What This Analysis Does
This analysis evaluates climate risk across all 335 New York City Housing Authority (NYCHA) developments. It combines information about environmental hazards (flooding and extreme heat), the physical and demographic vulnerability of each property, and existing mitigation measures to produce a composite Risk Index that helps prioritize climate adaptation investments.
Each property receives a score from 0 to 1 for every metric, where higher values indicate greater hazard, vulnerability, or risk. Properties are then ranked and classified into HIGH, MEDIUM, and LOW tiers (thirds of the ranked list).
Each development is also classified by its physical typology: High-Rise or Low-Rise (based on a mean building height threshold of 60 feet, approximately 6 stories) and Campus or Building (based on whether the development contains multiple buildings or a single structure).
The Risk Framework
Risk is calculated using a standard framework adapted from climate adaptation planning:
Risk = Hazard Exposure × Vulnerability − Mitigation
This means a property can have high hazard exposure but lower risk if its buildings are less vulnerable or if mitigation measures are already in place.
Hazard Exposure
×
Vulnerability
−
Mitigation
=
Risk
This formula is applied independently for each of four climate hazard domains, and the results are averaged into a single Risk Index:
Coastal & Tidal Flooding
Storm surge, sea level rise, and tidal inundation from the ocean and tidal waterways.
Stormwater Flooding
Inland flooding from overwhelmed drainage systems during heavy rainfall.
Groundwater Flooding
Rising water tables that can damage foundations and below-grade infrastructure.
Extreme Heat
Urban heat island effects from solar radiation, building density, and impervious surfaces.
The sections below explain each component of the framework — Hazard, Vulnerability, Mitigation, and Risk — broken down by domain.
How Scores Are Calculated
Every data input is first converted to a 0–1 scale so that different types of information (yes/no flags, numeric measurements, categorical timelines) can be compared and combined:
- Yes/No data (e.g., "Is this property in a flood zone?"): Yes = 1, No = 0
- Numeric data (e.g., groundwater depth): Scaled so the worst value across all properties = 1 and best = 0
- Timeline data (e.g., tidal flooding): Sooner onset scores higher (2050s = 1.0, 2080s = 0.67, 2100 = 0.33, no onset = 0)
These normalized inputs are then combined using weighted averages into sub-indices, and sub-indices are combined into the four main indices described below. A complete listing of all data inputs, encoding methods, and weights is available in the Technical Documentation.
Hazard Hazard Index
The Hazard Index measures how exposed each property is to climate-related hazards, regardless of building condition or mitigation. It is an equal-weighted average (25% each) of four domain sub-indices:
Coastal & Tidal Flooding
Combines four inputs with weights reflecting how certain and imminent each data source is:
- Tidal flooding timeline (35%) — when tidal flooding is projected to begin, per the New York City Panel on Climate Change (NPCC) 2023 mean higher high water flood extents (90th-percentile SLR). Earlier onset scores higher: 2050s = 1.0, 2080s = 0.67, 2100 = 0.33, no projected onset = 0.
- Current FEMA floodplain (30%) — whether any part of the property overlaps a FEMA PFIRM Special Flood Hazard Area (SFHA) or 500-year flood zone, determined from the FEMA National Flood Hazard Layer (NFHL) API.
- 2050 projected floodplain (20%) — whether the property intersects the NPCC-projected 2050 coastal surge zone.
- 2080 projected floodplain (15%) — whether the property intersects the NPCC-projected 2080 coastal surge zone.
- Source: New York City Panel on Climate Change (NPCC) sea-level-rise and coastal flood projections for tidal onset and 2050/2080 coastal surge; FEMA NFHL API for the current regulatory floodplain.
Stormwater Flooding
Combines three time-horizon scenarios, weighted toward current conditions:
- Current stormwater flooding (60%) — is any part of the property in a stormwater flood zone today?
- 2050 moderate scenario (30%) — projected stormwater flooding under a moderate climate scenario.
- 2080 extreme scenario (10%) — projected stormwater flooding under an extreme climate scenario.
- Source: NYC Department of Environmental Protection (DEP) Stormwater Flood Maps.
