Usgs geologic Investigations Series i-2761, Oahu

52

Oahu

Strong winds

W

inds on Oahu originate from three main sources: trade winds,

Kona winds, and hurricanes or tropical storms. Northeast  trade

winds are dominant throughout most (70%) 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 vari-

able 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 dur-

ing the winter and spring seasons and have reached velocities of 50 mph

for several days on end.

Damaging winds on Oahu and in Hawaii are most commonly associat-

ed with passing tropical cyclones (hurricanes, tropical storms, and tropi-

cal depressions). Historically, most tropical cyclones have passed the

Hawaiian Islands to the south and west. Because they spin counter-clock-

wise 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 localized microbursts and downdrafts accelerate

downslope as they descend over the palis (cliffs).As Hurricane Iwa passed

west of Oahu the highest winds were observed at the base of the Pali in

Kaneohe. Even so, coastlines  facing the passing storms usually are

adversely impacted by both wind and storm surge damage, like the

Waianae Coast was as Hurricane Iniki passed to the west, before slamming

into Kauai. History has shown that the islands do not have to take a direct

hit from a storm to sustain a high level of damage. Wind strength, storm

diameter, timing, and proximity, are important factors that control storm

impact to the coastal zone.

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53

Honolulu

KALIHI  CHANNEL

G

CS

T

SF

W

S

E

SL

V/S

Hazar

d 

Type:

OHA

S

Dfr

Se

D

D/Bfr

e

D

Se

Dfr

D/B

fr

Dfr

Kapiolani  Beach Park

2

3

2

3

3

4

2

3

5

HONOLULU

Hazard Intensity:

EXPLANATION

[For explanation of hazard types, see

 “Notes on Specific Hazards” in the Introduction]

Overall Hazard Assessment  (OHA)

G - Geology:

fr-fringing reef;  br-barrier reef;  e-embayed coast;  w-wetland

B-Beach;  S-Stream;  R-Rocky;  H-Headland;  D-Developed

Low - 1       2          3        4 - High

<20% -1      2       3 - >45%

W - High Waves

SF - Stream Flooding

S - Storms

E - Erosion

SL - Sea Level

V/S - Volcanic/Seismic

- No Data

T - Tsunami

CS - Coastal Slope

1         2          3         4          5         6          7

Low

High

Base Credit:  USGS 1:50,000 Honolulu, Hawaii  5420  IV  W733  Edition 1-DMA

Prepared in cooperation with Hawaii Coastal Zone Management Program, Hawaii Office of Planning and the National Oceanic & Atmospheric Administration

157°50'W

21°1

9'N

Cartography by Manoa Mapworks, Inc.

SCALE:  1:50,000

Contour Interval = 40 meters

0

0.5

1 mile

0

0.5

1.0

1.5 km

Island of Oahu

Coastal Hazard Intensity

H

onolulu

T

he heavily developed coastal metropolis of Honolulu and Waikiki is

built on a low-lying coastal plain, which was submerged by a higher

relative sea level approximately 125,000 yr ago and again as recently as

1,500 to 4,000 yr ago. In response to erosion and other hazards, an exten-

sive complex of shoreline protection structures, including groins, seawalls,

and revetments has been implemented to protect the densely developed

seaside. Once home to Hawaii’s legendary surfer and Olympic gold-medal

swimmer, Duke Kahanamoku, Waikiki attracts many visitors each year

who relax on the beautiful white sand beaches. Ironically, these beaches

are also largely a human fabrication. Early in the 1900s, sand was import-

ed to create an extensive artificial beach where only a narrow sand strip

fronting coastal wetlands and tidal marshes formerly existed. They con-

tinue today to be nourished by sands brought in from elsewhere on the

island.

The coast steepens at the foot of the prominent and geologically-young

crater of Diamond Head, known as Puu Leahi. A nearly continuous fring-

ing reef parallels the coast at Diamond Head, and widens to the west. The

bottom remains shallow far offshore along this entire region. The reef has

been dredged to form a channel at the outflow of the Ala Wai Canal, which

empties the Manoa and Palolo streams, and on both sides of Sand Island,

to provide for commercial shipping. Land reclamation on the reef has

occurred at the airport, Sand Island, and Ala Moana.

A moderate to high (5) Overall Hazard Assessment (OHA) for the

Honolulu coastal zone is principally dictated by the low coastal slope

which is especially susceptible to damage resulting from tsunami, stream

flooding, hurricane storm surge, and seasonal high-wave flooding.

Tsunami and storms are ranked high while stream flooding and high sea-

sonal waves are moderately high. These rankings are supported by a his-

tory in Honolulu of severe

flooding from both storm

surge and stream runoff

from the steep surrounding

hillsides of the Koolau

Range. Although Honolulu

has yet to experience a direct

hit from a major hurricane or

tsunami, a complacency may

exist among its inhabitants

that hurricanes and

tsunamis are not major

threats to this coast. Facing

southwest, however, coastal

Honolulu is extremely vul-

nerable to strong winds and waves generated by tropical storms that most

frequently pass the Hawaiian Islands just west of Oahu. While observa-

tions of tsunami flooding have not exceeded 8 ft, Honolulu has experi-

enced extraordinary coastal development within the elevation range of

historical tsunami runups. The threat from high waves is moderate to high

because this region regularly receives wave heights on the order of 6 ft

from south swell.As recent as the summer of 1995, however, waves as high

as 12 ft pounded the Honolulu shoreline causing significant flooding and

erosion along the waterfront of Waikiki’s seaside hotels and Kuhio Beach

Park. Erosion is high at the foot of Diamond Head (at the southeast corner

of this map) and moderately high throughout Waikiki, where seawalls and

groins have been placed to reduce coastal erosion.West of Ala Moana Park,

erosion is reduced to moderately low, because of the buffering effects of

the wider fringing reef offshore. Sea-level rise in this region is ranked

moderately low relative to the higher rates on Maui and the Big Island.

Seismicity along the Honolulu coastline, like the southern half of Oahu, is

ranked moderately high because it is within the Molokai Seismic Zone.

Honolulu

TOP: Waikiki, about 1958

BOTTOM: Waikiki, about 1984

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