HURRICANE RISK
ASSESSMENT FOR THE HAWAIIAN ISLANDS
George
Pararas-Carayannis
(©) Copyright
2007 George Pararas-Carayannis
INTRODUCTION
Weather-related disasters
that can adversely impact the Hawaiian Islands include hurricanes
and associated surge flooding, severe thunderstorms, tornadoes,
downbursts and high winds. Moderate to severe thunderstorms and
associated strong winds, small tornadoes and downbursts can occur.
Small scale tornadoes and downbursts can generate winds of more
than 100 miles per hour and can be loccally destructive.
 Although
infrequent, hurricanes and hurricane surges present the most
significant, larger-scale, weather-related disaster threat for
the Hawaiian Islands and pose environmental risks to public health
and safety. Hurricane Iniki in 1992, Hurricane Iwa in 1982 and
Hurricane Dot in 1959 are examples of what has happened in the
past and what could happen in the future. These hurricanes were
particularly destructive in Kauai, although all other islands
experienced their adverse impact.
In spite of the fact
that the island of Oahu never had a direct hit by a hurricane,
the probability that a tropical storm or a hurricane will pass
within 360 nautical miles (nmi) of Honolulu is estimated to be
about 80%. The probability that a hurricane will pass much closer
to Oahu or other Hawaiian Islands, or that it will make landfall
cannot be estimated in the absence of historical data. Although
this probability cannot be quantified, the potential risk cannot
be overlooked or ignored. All of the Hawaiian Islands are potentially
vulnerable to hurricanes and hurricane storm surge flooding.
Tracks of Central
Pacific Hurricanes passing over or near the Hawaiian Islands
from 1949 to 1998.
Flooding associated
with hurricane surges could be significant, even with hurricanes
passing distantly from the islands. Category 1 or 2 hurricanes
passing within 100 nautical miles from the islands can be expected
to flood the coastal areas by about 1-3 feet of water. Superimposed
will be the strorm waves. If a category 3 or 4 hurricane passes
by at the same distance or makes landfall on any of the islands,
coastal inundation of up to an elevation of 6-8 feet is possible.
Superimposed on such hurricane surge will be the storm 's waves
which could range from 20 to 35 feet and will contribute to
considerably more coastal flooding. If a hurricane similar to
Iniki makes landfall on Oahu or passes nearby, winds of more
than 100 mph are possible. The wind effects can be expected to
have a devastating effect on high rises with large glass windows
and glass facades. Flying broken glass presents a great danger.
HURRICANE
RISK ASSESSMENT FOR THE HAWAIIAN ISLANDS
Hurricanes and associated
storm surges present the greatest hazard risk for the islands.
A detailed description of recent historical hurricanes in Hawaii,
has been provided elsewhere at this web site (see links below).
The following analysis of the hurricane risk of the Hawaiian
Islands - and Oahu in particular - is based on tables, charts,
historical hurricane storm tracks and data (water levels/barometric
pressure, winds, waves and tides) obtained from numerous reliable
sources.
Historical
Hurricanes and Storm Systems in the Hawaiian Islands
The historical records
show that several storm systems develop every year in the Eastern
Pacific region. Destructive hurricanes have occurred in Hawaii
in the past and will occur again in the future. However, the
recurrence frequency of hurricanes in Hawaii is not as great
as that of other areas in the world (i.e. Caribbean, Gulf of
Texas). The diagrams above and below show the tracks of Central
Pacific Hurricanes from 1949 to 1998.
Most storm systems
pass south of the Islands. Some of these systems develop into
hurricanes. Infrequently, these storm systems turn north and
pass closer to the Hawaiian Islands, thus generating strong winds,
heavy rains and flooding. At least three hurricanes have passed
near or over the islands in the last 50 years. The diagram below
illustrates the path of hurricanes, tropical storms and depressions
near the Hawaiian Islands.
 Recent hurricanes,
tropical storms and depressions in the Hawaiian Island Region.
Hurricanes Iniki
(1992), Iwa (1982), and Dot (1959) were the most recent hurricanes
to impact the Hawaiian Islands. All three went right over or
close Kauai. There have been no direct hits of Oahu by any historical
hurricane in recorded times - although some meandered uncomfortably
close. Hurricane Iniki initially headed for Oahu but later changed
direction, missing the island. However, there was considerable
impact and damages - particularly along the southern and west
coasts of Oahu. Prior to hurricane Iniki and Iwa, tropical depressions
Hiki (1950) and Nina (1957) caused strong winds, heavy rains
and flooding on Oahu.
