What are my risks?
Do I live in a flood prone
area?
For information on flooding in your neighborhood,
refer to a Flood Insurance Rate Map (FIRM). These maps
are computed water surface elevations that are combined with topographic
mapping data to develop flood hazard maps. They provide information
on areas subject to flooding. They are used to guide future development
away from flood-prone areas and to regulate development that is
proposed to occur within such areas.
Call your Flood Coordinator (587-0248) for information
about flooding. Or, ask your regional librarian to see the Flood
Insurance Rate Map for your community.
Click
here for a general map of the flood zones on Oahu.
Where have stream floods been
recorded on Oahu?
Click
here to download an image map of stream floods.
The most frequent and severe flooding occurs where
steep sloping hillsides abruptly meet flat or low-lying coastal
plains, such as those found in Waimanalo, Kailua, Kaneohe (November
1992), and Laie (April 1994). The heaviest rainfall during the
last decade in Kaneohe occurred in October 1991, when 15 inches
fell in 48 hrs leading to intense flash flooding. Stream mouths
are also commonly susceptible to flooding, especially during marine
storm or high wave events, as runoff from streams reach a sea
that is partly elevated by the combination of high waves, winds,
and storm surges. Some of the largest rainfall counts and most
severe flooding events have occurred in the last several years.
During the first 15 days of November 1996, record-breaking rainfall
occurred along the Waianae Coast, where 21 inches fell in an area
where the average annual rainfall is 2 inches. In Ewa, 12.5 inches
fell in 7 hours on the 5th of that month, inducing flooding of
the low coastal plain. A series of slow moving storms with prolonged
rains that saturated the soils of south-central Oahu culminated
on New Years Day of 1988 in severe runoff and hillside erosion,
resulting in catastrophic damage to stream flood mitigation channels,
homes, and roads in Aina Haina and Niu Valleys. Other recent severe
events on Oahu include October 1981 flooding of Waiawa Stream
after heavy rains that lead to $786,000 damage and January 1968
flooding in Pearl City, which caused $1.2 million damage.
Where have high waves been
recorded on Oahu?
Click
here to download an image map of high waves.
The largest waves reach Oahu in winter. Along the
north shore, it is common to see wave heights between 15-20 feet
annually from winter swell. Wave heights of 50 feet have been
reported (December 1969 and January 1998). Often, winter north
and northeast swells wrap around Makapuu Point and generate waves
at Sandy Beach that are as high as the largest summer surf found
there. Trade wind waves can be high, but because of their shorter
wavelengths, they have less energy than north and south swell.
Trade wind swell has a greater easterly directional component,
which enables them to refract around to south and southwest-facing
shorelines producing wave heights of 1-4 feet. In the summer,
south-facing shorelines receive 4-6 foot swell. South swells tend
to have less energy and longer wave periods than winter swells.
Hurricane generated waves have exceeded 15 feet along east Oahu
and 10 feet on Oahu's southern shores. Combined with storm surge
and high tides, hurricane waves can overwash coastal roads and
properties, as they did along the Kaaawa and Kaneohe coasts during
Hurricane Fernanda in 1993 and along the Honolulu and Waianae
coasts during Hurricane Iniki in 1992.
Source: Fletcher, Charles, Eric Grossman, Bruce Richmond. Atlas
of Natural Hazards in the Hawaiian Coastal Zone. 2000.