Context
A rapidly urbanizing delta city faced worsening pluvial flooding despite an existing but undersized stormwater network. The municipal authority needed a technically defensible way to prioritize drainage investment across 14 sub-catchments.
Approach
We built a coupled EPA SWMM (network) and HEC-RAS 2D (overland) model calibrated against three historic flood events using CCTV-derived depth observations and resident-reported flood extents. The model was run against 5-year, 25-year, and 100-year design storms under both current and 2050 projected rainfall intensities.
Outcome
The resulting priority ranking directly informed the phasing of the city's $12M drainage upgrade program, with the two highest-risk sub-catchments moved into Phase 1 construction.
Methodology
DEM preprocessing from 5m LiDAR data; EPA SWMM network model construction from municipal GIS drainage records; HEC-RAS 2D overland coupling for surcharge and floodplain interaction; calibration against 3 historic flood events using CCTV depth observations and crowd-sourced flood reports; design storm simulation at 5/25/100-year return periods under current and 2050 rainfall projections (CMIP6 SSP2-4.5).
Results & Outcomes
- Ranked all 14 sub-catchments by flood risk and cost-per-hectare-protected
- Identified 2 sub-catchments responsible for 61% of modeled flood damage
- Directly informed phasing of a $12M municipal drainage upgrade program
- Reduced flood-extent mapping turnaround from an estimated 6 weeks to 9 days
Challenges
Limited historical gauge data required calibration against indirect evidence — CCTV footage timestamps and resident-reported flood depths — cross-validated against two independent events to build confidence in the calibration before proceeding to design-storm simulation.
“GeoVeris delivered a basin-wide flood hazard map that our field teams actually use during monsoon season — not just a report that sits on a shelf.”
Daniel Cruz, Program Director
A Regional Disaster Management Authority