HomeMy WebLinkAboutMulti-Hazard Mitigation Plan: 04. High Wind Storms CIVIL DEFENSE AGENCY
COUNTY OF HAWAII
920 ULULANI STREET HILO,HAWAII 96720
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4. High Wind Storms
Chapter 4:Hazard Analysis—High Wind Storms
CHAPTER 4 - HIGH WIND STORMS
4.1 Description of Hazard
Wind is one of the most costly property perils, causing more damage nationwide than
earthquakes, freezing, or other natural perils. (IIPLR, 1994) In wind storms and especially
hurricanes, windborne debris can be a major factor in causing damage. Flying objects such as
tree limbs, outdoor furniture, signs, roofs, gravel, and loose building components from
progressively failing adjacent buildings can impact the building envelope, creating openings
that allow internal pressure to build within. The internal pressures add to the external
pressures producing more severe pressures on the building components of the structure.
(IIPLR, 1994) The roof then is subjected to tremendous internal pressure building from
inside, together with the negative wind pressures lifting the roof from outside. The resulting
combined forces may cause roof system failure if the roof has not been adequately designed
and constructed. If the roof is breached, high winds and rain destroy the inside of the
building.
Winds in Hawaii originate from three main sources: trade winds, Kona winds, and hurricanes
or tropical storms. Northeast trade winds are dominant throughout most of the year and
generally range in velocity between 10 and 20 mph. However, trade winds of 40-60 mph
occasionally occur for several days at a time when the sub-tropical high-pressure cell located
in the central North Pacific Ocean intensifies. During the 1993-1994 and 1994-1995 winter
seasons, for example, strong and gusty trade winds of 40 to 50 mph lasted several days and
inflicted damage to roof tops, tree limbs, and telephone equipment. The east-facing
coastlines, as a result, are the windward coasts and most impacted by trade wind energy.
Kona winds are southerly winds and occur as light and variable winds during summer
months when trade wind circulation breaks down,but in winter they can be very strong when
storm systems moving across the central North Pacific draw air from the south toward their
low pressure troughs. Damaging Kona winds from storms generally occur during the winter
and spring seasons and have reached velocities of 50 mph for several days on end.
4.1.1 Utilities
Power distribution lines are susceptible to strong winds due to the relatively low design
standards for older portions of the grid which may also have preexisting damage in the wood
poles due to decay and termite attack. One of the most common impacts of a wind storm is
the loss of electrical service to some communities. [A comprehensive discussion of the
historic design criteria for electrical transmission and distribution systems in Hawaii County,
and recent updates, is provided in Chapter 5.]
4.1.2 Trade Winds
Trade winds are by far the most common winds over Hawaiian waters and play a major role
in defining the climatology of the region. (Kodama 1998) These persistent winds, which
blow from a NE to ENE direction, became known as trade winds long ago when clipper ships
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Chapter 4:Hazard Analysis—High Wind Storms
carrying cargo depended on the broad belt of Easterly winds encircling the globe in the
subtropics for fast passage.
High winds from trade winds, which blow 70% of the time (Kodama 1998), Kona winds
(30% of the time), and rare winds from hurricanes and tropical storms passing through
Hawaiian waters all affect the island of Hawaii. (Fletcher 2000) Tradewinds predominate
from the northeast and generally range from 10 — 25 miles per hour, although occasional
extreme events reach 40 - 50 miles per hour. North Pacific High pressure systems are
responsible for the majority of the gusty trade wind episodes over Hawaiian waters, which
commonly persist for several days before tapering off.
Strong, gusty trade winds can cause problems for mariners. These strong trades blowing from
the NE through East funnel through the major channels between the islands--Kauai, Kaiwi,
Pailolo, Kalohi, and Alenuihaha Channels--at speeds 5-20 knots faster than the speeds over
the open ocean. Mariners and beachgoers must exercise good judgment prior to entering the
waters exposed to strong trades, especially in the major channels.
2:00 a.m.HST 2:00 p.m.HST
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The winds during the day on l lawai'i Island vary in a
consistent fashion.These examples show mean wind flows
at 2:00 A.M.and 2:00 P.M.,based upon a 42-day sample
from 50 portable automated weather stations.The results
comply with meteorological predictions for the behavior
of air abort a heated island in a trade wind flow.
Figure 4-1. Typical diurnal wind patterns on Hawaii Island
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Chapter 4:Hazard Analysis—High Wind Storms
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Figure 4-2. Monthly wind roses for Hilo International Airport(shown for each quarter)
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Chapter 4:Hazard Analysis—High Wind Storms
4.1.3 Kona Storms
Kona winds occur as light and variable winds from the SW or SSW,most often during winter
months when trade wind circulation diminishes, and as strong generally southerly winds
when storm systems move across Hawaiian waters. Stormy Kona winds are most likely to
occur when a low pressure center is located within 500 miles NW of the islands and has an
unusually low central pressure, below 1000 millibars for the subtropics. Damaging Kona
winds have reached velocities of 50 miles per hour for several days on end. The western or
leeward sides of the islands, then, become windward in this case, as the predominant wind
pattern is reversed. Kona storms generally form in the region bounded by 15° - 35°N and
175° E — 140° W and move erratically, though with a slow tendency toward the west.
(Kodama 1998) These storms are persistent and can last up to two weeks.
