New Flood Maps for Hawai'i!  What are the Differences? 

by Eric Simmons, FEMA Region 9

My grandfather would say “use the right tool for the job” as we pulled many a coup in the workshop.  The right tool rule can also refer to communications as using a flood map for purposes which it was never intended is fraught with risk.  Similarly, using the right map incorrectly or adapting an unfit one to mimic the performance of the correct map is problematic.

State law effective May 1, 2022, requires real estate transaction disclosure in the area exposed to rising ocean levels.  Properties affected are those within 3.2-foot sea level rise projections by 2050.  An interactive mapping tool, screenshot for Waikiki below, supports adaption to sea level rise around Hawai'i as well as risk communication associated with ocean changes.

Properties with a higher score (Flood Factor of 9 or 10) face extreme risk and are more likely to flood.  According to First Street Foundation, more than 30% of all Hawai'ian properties face extreme to major flood risk today and losses in the future are increasing.  The Flood Factor scores, represented graphically by buildings below, are also provided on Redfin, Realtor.com, and Estately websites.

Flood Insurance Rate Maps, or FIRMs, are hazard maps published to support the National Flood Insurance Program.  They show zones of high flood hazard (for example, Zones AE, AO and VE as seen above) as well as medium, low and unknown flood hazard areas.  These flood zones can be viewed on Hawai'i’s Flood Hazard Assessment Tool (FHAT).  FIRMs are used to manage new development in floodplains and advance other flood loss reduction activities.  They help lenders determine who are required to purchase flood insurance.  FEMA has initiated an island wide restudy of flood hazards for Honolulu.  More information on FIRMs is available online. 

Tsunami evacuation maps identify areas that may be impacted by an upcoming event and must be evacuated under a warning.  Hawai'i residents and visitors can determine if they’re in a tsunami evacuation zone by checking their phone book or going online to the FHAT.  For Honolulu, the tsunami mapping now has an extreme evacuation zone. Similarly, the State Dam Safety Program provides on the FHAT if a property is in a dam failure evacuation zone.  Tsunami evacuation zones, as seen for Waikiki below, are established by each Hawai'ian County in conjunction with State public safety officials.  These maps are available online on a tsunami viewer.

Evacuation maps are not inundation maps, like a Flood Insurance Rate Map or hurricane storm surge inundation maps.  These storm surge maps for Hawai'i are viewable on a NOAA Viewer. The following screenshot shows expected storm surge and inundation depths from a Category 4 hurricane.

A major difference between evacuation maps and FIRMs are the FEMA hazard zones being based on flooding with a specific probability of occurring.  1% annual chance water elevations are a statistical measure and vary in height throughout a community.  Therefore, a 1% annual chance floodplain should not be directly related to a particular hurricane category or tsunami without extensive analysis.  Further comparison of these different flood maps is in the table.

		Sea Level Rise Exposure Area	Flood Factor	Flood Insurance Rate Maps	Tsunami Evacuation Zones	Storm Surge Maps
Purpose of Map		Online atlas to support disclosure and vulnerability / adaptation to sea level rise as mandated by State law	Understand current conditions flood hazards and how flood risk changes over time	Determination of what structures may require flood insurance and development criteria / floodplain management	Information to plan for evacuation during a tsunami or tropical storm	Identify land flooded by a hurricane.  A category 4 hurricane is like Hurricane Iniki in 1992
Map Source		Pacific Island Ocean Observing System (PacIOOS)	First Street Foundation	Federal Emergency Management Agency	County with data developed by the State of Hawai'i	National Oceanic and Atmospheric Administration
Types of Floods Depicted		An aggregate of 3 layers with sea level rise: passive flooding (still water high tide flooding), annual high wave flooding (over wash during the largest wave events of the year), and coastal erosion	Existing conditions and future flooding from rivers, rainfall and the oceans covering sizes from nuisance flooding to once-in-a-millennium.	Riverine and coastal flooding due to tropical storms, heavy rainfall, and –in some areas– tsunami or levee failure.  Flood zones are based on the 1% annual chance (100-year) event and, where available, the 0.2% annual chance (500-year) event.	Tsunami	Category 1, 2, 3 or 4 hurricanes with storm surge depths
Information on Map		Sea Level Rise Exposure Area (SLR-XA)	Grids attributed with inundation probability and flood depths associated with time (year 2020 and 2050).	Flood insurance zones, regulatory floodways, cross section and transect locations, base flood elevations or depths, hydraulic structures, etc.	Evacuation areas impacted by tsunami	Storm surge flooding for each hurricane category. Inundation areas subdivided by flood depth.

It is important to note that all five of these flood maps are not real-time products.  For active flooding or tropical cyclones, please go online and consult local products issued by the National Weather Service.  Prevention is the best protection and always use the right tool for the job.