🌊 Flood Risk Analysis
Two independent flood hazards assessed for every transit station nationwide. FEMA floodplain risk identifies stations in or near mapped riverine and coastal flood zones. Stormwater risk identifies stations vulnerable to heavy rainfall accumulation based on terrain, impervious surface coverage, and station design — hazards that standard flood maps do not capture.
About Flood Risk Analysis
This tool assesses two distinct flood hazard types independently and presents them as complementary dimensions of flood risk rather than a single blended score. A station may face high risk from one hazard type but not the other — understanding which hazard applies and why is essential for effective adaptation planning.
The Two-Axis Framework
FEMA Floodplain Risk is drawn from the National Flood Hazard Layer (NFHL), FEMA's official regulatory flood mapping database. It captures risk from rivers overflowing their banks and coastal storm surge. Stations in FEMA Special Flood Hazard Areas (SFHA) face a 1% or greater annual chance of flooding from these sources.
Stormwater / Heavy Rain Risk is a composite score developed specifically for this atlas. It captures vulnerability to rainfall accumulation — water that collects at a station due to terrain geometry, sealed surfaces, and station design. This hazard type is largely absent from standard flood maps but is the primary cause of transit disruption during intense rain events in urban environments.
Stormwater Score Formula
| Component | Weight | What It Measures |
|---|---|---|
| Topographic | 40% | Maximum of: bowl depth index, slope energy index, or flat ponding index. The dominant terrain pathway wins. |
| Impervious Surface | 35% | Total parking + roadway area as % of buffer, normalized to national distribution. Includes interaction term for bowl + imperviousness. |
| Station Vulnerability | 25% | Below-grade (1.00), at-grade (0.57), elevated (0.34). Reflects how station design translates surface water into operational impact. |
The Four Terrain Dimensions
Terrain-driven stormwater risk is assessed along four conceptual dimensions: How far (total elevation relief in the buffer), How fast (maximum slope angle), How deep (the station's position within the buffer's elevation range), and From how many directions (bowl vs. directional slope vs. flat ponding). All four must combine unfavorably for a station to receive a high terrain score.
Data Sources
Terrain data: USGS 3D Elevation Program (3DEP) 10m DEM, ½-mile radius buffer. Flood zones: FEMA National Flood Hazard Layer (NFHL) via ArcGIS REST API, queried at station coordinates. Impervious surfaces: OpenStreetMap-derived parking and roadway surface areas. Station metadata: National Transit Database (NTD).
Limitations
The ½-mile terrain buffer uses a fixed circle centered on the station, which may not correspond to the actual hydrological watershed boundary. Waterfront stations with bay or ocean floor in their buffer may show anomalous terrain metrics. FEMA maps reflect current regulatory determinations and do not incorporate future sea-level rise or increased precipitation intensity. This tool is a screening instrument and should not substitute for site-specific engineering analysis.