Flood Index
Layers
Event
By Island
Vulnerability Analysis
Flood Vulnerability Heatmap
9-factor FVI: iron oxide, event impact, evac zones & more
FVI vs FEMA: discrepancy map
Where the 9-factor model and FEMA zones agree or diverge
March 2026 Event: Hardest-Hit Areas
Note: All layers in this section are based on news reports, NWS bulletins, and Hawaii Civil Defense press releases. Not official GIS datasets. Locations are approximate.
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Flood impact zones
Communities with documented flooding, rescues, or damage
Source: NWS HFO, NPR, Hawaii News Now, NASA Earth Observatory
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Evacuation zones (March 20 to 22)
Areas under evacuation order or warning
Source: Hawaii Civil Defense press releases (approximate polygons)
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Dam risk site: Wahiawa
120-year-old earthen dam, 5,500 evacuated
Source: Gov. Green press release, NPR, Hawaii News Now
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Major rescue sites
Documented mass rescue locations
Source: Honolulu Fire Dept, National Guard (news reports)
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Extreme rainfall areas
>10" recorded March 10–24
Source: NWS HFO storm summaries, UH Hawaii Mesonet
Hydrology & Water Flow (Real Data)
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Waterway network (NHD+ HR)
787 real stream flowlines on O'ahu
Source: USGS NHDPlus High Resolution
Elevation zones
Source, transition, and accumulation areas for flood flow
Based on USGS DEM control points. See FVI methodology note.
Watershed boundaries (WBD HUC-10)
62 real watershed units across all Hawaii islands
Source: USGS Watershed Boundary Dataset
Potential Contamination Sources
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Contamination hazards
Septic, landfills, industrial, agricultural runoff, USTs
Environmental Catalysts & Risk Factors
Pineapple soils (high iron oxide)
USDA/NRCS: 24 to 30% Fe₂O₃, impacts infiltration & erosion
Atmospheric rivers (IVT forecasts)
CW3E/UCSD moisture transport events
Population exposure
2020 Census density & flood risk areas
Impervious surface coverage
NLCD urban runoff multiplier
FEMA & Hazard Zones (Real Data)
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FEMA Special Flood Hazard Areas
10,361 real DFIRM polygons across all Hawaii islands
Source: FEMA NFHL, DFIRM Dec 2025 (hazards.fema.gov)
Sea level rise coastal flood (3.2 ft)
HAZUS model, UH Coastal Geology
Approximate. See methodology note below.
Data sources:
· Hawaii Statewide GIS Program (geoportal.hawaii.gov)
· USGS National Map (elevation/hydro)
· NWS Honolulu & Hawaii Mesonet (precipitation)
· NASA Earth Observatory (satellite imagery)
· Hawaii DLNR (watersheds, OSDS, environmental data)
· FEMA & Esri GIS Hub (response mapping)
· CW3E/UCSD (Atmospheric rivers & IVT)
· NLCD/MRLC (Impervious surface)
· 2020 Census (Population)
· USDA/NRCS Soil Survey (Soils)
Contamination & environmental catalysts: OSDS, landfills, industrial facilities, agricultural runoff, USTs, Pineapple soils with high iron oxide content, atmospheric river data from CW3E, population exposure analysis.
· Hawaii Statewide GIS Program (geoportal.hawaii.gov)
· USGS National Map (elevation/hydro)
· NWS Honolulu & Hawaii Mesonet (precipitation)
· NASA Earth Observatory (satellite imagery)
· Hawaii DLNR (watersheds, OSDS, environmental data)
· FEMA & Esri GIS Hub (response mapping)
· CW3E/UCSD (Atmospheric rivers & IVT)
· NLCD/MRLC (Impervious surface)
· 2020 Census (Population)
· USDA/NRCS Soil Survey (Soils)
Contamination & environmental catalysts: OSDS, landfills, industrial facilities, agricultural runoff, USTs, Pineapple soils with high iron oxide content, atmospheric river data from CW3E, population exposure analysis.
Flood Risk Index
Mapping why O'ahu floods, and what changes it
Show flood risk heatmap
Color-coded grid: blue = low risk, purple = extreme risk
Stream proximity: loading real NHD+ stream network…
9 Factors That Drive Flood Risk · Total: 100%
Drag any slider to change how much each factor influences the score. Hit Recalculate when done.
15%
Low-lying land floods first. Coastal plains and valley floors like Waialua, Kailua, and Manoa score high. Mountain ridges score near zero. Elevation is determined by the landscape, not land use.
7%
Flat ground ponds water (high risk). Valley floors and coastal plains are almost completely flat. Steep slopes drain fast but can generate powerful flash floods in the valleys below them.
15%
O'ahu ranges from 18 inches per year (dry leeward Ewa side) to 200+ inches per year on the windward Ko'olau ridge. More rainfall means more frequent and more intense flood events. Climate change is making Kona Low storms more intense and more frequent.