On land, effects of strong Kona winds can be very dramatic. It is not uncommon for trees to
be uprooted, branches downed, and scores of roofs damaged. When reinforced by
mountainous topography, downslope winds can increase and can be very destructive to land
in low lying areas. During this time, considerable damage can be inflicted to boats caught in
the open ocean or boats anchored in SW exposed anchorages. Coastal erosion can also result
from the extended periods of heavy rain, strong surf and high winds.
4.1.4 Tropical cyclones:
In the Central Pacific Ocean tropical cyclones are classified according to their strength as
follows:
• Hurricane: An intense tropical weather system with a well defined circulation and
maximum sustained winds of 74 mph(64 knots) or higher.
• Tropical Storm: An organized system of strong thunderstorms with a defined
circulation and maximum sustained winds of 39 to 73 mph (34-63 knots).
• Tropical Depression: An organized system of clouds and thunderstorms with
defined circulation and maximum sustained winds of 38 mph(33 knots) or less.
Historically, most tropical cyclones have passed the Hawaiian Islands to the south. Because
they spin counter-clockwise in the Northern Hemisphere, east-facing coastlines in Hawaii
receive the brunt of strong onshore winds as storms approach the islands,while the south and
west coastlines feel onshore winds as the storms pass to the west. The highest wind speeds,
however, may occur on the side opposite the storm approach, as downdrafts accelerate
downslope as they descend over the mountainous terrain. Even so, coastlines facing the
passing storms usually are adversely impacted by both wind and storm surge damage.
History has shown that the islands do not have to take a direct landfall from a cyclone to
sustain a high level of damage. Wind strength, storm radius of maximum winds, timing, and
proximity, are important factors that control storm impact.
4.2 Significant Historical Events
By far the most notable documented high wind event from a winter storm to affect Hawai'i
County was that of January 1980, which caused damages of $42 million. (Disaster
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Chapter 4:Hazard Analysis—High Wind Storms
Declaration DR-613-HI) The loss on Hawaii Island was $11.7 million. Agriculture —
macadamia, coffee, foliage and flower farms — had major losses. Maui was also declared a
disaster area (Haraguchi, 1980).14 Other winter storms have caused much less, but more
localized effects, with flooding the main problem. The flood insurance maps are intended to
reflect the stream flooding from these events as well as coastal effects.
4.3 Probability of Occurrence
According to the National Weather Service (NWS) (Heffner, 2002), the monthly counts of
high wind events impacting the populated lower slopes and elevations of Hawaii County
over a ten year period indicate the months of"winter" storms to be the most active. A high
wind event by NWS definition includes sustained winds or frequent gusts of 40 mph or
greater.
For Hawaii County, 28 events were identified during the period January 1992 through Octo-
ber 2002. The typical breakdown of events by month is:
High Wind Events in Hawaii County:Jan. 1992-Oct. 2002
s
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LU
W
IOU 5
L
m 4
0
0 3
L
QJ
C0 2
E L
Z 1
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 4-3. High Wind Events for Hawaii County by Month,January 1992 to October 2002
Thus, the average is a little over 0.2 such events per month, but for the "season" of
occurrences (December through April), it averages 0.4 per month, island-wide.
4.4 Risk Assessment
The distinction between the tropical cyclone hazard and high wind hazard is illustrated by the
hazard curves for the Hawaiian Islands shown in Figure 4-4. The figure shows that the
relatively low wind speeds that occur more frequently are more likely to be from non-tropical
cyclonic winds while the relatively high but less frequent wind speeds are more likely to be
14 Haraguchi,Paul, 1980,`Storm of January 8- 10, 1980,State of Hawai'i,"DLNR,December 1980.
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Chapter 4:Hazard Analysis—High Wind Storms
caused by tropical cyclones. The transition is for peak gusts around 68 mph and an average
return period of around 30 years,therefore just below hurricane strength.
160
140 Tropical
Cyclone
Events
120
c 100
E
80
Y
Non-Tropical
R
Wind Events
a. 60
40
20
0
10 100 1000
Return Period,Years
Figure 4-4. Wind hazard curves for the Hawaiian Islands for Tropical Cyclone and Non-Tropical Winds
There is no objective model that is used solely for the purpose of evaluating economic losses
due to high wind events without regard to tropical cyclone occurrences. The future risk of
such events is projected to be roughly equivalent to the historic losses of past events.
4.5 Mitigation Strategies
4.5.1 Previous/Current Efforts
Hawaii design wind pressures have changed over the years in the building code. The
Uniform Building Code (UBC) design windspeed was based on an analysis of Honolulu
weather station data by H.C.S. Thom in 1968 without consideration of hurricane history. The
UBC wind loadings have historically lagged the ASCE 7 standard with respect to hurricane
hazard because almost all the UBC constituent states were not in hurricane regions (except
for Hawaii,which was only recently recognized after Hurricane Iwa and Iniki).
Table 4-1. Design Wind Pressures per UBC/IBC Code Years
Building Code Years Design Wind Pressure at IOM height*
IBC 2003 26 psf
UBC 1991 to 1997 30 psf
UBC 1982 to 1988 26.5 psf
UBC 1958 to 1979 15 psf
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Chapter 4:Hazard Analysis—High Wind Storms
*Based on component and cladding wall pressures,for an enclosed building,near the
coastline,not near a comer of the building.
The critical benchmark year identifying structures previously designed to an inadequate wind
pressure would be 1985,the date of Hawaii's adoption of the 1982 UBC edition.
4.5.2 Future Plans
Future mitigation plans for high wind storms will be discussed in the proceeding chapter on
Hurricanes, as Hurricanes pose a much greater threat to loss of property, infrastructure, and
life.
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