Covers acquisition + analysis of rainfall data for design — IMD records, station selection, frequency analysis (Gumbel, log-Pearson III), Intensity-Duration-Frequency (IDF) curve construction, design storm hyetograph (uniform, triangular, Chicago method), areal reduction factor for catchments > 25 km², and city-specific rainfall intensities for major Indian cities.
Compute design rainfall intensity for a given duration + return period using Sherman's IDF equation i = a/(t+b)^n. Apply climate uplift per CPHEEO 2019 / AMRUT 2.0 (10–30 %). Constants a, b, n are city-specific — request from IMD Hydromet circle or use city PWD storm water manual.
Rainfall analysis is where the entire drainage design starts — get the IDF wrong and every downstream pipe size, inlet spacing, and detention pond is wrong. Yet this is exactly where most Indian municipal drainage designs cut corners.
The classical approach: Take ≥ 25 years of station rainfall, fit a Gumbel or log-Pearson III distribution to the annual maximum series, derive intensity for design return period, build an IDF curve via Sherman's equation. Use it to compute design intensity at duration = time of concentration. Standard textbook procedure.
The practical reality in India: Quality rainfall data is patchy. IMD has good daily data; sub-daily is paid + bureaucratic. Many cities use IDF curves derived in the 1960s-80s that no longer reflect current climate. Mumbai's July 2005 event (944 mm in 24 hours) and Chennai's December 2015 floods (over 1000 mm in a week) demonstrated that historical IDF curves dramatically under-predict current extremes.
Climate uplift is now standard professional practice. CPHEEO 2019 recommends 10-30 % uplift; coastal cities push the upper end. Smart Cities Mission projects fund new sub-daily rainfall analysis using modern records — leverage these where available.
Areal reduction matters for catchments > 25 km² — point intensity at the rain gauge always overstates the average intensity over a large area. Skip ARF and you oversize the trunk drain.
Design hyetograph choice matters for hydraulic modeling — Chicago method (peak at 1/3 of storm duration) better matches Indian convective storms than uniform rainfall. For simple Rational-method designs, single intensity at tc duration suffices.
Where this chapter sits: rainfall is the boundary condition for the whole drainage system. The IDF + design storm decisions made here propagate through every chapter that follows.