12%
Pavement and rooftops block rain from soaking into the ground. Downtown Honolulu is 75 to 85% covered by impervious surfaces, meaning nearly all rainfall runs off into streets and streams instead of soaking in. Each new development that paves over open land increases downstream flood risk for existing neighborhoods.
11%
The USDA (United States Department of Agriculture) rates soils A through D. A soils drain fast and absorb rain well. D soils drain very slowly and saturate quickly, so even moderate rain causes runoff. Coastal lowland soils are almost all D-class. Healthy native soils and lo'i kalo (taro) systems can improve drainage over time.
15%
How close you are to the nearest stream channel. Land within 0.5 km scores near 1 (highest risk); beyond 8 km scores near 0. Kaukonahua Stream overflowed its banks in Waialua during the March 2026 event, sweeping away homes and cars. Traditional ahupua'a systems managed these same streams with terraced lo'i kalo that absorbed peak flows before they could flood communities.
10%
Former pineapple plantation soils (Waialua and Ala series) contain 18 to 28% iron oxide. Decades of heavy machinery compacted these soils into a hardpan layer just below the surface that blocks water from draining down. Rain instead moves sideways into streams 30 to 50% faster than in healthy soils. This is a direct, measurable legacy of Dole and Del Monte pineapple operations that ended decades ago but left behind altered soils. Native ground cover and lo'i kalo restoration have been shown to break up this hardpan over time.
10%
The reality check layer: areas where flooding, structural damage, or rescues were actually documented during the March 2026 Kona Low storms. A high score here means real flood events have already confirmed what the model predicts. These areas need the most urgent policy attention.
5%
Whether this area fell under an official civil-defense evacuation order March 20 to 22, 2026. The Wahiawa Dam, built in 1906 to supply water for sugar cane plantations, threatened to overflow during the storm. Infrastructure designed for a plantation economy was not built to protect communities from climate-intensified storms. 5,500 people were evacuated.
Live formula
Reading the Colors
0.00 to 0.30 · Low risk
Higher ground, fast-draining soils, or far from streams. Little flood history. Generally safe under most storm conditions.
Higher ground, fast-draining soils, or far from streams. Little flood history. Generally safe under most storm conditions.
0.30 to 0.45 · Moderate risk
Some combination of risk factors present. Flooding is possible in intense storms. Flood insurance worth considering.
Some combination of risk factors present. Flooding is possible in intense storms. Flood insurance worth considering.
0.45 to 0.60 · High risk
Multiple compounding factors: low elevation, poor soil drainage, and stream proximity. Flood insurance strongly recommended. These areas appear regularly in NWS flood advisories.
Multiple compounding factors: low elevation, poor soil drainage, and stream proximity. Flood insurance strongly recommended. These areas appear regularly in NWS flood advisories.
0.60 to 0.75 · Very High risk
Areas with documented flooding history where multiple severe factors stack together. Often former plantation land or dense urban valleys. Active policy intervention is warranted here.
Areas with documented flooding history where multiple severe factors stack together. Often former plantation land or dense urban valleys. Active policy intervention is warranted here.
0.75 to 1.00 · Extreme risk
Areas that actually flooded in March 2026 and sit inside official evacuation zones. These zones are the strongest case for flood mitigation funding, infrastructure investment, and Indigenous land restoration programs.
Areas that actually flooded in March 2026 and sit inside official evacuation zones. These zones are the strongest case for flood mitigation funding, infrastructure investment, and Indigenous land restoration programs.
Policy Scenarios: What If?
Each scenario models what happens to flood risk when land use or climate changes. Click one, then hit Recalculate to see the shift across O'ahu.
Start here
Indigenous land restoration
Restore loʻi kalo (taro wetlands), break up plantation iron-hardpan through native plantings, return ahupuaʻa watershed management. Based on measured runoff reductions at restored sites in Hawaii.
Current conditions (2026 baseline)
Today's land cover, soils, and development patterns. Compare against the restoration scenarios.
Native forest and watershed restoration
Restore native forest in upper watersheds above 300 ft elevation. Reduces runoff and improves soil infiltration.
Urban expansion (+20% impervious)
Continued development in Ewa/Kapolei corridors. Each paved acre increases flood risk downstream.
Sea level rise +3.2 ft
Raised groundwater and reduced coastal drainage capacity compound existing flood risk
Extreme rainfall (March 2026 repeat)
Doubles the rainfall intensity factor. Climate research suggests events like March 2026 will become more frequent.
About this tool: This map uses a Flood Vulnerability Index (FVI), a research method that combines 9 physical factors into a single flood risk score for each area. Think of it like a report card for flood risk: each factor contributes a share, and together they explain why some neighborhoods flood more than others.
