Usgs geologic Investigations Series i-2761, Oahu

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Oahu

Stream flooding

F

loods from stream overflow and high surface runoff (non-channel-

ized flow) are common on all of the Hawaiian Islands and are pri-

marily a result of torrential rains that fall on the steep slopes and small

drainage basins characteristic of island drainage systems. The most fre-

quent and severe flooding occurs where steep sloping hillsides abruptly

meet flat or low-lying coastal plains, such as those found in Waimanalo,

Kailua, Kaneohe, and Laie. 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. Nearly every year flash floods and pro-

longed flooding damage property, homes, highways, and crops on each

island.Although floods are caused by natural events, most flood damage is

a result of human occupation and development of lands that are suscepti-

ble to flooding without having provided for adequate protection. As of

1983, floods in Hawaii had claimed more than 350 lives and caused

more than $475 million in damage.

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 in fell in an area where the average annual rainfall is only

2 in. In Ewa, 12.5 in fell in 7 hr on the 5th of that month, inducing flooding

of the low coastal plain. On October 25, 1993, 2–4 in of rainfall caused flash

flooding and extensive street flooding throughout the Honolulu area. On

the windward side of Oahu, flooding has been common after heavy pre-

cipitation such as on April 12, 1994, in Kahuku, November 26, 1992, in

Kaneohe, and  October 11, 1992. The heaviest rainfall during the last

decade in Kaneohe occurred on October 15–16, 1991, when 15 in fell in 48

hr leading to intense flash flooding. 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 led

to $786,000 damage and January 1968 flooding in Pearl City, which caused

$1.2 million damage. The hazards resulting from stream flooding are sig-

nificant on all of the islands and should be seriously considered for plan-

ning development, as well as for recreation activities.

Many formerly flood-prone streams on Oahu have now been artificial-

ly channelized to protect development situated on the adjacent flood plain.

Although the threat of flood hazard is reduced by this measure, there is a

resultant destruction of wetland and stream channel ecosystems that

occur. The result is to promote building on former floodplains and the

destruction of the riverine environments and estuaries that connect the

island’s watersheds and reefs.

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51

High waves

H

igh waves are common along Hawaiian shores, making the islands

perhaps the most popular destination for big wave surfing in the

world. Lying in the center of the North Pacific Ocean, Hawaii receives high

waves from distant storms in the northern and southern hemispheres and

from tropical cyclones passing in the vicinity. The hazards associated with

high waves include debris overwash, flooding, erosion, high wave energy

and turbulence in the nearshore zone, and strong currents. Waves that

reach the shoreline are determined by the energy inherent in the

approaching swell (a function of wave height and wave length-the distance

between successive wave crests), shoreline aspect, slope, morphology and

geology, and offshore characteristics including seafloor depth, morpholo-

gy, and barriers (islands, rocks, reefs, sandbars). When deep-water ocean

swells encounter the shallow island margins they rise to great heights

because their tops stack up on their slower moving bottoms due to friction

along the shallower seafloor. Because the contact between deep water and

the shallow margins around the Hawaiian Islands is abrupt, surface waves

can grow very tall, very rapidly. Large waves tend to travel in sets, and after

breaking they rush up onto the beach temporarily elevating the sea surface

near the shoreline. Rip currents form as the water that is pushed up on the

shore by successive large waves, tries to flow back to the sea.

The largest waves that reach Oahu generally arrive in winter as a result

of intense storm activity in the North and Northwest Pacific. The high

amplitude and long wavelength associated with these swells create very

large waves with considerable energy. Along the north shore of Oahu, it is

common to see wave heights between 15–20 ft annually from winter swell.

However, wave heights of 50 ft have been reported, for instance in

December 1969 and January 1998. This high wave energy pounds the

north and west shorelines as it hits head on, but it can also refract and pro-

duce high waves around the entire island. Often, winter north and north-

east swells wrap around Makapuu Point and generate waves at Sandy

Beach that are as high as the largest summer surf found there. Periodically,

as winter mid-latitude storms track northeast of the islands toward the

mainland, they will generate swell that impacts the east sides of the

islands. In the summer, south-facing shorelines receive 4–6 ft swell from

distant storms in the Southern Hemisphere. South swells tend to have less

energy than winter swells, but because their source can be as far away as

New Zealand, they can have very long periods. 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 ft.

High waves from hurricanes present a more complex hazard, as they

may coincide with high tide, storm surge, and wind and wave setup, to pro-

duce a combined threat. High waves from hurricanes generally occur dur-

ing hurricane season between June 1 and December 1. High waves from

hurricanes most often hit the eastern shores as hurricanes approach the

islands from the east, and south- and west-facing shorelines as the storm

passes to the south and west. Hurricane generated waves have exceeded 15

ft along east Oahu and 20 ft 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.

Oahu

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