Groundwater Flooding
Based on a single measurement: depth to groundwater at each property. Shallower water tables mean higher hazard. Depths of 16 feet or more are treated as having no groundwater hazard (based on USGS HERA thresholds). The depth value is inverted and scaled so that the shallowest property scores 1 and the deepest scores 0.
Extreme Heat
Combines three physical characteristics of each property:
- Ground-level solar radiation area (70%) — how much of the property receives direct solar radiation, measured in square feet. Larger exposed areas mean more heat.
- Mean building height (20%) — taller buildings trap heat in urban canyons and radiate stored heat.
- Total roof area (10%) — larger roof surfaces absorb and re-radiate more solar energy.
Vulnerability Vulnerability Index
The Vulnerability Index measures how susceptible each property is to damage or harm from the hazards it faces. It is an equal-weighted average (0.25 each) of four domain sub-indices: Coastal/Tidal, Stormwater, Groundwater, and Heat. Groundwater and Heat vulnerability are only evaluated when the corresponding hazard is present — if a property has no exposure, its vulnerability for that domain shows as “Not applicable” and is excluded from the per-site average.
Coastal & Tidal Flood Vulnerability
Vulnerability is measured by two factors that together capture how many buildings on a property are at risk from coastal flooding:
- Inhabited floors below coastal DFE (67% weight) — the number of buildings with their first inhabited floor below the Design Flood Elevation. This captures direct risk to residents.
- Basements below coastal DFE (33% weight) — the number of buildings with estimated basement elevation below the DFE. This captures risk to building mechanical systems, electrical infrastructure, and storage even when inhabited floors remain above flood level.
The DFE is derived from the baseline flood elevation (BFE) — the highest ground elevation within the flood zone at the property, determined by intersecting FEMA flood zone polygons (from the NFHL API) or coastal surge polygons with the development boundary, then sampling a high-resolution LiDAR elevation model — plus a freeboard buffer of 40 inches (3.3 ft):
DFE = BFE + 40″ freeboard
Basement elevation is estimated as grade elevation minus 10 feet, a conservative assumption applied universally pending availability of site-specific data.
Stormwater Flood Vulnerability
Uses the same two-factor approach as coastal vulnerability:
- Inhabited floors below stormwater DFE (67% weight) — buildings with first inhabited floor below the stormwater flood elevation.
- Basements below stormwater DFE (33% weight) — buildings with estimated basement below the stormwater flood elevation.
The stormwater DFE uses a freeboard buffer of 24 inches (2 ft) above the BFE:
DFE = BFE + 24″ freeboard
More buildings below these levels = higher vulnerability.
Groundwater Vulnerability
Building elevation data cannot reliably capture below-grade infrastructure condition, so groundwater vulnerability is estimated from building age as a proxy. Only properties with actual groundwater hazard (depth shallower than 16 feet) receive a vulnerability score; properties over deep groundwater are marked “Not applicable.”
- Building age (100% weight) — older buildings are more likely to have deteriorated waterproofing, outdated drainage systems, and foundation systems vulnerable to groundwater intrusion.
Heat Vulnerability
Based on resident demographics, since heat-related health impacts disproportionately affect certain age groups:
- Senior population (70% weight) — percentage of residents aged 62 and over. Seniors face the highest risk of heat-related illness.
- Youth population (30% weight) — percentage of residents under 18. Children are also more vulnerable to extreme heat.
Mitigation Mitigation Index
The Mitigation Index measures what protective measures are already in place at each property. Higher mitigation scores mean more protection is present, which reduces the final risk score. Mitigation is only evaluated for properties exposed to the corresponding hazard.
Coastal & Tidal Flood Mitigation
Three factors, with post-Sandy resilience investments weighted most heavily:
- Sandy recovery and resiliency work (45%) — whether the property received infrastructure improvements through the post-Hurricane Sandy recovery program.
- IDA recovery work (30%) — whether the property received infrastructure improvements through the post-Hurricane Ida recovery program.
- Elevated or protected critical equipment (25%) — whether boilers, electrical systems, and other critical infrastructure have been raised above flood levels or otherwise protected. Scored as full, partial, or none.
