hurricane Iniki Hawaiian Islands Dr. George Pararas Carayannis

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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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