Translation of catchment + hydrology into a buried pipe network — layout principles (grid vs herringbone vs radial), pipe diameter selection from Q + Manning, slope determination, depth + cover requirements, manhole spacing + sizing, junction design, pipe material selection, alignment with road geometry + utilities.
This is where catchment + hydrology + hydraulics translate into actual buried infrastructure. Pipe sizes, slopes, depths, manholes, materials — the bill of quantities you'll tender for crores.
Network layout follows roads almost universally. Why: the road already has the right-of-way + cross-fall + utility corridor; building a parallel storm sewer beneath the carriageway is operationally efficient. Cross-country alignments add easement complications + acquisition cost. Only deviate when road alignment forces excessive depth (> 4 m cover) or doesn't match catchment fall.
Pipe sizing is iterative: from Manning's, given Q (from chapter 3-4), trial diameter, compute slope to achieve 0.6 m/s self-cleansing at 80 % depth, check max velocity not exceeded, check HGL stays below road invert. Software (SewerCAD, SWMM) automates this; hand-design uses standard sizes 300/450/600/750/900/1050/1200/1500/1800/2000 mm RCC.
Monotonic diameter rule: never reduce pipe diameter in the direction of flow, even if hydraulic capacity allows. Sediment management + maintenance access require monotonic growth. A 'pinch' downstream blocks debris + creates upstream surcharge.
Cover = 0.90 m minimum to pipe crown under trafficked road (IS 458 + HS-25 loading). Less and the pipe cracks under truck loading; more and excavation cost balloons. Aim for 1.5-2.5 m cover for typical urban; up to 4 m at low points + outfalls.
Manhole spacing = function of cleaning equipment reach. 30 m for ≤ 600 mm pipe (rod + jet hose limit); 60 m for 600-1200 mm; 100 m for > 1200 mm where man-entry inspection is feasible. Mandatory at every junction, change of direction, change of slope, or change of diameter.
Drop manholes when invert drop > 600 mm — vertical drop pipe inside the manhole prevents splash erosion at the junction floor. Standard detail per IS 1742.
Material: RCC NP3 dominates Indian practice for 300-2400 mm — mature supply chain, IS 458 compliant, cost-effective. HDPE for low-cover or corrosive-soil situations. PVC for short laterals only. Brick arch for legacy systems only — replace where economically feasible.
Utility coordination is the silent killer. Water main + sewer main + gas + telecom + electrical all compete for the road corridor depth. Pre-construction utility survey + coordination meeting + clash detection mandatory. Most project delays + cost overruns trace back to undocumented utilities discovered during excavation.
Where this chapter sits: this is the deliverable chapter — it translates everything before into a tender-able BOQ. Pipe schedule, manhole schedule, alignment drawings + profiles, BOQ. The output of this chapter is what the contractor builds.