Assessment
of Potential Hurricane and Storm Surge Impact on Oahu
Strong hurricane winds
and surges have the potential to impact Oahu and result in extensive
destruction in the Honolulu Metropolitan area and elsewhere.
Flooding due to storm surge presents a very high risk for potential
damage if a hurricane makes landfall on Oahu or passes near the
island. The following is a brief overview of basic concepts used
to predict and quantify surge components that cumulatively contribute
to the generation of hurricane surge flooding.
Hurricane
Surge
The diagram below
portrays graphically the various components, which combine and
contribute to the cumulative total of hurricane surge height
on an open-ocean coast (Pararas-Carayannis, 1975, 2002). However,
coastal morphology, direction of hurricane approach, radius of
maximum winds, coastal configuration and geometry of a basin,
may also affect the extent of rise of water level and degree
of surge flooding. This occurred with hurricane Katrina when
the New Orleans levees were overtopped and failed from higher
surge approaching from the direction of Lake Pontchartain, from
the north of the city, rather than from the Gulf of Mexico.
 Hurricane Surge Components caused
by the atmospheric pressure field, wind stress on the water surface.(After
Pararas-Carayannis, 1975, 2002).
Specific factors which
can combine to produce extreme water fluctuations at a coast
during the passage of a hurricane include: storm intensity, size,
path, duration over water, atmospheric pressure variation, speed
of translation, winds and rainfall, bathymetry of the offshore
region, astronomical tides, initial water level rise, surface
waves and associated wave setup and run up due to wind frictional
effects. The cumulative surge height results from frictional
wind effects, atmospheric pressure changes, the bathystrophic
component (due to the earth's rotation), the phase of astronomical
tide, and the superimposed storm waves.
The bathystrophic contribution is an important parameter of the
hurricane surge. A brief explanation of what causes the hurricane
surge flooding is provided at http://drgeorgepc.com/HurricaneSurge.html
In brief, in the northern
hemisphere hurricane winds approaching a coast have a counterclockwise
motion. Because of the Coriolis effect due to the earth's rotation,
the flow of water induced by the cyclonic winds will deflect
to the right, causing a rise in the water level. Therefore, the
bathystrophic storm tide is important in producing maximum surge
even when the winds blow parallel to the coast. To what extent
the bathystrophic component will add to the flooding on the
coast will depend on the storm's direction of approach. Some
coastal areas could be flooded to a greater extent. If for
example a hurricane makes landfall west of Honolulul, rather
than to the east, greater flooding could be expected. However,
even if a hurricane does not make landfall on Oahu but passes
considerably south of the island and is moving in a west/northwest
direction at a distance of 150 miles or less, considerable flooding
could also occur.
Prediction
and Quantification of Flooding due to Hurricane Surge
Quantitatively calculating
the degree of surge flooding resulting from the combined meteorological,
oceanic and astronomic effects - coincident with the arrival
of a hurricane at the coast - is a rather difficult problem to
solve. Difficulties arise because a hurricane is a three dimensional
weather system, with ever changing dynamic conditions of wind
speeds, directions, and atmospheric pressures.- and these conditions
change.
The capability to predict and quantify is based primarily on
the use of analytic and mathematical models, which estimate interactions
between winds and ocean. The models develop the three dimensional
wind field of a hurricane, the radius and changing direction
of maximum winds, the landfall and the resulting storm surge
flooding. The models can be quasi-one-dimensional, two dimensional,
or three-dimensional numerical schemes. The simplest would be
a quasi-one-dimensional model, which is a steady-state integration
of stresses of the hurricane winds on the surface of the water
from the edge of the Continental Shelf to the shore.
Many sophisticated mathematical models have been developed in
recent years to provide more accurate three-dimensional estimates
of energy flux and flooding that can be caused by a passing hurricane.
As indicated, the cumulative hurricane surge must include the
combined effects of direct on-shore and along shore wind-stress
components on the surface of the sea, the effect of the earth's
rotation (the bathystrophic component), the changing atmospheric
pressures, the ocean surface/wind coupling, and the bottom frictional
effects. All mathematical models, regardless of sophistication
of methodology, must use the Bathystrophic Storm Tide Theory.
Numerical models have been routinely used in the past. Briefly,
to model a hurricane and calculate maximum surge heights, the
following meteorological parameters must be first determined.
