this; hand calculation is feasible for small networks.
Junction losses are often neglected in textbook designs but accumulate significantly. K = 0.05 for smooth in-line, 0.3-0.5 for typical 4-way, 1.0 for sharp 90°. Detailed manhole + junction design (bell-mouth transitions, deflectors) can halve K — a worthwhile investment for high-velocity outfalls.
Where this chapter sits: Manning's equation + design conventions are the bridge from peak Q (chapter 3-4) to actual pipe sizes (chapter 7). All hydraulic decisions trace back to this chapter's parameters.