Usgs geologic Investigations Series i-2761, Oahu

Nanakuli

Ko Olina

Beach Park

56

Nanakuli

T

he Nanakuli coastline from Maili Point to the oil refinery and power plant south of

Honokai Hale, marks the southernmost portion of the west coast of Oahu. Small pock-

et beaches of sand and/or basalt boulders lie between lengthy expanses of 6-10 ft high rocky

cliffs of old reefrock and/or beachrock. Several sandy beaches are perched on top of reefrock

and beachrock at the water’s edge. Offshore, the seafloor is largely a hardground of reef sepa-

rated by occasional small sand channels and a few larger sand fields inside an extensive com-

plex of submerged fringing reefs. The slopes are relatively gentle along this coast but steepen

around Maili Point, where signs of headland slumping can be seen on Puu o Hulu Kai.

Seemingly protected behind Maili Point, the town of Nanakuli actually has experienced con-

siderable inundation from high waves. Tsunami heights of 20 ft from the 1946 Alaskan earth-

quake were recorded in Nanakuli, whereas farther northwest in Waianae (see Waianae map)

the same event produced only 12 ft floodwaters.

The Overall Hazard Assessment (OHA) for the Nanakuli coastline is moderate to high (5)

in the low-lying coastal town of Nanakuli and at the two isolated beaches just north and south

of Kahe Point Beach Park. These areas are most susceptible to tsunami, stream flooding, high

winds, and storm surge. The OHA is moderate to low (3) at Kahe Point and along the small

rocky shore where Pili o

Kahe Gulch meets the sea.

The OHA is moderate (4)

south of Kahe Point Beach

Park to Ko Olina, and along

Maili Point. Tsunami and

storm hazards along this

coast are ranked high from

Ulehawa to Nanakuli Beach

Park, just north of Kahe

Point Beach Park, and the

southern region near the

oil refinery and power

plant, where the coastal

slopes are lowest. Both are

ranked moderately high

along the steeper rocky

areas between. Stream

flooding is ranked moder-

ately high for the Nanakuli coastal plain and the beach seaward of Honokai Hale, and moder-

ately low and low for the steeper areas. The high wave hazard is ranked moderately high along

low-lying Nanakuli, which receives larger winter waves, and moderately low to the south

where the slope is relatively steeper and the coast farther removed from influence of winter

swell. Erosion is moderately high for sandy and rocky low-lying beaches and moderately low

where rocky cliffs help mitigate loss of coastal land. Sea-level rise is moderately low through-

out this coast, except at the isolated and steep rocky outcrop just south of Nanakuli Beach Park

where it is ranked low. The volcanic/seismic hazard here, like the southern half of Oahu, is

ranked moderately high because of its proximity to the Molokai Seismic Zone and history of

seismicity during the last 200 yr.

The artificial coves of Ko Olina provide sandy beaches along an

otherwise rocky shoreline.

---

B

eautiful sandy beach embayments separated by spectacular basaltic headlands mark

the Waianae coast. Streams, most channelized into drainage canals, empty deeply

incised valleys on a low-lying but narrow coastal plain of emerged fossil limestone reef rock

that formed about 125,000 yr ago when sea level on Oahu was higher than present. Seawalls

have been built along many of the seaside properties, and large boulder breakwaters guard

Pokai Bay and the Waianae Small Boat Harbor. Beach widths at Maili and Makaha can vary by

145 ft annually due to seasonal changes in wave energy. In the long term it appears that the

central portions of these beaches have accreted at the expense of their margins. Fossil reefs

separated by scattered sand-rich channels and scoured surge channels lie offshore just land-

ward of a relatively extensive fringing reef. Except for Lahilahi Point (Black Rock), a volcanic

headland, the majority of this coast maintains very low slopes, and as a result, has experi-

enced damaging floods from streams and inundation by hurricane storm waves. Facing

southwest, the Waianae coast has historically received significant wind and wave energy asso-

ciated with passing tropical storms that tend to track just west of Oahu as they pass the

Hawaiian Islands. Two storms of recent memory, Hurricanes Iwa (1982) and Iniki (1992) gen-

erated damaging high waves, and the associated storm surge produced coastal flooding to an

elevation of 11 ft above mean sea level and higher.

From Makaha to Maili

Point, the Overall Hazard

Assessment (OHA) is high

(6) where hazards due to

tsunami, stream flooding,

high waves, and storms are

individually ranked high,

largely because of the low-

lying, gradually sloping

coastal plain. Flood inun-

dation heights of 12 and

14 ft were recorded in this

region  during  the 1946

and 1957 tsunamis,

respectively. Flash floods

and stream floods, some-

times  lasting two or three

days, have occurred rela-

tively frequently along the Waianae coast. As a result, stream flooding hazard is ranked mod-

erately high south of Maili, and high north of Maili Beach Park, where several streams drain

inland valleys along the low coastal plain. Only at the headlands of Lahilahi and Maili Points

is the overall hazard assessment reduced to moderate to high (5), due to the moderately high

tsunami threat and moderately low erosion hazard there. South of Maili Point, where the

tsunami, high waves, and storm hazards are reduced to moderately high, and stream flooding

is moderately low, the overall hazard is ranked moderate (4). Erosion is ranked high at the

west end of Ulehawa Beach Park and Makaha, and moderately high along the low-lying beach

embayments on either side of Lahilahi Point. Sea-level rise is ranked moderately low and low

throughout the region. The volcanic/seismic hazard here in the southern half of Oahu is

ranked moderately high.

57

Waianae

Waianae

Waianae Small Boat Harbor and Lahilahi Point (Black Rock) to

the west.

---