These are the hurricane's central pressure index, its peripheral
pressure, the radius to maximum winds, the maximum gradient wind
speed, the maximum wind speed, and the speed of hurricane translation
(overall speed of the system). The models must integrate also
the astronomical tide, existing ambient wave conditions, surface
and bottom friction and coastal topography. Only then one can
proceed with the solution of the complex hydrodynamic equations
of motion and continuity that will allow determination of the
time history of expected changes in sea level associated with
the hurricane, at any given point along a shore. Most of the
numerical models for hurricane surge prediction are fairly accurate
and have been verified with historical data. Recently developed
numerical models using a three dimensional approach, faster and
more efficient computers, and more accurate weather data from
satellites, have greater potential for more accurate predictions.
 Statistical Probability of a Tropical
Storm or Hurricane Striking Oahu.
Hurricanes do not
occur with frequency in Hawaii. Most of them bypass the Islands.
There has not been a known historical hurricane that has struck
the southern coast of Oahu, with winds and waves similar in intensity
as Iniki in Kauai. However, this should not be assumed as proof
that such a hurricane cannot strike Oahu. Hurricanes, similar
in intensity to Iniki or Iwa, can be expected to occur again
near the islands and some could make landfall. As Iniki's track
showed, the hurricane was heading for an almost direct hit of
Oahu 24 hours before changing direction. Had this hurricane hit
the island, damage would have been much greater and many deaths
could have occurred.
Probability
that a tropical storm or hurricane will pass within 360 nmi of
Pearl Harbor, and approximate point of approach (CPA) (Pearl
Harbor study).
There is no historical data to help determine the statistical
probability of a direct strike by a hurricane on Oahu. However,
there is sufficient data to determine the probability of a hurricane
passing near the island. A U.S, Navy study of 27 tropical storms
and hurricanes during a 47-year period from 1949-1995 estimated
that the probability of a tropical storm or hurricane passing
within 360 nautical miles of Pearl Harbor is about 80% (see map
above). The probability that a hurricane will pass much closer,
or that it will make landfall, cannot be estimated in the absence
of historical data. Although this probability is relatively small
and cannot be quantified, the potential risk cannot be overlooked
or ignored.
The same U.S, Navy
study of 27 tropical storms and hurricanes passing within 360
nmi of Pearl Harbor during a 47-year period from 1949-1995, indicates
that there is a 20% probability that storm systems will approach
Oahu from the east southeast direction.
 Directions of approach of 27
tropical storms and hurricanes passing within 360 nmi of Pearl
Harbor during a 47-year period from 1949-1995. The length of
each arrow is proportional to the number of storms approaching
from the indicated direction. (From U.S. Navy study)
Worse Case
Hurricane Impact Scenario for Honolulu
The following is the
worst-case scenario of a hurricane that could potentially impact
Honolulu. This hurricane, similar to Iniki in intensity (category
4), approaching Honolulu from a southern or an east/southeast
direction, making landfall west of the airport near Ewa or Barbers
Point is the worse possible in terms of anticipated damage by
strong winds and surge flooding. This hypothetical maximum probable
hurricane on Oahu is abbreviated as MPH.
The following analysis
provides documentation in support of such hurricane occurrence
and estimates of expected winds and surge inundation.
Potential
Direct Hit of Hurricane on Oahu: It
may be argued that no hurricane on record ever had a direct landfall
on Oahu - thus there is no danger. However, it should be reiterated
that Iniki had the potential of striking directly Oahu. What
deflected Iniki's path from making landfall on the island in
1992 was the western edge of the subtropical high-pressure ridge,
a semi permanent feature found north of Hawaii. Normally, this
pressure ridge keeps hurricanes south of the islands. However,
in September 1992, a weakness in this ridge developed west of
160W when a large low system or trough began to drift south along
and just east of the International Dateline. This air mass flow
change caused Iniki to change its path in the early hours of
September 10 and head somewhat north of its previous west-northwest
track - bringing it closer to the islands.
At that time Iniki was located near 15N 159W, or 410 miles south
of Honolulu, but still moving in a generally west direction at
10 knots and looked as though it would miss the islands. However,
subsequent unpredictable flow changes in the overall Central
Pacific circulation changed Iniki's path to a northward direction.
If the large low system had been further east of the International
Date Line, or if there was additional weakness of the Pacific
High - and if these anomalies had occurred a day earlier - Iniki
could have clearly headed northward sooner and could have made
landfall on Oahu. A more recent example would be Hurricane Daniel
of July 2006. Fortunately, Daniel weakened very rapidly when
it encountered cold water. However, its predicted track shows
that it could have passed closer than 100 miles south of Honolulu.
