What are the Differences Between Various Flood Maps for Hawaii?

Flood Insurance Rate Maps, or FIRMs, are hazard maps published by the Federal Emergency Management Agency (FEMA) to support the National Flood Insurance Program.  They show zones of high flood hazard (for example, Zones AE, AO and VE as seen below for Waikiki) as well as medium, low and unknown flood hazard areas. 

Flood Insurance Rate Map for Waikiki (Source: FEMA from 2011)

These flood zones can be viewed on Hawaii’s Flood Hazard Assessment Tool (FHAT).  FIRMs are used to manage new development in floodplains and advance other flood loss reduction activities.  They determine who are required to purchase flood insurance and may impact the rating of insurance premiums.  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.  Hawaii 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 evacuation zone.  Tsunami evacuation zones, as seen for Waikiki below, are established by the individual Hawaiian Counties in conjunction with State public safety officials.  These maps are available online on a tsunami viewer.

Tsunami Evacuation Zones for Waikiki (Source: City and County of Honolulu from 2011 and 2018)

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

Hurricane Storm Surge Hazard Map for Waikiki (Source: NOAA from 2019)

A major difference between evacuation maps and Flood Insurance Rate Maps are the FEMA 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 the different flood map types is shown in the table below.

  Tsunami Evacuation Zones Storm Surge Maps Flood Insurance Rate Maps
Purpose of Map 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 which flooded Kauai in 1992 Determination of what structures may require flood insurance, insurance rating, and floodplain management.
Map Source County with data developed by the State of Hawaii National Oceanic and Atmospheric Administration Federal Emergency Management Agency
Types of Flood Depicted Tsunami Category 1, 2, 3 or 4 hurricanes with storm surge depths 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.
Information on Map Evacuation area impacted by tsunami Storm surge flooding for each hurricane category. Inundation areas subdivided by flood depth: greater than 9’, between 9 and 6’, between 6 and 3’, and less than 3’ above ground. Flood insurance zones, regulatory floodways, cross section and transect locations, base flood elevations or depths, hydraulic structures, etc.

This article updates one from the January 2011 edition of Wai Halana.  Please note that all three 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.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Powered by WordPress.com.

Up ↑

%d bloggers like this: