🌊 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 risk rather than a single blended score. A station may face high risk from one hazard type but not the other — understanding which applies and why is essential for effective adaptation planning.
What We Measure
Every station in the Atlas is evaluated on two independent flood hazard dimensions, presented together as a 2×2 risk matrix.
FEMA Floodplain Risk is drawn from the National Flood Hazard Layer (NFHL), FEMA's official regulatory flood mapping database. It captures risk from riverine overflow and coastal storm surge. Each station is assigned the flood zone designation at its coordinates — ranging from Zone A and AE (high-risk Special Flood Hazard Areas with a 1% or greater annual flood probability) through Zone X (minimal risk). The analysis also records whether the station falls within NFHL coverage at all, since significant portions of the country remain unmapped.
Stormwater / Heavy Rain Risk is a composite score developed specifically for this Atlas. It captures vulnerability to rainfall accumulation — water that pools at or enters a station due to terrain geometry, the volume of sealed surfaces in the surrounding area, and the station's physical design. This hazard is largely absent from standard flood maps but is the primary driver of transit disruption during intense rain events in urban environments. Scores range from 0 to 100 and are normalized against the national distribution of all 5,100+ stations.
How the Analysis Works
Flood data is pre-computed for every station and stored in two source files that are merged at query time: stormwater risk scores derived from terrain and surface analysis, and FEMA zone assignments queried at station coordinates.
Stormwater Composite Score — The score is a weighted sum of three components, each normalized to a 0–1 scale before weighting:
| Component | Weight | What It Measures |
|---|---|---|
| Topographic | 40% | The dominant terrain pathway wins: maximum of the bowl depth index (station sits in a depression relative to its surroundings), the slope energy index (water drains toward the station from higher ground), or the flat ponding index (near-level terrain with no drainage outlet). |
| Impervious Surface | 35% | Total parking, roadway, and building footprint area as a share of the 800-meter buffer, normalized to the national distribution. An interaction term amplifies the score when high imperviousness combines with a bowl terrain geometry — the worst-case pairing for runoff accumulation. |
| Station Vulnerability | 25% | Reflects how station design translates surface water into operational impact: below-grade stations (underground) score 1.00, at-grade 0.57, elevated 0.34. An underground station in a topographic bowl surrounded by pavement represents the highest possible combined risk. |
Terrain Dimensions — The topographic component is assessed along four conceptual axes: How far (total elevation relief within the buffer), How fast (maximum slope angle), How deep (the station's position within the buffer's full elevation range — a low position_ratio indicates the station sits near the bottom of its local terrain), and From how many directions (bowl convergence vs. directional channeling vs. flat ponding). All four must combine unfavorably for a station to receive a high terrain score.
The 2×2 Matrix — Each station is placed in one of four quadrants by crossing its stormwater score (low / high, split at score 50) against its FEMA status (in or out of a Special Flood Hazard Area). The quadrant placement captures the nature of the risk: a station in the upper-right faces compounding hazards from both sources; upper-left faces stormwater risk that standard flood maps miss entirely; lower-right is mapped for floodplain risk but may be underestimated for rainfall events; lower-left has the lowest overall flood exposure.
Data sources: USGS 3D Elevation Program (3DEP) 10-meter DEM for terrain metrics; FEMA National Flood Hazard Layer (NFHL) via ArcGIS REST API for flood zone designations; OpenStreetMap-derived polygons for parking, roadway, and building footprint areas; National Transit Database (NTD) for station type and facility classification.
Why This Matters
Transit infrastructure faces a compounding flood problem that standard risk frameworks are poorly suited to capture. Research on subway system flooding finds that below-grade stations are uniquely vulnerable: a single inundation event can simultaneously disable fare equipment, traction power, signal systems, and communications, causing disruptions that cascade across an entire network. A 2025 Washington Post analysis found New York City subway service was disrupted by flooding at least 75 times between January 2020 and September 2025 alone.
The gap between FEMA floodplain mapping and actual urban flood risk is the central problem this analysis addresses. FEMA's National Flood Hazard Layer was designed to identify riverine and coastal surge risk for insurance and land-use regulation purposes — not to assess how rainfall behaves on impervious urban surfaces during intense storms. Studies of urban subway flood risk consistently find that stormwater intrusion from surface runoff — driven by terrain geometry and impervious surface density — is the mechanism most responsible for operational disruptions, yet it is the hazard least visible in standard regulatory maps.
The equity dimension compounds the planning challenge. Research on transit accessibility under flooding finds that socioeconomically vulnerable populations — those least likely to have access to a private vehicle — are disproportionately reliant on fixed-route transit and therefore bear a greater share of the disruption burden when stations flood. Unlike cars, fixed-route transit cannot detour around flooded infrastructure. Identifying which stations face the highest risk, and what type of risk, is a prerequisite for equitable climate adaptation planning.
Important Notes and Caveats
- The stormwater terrain buffer uses a fixed ½-mile circle centered on the station coordinate. This may not correspond to the actual hydrological watershed boundary, which can be larger, smaller, or asymmetric depending on local topography.
- Waterfront stations with open water (bay, river, or ocean) within their buffer may show anomalous terrain metrics, particularly for elevation range and slope values, because bathymetric elevation is included in the DEM.
- FEMA flood zone designations reflect current regulatory determinations based on historical hydrology. They do not incorporate future sea-level rise projections or increased precipitation intensity under climate change scenarios. A station in Zone X today may face materially higher floodplain risk by mid-century.
- Significant portions of the country are not yet covered by the NFHL. Where NFHL coverage is absent, the FEMA risk field is marked accordingly — the absence of a flood zone designation does not mean the area is low-risk.
- The stormwater score is a screening instrument based on static physical characteristics. It does not model actual storm events, drainage capacity, or subsurface infrastructure. It should not substitute for site-specific hydrological engineering analysis when decisions about capital investment are at stake.
- Impervious surface data is derived from OpenStreetMap and may be incomplete or outdated in areas with sparse OSM coverage, particularly in smaller cities and rural transit systems.