In summary, there
is no warrantee that the subtropical high-pressure ridge northwest
of the Hawaiian Islands will always be strong enough to deflect
hurricanes south of the islands. An inspection of Iwa' track
illustrates how unexpected steering flow changes can occur. Similarly
to Iniki, Iwa appeared to be too far to the west of the islands
and heading north, when its path changed suddenly to a northeast
direction, thus making landfall on Kauai. Such abrupt changes
in atmospheric circulation have become more frequent in recent
years, perhaps because of global warming and a more intense El
Nino ocean circulation. Recent anomalies in the flow of the jet
stream caused atmospheric changes in the Central Pacific, which
were responsible for the heavy rains and floods of 2006 in the
Hawaiian Islands. In conclusion, although a hurricane has not
made landfall on Oahu within historic times, this does not mean
that an intense hurricane will not hit the island or pass nearer
in the future.
Sequence
of Potential Winds and Surge Flooding if a Maximum Probable Hurricane
(MPH) makes Landfall on Oahu
As previously indicated,
the wind sequence and chronology of potential flooding effects
of the MHP can only be quantified with the use of mathematical
modeling. Without modeling, only qualitative estimates can be
provided, based on wind fields and resulting storm surges of
destructive historical hurricanes - which have been well documented
(Pararas-Carayannis, 1975, 1992). The following is a probable
time history of wind and surge flooding effects that could be
expected if the postulated MPH (similar to Iniki), approaches
Honolulu from a South or South/Southeast direction and makes
landfall near Barber's Point. Such a hurricane would place Oahu
in the dangerous semicircle of its impact. Winds of up to 130
mph with gusts up to 160 mph are possible. The effects of the
winds and of the flooding would be severe.
Potential
Winds: When the center
of the MPH is about 180-200 miles south or southeast of Honolulu,
there will be strong winds, with gusts up to 35-40 mph. When
the hurricane's center is about 130 miles south of Honolulu,
the gusts could potentially increase to about 55 mph. As the
MPH moves closer, winds will be from the east northeast with
sustained speeds of 55 mph, gusting to about 60-65. Wind damage
will begin and sea level will start rising along Oahu's eastern
and southern coastlines.
As the MPH gets even
closer to Honolulu, the winds will be from the east (090) with
average sustained speeds of about 80 mph and gusts ranging from
115 mph to 140 mph. The gusts will begin to exceed the wind design
threshold of 80 to 100 mph of most buildings constructed in the
Honolulu area and gradual damage will begin. Glass windows at
high rises will start popping out. High storm waves wil start
pounding the shoreline and major flooding will begin.
As the center of
MPH nears the Honolulu coastline (perhaps 40 miles away or closer),
the winds will be strongest as the flow will be down slope. Maximum
winds can be expected along the southern coast of Oahu , before
the passage of the hurricane's eye makes landfall or passes near
the island. These maximum winds will be from an east-southeast
direction at speeds of about 100 miles per hour with peak gusts
estimated to range between 100 to 140 miles per hour. At this
time, major damage to buildings can be expected. Also, during
this particular period of time, the frictional effects of the
wind will be in a landward direction and major flooding will
begin.
Potential
Hurricane Surge Flooding Effects: The
flooding effects will vary depending on the hurricane's speed
of translation when it is near or over the island. If the hurricane
moves fast, the flooding effects may not be as great. If the
hurricane moves slowly over the island and the central barometric
pressure is a very low (950 mm or less), the level of flooding
along the entire southern and eastern coast of Oahu could be
further augmented. If the hurricane makes landfall near the time
the astronomical tide is the highest (spring tide), maximum flooding
can be expected.
After the center
of MHP crosses the southern coast of Oahu near Barber's Point,
the wind direction can be expected to change rapidly from the
eastern direction to a direction from the south-southeast and
then to a southern direction. Although the winds will not be
as strong, maximum surge flooding will begin to occur and the
waves will start breaching the protective seawalls of small
boat harbors. Wind friction, the bathystrophic component and
the wave setup will be at a maximum. Coupled with the astronomical
tide (assuming to be maximum), and the rise in sea level due
to reduction of atmospheric pressure (as the hurricane center
passes), maximum flooding will result along the southern coast
of Oahu - east of the hurricane's trajectory path. Superimposed
on the higher elevation of the sea will be the storm waves, which
will intensify.
Conclusions
Both winds and flooding
from a severe hurricane can be expected to impact adversely and
to result in major structural damage to property in Honolulu
and the rest of Oahu. Hurricane winds and surges can be expected
to uproot trees and to damage structures. Missiles and broken
glass will be flying through the air. Venturi effects will amplify
the speed of the winds and cause destruction to homes on ridges
and valleys. Isolated downburst events will demolish structures.
