Establishes the framework for designing urban storm water drainage systems — design return periods by land use, design philosophy (separate vs combined), planning horizon, climate-resilient sizing, integration with sewerage and SUDS. Covers institutional framework (ULB, AMRUT 2.0, Smart Cities Mission) and outlines the difference between conveyance-based and source-control approaches.
Storm water drainage is one of the most under-invested municipal services in Indian cities — and one of the most visible when it fails. Every monsoon brings news of waterlogged underpasses, marooned colonies, and traffic chaos. The CPHEEO Manual on Storm Water Drainage Systems (2019, the first dedicated edition) finally gave engineers a unified design framework after decades of mixing IRC SP 50, IS codes, and ad-hoc municipal practice.
The core design decision is the return period — how often the system is allowed to be overwhelmed. A 2-year design accepts overflow every monsoon for residential streets where flooding is inconvenient but not catastrophic; a 100-year design protects underpasses, hospitals, and substations where failure costs lives or critical service. Pick the return period before you size anything — overdesigning a residential drain wastes ₹crores; underdesigning a metro underpass kills people.
The separate vs combined debate is largely settled in CPHEEO 2019 — separate is the future. Combined sewers were a 19th-century convenience that became a 21st-century liability: they overwhelm sewage treatment plants in monsoon, cause combined sewer overflows that pollute receiving waters, and complicate O&M. AMRUT 2.0 explicitly funds separation in legacy areas, though the ₹3-8 crore/km cost means it'll take decades.
Climate uplift is the new normal. Cities that historically saw 50-60 mm/hr peak intensities now record 80-120 mm/hr events almost every monsoon. Designing to 1980s IDF curves is professional negligence in 2026 — apply at least 10 % uplift, more for coastal/cyclone zones. The CPHEEO 2019 manual is explicit on this; AMRUT 2.0 reinforces it.
Where this chapter sits: it's the first decision in any drainage project — return period × climate uplift × separation philosophy define the entire downstream design. Get this wrong and no amount of clever hydraulic detail saves you.