Capture of runoff from road surface into the buried storm sewer — inlet types (kerb opening, grated, combination, side-entry, top-entry), capacity equations, inlet spacing along longitudinal road profile, gutter flow analysis, by-pass flow, integration with road geometry (cross-fall, longitudinal slope, super-elevation), sediment + debris management.
Inlets are the most-failed component of urban drainage — and the most overlooked at design stage. A perfectly-sized pipe network with clogged or insufficient inlets is useless: water accumulates on the road surface instead of entering the buried system.
The physics: water flowing along the gutter (between kerb + driving lane) is captured by an inlet — a kerb opening (side-entry) for inconspicuous capture, a grated inlet (top-entry) for high-capacity capture, or a combination of both. The captured flow drops via a 200-300 mm lateral pipe to a manhole on the buried trunk drain.
Spread T is the design constraint: the width of water flowing along the gutter, measured perpendicular to the kerb. Max spread = 1.5 m on collectors (water doesn't encroach into the driving lane), 1.0 m on arterials, 0.5 m on underpasses (where any encroachment risks vehicle hydroplaning + standing water). Inlet spacing is sized so cumulative gutter flow never exceeds the capacity that limits spread to design max.
Clogging factor 0.50 — assume 50 % of grate area is blocked by leaves, plastic bags, sand, silt. This is not pessimism; it's reality in Indian cities. Designs that ignore clogging are why every monsoon brings news of flooded streets despite 'adequate' pipe network.
Inlet location rules: (1) every sag point in the road profile (water naturally accumulates), (2) every intersection (don't let upstream water cross), (3) every change of grade (transition water concentration), (4) uniform spacing along longitudinal profile (controlled by spread limit).
Inlet types:
- Kerb opening (side-entry): inconspicuous, no traffic obstruction, limited capacity. Best for low-medium-flow residential streets.
- Grated (top-entry): high capacity, vibration risk, bicycle/pedestrian safety concern. Use for arterial + high-flow.
- Combination (kerb + grated): max capture, used at sags + intersections + critical points.
O&M reality: annual cleaning + jetting is mandatory. ULBs that skip this lose 50-70 % effective capacity within 3-5 years. Smart inlet covers with anti-theft + flood-level sensors are emerging through Smart Cities Mission funding — ₹15-30K per smart inlet vs ₹3-5K for conventional cast-iron grate.
The standards: IS 13979 covers gully pit / catch basin specification; IS 5961 covers cast-iron grates + frames; CPHEEO 2019 + IRC SP 50 give the design methodology.
Where this chapter sits: inlets are the interface between surface (road) and subsurface (storm sewer). Get inlet design + spacing right and the buried system actually receives the runoff it was sized for; get it wrong and the buried network is academic — water still ponds on the road.