Data sources: Elevation from USGS national DEM, rainfall from the Hawaii Rainfall Atlas, soils from USDA SSURGO/NRCS, land cover from NLCD, stream network from USGS NHD+ High Resolution, flood zones from FEMA NFHL. Each grid cell covers roughly 400 meters of land.
Important limits: This is a screening-level planning tool, not an official flood insurance rate map. It does not replace FEMA assessments and cannot predict exact flood depths. It is best used to understand relative risk across areas, identify where land-use decisions matter most, and simulate how restoration or development would shift risk patterns. For legal flood insurance determinations, use FEMA's official Flood Map Service Center.
Data sources: Elevation from USGS national DEM, rainfall from the Hawaii Rainfall Atlas, soils from USDA SSURGO/NRCS, land cover from NLCD, stream network from USGS NHD+ High Resolution, flood zones from FEMA NFHL. Each grid cell covers roughly 400 meters of land.
Important limits: This is a screening-level planning tool, not an official flood insurance rate map. It does not replace FEMA assessments and cannot predict exact flood depths. It is best used to understand relative risk across areas, identify where land-use decisions matter most, and simulate how restoration or development would shift risk patterns. For legal flood insurance determinations, use FEMA's official Flood Map Service Center.
March 10–16, 2026 · Storm 1: Kona Low #1
Hurricane-force winds, record rainfall across all islands
The first Kona low system siphoned moisture from the tropics and stalled over the island chain. Wind gusts reached 135.4 mph on Hawaii Island. Rainfall broke daily records at every major NWS station.
62"
Max rain (Maui)
135mph
Wind gust (HI Island)
4
Islands impacted
Source: NASA Earth Observatory · NWS Honolulu storm summaries
March 19–24, 2026 · Storm 2: Kona Low #2
Worst Hawaii flooding in 20 years: 236 rescued, 5,500 evacuated
The second system struck already-saturated ground. North Shore Oahu (Waialua, Haleiwa) bore the worst damage with homes and cars swept away. Wahiawa Dam (1906) threatened failure. Gov. Green declared >$1 billion in estimated damage. 115,000 customers lost power.
236
People rescued
5,500
Evacuated
>$1B
Est. damage
Source: NPR · Hawaii Civil Defense press releases · Gov. Josh Green statement
March 23, 2026 · "Rain Bomb" over Mānoa/Pālolo
Stationary storm cell drops 2–4 inches per hour over Honolulu valleys
Mayor Blangiardi described it as a "classic rain bomb." Six Hawaii Mesonet stations recorded 3.5–6.5 inches total, most within a three-hour window. Mānoa Stream overflowed into shopping centers, roads, and an elementary school. The Molokai radar was offline throughout the entire event period.
4"/hr
Peak rain rate
3hrs
Peak duration
6.5"
Max station total
Source: Mayor Blangiardi press conference · UH Hawaii Mesonet station data
March 1–24 Cumulative · University of Hawaii Mesonet
2 trillion gallons total: 3,000% of normal rainfall
UH Mānoa's Hawaii Mesonet (77-station statewide network) documented the full scope. Statewide average: 18.25 inches over 23 days, 2.6× the March average of 6.85 inches. Some locations recorded 14-day totals at 3,000% of historical norms. The Molokai radar outage made real-time response harder; the Mesonet network filled the gap with 5-minute rainfall readings.
2T gal
Total rainfall
18.25"
Statewide avg
3,000%
Above normal
Source: Hawaii News Now · UH Mesonet / Phys.org
Context
Why was this so catastrophic?
Saturated ground: Storm 2 hit soil already at capacity from Storm 1. Even moderate rain caused immediate runoff and flooding.
Aging infrastructure: The Wahiawa Dam was built in 1906. North Shore drainage systems were not designed for this volume.
Radar gap: The Molokai WSR-88D radar was offline, removing a key tool for real-time forecasting during the event.
Climate signal: Kona lows are a normal feature of Hawaii's winter, but this back-to-back sequence with tropical moisture at this intensity is consistent with climate-change-driven intensification of Pacific weather systems.
Aging infrastructure: The Wahiawa Dam was built in 1906. North Shore drainage systems were not designed for this volume.
Radar gap: The Molokai WSR-88D radar was offline, removing a key tool for real-time forecasting during the event.
Climate signal: Kona lows are a normal feature of Hawaii's winter, but this back-to-back sequence with tropical moisture at this intensity is consistent with climate-change-driven intensification of Pacific weather systems.
March 2026 Legend
Event Impact
Flood impact zone
Evacuation zone
Extreme rainfall area
Dam risk site
FEMA / Hazard
FEMA Special Flood Hazard
Hydrology
Major stream / river
FVI vs FEMA
Both flag high risk
FVI high, no FEMA zone
FEMA zone, FVI lower
Vulnerability Index
Low (0–0.30)
Moderate (0.30–0.45)
High (0.45–0.60)
Very high (0.60–0.75)
Extreme (0.75+)