FloodReady Delhi

Rainfall exceeds drain conveyance capacity. IIT Delhi DMP found 3,854 flooded junctions in Najafgarh basin alone under a 2-year return period storm (Scenario 1, Table 3.3-3). This is the most frequent mechanism in Delhi.

C_eff — Effective drain capacity (Manning's n: 0.014 impervious, 0.012 RCC box, 0.025 open drain; Table 4.1-3). Reduced by Horton soil saturation factor (f_∞ = 0.635 mm/hr for Group D loam; Table 4.3-1).
R_peak — Peak 3-hour rainfall intensity (IDF-based; IIT Delhi Ch 4.4).
T_runoff — Terrain runoff multiplier from imperviousness (NCR Functional Drainage Plan Table 5.4-5.5).

Yamuna overtops embankments or submerges drain outfalls. CWC 2023: stage 208.66 m flooded Civil Lines, Mayur Vihar, Patparganj entirely — different mechanism from rain. IIT Delhi DMP: “final disposal of majority of storm water is into Yamuna through outfall points” (Ch 1.2).

Elevation — SRTM v4.1 ward mean elevation. Full penalty ≤207 m (Yamuna danger zone), zero ≥215 m, linear taper.
Terrain class — IIT Delhi basin delineation: floodplain (Trans-Yamuna alluvial clay), low, flat, ridge (Delhi Ridge/Aravalli).
Yamuna discharge — CWC Hathnikund gauge: Normal <100k cusecs, Danger >300k cusecs (48-72h travel time to Delhi).

The critical interaction: high Yamuna submerges drain outfalls, stormwater has nowhere to go. Even 20 mm/hr rain causes severe flooding. The July 2023 disaster was compound — heavy local rain AND Yamuna in flood simultaneously.

DMP Executive Summary: “storm water has to be managed so it does not interfere with drainage.” SWMM modelling shows junction flooding increases 3-5× when outfalls are submerged. Multiplicative interaction ensures this component only activates when BOTH rain AND river are elevated AND ward is low-lying.