Methodology for delineating drainage catchments + sub-catchments — topographic survey, digital elevation models (DEM/SRTM), GIS-based catchment derivation, drainage district identification, imperviousness mapping, future build-out projection, integration with city master plan + land-use zoning.
Catchment characterisation is the spatial foundation of every drainage design. Get the catchment boundaries wrong and the design serves the wrong area; get the imperviousness wrong and you over- or under-size the entire network.
The modern workflow is GIS-based: start with a DEM (Digital Elevation Model), derive catchments using flow-accumulation + flow-direction algorithms in QGIS+GRASS or ArcGIS, then cross-check against existing drain alignments + a topographic walk-down. Free SRTM 30 m is screening-grade; project-grade designs need 5-10 m DEM (paid Cartosat) or LiDAR-derived 0.5-1 m raster (Smart Cities Mission datasets where available).
For critical infrastructure — railway underpass, metro entry, hospital — use LiDAR + topographic survey at sub-decimeter accuracy. The cost of getting the contour wrong at a low point is the cost of flooding it every monsoon.
Drainage districts are the planning units — typically 10-50 ha urban, 50-200 ha suburban. Each district has one outfall to a trunk drain or natural receiving water. Districts must be delineated by natural topography, not by administrative boundaries (wards, zones). Cross-jurisdictional coordination at master-plan stage is essential — fragmented planning = fragmented infrastructure.
Imperviousness mapping is where reality diverges from idealised land-use plans. The master plan says 40 % FAR + 30 % open space; the reality is 65 % paved + 15 % open + 5 % water-harvested + 15 % illegally encroached. Map current imperviousness from satellite (Sentinel-2 10 m free, Cartosat 2.5 m paid), and project future build-out using FAR × imperviousness factor (0.65 residential, 0.85 commercial). Add a 10-20 % margin for inevitable over-development.
Effective Imperviousness (EIA) — only the impervious area that drains directly to the storm drain — is the correct input for runoff calculation. Soakaway-drained patios, rain-garden-fed roof drains, and rainwater-harvested driveways don't add to peak flow. For low-density residential with mandatory RWH, EIA can be 10-20 % less than gross.
Where this chapter sits: catchment delineation + imperviousness mapping define the spatial inputs (A, C, sub-catchment topology) for runoff calculation in chapters 3-4. Get this chapter right and the rest of the design has a chance; get it wrong and no amount of clever hydraulics fixes it.