Stormwater Flood Mitigation
Four factors combining post-disaster resilience work, dedicated stormwater programs, and cross-domain credit from coastal mitigation:
- Sandy recovery cross-domain credit (40%) — comprehensive flood resilience work also benefits stormwater management.
- IDA recovery cross-domain credit (25%) — IDA recovery investments similarly provide stormwater resilience benefits.
- Cloudburst site (20%) — whether the property is part of NYC’s cloudburst management program, with stormwater retention and conveyance infrastructure for extreme rainfall.
- Critical equipment protection credit (15%) — elevated equipment also protects against stormwater damage.
Groundwater Mitigation
Three factors that reduce vulnerability to chronic groundwater intrusion:
- Sandy resiliency retrofits (45%) — comprehensive resilience work that included below-grade waterproofing, foundation repairs, and drainage improvements which reduce groundwater intrusion in addition to flood resilience.
- IDA recovery work (30%) — post-Ida recovery addressed basement flooding and sewer-backup failures that correlate strongly with shallow groundwater.
- Critical equipment protection (25%) — elevating boilers, electrical switchgear, and other critical building systems above grade protects against chronic groundwater seepage and rising water tables.
Heat Mitigation
Two equally weighted (50/50) factors:
- Cool roof — whether the property has had reflective roofing installed, which reduces heat absorption.
- On-site cooling centers — whether an on-campus cooling center is present (union of DFTA senior centers and DYCD community/youth centers), providing residents with a refuge during heat emergencies without off-site travel.
Risk Risk Index
The final Risk Index combines hazard, vulnerability, and mitigation into a single score for each property. Rather than simply multiplying the three overall indices together, risk is calculated independently for each domain and then averaged. This ensures that each type of climate hazard contributes meaningfully to the final score.
How Domain-Level Risk Is Calculated
For each of the four domains:
- Raw risk = Hazard sub-index × Vulnerability sub-index
- Adjusted risk = Raw risk × (1 − Effectiveness × Mitigation score)
The mitigation effectiveness controls how much protection mitigation can provide. It varies by domain to reflect the real-world impact of different types of interventions:
| Domain | Max Mitigation Reduction | Rationale |
|---|
| Coastal/Tidal |
90% |
Engineered flood barriers and elevated equipment are highly effective at preventing flood damage |
| Stormwater |
90% |
Green infrastructure, cloudburst measures, and resilience investments significantly reduce stormwater impacts |
| Groundwater |
90% |
Below-grade waterproofing, foundation repairs, elevated critical equipment, and post-Ida drainage retrofits substantially reduce chronic groundwater intrusion impacts |
| Heat |
60% |
Cool roofs and cooling centers reduce heat risk but cannot fully eliminate exposure |
The four domain-level adjusted risks are then averaged equally (25% each) to produce the final Risk Index.
HIGH / MEDIUM / LOW Classification
Properties are ranked from 1 (highest/worst) to N (lowest/best) for the Risk Index and each sub-index. Rankings are divided into three tiers:
- HIGH — top third of ranked properties
- MEDIUM — middle third
- LOW — bottom third
For flood-related sub-indices (coastal, stormwater, and groundwater), only properties with actual hazard exposure are ranked. Properties with zero exposure are excluded from the ranking entirely and shown as "N/A" rather than being given a misleading low rank.
What this means in practice: A property with high coastal flood hazard but strong Sandy- or IDA-funded mitigation may see its coastal risk reduced by up to 90%. Groundwater risk is similarly eligible for up to a 90% reduction where Sandy, IDA, or elevated-equipment mitigation is in place. Heat mitigation is capped at 60% because cool roofs and cooling centers cannot fully eliminate heat exposure. The final Risk Index reflects the combined picture across all four domains.
Conditional Logic
Vulnerability and mitigation scores are only evaluated when the corresponding hazard is actually present at a property. For example:
- A property with no coastal flood exposure does not receive a coastal vulnerability or mitigation score — it shows as "Not applicable" in the map.
- A property with no stormwater flooding in any time horizon is not assessed for stormwater mitigation.