Fires may ignite. There will be disruptions of power and telephone
service.
Even with a Category 1 or 2 hurricanes, Honolulu and the rest
of Oahu will be vulnerable. Such hurricanes can be expected
to create flood the site by about 1-3 feet of water. If a category
3 or 4 hurricane strikes, inundation of up to an elevation of
5-7 feet is possible, because superimposed on the surge will
be storm waves. As documented, even with Iniki passing far from
Oahu, the Waianae coastline experienced the most damage from
the storm surges with flooding to the second floor of beachside
apartments.
REFERENCES
AND ADDITIONAL READING
Adams, W.M., and Pararas-Carayannis
G., Relative Susceptibility of the Hawaiian Islands to Waves
Generated by Storms and Nuclear Explosions. (Hawaii institute
of Geophysics - Report to the U.S. Atomic Energy Commission.)
Oct. 1966.
Brown R. H. (1993). Natural Disaster Survey Report: Hurricane
Iniki. URL accessed on 2006-03-13.
Central Pacific Hurricane
Center (1992). The 1992 Central Pacific Tropical Cyclone Season.
URL accessed on 2006-03-13.
National Hurricane
Center (2004). Costliest U.S. Hurricanes 1900-2004 (adjusted).
URL accessed on 2006-03-18.
Pararas-Carayannis
G. Hurricane Surge Prediction - Understanding the Destructive
Flooding Associated with Hurricanes http://drgeorgepc.com/HurricaneSurge.html
Pararas-Carayannis
G. HURRICANE INIKI IN THE HAWAIIAN ISLANDS September 11, 1992
http://drgeorgepc.com/HurricaneIniki.html
Pararas-Carayannis, G., 1973. Offshore Nuclear Power Plants:
Major Considerations and Policy Issues. Chap. VIII: Direct Environmental
Impacts of Offshore Plants, 8 Nov. 1973, President's Council
on Environmental Quality (CEQ), Task Force on Offshore Nuclear
Power Plants, Washington D.C.
Pararas-Carayannis, G., 1975. "Verification Study of a Bathystrophic
Storm Surge Model". U.S. Army, Corps of Engineers - Coastal
Engineering Research Center, Washington, D.C., Technical Memorandum
No. 50, May 1975 (Study performed for the U.S. Nuclear Regulatory
Commission for the licencing of the Crystal River (Florida) nuclear
plant).
Pararas-Carayannis,
G. Proposed American National Standard - Aquatic Ecological Survey
Guidelines For the Siting, Design. Construction, and Operation
of Thermal Power Plants. American Nuclear Society, Monogram,
September, 1979.__
Pararas-Carayannis, G.,1993. The Wind and Water Effects from
Hurricane Iniki on September 11, 1992, at Lawai Beach Resort,
Poipu, Island of Kauai, Hawaiian Islands. A study prepared for
Metropolitan Mortgage & Securities Co., Inc, Spokane, Washington,and
the Ritter Group of Companies, Chicago, June, 1993.
Pararas-Carayannis, G., 2004. Natural Disasters in Oceania, Chapter
10, in International Perspectives on Natural Disasters: Occurence,
Mitigation, and Consequences -Book Series: ADVANCES IN NATURAL
AND TECHNOLOGICAL HAZARDS RESEARCH, Western Michigan University,
ISBN: 1-4020-2850-4, Nov. 2004, Springer Publishing, Netherlands.
Pararas-Carayannis, G., Climate Change, Natural and Man-Made
Disasters - Assessment of Risks, Preparedness and Mitigation,
Keynote Presentation _ CLIMATE CHANGE _ DISASTER PREPAREDNESS_30th
Pacem in Maribus (PEACE IN THE OCEANS). A Year after Johannesburg.
Ocean Governance and Sustainable Development: Ocean and Coasts
- a Glimpse into the Future, Kiev, Ukraine, October 26-30, 2003
See also http://drgeorgepc.com/ClimateChange.html
US Army Corps of Engineers
(1993). Hurricane Iniki Assessment. US Military. URL accessed
on 2006-03-13.
SEE ALSO
HURRICANE INIKI IN THE HAWAIIAN ISLANDS
Hurricane Iniki of September 5 - 13, 1992 in
the Hawaiian Islands
Examination of its Anomalous Path
Hurricane Iwa of November 19- 25, 1982 in the
Hawaiian Islands
Hurricane Surge Prediction - Understanding the
Destructive Flooding Associated with Hurricanes
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