Usgs geologic Investigations Series i-2761, Oahu

54

Pearl Harbor

T

he entrance to Pearl Harbor separates the beaches along the coastal

plain of Ewa to the west from the isolated and less accessible shores

of Hickam Air Force Base and the Honolulu International Airport to the

east. Fringing reefs parallel the coastline extending nearly one mile off-

shore, except at the mouth of the harbor. Here, sediments transported from

the central plain of Oahu by streams descending from both the Koolau and

Waianae ranges meet the sea and form an embayed wetland environment

ringing the shores of Pearl Harbor. The shallow nature of the broad fring-

ing reef at Ewa is effective at intercepting waves and dissipating their ener-

gy far offshore of this low-lying and very gently sloping coast. Extensive

land reclamation has transformed the coastline near the airport and Pearl

Harbor entrance. The coastline in the vicinity of Ewa Beach appears to be

actively eroding even under relatively low wave activity. Seawalls and rock-

piles surround the harbor entrance and airport runways in the east, while

small jetties, groins, and seawalls have been emplaced along the narrow

sandy beaches of Ewa.

The Overall Hazard Assessment (OHA) for the Pearl Harbor coastal

region ranges from moderate to high (5) at the mouth of Pearl Harbor to

moderate (4) west of Oneula Beach Park, primarily as a result of the low

slopes found along the seaward edge of Oahu’s largest drainage basin. The

low-lying coastal plain surrounding Pearl Harbor is susceptible to wave

inundation so the tsunami threat is ranked high. This coast becomes

increasingly arid to the west, and as a result, the stream flooding hazard is

reduced from high near the harbor mouth to moderately high on each side

and moderately low west of Oneula. Although annual wave heights reach

4-6 ft from south, southwest, and refracting northwest swell, the hazard

from high waves is reduced to the west of Oneula Beach Park, where shal-

low fringing reefs extend far offshore and help to dissipate wave energy. As

a result, the high-wave hazard is moderately high east of Oneula and mod-

erately low to the west. Historically, the region has yet to experience the

direct hit of a hurricane, however, because it faces south it is vulnerable to

high winds and waves generated by tropical storms that commonly pass to

the west of Oahu. Erosion is moderately low east of the harbor mouth, and

high along the Ewa Beach coastline, where a recent proliferation in sea-

walls and revetments has exacerbated beach loss. It is moderately high

along Oneula and moderately low to the west. The threat of sea-level rise

is ranked moderately low, while the seismic hazard is moderately high due

to the nature of the unconsolidated sediments upon which portions of this

area are developed. The volcanic/seismic hazard along the Pearl Harbor

coastline is moderately high.

Pearl Harbor

The low Ewa coastal

plain is fringed by

narrow sandy beaches

and a wide, shallow

reef flat offshore.

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55

Barbers Point

T

he remote coast of Barbers Point is a broad nearly flat marine terrace

underlain by reefs formed by the Waimanalo Stand of the Sea,

125,000 yr ago. It is also one of the driest areas on Oahu. Small 3-6 ft rocky

outcroppings of fossil reefrock extend from Nimitz Beach in front of

Barbers Point Naval Air Station around the point to the oil refinery in the

west. This coastal zone has a very low slope. Poorly defined fringing reefs

run the length of this coast where the nearshore zone is largely comprised

of a hard coral and calcareous algae reef bottom interrupted in a few loca-

tions by small sand channels. Because Barbers Point faces due south, it is

susceptible to Kona storms, tropical storms, and waves coming from the

south. It also extends sufficiently to the west that it can experience signif-

icant swell wave energy refracting around the island from the north. The

Alaskan earthquake of 1946 brought 12 ft tsunami waves to Barbers Point,

while just to the east at Ewa Beach, the highest recorded tsunami runup

was 3 ft.

The Overall Hazard Assessment (OHA) for the Barbers Point coast is

moderate (4), primarily due to the high storm and tsunami hazards. The

low coastal slope of Barbers Point makes this region vulnerable to inun-

dation and flooding associated with high waves and tsunamis. As a result,

the hazard due to tsunami is high. The storm threat is also ranked high

along this coast, as hurricane inundation, including that from Iniki (1992)

and Iwa (1982), has historically been damaging to this and other south-

facing coastlines. Stream flooding, however, is ranked moderately low to

low because it is very arid in this region and far removed from the

drainages of the Koolau and Waianae ranges. The hazard due to high sea-

sonal waves is ranked moderately low because this region largely experi-

ences moderate wave energy from south swell. The hazard due to erosion

is slight and is ranked moderately low along the rocky cliffs of Barbers

Point and low in front of the isolated sandy beach at Nimitz Beach. The

threat from sea-level rise is somewhat mitigated by the rocky coastline and

is ranked moderately low here as in most of Oahu. The volcanic/seismic

hazard is ranked moderately high, as is the southern half of Oahu, due to

a history of occasional significant seismic activity.

Barbers Point

Oahu’s oil refineries are built on this low-lying Barbers Point coastal plain,

which rises only ~1–2 m above sea level.

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