- Heat vulnerability and mitigation are evaluated for any property with solar radiation exposure, building height, or roof area data (which in practice is all properties).
This prevents properties from appearing artificially low-risk simply because they lack the infrastructure to evaluate for a hazard they do not face.
Key Assumptions and Limitations
Core Assumptions
- The model prioritizes relative comparison rather than absolute risk prediction. All scores are normalized across the 335 NYCHA developments — they rank properties against each other, not against an external safety standard. A "LOW" risk property is low relative to other NYCHA properties, not necessarily low in absolute terms.
- Equal domain weighting. Each of the four hazard domains (coastal, stormwater, groundwater, heat) contributes equally (25%) to the final Risk Index. This assumes no single hazard type is inherently more important than another.
- Hazard-present gating. Vulnerability and mitigation are only assessed when a property has exposure to the corresponding hazard. Properties without exposure receive "Not applicable" rather than a zero score.
- Mitigation effectiveness caps. Even with perfect mitigation, some residual risk remains. Engineered flood protection (coastal, stormwater, groundwater) can reduce risk by up to 90%; heat mitigation by up to 60%. These caps reflect the practical limits of current mitigation strategies.
- Freeboard buffers. The design flood elevation (DFE) includes a safety buffer above the baseline flood elevation (BFE): 40 inches for coastal flooding and 24 inches for stormwater flooding.
- Groundwater depth cap. Groundwater depths of 16 feet or more are treated as posing no hazard, based on the USGS HERA (Hazard Exposure Reporting and Analytics) threshold.
- Groundwater vulnerability proxy. Because direct below-grade infrastructure condition data is not available, groundwater vulnerability is estimated from building age as a proxy.
- Basement depth assumption. Every building is assumed to have a basement 10 feet below grade elevation. This universal assumption will be refined when site-specific basement data becomes available from NYCHA engineering teams.
- Vulnerability floor. Properties exposed to coastal, stormwater, or groundwater flooding but with no buildings below the design flood elevation (or no building-level vulnerability signal) still receive a minimum vulnerability score (0.1 on the 0–1 scale). This reflects site-level risks — flooding can damage grounds, disrupt access, and affect infrastructure even when building interiors remain above flood level.
- Current and near-term data is weighted more heavily than distant projections within each hazard sub-index (e.g., current stormwater flooding at 60% vs. 2080 at 10%).
Limitations
- Groundwater depth data is interpolated from monitoring wells and may not capture localized conditions.
- Mitigation data reflects known programs (Sandy recovery, IDA recovery, Cloudburst, elevated critical equipment, Cool Roofs, DFTA/DYCD Cooling Centers) and may not capture all improvements made at individual properties.
- Future flood projections are based on climate models with inherent uncertainty.
- The analysis does not account for temporary or emergency protective measures.
Outputs
The analysis produces two primary deliverables:
- Interactive webmap — an HTML portfolio map showing all 335 developments color-coded by risk level, with clickable popups displaying index breakdowns, rankings, and hazard layer overlays.
- Site-level PDF reports — individual climate risk reports for each development, available through the webmap. Each report includes:
- Coastal and stormwater BFE/DFE per development
- Risk exposure level per hazard domain
- Baseline and betterment mitigation action recommendations per hazard, provided directly by NYCHA
Updating the Analysis
The analysis pipeline is designed to be re-run as conditions change. Input datasets can be replaced in the project directory and the pipeline re-executed to produce updated results. Anticipated update scenarios include:
- Revised climate projections — updated flood maps, stormwater models, or sea level rise scenarios can replace existing raster and vector inputs.
- Completed mitigation projects — as new Sandy, IDA, Cloudburst, Cool Roof, critical-equipment elevation, or cooling-center work is completed, the source data can be updated to reflect current mitigation status.
- Expanded risk factors — new hazard domains, vulnerability metrics, or mitigation programs can be added to the configuration and data pipeline.
- Revised mitigation recommendations — the baseline and betterment action recommendations in the PDF reports can be updated as NYCHA's standards and strategies evolve.
Specific procedures for each of these update types are documented in the Technical Documentation.